Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
Be sure to check all labels carefully before combining insecticides and herbicides. Thresholds are based on sampling 100 plants (10 plants x 10 locations).
Alfalfa
Begin scouting for potato leafhoppers (PLH). Stubble insecticide applications are rarely needed and seldom provide reasonable control since adult leafhoppers will move out of the field after cutting. 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 Penn State Extension website https://extension.psu.edu/potato-leafhopper-on-alfalfa.
Field Corn
Scout for armyworms and cutworm if growing a hybrid that doesn’t control for them. Treatment threshold for armyworms in corn is 25% infested plants with larvae less than one inch long. Large larvae feeding deep in the whorls will be challenging to control. When scouting for cutworm, check/dig around the base of the plants to determine if cutworms are present and alive. Cutworms tend to be more common in late terminated cover crop, late planted corn fields. Cutworm thresholds are 5% cut plants at V2-V4 or 10% of plants with signs of fresh leaf feeding.
Once plants start to silk, 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). Cereal leaf beetle, stink bugs, and Japanese beetles are ‘edge’ pests, so treatment may only be needed around field edges and pivot tracks.
David Owens, Extension Entomologist University of Delaware
If you have not yet scouted alfalfa for weevils, now is the time! Alfalfa weevil eggs began hatching earlier this year than usual, and we found active weevil larvae almost three weeks ago in Hebron and Greenwood (Figure 1). Alfalfa weevil adults fly into fields in the fall, mate, and begin laying eggs. In our area, fall weather is warm enough for weevils to mate and have enough time to lay a significant number of eggs before advancing cold weather causes them to seek overwintering shelter outside of the field. Weevils are not active below 48 °F. Once warmer weather returns in March, weevils will come out of overwintering habitat in woods and leaf litter and fly back to alfalfa to continue laying eggs. Egg laying scars on stems will appear as small round circular punches in the stem.
Figure 1. Alfalfa weevil larvae feeding on alfalfa.
There are three implications our warm fall weather has when comparing Delmarva alfalfa weevil activity to other states. First, we can have extremely early weevil hatch, and I have heard rumor of weevil damage to alfalfa in December in some states. Second, it means that degree day models used in more northern and western regions might not be accurate enough and fields may need to be treated before the degree day targets have been reached! Third, it means we can have an extended spring activity period lasting 8 weeks or more.
To scout your field, select 30 stems by randomly walking to six locations and plucking stems from those locations. Take a few of the stems at a time and beat them vigorously against the side wall of a bucket to dislodge weevil larvae. Also measure the length of a few of those stems to get an average stem height. Penn State has a great alfalfa weevil control fact sheet with a dynamic threshold table based on stem height, value of the hay, and control costs. We incorporated the table into the UD Insect Control in Alfalfa document which can be found here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/commercial-field-crop-pest-management/alfalfa/. Be advised that small weevil larvae often hide in the half folded terminal leaflets and are not easy to dislodge. You may want to visually examine the terminals after shaking stems.
When making an insecticide application, use higher water volume rates to ensure good coverage, especially into the terminals. Do not apply an insecticide a couple of days before or after a significant cold spell. Weevil larvae need to be active to come into contact with treated foliage. Keep good records of previous applications and be sure to scout fields after application. We can have new weevil hatch after a spray, justifying a second and sometimes even a third application prior to cutting. Our insecticide selection is very limited in alfalfa, and for this reason it is possible that besides environmental factors there can be some intrinsic factors in your local population that could render them less susceptible to a given insecticide if it has been used continuously for a long period of time. Adding to this concern is the chlorpyrifos tolerance revocation effective February 28, 2022. In last year’s spray trials, pyrethroids provided between 40 and 70% efficacy three weeks after application. Carbaryl initially provided a 70% reduction in alfalfa weevil but residual activity was very short and weevil counts increased afterwards. The most consistent active ingredient has been indoxacarb (Steward). It has a supplemental 2ee label for reduced rates, 4.0 to 11.3 fl oz per acre as opposed to the main label rate range of 6.7 to 11.3 fl oz. It also provided excellent residual control in last year’s trial at the 6.7 fl oz rate. FMC advises lower rates should have about 8-14 days residual, higher rates up to 21 days. When making an application to alfalfa, industry recommends including an spreader sticker type adjuvant such as an organosilicone to improve coverage. As always, make sure to read the labels of all products applied or tank-mixed to make sure there are no insecticide/adjuvant/herbicide concerns.
Please note that Besiege is labeled for alfalfa but does not appear in the 2020 control guide. A high rate of Besiege is equivalent to a high rate of WarriorII in terms of lambda cyhalothrin active ingredient. The addition of chlorantraniliprole is not effective on alfalfa weevil.
Finally, be aware that pyrethroids can flare aphids. Dimethoate, while not consistent in its ability to suppress alfalfa weevil, is excellent on aphids, and you may see some advice regarding tank mixing the two active ingredients. While aphid outbreaks are unusual, they can happen. Last year we had a spray trial on aphids following a lambda cyhalothrin application. A repeat application in the trial had no impact on aphids. There are two newer products that need to be updated into the guide but are registered and are excellent on aphids: Sivanto and Sefina (see supplemental labels). Neither has alfalfa weevil efficacy but will just about zero out aphids should they be a concern. Otherwise, if you do experience an aphid outbreak, they are usually late enough that the field can be cut early.
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.
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.
Darsy Smith, Graduate Student & Dr. William Lamp, Professor University of Maryland, Department of Entomology
Figure 1. Yellow appearance of the alfalfa field that lead to the discovery of the cowpea aphid outbreak at BARC. Photo courtesy of Russell Griffith.
An unexpected outbreak of cowpea aphid, Aphid craccivora Koch, in Maryland was discovered last month by Terry Patton, who was contacted by Russell Griffith, tractor operator leader at Beltsville Agricultural Research Center (BARC), because of the yellow appearance of an alfalfa field (Figure 1) and the infestation of dark aphids (Figure 4). Since the 1990s, infestations of the cowpea aphid have been observed in Maryland alfalfa, but this is an unusually large outbreak. Stay alert to this emerging pest and learn how to identify it since it has a wide range of hosts and may damage crops.
Figure 2. Adult cowpea aphid. Note the cornicle (yellow arrow) is dark and long and the abdomen (red arrow) is distinctive dark and shiny. Photo courtesy of influentialpoints.comFigure 3. Adult and nymph cowpea aphids. Nymph color is opaque and varies (yellow arrow) from brown to gray while the adult (red arrow) has the distinctive dark, shiny abdomen. Photo courtesy of Andrew Jensen, https://aphidtrek.org/
Cowpea aphid identification and injury. Cowpea aphid is not generally an economic pest in alfalfa but learning how to identify the aphid and its injury can help you prevent losses. Cowpea aphid is easily differentiated from other aphids in alfalfa because its dark coloration, with the abdomen of the adults much darker and shinier than the rest of its body (Figure 2). In addition, the cornicle (or siphuncule) is dark and long (Figure 2). The nymph is less shiny (opaque) and varies from brown to gray (Figure 3). The legs and antennae of both adults and nymphs are pale with dark tips (this characteristic is more distinctive in adults).
The cowpea aphid is a sap-sucking feeder and damage caused in alfalfa by this pest results from the injection of a toxin into the phloem of the plant. With high population densities on plants, the aphid can cause stunting or plant death. In addition, it can cause yellowing in alfalfa leaves (Figure 1). Like other aphids, this insect produces honeydew that will benefit fungus growth and eventually cause sooty mold.
How to find them? Cowpea aphids are usually found in clusters on the alfalfa leaf and stems (Figure 4). It can also be found in vegetative growth and flower parts of a wide range of hosts. They are readily sampled with sweep nets.
Figure 4. Cowpea aphids on growing tip of alfalfa at BARC. Photo courtesy of Russell Griffith.
Host plants. Cowpea aphid is most commonly found in alfalfa, but may be found on other legumes, such as clovers. More uncommonly, the aphids occur on a variety of weeds and other plants in other plant families.
Management options. Unfortunately, there is not an economic threshold specified for this pest in Maryland alfalfa at this point. However, here are general guidelines for responding to the pest:
Conserve natural enemies. Natural enemies such as lady beetles, damsel bugs, and parasitoid wasps often locate, feed, and reproduce in conjunction with high densities of aphid in alfalfa. If you conserve natural enemies you might find aphids parasitized by parasitoid wasps (Figure 5). When scouting for aphids, watch for natural enemies to help control aphid populations. To help natural enemies stay in your alfalfa field you can use border-strip cutting while harvesting to provide refuge habitats. For more information of this practice, see “Harvest Scheduling and Harvest Impacts on IPM” at the end of this report.
Monitoring for decision-making. Early infestations in alfalfa can result from migration from southern areas. Pay attention to alfalfa fields in March and continue to monitor until fall. Since there are no thresholds developed for cowpea aphid, the thresholds for insecticide applications developed for the blue alfalfa aphid can be used: if alfalfa is 10 inches, then treat if there are 20 or more aphids per stem; if alfalfa is 20 inches tall, then treat if 50 or more aphids per stem.
Figure 5. Aphids parasitized or aphid mummies by a parasitoid wasp. Note the emergence hole (yellow arrows) of the parasitoid wasp in the mummy. Photo courtesy of Darsy Smith.
Potential reasons for the outbreak.
Researchers in the Lamp Lab, University of Maryland, have noted this pest in greenhouse settings but rarely observed them in alfalfa fields. Dr. Lamp suggests that the cowpea aphid may have migrated into Maryland because this species is more common in southern and western areas. Also, the mild winter may have allowed individuals that migrated last year to overwinter in Maryland. Additionally, lack of natural enemies in early spring can potentially lead to an outbreak. Conserving the natural enemies in alfalfa fields and neighboring areas can help decrease aphid abundance.
How can you help?
Records from your alfalfa fields and surrounding crops are valuable sources of information. The information is helpful to not just explain an outbreak but also to provide useful guidelines for farmers to manage the crop and avoid a future outbreak. Practical information that you can provide include any of the following:
Date of first time you have observed the pest
Date of outbreak
Plant height and phenology stage: when you observed the pest for the first time and during outbreak
Presence of natural enemies
Pesticide use and efficacy of application
Alfalfa cultivars/varieties planted during outbreak and previous year
Pictures of damage and estimate of loss
If you find cowpea aphid in your alfalfa field, please contact the nearest county extension office.
Morgan N. Thompson & William O. Lamp University of Maryland, Department of Entomology
Nitrogen is a critical nutrient for forage crop growth and quality. Typically, farmers need to apply additional nitrogen fertilizers to meet the nitrogen demand of crops. Nitrogen-fixing crops, however, do not require nitrogen fertilizer inputs, providing their own nitrogen supply through symbiotic interactions with soil microbes (rhizobia). Rhizobia induce the formation of root nodules in nitrogen-fixing crops, predominantly legumes, and extract inert nitrogen gas from the atmosphere to produce ammonium. In exchange for ammonium, legumes provide the rhizobia carbohydrates to fuel the microbe’s metabolism. Alfalfa is a leguminous forage crop that relies on symbiotic interactions with rhizobia to obtain nitrogen. As a perennial crop, alfalfa stands can last from 3-7 years and typically require no nitrogen fertilizer inputs, making alfalfa a sustainable and high-quality option for forage growers.
Pest pressure can decrease the economic viability of an alfalfa harvest. One particularly devastating pest of alfalfa in Maryland is the potato leafhopper (Empoasca fabae). Potato leafhoppers migrate northward from the southern United States every spring, making the timing of management in the northeast very difficult. Additionally, potato leafhoppers can utilize many alternative host plants, some of which are also of agroeconomic value, such as soybeans and several other fruit and vegetable crops, and leafhoppers can reproduce multiple times during the growing season. To protect alfalfa from potato leafhopper damage (termed ‘hopperburn’), insecticides are often the only option for growers. As a perennial crop, serious pest pressure in one growing season could impact nitrogen fixation in subsequent growing seasons, further accelerating economic losses for growers.
Figure 1. Amount of fixed nitrogen in alfalfa stems and leaves. * represents significant differences between treatments. No Nitrate = No Nitrogen Fertilizer, Moderate Nitrate = Nitrogen Fertilizer Applied; E. fabae- = No Leafhopper Pressure, E. fabae+ = Leafhopper Pressure.
Therefore, in recent field and greenhouse experiments, we sought to determine the effect of potato leafhopper pest pressure on nitrogen fixation in alfalfa. We predicted pest pressure would negatively impact plant growth and carbohydrate production, resulting in reduced nitrogen fixation by rhizobia and uptake of fixed nitrogen by alfalfa. We also predicted losses in nitrogen content of alfalfa due to pest pressure could be offset by nitrogen fertilizer applications. To test our predictions in a field setting, we planted four combinations of small plots: 1) Fixing Cultivar + Nitrogen Fertilizer, 2) Non-Fixing Cultivar + Nitrogen Fertilizer, 3) Fixing Cultivar No Nitrogen Fertilizer, and 4) Non-Fixing Cultivar No Nitrogen Fertilizer. Fixing and non-fixing alfalfa cultivars were utilized to compare plants reliant on both nitrogen fixation and soil nitrogen with plants completely reliant on soil nitrogen. We split each plot in half, applying cages with leafhoppers to one side and cages without leafhoppers to the other. We analyzed the amount of fixed nitrogen in aboveground plant tissue. Results from the field experiment contradicted our predictions, showing nitrogen fertilizer did not increase aboveground nitrogen content of alfalfa under pest pressure. Nitrogen fertilizer (Moderate Nitrate) also decreased aboveground fixed nitrogen content in plants with and without pest pressure (Fig. 1). Unfertilized plants (No Nitrate), in contrast, showed significantly increased amounts of fixed nitrogen content when under pest pressure (Fig. 1). These results contradicted our predictions and suggest alfalfa interactions with rhizobia play a role in helping plants withstand pest damage.
We also examined leafhopper-alfalfa interactions in a greenhouse setting. Here, we analyzed the response of two different cultivars of alfalfa: leafhopper-susceptible (Pioneer 55V50) and leafhopper-resistant (Pioneer 55H94). Nitrogen fertilizer treatments were applied to both cultivars, as well as cages with or without leafhoppers. Results indicate that additional nitrogen fertilizer did not increase the percent nitrogen of plants under pest pressure, regardless of the cultivar (Table 1).
Overall, we conclude leafhopper pest pressure decreases total nitrogen content of alfalfa across all four cultivars tested in both field and greenhouse settings. Amending soils with additional nitrogen fertilizer did not offset losses to leafhopper pressure and we do not recommend this as a management strategy to growers. In our field experiment, however, we found evidence that leafhopper pressure enhances aboveground fixed nitrogen content of alfalfa grown in soils without additional nitrogen. Rhizobia may play an unexamined role in the response of alfalfa to leafhopper pressure. Broader implications of our results highlight how pest damage may increase nitrogen fixation, which may benefit farmers interested in utilizing nitrogen-fixing cover crops.
Acknowledgements: Many thanks to the Western Maryland Research and Education Center staff and greenhouse staff at the University of Maryland aiding in the execution of these experiments, as well as members of the Lamp Lab. This study was funded by Northeastern Sustainable Agriculture Research and Education (Award Number GNE18-187-32231) and the Hatch Project MD-ENTM-1802.
Table 1. Systemic (shoots, crowns, roots) percent nitrogen content of susceptible and resistant alfalfa cultivars in the greenhouse. No Nitrogen Added = No Nitrogen Fertilizer, Nitrogen Added = Nitrogen Fertilizer Applied; Healthy = No Leafhopper Pressure, Injured = Leafhopper Pressure.
The U.S. Environmental Protection Agency (EPA) has just issued a long-term approval for the insecticide sulfoxaflor, which the Agency has characterized as “an effective tool to control challenging pests with fewer environmental impacts.” The following information is from today’s EPA OPP Update.
“After conducting an extensive risk analysis, including the review of one of the agency’s largest datasets on the effects of a pesticide on bees, EPA is approving the use of sulfoxaflor on alfalfa, corn, cacao, grains (millet, oats), pineapple, sorghum, teff, teosinte, tree plantations, citrus, cotton, cucurbits (squash, cucumbers, watermelons, some gourds), soybeans, and strawberries.
EPA is providing long-term certainty for U.S. growers to use an important tool to protect crops and avoid potentially significant economic losses, while maintaining strong protection for pollinators,” said Alexandra Dapolito Dunn, assistant administrator for EPA’s Office of Chemical Safety and Pollution Prevention. “Today’s decision shows the agency’s commitment to making decisions that are based on sound science.”
Sulfoxaflor is an important and highly effective tool for growers that targets difficult pests such as sugarcane aphids and tarnished plant bugs, also known as lygus. These pests can damage crops and cause significant economic loss. Additionally, there are few viable alternatives for sulfoxaflor for these pests. In many cases, alternative insecticides may be effective only if applied repeatedly or in a tank mix, whereas sulfoxaflor often requires fewer applications, resulting in less risk to aquatic and terrestrial wildlife.
EPA’s registration also includes updated requirements for product labels, which will include crop-specific restrictions and pollinator protection language.
*Background*
In 2016, following a 2015 decision of the Ninth Circuit Court of Appeals vacating the registration of sulfoxaflor citing inadequate data on the effects on bees, EPA reevaluated the data and approved registrations that did not include crops that attract bees. The 2016 registration allowed fewer uses than the initial registration and included additional interim restrictions on application while new data on bees were being obtained. Today’s action, adding new uses, restoring previous uses, and removing certain application restrictions is backed by substantial data supporting the use of sulfoxaflor.
For additional information, please visit the EPA website.
