Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
Soybeans: Continue scouting for corn earworm, stink bugs, and soybean looper in double-crop fields. Defoliation thresholds at R5 are 15% and between 20 and 30% at R6. Corn earworm pheromone trapping information for Maryland can be found at https://extension.umd.edu/resource/corn-earworm-pheromone-trapping. NC State Extension has a good CEW threshold calculator can be used to help decide if it is worth treating: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html. Stink bug thresholds are 5 bugs per 15 sweeps until the soybeans reach the R7 stage, after which treatment for stink bugs is not necessary.
As the month progresses and fields get closer to harvest, sample stems in any field with a history of Dectes stem borer issues. If the field has a large stem infestation, prioritize that field for as timely a harvest as possible to reduce loss due to lodging.
Sorghum: Sugarcane aphids have been found on the Eastern Shore. This week’s cooler weather should slow down their reproduction rate, but if we get some warm days in September, that might change. To scout, check the 50 plants in 4 different locations at least 25 feet into the field. If honeydew is present, check the upper and lower canopy of 15-20 plants for aphids. Treatment thresholds are when the field has an average of 50 and 125 aphids per leaf or 30% infested plants with localized areas with honeydew present. David Owen from UDel recommends using Sefina, Sivanto, and Transform for control. See Texas A&M “Scouting Sugarcane Aphids” handout for more information.
Reports are for crop conditions up until 8/5/2021.
Western Maryland
We had a great wheat harvest and many optimists planted double-crop beans. It has been very dry in most of the county. Depending on where in the county you are located is the difference between chopping corn now and admiring your crop. Hay will be in short supply so many will plant forage oats for fall forage. Full season beans are looking good most places. We will continue to pray for rain. —Jeff Semler, Washington Co.
Central Maryland
In general, Central Maryland has had a good combination of heat and rain, and the crops are looking great. Most of the region is close to or above the normal rainfall over the last 30 days. Fall armyworm and western bean cutworm traps near Poolesville have been empty this past month. Corn silage harvest will begin soon, especially if August turns up the heat.—Kelly Nichols, Montgomery Co.
Northern Maryland
We’ve been fortunate to have excellent pollination conditions over the past 3 weeks when the vast majority of the corn crop was pollinating. Temperatures were in the upper 80s and occasionally low 90s with frequent storms that brought timely moisture. The majority of the corn crop is looking above average. Full season soybeans are beginning to set pots and look good; some fields had very spotty emergence due to slug damage early; we will see what August brings in terms of rain to help make the bean crop. Double crops had good soil moisture to germinate and are off to a decent start. Insect and disease pest pressure in both corn and beans has been minimal so far. Small grain harvest wrapped up a couple of weeks ago with big wheat yields.—Andy Kness, Harford Co.
Upper Eastern Shore
Soil moisture levels are all over the board. Some areas have been receiving rains while others have not. None of the region is too wet. Therefore, corn and early bean yield predictions also vary, but not disastrous. Corn will be 100-250 bu/acre plus. Early beans will be 40-80 bu/acre plus. Later maturing beans still have a long ways to go. Corn leaf diseases have been expanding/moving up the plant in the last few weeks. Soybean diseases have been minimal, but foliage feeding insects are increasing. Luckily most of the beans have plenty of foliage and can withstand some feeding. Trouble weeds, especially the glyphosate resistant weeds are starting to outgrow beans and showing their ugly heads(growing points). As with past months, there has been good quality hay harvested.—Jim Lewis, Caroline Co.
Lower Eastern Shore
Weather has been favorable and grain crops are looking good. Corn is around R3 reproductive stage. Full-season soybean is flowering. Deer are regularly observed grazing in soybean fields. Corn Earworm numbers are currently low. Herbicide-resistant weeds such as ragweed, marestail and Palmer amaranth are present and posing management challenges.—Sarah Hirsh, Somerset Co.
Southern Maryland
The story this month is squarely focused on moisture. Temperatures across the region are reaching into the 90’s with sporadic isolated storms hitting here and there. Crops are now showing moisture stress. The corn crop is past pollination and approaching dough stage on earliest plantings. We could really use another rain to help fill corn out, but it’s looking to be at least an average crop. Soybeans are setting pods, with full season beans at R3-R5. We have not seen many pest outbreaks in beans yet. We are on the lookout for spider mites with the hot dry conditions. Cool season grasses are going dormant in the hot dry weather.—Ben Beale, St. Mary’s Co.
Alan Leslie, Agriculture Agent | aleslie@umd.edu University of Maryland Extension, Charles County
Figure 1. Corn earworm moths in a pheromone trap at the Central Maryland Research and Education Center in Beltsville.
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. The relatively mild 2020-2021 winter allowed adult moths to overwinter in Maryland, and some parts of the state experienced a higher than normal first flight in early June. Now that we are heading towards the end of summer, the next generation of moths are emerging as adults and have begun flight. Some areas continue to capture few moths and are experiencing low pressure, while others have been experiencing moderate pressure that has increased to heavy pressure (Fig. 1). See Figure 2 for pressure at select sites in Maryland. Monitoring pressure using on-farm traps provides the most accurate information for making management decisions. We recommend using two pheromone traps and replacing the lures frequently, especially during periods of hot weather. For weekly updates on pheromone trap captures across the state and information on how to scout for and manage corn earworm in vegetable and grain crops, visit the new webpage: https://extension.umd.edu/resource/corn-earworm-pheromone-trapping.
Figure 2. Weekly captures of corn earworm moths using pheromone traps at select sites across the state. Dashed line represents the cutoff for what is considered heavy moth pressure.
Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
Soybean: Continue to scout for stink bugs, dectes stem borer, and defoliators (bean leaf beetle, Japanese beetle, grasshoppers, and caterpillars). Control may be needed if there is 15% defoliation from bloom through pod fill. Chemical control is not recommended for dectes stem borer 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.
With the upcoming hot weather there is an increased chance of spider mite outbreak in vegetable and soybean fields (Fig 1). Scout by using a hand lens to examine mid to upper canopy leaflets on two plants in twenty locations along the edges of the field. Consider treating if 10 % of plants have heavy stippling feeding damage on ⅓ of their mid and lower leaves.
The next flight of corn earworm (CEW) will be taking place soon. Bean fields with open canopies (wide rows), are drought-stressed, or have recently had an insecticide applied are at higher risk for CEW. CEW larvae can feed on flowers without impacting yields. NC State has a good economic threshold calculator to assist with management decisions: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.
Field Corn: Check for stink bugs around the edges. The threshold is 1 stink bug per 2 plants from pollen shed to blister stage. Treatment is not recommended past the blister stage.
Sorghum: Sugarcane aphids 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.
Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
Be sure to check all labels carefully before applying and combining insecticides and herbicides.
Soybean: Scout for the usual defoliators, including bean leaf beetle, Japanese beetle, and caterpillars. The treatment threshold is 15-20% defoliation for bloom to pod fill. Note that defoliation percentages should be based on the entire soybean plant or canopy, not just the top leaves or worst leaves. University of Nebraska Extension has a nice guide for defoliating insects in soybeans (https://croptechcafe.org/defoliating-insects-in-soybeans-thresholds-training-and-tools/).
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. However, if a high number of adults are found, harvesting that field as soon as it matures will reduce losses associated with lodged plants.
Field Corn: At the start of silking, scout for Japanese beetles and stink bugs. For Japanese beetles, the treatment threshold is when there is less than ½ inch of silk and less than 50% pollination, and an average of 2 or more beetles per ear. For stink bugs, the threshold is 1 bug per 10 plants (V1 to V6), 1 bug per 8 plants (V14 to VT), and 1 bug per 4 plants (R1 to R2). Stink bugs and Japanese beetles are ‘edge’ pests, so treatment may only be needed around field edges and pivot tracks.
Alfalfa: Continue scouting for potato leafhoppers (PLH). Cutting will destroy many of the nymphs, and adults will often move elsewhere, but be sure to scout the regrowth. A rough threshold estimate is 20 PLH per 100 sweeps on alfalfa 3 inches or less in height, 50 PLH per 100 sweeps in 4-6 inch tall alfalfa, and 100 PLH per 100 sweeps in 7-11 inch tall alfalfa. A more precise threshold chart can be found on the Penn State Extension website https://extension.psu.edu/potato-leafhopper-on-alfalfa.
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
