Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
Alfalfa
Alfalfa weevils emerge and lay eggs in alfalfa stems in Mid-April. The larvae are yellowish-green with blackheads. The easiest way to scout for this pest is to randomly collect 30 stems from the field and shake them into a bucket. The economic threshold for alfalfa weevil is determined based on the height of plants, the value of the forage, and the cost of insecticidal treatment. A threshold chart can be found on Penn State Extension website https://extension.psu.edu/alfalfa-weevil (Figure 1).
Figure 1. Economic threshold for Alfalfa Weevil. Source: Penn State University
Small Grain
Cereal leaf beetle adults become active in Maryland around mid-April. Adults will lay eggs and larvae will start feeding around the end of the month. Scouting should be done away from the field edge since they tend to clump in fields and near edges. Check tillers of 50-60 randomly selected plants per field and count the number of eggs and larvae. The economic threshold is reached when there is an average of one or more larvae on 25% of tillers. The populations can be spotty, so checking individual fields is often necessary.
Several species of aphids will start to appear in small grains and other spring crops in April, depending on the weather. The most common species in our area are English grain aphid, bird cherry-oat aphid, corn leaf aphid, and the greenbug. Springtime feeding damage can cause discoloration on the leaves and shriveled heads. To scout for aphids, examine one linear row-foot at ten sites within the field. The economic threshold for aphids in wheat in pre-heading stages varies based on the aphid species present. Still, the general rule is treatment is recommended if there is an average of 150 aphids per linear foot of row, with no natural enemies present. For information about species identification and thresholds, check out the “Early Aphid Occurrences: a Possible Result of Warmer Winter Temperatures” article on the Maryland Agronomy News Blog, or contact your local Extension agent.
Andrew Kness and Erika Crowl University of Maryland Extension
Drones are becoming increasingly popular in agriculture for not only imagery, but product application. As a result, startup companies offering aerial pesticide application via drone are emerging. Farmers have taken interest in the technology and service for several reasons, but the main benefit to using a drone to apply crop production products in soybean and corn is that it offers a feasible method for in-season foliar product application to fields that are smaller, fragmented, or irregularly shaped, without the potential for damaging the crop with a ground spray rig. Additionally, drones may have an advantage over helicopters or fixed-winged aircraft in small fields because they are more nimble and have the potential to achieve application to field edges that would be missed by aircraft. Finally, drones are much less intrusive to curious neighbors who often raise concerns when they see an aircraft applying products to fields.
Although drones offer a lot of potential, there is very little published data on their efficacy to apply products, which is cause for question and concern as to if drones are a viable and worthwhile means of applying products, such as pesticides, to corn and soybeans. Additionally, drones tend to lack spray tank capacity, so spray volumes with drone applications are low (1-2 gallons per acre). These low-volumes may pose challenges in achieving adequate coverage; however, products are applied at a greater concentration. This research project, funded by the Maryland Soybean Board, aims to collect data regarding drone spray efficacy in corn and soybean.
Methods
Two different drones (Figure 1) were used to apply water to standing corn and soybeans in fields located in Harford County, Maryland on August 27, 2020. Corn was planted on 30-inch rows and at the R4 growth stage during application. Full season soybeans were planted on 15-inch rows and at R6 during application.
The drones were operated by certified pilots who offer custom pesticide application in field crop and nurseries. Weather conditions were sunny and 91°F with variable winds out of the Northeast at 5 mph, gusting to 25 mph.
Prior to spraying, water-sensitive cards (Syngenta AG®, Basel, Switzerland) were placed within the crop canopy at various heights in sets of five replicates. Three heights were used for both corn (top: third leaf from tassel; middle: ear leaf; and bottom: third leaf from bottom) and soybean (top: upper most fully expanded leaf; mid: middle of canopy; and bottom; 1 foot up from soil). When water droplets hit the card, the card arrests the spread of the droplet and stains the card blue. The cards were retrieved then scanned into the computer and analyzed using the software DepositScan (USDA-ARS, Wooster, OH), which calculates percent spray coverage, droplet density, and droplet size.
Each drone applicator flew on water at approximately 4 feet above the canopy at 16 mph onto corn and soybeans at a rate of 1 gallon per acre for Drone 1 and 1.5 gallons per acre for Drone 2 using TeeJet® TXA8002VK nozzles.
Results & Discussion
Due to the extremely high humidity, close canopy, and transpiration rate of the soybean plants, the middle and bottom spray cards in the soybean plots were difficult to read. As a result, data for bottom soybean cards were omitted.
As to be expected, spray coverage was highest in the top of the canopy for both corn and soybean for both drones (Figure 2). Drone 1 achieved 1.61% and 2.40% coverage in the top canopy of corn and soybean respectively. Drone 2 also achieved greater coverage in the top canopy of soybean (1.50%) than corn (1.10%). Both drones delivered nearly identical droplet density (Figure 3) in the top canopy of both corn (18.9 drops/cm2) and soybean (22.0 drops/cm2). Droplet size (Figure 4) in the top canopy of corn for Drone 1 was 146 µm vs. 134 µm for Drone 2 and 171 µm for Drone 1 vs 148 µm for Drone 2 in soybean top canopy.
Figure 2. Average percent spray coverage for water sensitive spray cards placed in the top, middle, and bottom canopy of corn and soybean for two drones.Figure 3. Average droplet density on water sensitive spray cards placed in the top, middle, and bottom canopy of corn and soybean for two drones.
Spray coverage dropped only slightly for Drone 1 (to 1.57%) in corn from top to middle, but much more in soybean (down to 0.68%), likely due to the dense canopy. Coverage dropped to 0.42% for Drone 2 at the ear leaf in corn and down to 0.20% in the mid canopy soybean. Drone 1 achieved 21.65 drops/cm2 for the ear leaf in corn whereas Drone 2 delivered 7.00 drops/cm2. In mid canopy soybean, droplet density was similar for both drones (13.5 drops/cm2 for Drone 1 and 10.17 drops/cm2 for Drone 2). Droplet size for both drones were similar between top, middle, and bottom corn. Droplet size decreased for both drones in middle canopy soybean to 122 µm for Drone 1 and 87 µm for Drone 2.
Figure 4. Average droplet size for Drones 1 and 2 on water sensitive spray cards placed in the top, middle, and bottom canopy of corn and soybean.
Spray coverage and droplet density were similar for both drones in bottom canopy corn. Spray cards for bottom canopy soybeans were not analyzed due to artifacts created by canopy humidity.
It is generally recommended that droplet density should be between 20-30 droplets/cm2 for adequate insecticide application, between 20-40 droplets/cm2 for herbicide application, and at least 50 droplets/cm2 for fungicide applications. Based on these assumptions, both Drones have the capability to deliver densities at or over 20 droplets/cm2 in the upper canopy, which may be adequate for herbicide or insecticide applications. However, a greater density of droplets needs to be achieved for adequate fungicide application. Previous research has shown that flight velocity plays a significant role in droplet density and spray coverage (Hunter III et. al., 2020). Spraying slower would likely improve coverage and efficacy of fungicides applied via drones, which will be an area of future research for this project.
Example Spray Card Images (click to enlarge)
Acknowledgements
We would like to thank the Maryland Soybean Board for funding this research, A1-Aerials, K Drone, and Crowl Brothers, Inc. for collaborating on this research project.
Literature Cited
Hunter JE III, Gannon TW, Richardson RJ, Yelverton FH, Leon RG (2020) Coverage and drift potential associated with nozzle and speed selection for herbicide applications using an unmanned aerial sprayer. Weed Technol. 34: 235–240. doi: 10.1017/ wet.2019.101
Emily Zobel, Agriculture Agent Associate University of Maryland Extension, Dorchester County
Soybean: Late double-crop soybean fields that are next to corn may still be at risk for defoliation and stink bug. The thresholds for stink bugs through R6 are 5 bugs per 15 sweeps, and defoliation in R6 needs to be approaching 20% before treatment is advised.
Wheat: The Hessian fly is not a significant pest in the Mid-Atlantic States because small grains usually are planted after the adult “fly-safe” date. If planting early, consider planting a resistant variety, since there is no insecticidal control that can be applied once the field becomes infested. The “fly-safe” date for areas across Maryland is the following: September 30 for the Mountain region, the first week in October for the Piedmont region, and the second week in October for the coastal plains.
Spotted Lanternfly adults are out and are laying eggs. If you are moving equipment in and out of quarantine areas, please check equipment for this invasive insect to reduce the spread. While it has been found in corn, soybean, and alfalfa, it is not considered a pest on these crops. However, their feeding has been harmful to grapes, hops, and tree fruits. If you observe any egg masses or insects which look similar to this, please try to collect them, and inform the Maryland Department of Agriculture at (410) 841-5920 or DontBug.MD@maryland.gov as soon as possible. For more information about spotted lanternfly can found on the MDA website.
Kelly Hamby and Galen Dively University of Maryland, Department of Entomology
Insect Resistance Management in Bt Crops: Transgenic crops expressing insecticidal toxins sourced from Bacillus thuringiensis (Bt) bacteria reduce yield loss and insecticide use, delivering economic benefits for growers. Because this breakthrough in pest management is considered a public good and insect resistance is the largest threat to Bt crops’ durability, insect resistance management programs were developed and mandated by the EPA prior to the release of Bt crops. These plans included planting untreated refuge crops at high enough acreage to produce many susceptible adult insects that could interbreed with and dilute the resistance from insects surviving Bt crops (Figure 1).
Figure 1. Susceptible (white) corn borers emerge from the untreated block refuge (yellow) planted on the side of the Bt field (green). Resistant (red) corn borers emerge from the Bt field (green) and interbreed with susceptible moths to produce moths with diluted (white and red) resistance genes.
In addition, crops were supposed to express Bt toxins at a high enough dose that insects with diluted resistance genes (white and red) would be killed, called a “high dose” strategy. Finally, pyramided hybrids that contain multiple toxins targeting the same pest were developed to make it more difficult for pests to overcome the toxins. EPA also required monitoring for insect resistance and mitigation strategies to implement once resistance was detected.
The Issue: When best management practices for Bt insect resistance management are followed, for example, European corn borer (Ostrinia nubilalis) management in the U.S., resistance development has been slowed. In fact, all single and pyramided Bt traited corn hybrids still provide 100% control of corn borers. However, for some pests [corn earworm (Helicoverpa zea) and Cry toxins] Bt toxins were less effective and products were not high dose. This issue was further compounded by poor refuge compliance, which lead to the development of refuge-in-a-bag (RIB) seed mixes to increase refuge acreage. This technology was designed based on corn rootworm biology and is not as good as a separate block refuge for most other target pests. Finally, while pyramided multi-toxin hybrids were developed, hybrids that contained a single effective toxin for the management of some pests continued to be marketed. This enables insects to develop resistance to a single toxin first providing a “stepping stone” to resistance in pyramided hybrids that contain the same or similar toxins because they can already survive on some of the toxins that are being expressed. In addition, the same Bt toxins are used in both corn and cotton, so corn earworm (also known as bollworm) goes through multiple generations of selection pressure in the same year, increasing resistance. Reports of caterpillar pests resistant to Bt corn and cotton in the U.S. have occurred since 2014 for fall armyworm, since 2016 for corn earworm, and since 2017 for western bean cutworm. However, none of these resistance reports triggered EPA’s current regulatory definition of pest resistance and no mitigation actions were taken. Therefore, the EPA released a draft document outlining proposed changes to reduce resistance risks (especially for non-high dose pests at heightened risk of resistance), to increase the longevity of currently functional Bt traits and future technologies, and to improve the current caterpillar pest (Lepidopteran) resistance management program for Bt corn and cotton (USEPA 2020).
Proposed Changes: Changes build off current insect resistance management plans and incorporate feedback and recommendations developed by a July 2018 Scientific Advisory Panel, independent academic scientists, the Agricultural Biotechnology Stewardship Technical Committee, the National Alliance of Independent Crop Consultants, and Syngenta Crop Protection, LLC (USEPA 2020). The EPA has 1) confirmed Bt resistance to specific Bt toxins in corn earworm, fall armyworm, and western bean cutworm, 2) proposed a new resistance definition for non-high dose pests that acknowledges their heightened risk of resistance and enables more rapid response to unexpected injury, 3) proposed a resistance monitoring approach that will use sentinel plots to monitor unexpected injury in addition to reported cases of unexpected injury in Bt crops, 4) proposed an improved resistance mitigation strategy with best management practices to respond to unexpected injury within the growing season and moving forward, and 5) will continue to require reporting on refuge compliance, unexpected injury, and insecticides targeting the pests that are also targeted by Bt (USEPA 2020).
Changes Under Discussion: In addition to the above changes, three additional changes have been proposed that require further discussion and stakeholder comment (USEPA 2020). The first focuses on reducing the acreage of products that no longer effectively manage resistant caterpillar pests and that share or have similar toxins as multi-toxin pyramided hybrids that still provide control. Therefore, the EPA is proposing a short term (~ 3 year time frame) phase down of hybrids that contain a single toxin for control of caterpillar pests, capping acreage planted in these hybrids to a minimum. These include field corn (Table 1), sweet corn (Table 1), and cotton products. In addition, non-functional pyramids that do not contain effective toxins for control of resistant caterpillar pests would have a longer term (~ 5 year time frame) phase down to minimal acreage (Table 2). Even with the potential phase downs Cry toxins will still be available for planting in pyramided hybrids that include the Vip3A trait.
To improve refuge compliance nationwide, the EPA proposes to increase refuge-in-the-bag (RIB) seed blend technologies to 10% refuge and maintain current requirements to plant a separate 20% block refuge in cotton producing areas (USEPA 2020). This should help insect resistance management for all pests managed by Bt and may be especially important for pests at heightened risk of developing resistance.
To further increase refuge compliance, especially in cotton producing areas, additional strategies have been proposed. For example, sale of Bt corn products requiring block refuge must be followed up with mandatory on-farm visits [conducted by industry (registrants)] to assess refuge compliance during the growing season, which will be conveyed to growers at the point of sale and be included in the grower insect resistance management agreement (USEPA 2020). Visits will be reported to the EPA. Farmers out of compliance with block refuge standards in cotton producing regions for one year will not be allowed to purchase Bt products, including RIB and block refuge products, for two years. Seed dealers will be required to keep grower IRM agreement records for 3 years, with audits that could result in losing the opportunity to sell Bt seed if signature rates or record keeping are noncompliant [conducted and enforced by industry (registrants)]. The industry (registrants) must ensure the availability of non-Bt elite corn hybrids for refuge plantings (USEPA 2020), which should improve the quality and yield of these plantings.
Potential Impacts to Mid-Atlantic Seed Dealers and Growers: Phase downs of single toxin and non-functional pyramid hybrids will impact hybrid availability and selection; however, these toxins (which control corn borers) will be available pyramided with Vip3A. If you are planting hybrids that require 20% block refuge (such as is the case with the single traited hybrids that are being phased out), a mandatory on-farm visit by the registrants and/or seed dealers may be required. Non-Bt elite corn hybrids will have to be made available for block refuge and refuge-in-a-bag seed mixes which should make yield more comparable to Bt plants.
US EPA. 2020. EPA draft proposal to address resistance risks to Lepidopteran pests of Bt following the July 2018 FIFRA scientific advisory panel recommendation. Memorandum EPA-HQ-OPP-2019-0682-0007. https://www.regulations.gov/document?D=EPA-HQ-OPP-2019-0682-0007
Maria Cramer, Galen Dively, and Kelly Hamby University of Maryland, Department of Entomology
It is not unusual to see groups of Japanese beetles feeding on corn silks, which is known as “silk clipping” Figs.1 and 2). While Japanese beetle numbers tend to peak in July, there are multiple beetles that may clip corn silks, and with later maturity field and sweet corn silking in August, it is important to still be on the lookout But how much of a concern is silk clipping, what should you be looking for, and what should you do about it?
Figure 1 (left). Japanese beetles feeding on corn silks. Figure 2 (right). Silk regrowth after clipping. Images: M. Cramer, University of Maryland
Silk clipping is often not as much of a concern as it initially appears. If silks are clipped after pollination, which occurs within the first 4-5 days of silk emergence1, kernel set will not be affected2. If clipping reduces the number of kernels, the kernels may develop to be larger and offset the reduction in number2. However, under drought conditions, yield loss from silk clipping is more likely2,3.
Drought slows silk emergence and pollination, which means there is a longer window where silk clipping can hurt yield. Indeed, severe drought stress can cause incomplete silk emergence and cause a mismatch between pollen shed and silks that results in nearly blank cobs1. Drought can also make it harder for plants to compensate for poor pollination1. If leaf rolling begins in the early morning and continues until evening1, the field is stressed enough to be of concern and it is important to scout for silk clipping beetles during the first several days of silk emergence.
The culprits. Japanese beetles are the most noticeable silk clippers in Maryland because they are large, shiny, and congregate in groups (Fig. 3). They are a sporadic pest4 and their populations will vary yearly. However, their populations may be higher in corn following sod, soybean, or perennial ryegrass or clover covercrops4. Other beetles that may clip silk include the western, northern, and southern corn rootworm adults (Fig. 4)5. Western corn rootworm (WCR) has several look-alikes that do not clip silks, so make sure check the stripes; WCR will not have crisp black stripes, but instead has smudged stripes.
Figure 3 (left). Japanese beetle. Image: E. Hodgson, Iowa State. Figure 4 (right). Adults of southern corn rootworm (left), western corn rootworm (middle), and northern corn rootworm (right). Image: Varenhorst, South Dakota State
Scouting. Silking typically begins 3 days after tasseling5, so plan your scouting accordingly. You want to evaluate the silk stage and pollination. Silks naturally senesce about 10 days after emergence, browning and drying out. At this point, pollination can no longer occur1. To determine if green silks have been successfully pollinated, you can dissect the ear and do a shake test. Pollinated silk starts to discolor and drop away at the base of the silk where it attaches to the ear. Bob Nielson with Purdue Extension has produced a great video describing the pollination shake test: https://www.youtube.com/watch?v=K7DiwD4N0T0&feature=youtu.be
You should scout if pollination is incomplete. When scouting, make sure you sample both the edges and the interior (at least 40 feet into the field); while you may see alarming numbers of Japanese beetles on the edge of the field, there are usually much fewer inside the field2. Sample a minimum of 20 corn plants in 5 locations spaced evenly though the field. Count the number of beetles per ear and measure the length of the silks.
Thresholds. For Japanese beetles, three conditions need to be met to before an insecticide application will pay off: 1) there are three or more beetles per ear, 2) silks are clipped to less than ½ inch in length, 3) and pollination is less than 50% complete4 (most silks in the field are still green and/or shake test indicates about half of the silks are still attached). Conditions are similar for rootworm beetles, but the threshold is five or more beetles per ear.
Treatments. Because broad-spectrum insecticides may cause flare ups of other pests (for example, aphids or spider mites), only spray if thresholds are met. Pollen-shed is a time when there are large numbers of beneficials in the corn field doing important pest control work (Fig. 5), and foliar sprays may decrease their numbers.
Figure 5. Lady beetle larva eating corn pollen. Image: M. Cramer, University of Maryland
For Japanese beetles, consider a perimeter spray if most of the damage is on field edges (where they tend to feed more heavily). Japanese beetles are difficult to control, but pyrethroids should provide some control (e.g., Baythroid®, Brigade®, Warrior II®, Hero®, etc.). Good adult corn rootworm control has been found for indoxacarb products (e.g., Steward®), pyrethroids (e.g., Warrior II®, Brigade, etc.), and neonicotinoid pyrethroid mixes (e.g., Endigo®)6,7. When using insecticides, always consult and follow the label.
If silk clipping by Japanese beetles is a consistent problem, consider cultural controls like avoiding ryegrass and clover cover crops. Because female beetles lay eggs more easily into soft ground, it is also possible to reduce egg laying in nearby fields by pausing irrigation during the peak of Japanese beetle activity4.
Steckel, S., Stewart, S. D. & Tindall, K. V. Effects of japanese beetle (Coleoptera: Scarabaeidae) and silk clipping in field corn. J. Econ. Entomol.106, 2048–2054 (2013). https://academic.oup.com/jee/article/106/5/2048/878220
Shanovich, H. N., Dean, A. N., Koch, R. L. & Hodgson, E. W. Biology and Management of Japanese Beetle (Coleoptera: Scarabaeidae) in Corn and Soybean. J. Integr. Pest Manag.10, (2019). https://academic.oup.com/jipm/article/10/1/9/5454734
DeVries, T. A. & Wright, R. J. Evaluation of Foliar Applied Insecticides for Control of Adult Corn Rootworm in Corn, 2015: Table 1. Arthropod Manag. Tests41, tsw080 (2016). https://academic.oup.com/amt/article/41/1/tsw080/2658080
DeVries, T. A. & Wright, R. J. Evaluation of Foliar-Applied Insecticides for Control of Adult Corn Rootworm in Corn, 2015C: Table 1. Arthropod Manag. Tests41, tsw096 (2016). https://academic.oup.com/amt/article/41/1/tsw096/2658095
Read and follow all label requirements for insecticides.
Soybean: Continue checking for defoliators, such as bean leaf beetle, Japanese beetle, grasshoppers, and caterpillars. Control may be needed if there is 15% defoliation on plants from bloom through pod fill. As we move into the heat of the summer, keep an eye out on your aphid population, which may increase quickly. The summer generation of soybean podworms are emerging. Fields next to maturing corn, have open canopies, are drought-stressed, or have recently had insecticide applied are at high risk for podworms. While worm feeding on flowers will not impact yields, feeding during pod development can. North Carolina State University has a great economic threshold calculator (https://soybeans.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html) .
Alfalfa: Continue scouting for leafhopper and blister beetles.
Sorghum: Sugarcane aphids showed up in fields last year in August. Check the underside of leaves for insects. The threshold is 50 aphids per leaf on 25 – 30% of plants. They have shown some resistance to pyrethroids.
Check head for head worms and fall armyworms once heads have started to flower. Check 10 spots per field, 5 plants per spot. An easy scouting method is to use a 5-gallon bucket and shake the head into it and then count the number of medium (1/4 – 1/2 inches) and large (> 1/2 inch) dislodged caterpillars. Texas A&M has a great threshold calculator that takes the grain price and treatment into consideration (https://extensionentomology.tamu.edu/sorghum-headworm-calculator/).
Adapted from Maryland State Horticulture Society Newsletter
After several years of debate in Annapolis to ban the use of this product in Maryland, the final decisions was made to allow the phase out of this product. The manufacturer will discontinue production of this product. Instead of an outright ban, MDA has developed the phase out process which is listed below. This can be found on page 442 of the Maryland Register, Volume 47, Issue 8 dated April 10, 2020.
.02 General Requirements for Applying or Recommending Pesticides.
A.—D. (text unchanged)
Restrictions on Use of Insecticides that Contain Chlorpyrifos.
(1) Aerial Applications Prohibited. A person may not conduct an aerial application of any insecticide containing Chlorpyrifos in the State.
(2) Other Applications Generally Prohibited After December 31, 2020.
(a) Except as provided in §E(2)(b) and (c) of this regulation, after December 31, 2020, a person may not apply an insecticide containing Chlorpyrifos or seeds that have been treated with Chlorpyrifos in the State for any use.
(b) Fruit Trees and Snap Bean Seeds. Until June 30, 2021, a person may use an insecticide containing Chlorpyrifos or seeds that have been treated with Chlorpyrifos in the State to treat snap bean seeds and the trunks and lower limbs of fruit trees. After June 30, 2021, such applications are prohibited unless authorized by the Secretary under §E(2)(c) of this regulation.
(c) Limited Particular Use Authorization. After December 31, 2020, a person may file a written application with the Department requesting authorization to use an insecticide that contains Chlorpyrifos or seeds that have been treated with Chlorpyrifos for a particular use. If the Secretary has determined that there are no effective alternatives for the particular use noted in the application, the Secretary may authorize such use for a specified period of time, which may not extend beyond December 31, 2021.
(3) Establishment of Committee. The Secretary shall establish a committee, with members appointed by the Secretary, to determine alternatives to using Chlorpyrifos or seeds that have been treated with Chlorpyrifos, which shall dissolve on December 31, 2021.
This allows the use of this product as listed above. Please note the important dates. Until June 30, 2021, a person may use an insecticide containing Chlorpyrifos or seeds that have been treated with Chlorpyrifos in the State to treat snap bean seeds and the trunks and lower limbs of fruit trees. After June 30,2021, such applications are prohibited unless authorized by the Secretary under §E(2)(c) of this regulation. Use these products carefully .
Kelly Hamby1, Maria Cramer1, Galen Dively1, Sarah Hirsh2, Andrew Kness2 Alan Leslie2, Kelly Nichols2, Emily Zobel2, and David Owens3 1University of Maryland, Department of Entomology | 2University of Maryland Extension 3University of Delaware Extension
A few hot spots where corn earworm (also known as tomato fruitworm, soybean podworm, and sorghum headworm) activity is starting to rise have been identified in central Maryland and Delaware. The warm 2019-2020 winter allowed for overwintering in our area, and some parts of the state experienced a higher than normal first flight in early June. The warm weather through June and July made for speedy development and earlier activity for the second summer generation. Because corn earworm has developed resistance to most Bt hybrids, significantly more adult moths are emerging compared to levels a decade ago. Some areas continue to capture few moths and are experiencing low pressure, while others have been experiencing moderate pressure that may continue to increase towards heavy pressure (>65 moths captured per 5 days). Captures for select sites in Maryland and Delaware are pictured below, and values within the gray box indicate low pressure (<7 for weekly captures, and <5 for four to five day captures).
Corn earworm larva feeding damage to corn
Although corn earworm prefer fresh corn silks for egg laying, they will lay eggs on wilted and brown silks if the plants remain green and unstressed. As corn matures further over the next several weeks, corn earworm activity will shift to other host plants including soybeans and vegetables. See last summer’s articles for scouting and management recommendations in vegetables as well as sorghum and soybeans.
Podworm outbreaks have historically occurred in growing seasons where the corn crop was drought and heat stressed, with corn senescing earlier than normal. However, narrow row spacing in soybeans makes the plants less attractive to female moths and increases the likelihood that fungal pathogens will infect the larvae. Therefore, it is important to scout bean fields, especially paying attention to those fields with a more open canopy in areas where the nearby maturing corn is no longer attractive to earworm moths. North Carolina State University has produced a helpful economic threshold calculator for podworm in soybean: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html
Acknowledgements: Corn earworm trapping efforts in were supported by the Crop Protection and Pest Management Program [grant numbers 2017-70006-27171 and 2017-70006-27286] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
Emily Zobel, Agriculture Agent Associate University of Maryland Extension, Dorchester County
Soybean: The usual defoliators are starting to arrive, including bean leaf beetle, Japanese beetle, grasshoppers, and caterpillars. Control may be needed if there is 30% defoliation during the seedling and vegetative stages and 15% defoliation once plants start to bloom through pod fill.
Adult Dectes Stem borer will be emerging over the next several weeks. Chemical control is not recommended since it would require multiple applications to reduce larval infestations, which is not economical. If a high number of adults are found, harvesting that field as soon as it matures will reduce losses associated with lodged plants.
Fields that have an open canopy, drought-stressed, or have recently had an insecticide applied are at higher risk for corn earworm (CEW). CEW larva can feed on flowers without impacting yields because soybeans overproduce flowers. However, feeding during pod development can affect yield. An economic threshold calculator is available to assist with management decisions: https://soybeans.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.
Field Corn: As corn ears begin to form, check for stink bugs. Stink bugs will gather around the edges of fields, so scouting should be done at least 15 rows in. Thresholds are 1 stink bug per 4 plants when the ear is forming, and 1 stink bug per 2 plants from pollen shed to blister stage. Treatment is not recommended past the blister stage. Japanese beetles are minor defoliators and will clip corn silks, but control is not needed unless silks are cut back to less than ½ inch, and less than half the field has been pollinated.
Alfalfa: Once plants have hopper burn, there is no way to undo it, so continue scouting for leafhopper. Since infestations are highly variable, individual fields should be scouted. If you are planning on selling your hay for horse feed, check for blister beetle as well since they produce cantharidin, which causes skin blisters on humans and can make horses sick.
Sorghum: Sugarcane aphids were found on the Eastern Shore last year and typically show up in fields late July and August. Check underside of leaves for insects. Honeydew will turn leaves shiny and is an easy to see indicator that aphids are present. Sugarcane aphids are light yellow with black cornicles, antennae, and feet. Thresholds depend on plant growth stage; at boot to milk, thresholds are 50 aphids per leaf on 25 – 30% of plants. There is documented resistance to resistance to pyrethroids.
Laura C. Moore^,*, Alan W. Leslie#, Cerruti RR Hooks$,* and Galen P. Dively+,* Former graduate student^, Associate Professor and Extension Specialist$, Professor Emeritus+, CMNS, Department of Entomology*, Agriculture Extension Agent, Charles County#
Introduction
Increasing floral diversity within agricultural fields has been proposed as a method to bolster natural enemies and subsequently reduce pest populations. A key factor that enhances predator and parasitoid populations is the availability of nectar and/or pollen food subsidies from flowering plants. Many natural enemies, particularly hymenopteran (wasps) parasitoids, require carbohydrates for successful reproduction and overall fitness. However, monoculture cropping systems are relatively weed-free and generally lack floral resources required by many natural enemies. A literature review showed that the successful establishment of certain parasitoids in cropping systems depended on the presence of nectar-bearing weeds. In addition to providing natural enemies nectar and pollen to eat, flowering plants can supply alternative hosts or prey, shelter, overwintering sites and a more suitable microclimate.
Many conservation projects have been implemented by farmers to increase beneficial services on arable lands. In Maryland, the opportunity to practice conservation biological control exists within the Conservation Reserve Enhancement Program (CREP). Conservation biological control is a pest management approach that manipulates agricultural systems so as to promote pest suppression by naturally occurring predators, parasitoids and pathogens. The CREP seeks to establish riparian buffers in Maryland to improve water quality, filter sediments and nutrients from runoff and provide wildlife habitat. However, these buffers can be engineered to support communities of natural enemies and serve as corridors for their movement into neighboring crops.
The aim of this study was to determine whether buffer strips could be used as insectary plants to enhance beneficial arthropods (insects and spiders) within neighboring soybeans. Insectary plants are plants grown with cash crops to attract, feed and shelter insect parasitoids and predators so as to enhance their natural control of insect pests. We monitored pest and beneficial arthropods in the buffer insectary plants and neighboring soybean plantings and tried to link arthropods found in buffer plants with pest management in neighboring soybeans.
Abbreviated Experimental Procedures
Insectary buffer test plants. Partridge pea and purple tansy are commonly used to enhance floral resources along field margins for pollinator plantings and to enhance communities of natural enemies in adjacent crops. Partridge pea is a native annual legume and is widely used in seed mixes of CREP riparian buffers because it readily reseeds itself, is competitive when grown with grass mixes, and provides nutritional seed for game birds. As an insectary plant, partridge pea has a long bloom period and each leaf petiole has an extrafloral nectary at its base, which produces nectar throughout the growing season. A diverse assemblage of pollinators and natural enemies are attracted to partridge pea. Purple tansy has a long flowering period, high-quality nectar and pollen production, and is reported as being a valuable insectary plant. Proso millet is a warm season annual grass that lacks floral resources. As such, it served as a grass control to determine how added vegetation diversity in the absence of floral resources would impact natural enemies.
Experimental design. Field experiments were conducted over two years at the Central Maryland Research and Education Center in Beltsville, MD. In year 1, 16 plots of soybean were seeded on May 11. Each plot consisted of 20 soybean rows spaced 35 cm (15 in) apart and bordered on each side by an insectary buffer strip (Fig. 1). The test buffer strips consisted of 1) purple tansy, 2) partridge pea, 3) 50:50 seed mixture of purple tansy + partridge pea, or 4) proso millet. Each soybean plot-buffer combination was replicated four times. Seeds of partridge pea, purple tansy, and proso millet were planted with a no-till drill in rows 23 cm (9 in) apart at a rate of roughly 12,000 seeds per ha (4856 per ac) on the day soybeans were planted.
Fig. 1. Illustration of a soybean-buffer treatment plot in year 1. Soybean plots were bordered on each side with buffer insectary plants. Buffers included purple tansy, partridge pea, 50:50 seed mixture of partridge pea/purple tansy or proso millet.
The year 1 study showed that purple tansy was unsuitable for the hot summer conditions in Maryland Thus, it was not used in the year 2 experiment, which focused solely on partridge pea as the insectary buffer plant. The year 2 experiment included 14 strip plantings of full season soybean at five different locations (Fig. 2). Soybeans were planted no-till in 75 cm (30 in) wide rows during May. Each strip was bordered at one end with a partridge pea buffer and at the other end with a mixed grass border of fescue (Festuca spp.) and orchardgrass (Dactylis spp.).
Fig. 2. Aerial view of experimental layout in year 2. Study consisted of 14 contour strips of full-season soybeans and adjoining partridge pea buffers (indicated by black polygons) at one end of each strip. Grassy areas were on opposite ends of soybean strips without a buffer.
Arthropod population assessments. Abundances of arthropods active in the plant canopy were measured with yellow sticky cards secured to bamboo poles. Further, sweep-net samples were taken in July and August to estimate green cloverworm (Hypena scabra) numbers. The green cloverworm served as a bioindicator of changes in pest populations potentially caused by enhanced natural enemy activity. The larger field size in year 2 allowed sticky cards to be placed throughout the soybean strip. One card was placed in the center of each partridge pea buffer, and additional cards were placed at distances of 3, 6, 12, 18 and 24 m (10 ft to 79 ft) from the border on both sides of each soybean strip (total of 10 sticky cards per strip). Sampling was conducted weekly or biweekly. In year 2, pitfall traps were also installed in the ground adjacent to each sticky card to estimate the abundance of surface-dwelling arthropods over 7-day intervals.
Summary of Results
Year 1 Study – comparison of four insectary buffers parasitoid abundance. Three families of parasitoids Mymaridae, Scelionidae and Trichogrammatidae comprised 83.9% of the total of parasitic wasps captured on sticky cards. Families Ceraphronidae, Braconidae and Eulophidae comprised an additional 12.5% of the wasp parasitoid group. Of these parasitoids, mymarids were the most abundant and there were 73-78% higher sticky card captures of this wasp in partridge pea compared to purple tansy and millet buffers. However, significantly fewer mymarids were captured in soybeans adjacent to partridge pea than adjacent to purple tansy or millet. Scelionid parasitoids were more abundant in millet and purple tansy buffers but their numbers were similar in soybeans regardless of the neighboring buffer type. Trichogrammatid abundance was greatest in millet early in the season and in buffers with partridge pea by season end. Two families of fly parasitoids (Tachinidae and Sarcophagidae) averaged 9.4 and 4.4 flies per sticky card in insectary buffers and soybean plots, respectively. The abundance of sarcophagid flies was significantly higher in buffers with partridge pea than millet or purple tansy alone. Similarly, soybeans adjacent to partridge pea were inhabited by more tachinids and sarcophagids than soybeans adjacent to millet or purple tansy.
Predator abundance. Overall predator abundance was significantly higher in purple tansy and millet compared to partridge pea or mixed (partridge pea + purple tansy) buffers. Mean captures per card were 5.0 in partridge pea, 6.8 in mixed, 8.5 in purple tansy, and 10.3 in millet. However. similar predator numbers were captured in soybean plots adjacent to all four buffer types.
Insect herbivores (plant feeders). Sweep net counts of green cloverworm were statistically similar in soybean plots adjacent to the four different buffer types. Overall numbers per 10 sweeps averaged 24.6, 27.0, 18.0, and 23.0 in soybeans adjacent to millet, purple tansy, mixed and partridge pea buffers, respectively. The bulk of other insect herbivores captured on sticky cards were mainly aphids, leafhoppers, planthoppers and plant bugs. Mean numbers captured per card were 86.1 (millet), 113.2 (purple tansy), 57.6 (mixed) and 53.7 (partridge pea).
3.2. Year 2 Study partridge pea vs. natural grass vegetation
Parasitoid abundance. The most abundant parasitoids belonged to families Mymaridae, Trichogrammatidae and Scelionidae in order of abundance, and together comprised 84.3% of the total hymenopteran parasitoids captured. Each family responded differently to the partridge pea treatment. Mymarid abundance was higher overall in partridge pea buffers but did not enhance their abundance in neighboring soybeans (Fig. 3). Significantly fewer trichogrammatids were captured in partridge pea compared to numbers captured in soybean with and without the partridge pea buffer. Mean captures of dipteran parasitoids per sticky card abundance were significantly higher in soybean neighboring partridge pea, with the exception of the first and last sampling dates.
Fig. 3. A) Mean number (±SE) of mymarid parasitoids captured per sticky card in partridge pea buffer, soybean neighboring buffer, and soybean without buffer in year 2. Data for soybean were averaged over all sampling distances (3, 6, 12, 18 and 24 m) from the field edges. B) Mean number in soybean at different distances from field edges with and without a partridge pea buffer.
Predator abundance. Long-legged flies, minute pirate bugs, and big-eyed bugs comprised 81.4% of the total predatory arthropods captured. Soldier beetles, fireflies and lady beetles represented an additional 11.6%. Mean abundance of predators per sticky card was 11.5 ± 1.1 in buffer, 4.1 ± 0.16 in soybean neighboring buffer and 4.9 ± 0.18 in soybean without buffer. Abundance of predators was significantly lower in soybean neighboring the partridge pea buffer (Fig. 4). However, this was largely due to the activity of long-legged flies, which were more attracted to the partridge pea buffer. Still, their numbers were significantly lower in soybean strips neighboring partridge pea compared to soybeans without partridge pea buffers.
Fig. 4. A) Mean number (±SE) of arthropod predators captured per sticky card in partridge pea buffer, soybean neighboring buffer and soybean without buffer in year 2. Data for soybean were averaged over all sampling site distances (3, 6, 12, 18 and 24 m) from the field edges. B) Mean number in soybean at different distances from field edges with and without a partridge pea buffer. Arthropod predator guild consisted of long-legged flies, minute pirate bugs, big-eyed bug, soldier beetles, fireflies and lady beetles.
Insect herbivores/pests. Thrips, leafhoppers, treehoppers, froghoppers and planthoppers comprised over 95% of herbivores captured on sticky cards. The total number of herbivores per sticky card averaged 108.2 in the partridge pea buffer, 96.3 in soybean neighboring buffer, and 96.4 in soybean without buffer. Thus, herbivore numbers did not differ significantly in the buffer and soybeans.
Pitfall trap predators. A total of 56,296 arthropods were identified from pitfall trap samples. Of predators captured in pitfall traps, ants, spiders, soldier beetle larvae, rove beetle adults and larvae, and ground beetle adults and larvae were the most abundant. Ant numbers were significantly lower in soybeans neighboring partridge pea on all sampling dates.
Discussion
Year 1 study was conducted to determine if pure and mixed buffer strips of partridge pea and purple tansy could attract greater numbers of beneficial arthropods than non-floral strips of millet, and whether these buffers enhance beneficial arthropod abundance in neighboring soybeans. Purple tansy was not a suitable insectary plant as it was not well adapted to the seasonal period of the study in Maryland. Furthermore, purple tansy would probably be less desirable to establish and maintain as a buffer strip due to its relatively high seed price, slow growth characteristic and greater susceptibility to weed competition. Moreover, purple tansy was quickly out-competed by partridge pea in the mixed planting to the extent that the pure and mixed buffers containing partridge pea attracted similar arthropod communities.
Overall, results consistently showed that partridge pea attracted and supported high populations of natural enemies and potential hosts and prey, with abundances significantly greater than levels found in adjacent soybeans. Sticky card captures of wasp and fly parasitoids in year 1 were more than 70% higher overall in buffers containing partridge pea compared to other buffer types. Similarly, populations of all beneficial arthropods captured by sticky card and pitfall sampling in year 2 were approximately 80 to 72% higher, respectively, in partridge pea buffers compared to the soybean crop.
Parasitoids. Mymarid wasps were notably the most common parasitoids captured on sticky cards and consistently more abundant in partridge pea compared to soybean. These tiny wasps parasitize insect eggs in concealed sites within plant tissues or the soil and are important natural control agents of economically important leafhopper pests. In year 1, mymarids reached levels in partridge pea buffers that were four-fold higher than those in soybean plots, yet significantly lower levels of mymarids were captured in soybean adjoining these buffers. This suggests that the partridge pea lured mymarids from neighboring soybeans. High numbers of mymarids were also captured in partridge pea in year 2 but their abundance in soybeans was not enhanced. This suggests that partridge pea may provide some parasitoids and their associated hosts with all resources required for survival and reproduction. This would in effect provide no incentive for these parasitoids to forage within neighboring crops.
Most fly parasitoids found on sticky cards were tachinids or sarcophagids. The vast majority of hosts of tachinid flies are plant-feeding insects. Their level of parasitism can vary greatly, from less than 1% to approaching 100%, depending on such factors as the size of a host and parasitoid population, and environmental conditions. During both study years, their overall abundance in partridge pea was 62.3% higher than levels in soybean. In year 2, this effect was heightened at the field edge next to buffers, suggesting that higher numbers of parasitic flies encroached into the neighboring soybeans but enter only a short distance within the crop.
Predators. In year 1, predators captured on sticky cards were 65% more abundant in the millet and purple tansy buffers. This response was mainly attributed to the abundance of long-legged flies. These predatory flies hover while searching for small, soft-bodied arthropods, particularly other flies, aphids, spider mites, larvae of small insects and thrips. However, abundances of long-legged flies in soybean plots were not affected by buffer type in year 1. Long-legged flies were also the predominant predators active in the plant canopy in year 2, with overall numbers 2-3 times higher in partridge pea buffers compared to levels found in soybeans. However, their abundance was significantly lower in soybean neighboring partridge pea, particularly at sampling sites closest to the field edge. This is further evidence that the partridge pea acted as a natural enemy sink.
Of the ground-dwelling predators captured by pitfall traps, ants were the predominant group and their abundance was significantly higher in partridge pea than adjoining soybeans. Their numbers were significantly lower in soybean plantings adjacent to partridge pea than grassy check treatment on all sampling dates, implying again that partridge pea acted as a natural enemy sink by luring ants away from soybean. Populations of other ground-dwelling predators, which consisted mainly of spiders, rove beetles, soldier beetles and ground beetles, showed a definite preference for partridge pea compared to soybeans. However, their abundances in the crop were not affected by partridge pea presence.
Herbivores. Sticky card captures each study year indicated that partridge pea harbored significantly more insect herbivores compared to soybean. The majority of herbivores were aphids, leafhoppers, planthoppers and plant bugs. In year 1, number of green cloverworm, as well as other herbivores in soybean were similar regardless of the buffer treatment.
Conclusion
This study demonstrated that partridge pea provides floral resources and alternative food for a diverse community of natural enemies and herbivores. However, its presence as a monoculture buffer did not result in increased number of major natural enemies in neighboring soybeans. Taken together, partridge pea planted as a monoculture acted more as a natural enemy sink by attracting beneficial arthropods away from soybean, potentially decreasing natural control efforts. For this reason, a monoculture of partridge pea may not be an ideal insectary planting if the ultimate goal is to maximize natural enemy efficacy in neighboring soybean fields.
In conservation reserve practices, monocultures of partridge pea are more commonly planted as a wildlife habitat to provide food for bobwhite quail and other wildlife and as flowering habitat for different pollinator taxa. Because the foliage is potentially poisonous to cattle and re-seeding plants can aggressively fill in voids when used as part of a seed mix, conservationists recommend for herbaceous riparian buffers that the total seed mix consist of no less than 1% and no more than 4% partridge pea. However, decisions about the deployment of insectary plants as a monoculture or part of a riparian buffer mix planting should take into consideration the attractiveness and resources provided to natural enemies and their hosts/prey by the insectary habitat in comparison to those provided by the neighboring cash crop. Simple addition of a highly attractive flowering buffer adjacent to a crop could be counterintuitive to natural biological control efforts.
Acknowledgements
Financial support for field studies and publishing results was provided by the Northeast Sustainable Agriculture Research and Education Grants Program, Maryland Soybean Board and USDA NIFA EIPM grant number 2017-70006-27171.
