Soybean aphids have been spotted in several fields across Southern Maryland this week, with notable activity in St. Mary’s County and Charles County. Infestations were first observed along field edges but have since been found deeper into the canopy in some locations. As you can see from the pictures, the fields look relatively healthy from the road, and it isn’t until you enter the field that you notice the large aphid populations, which is why it is important to keep scouting your fields regularly. Aphids were clustered on the undersides of leaves, along petioles, and around new growth, with many plants showing multiple colonies. In some spots, the density was high enough that honeydew, a sticky sweet substance aphids excrete, was visible creating a sheen on leaves and causing fungus to grow.Soybean Field at threshold for aphids with no obvious visual signs. Image. H. Schug, Univ. of Maryland.
The soybean aphid is a small, yellow-green insect with black cornicles protruding from the rear of its body. Winged forms are dark with clear wings, while wingless forms are pale and slow-moving. Aphids feed by piercing plant tissue and removing sap, which can stress plants, reduce photosynthetic efficiency, and in high numbers, lead to leaf curling, yellowing, and stunting. Prolonged feeding during the reproductive stages (until R6) can lower pod set, reduce seed fill, and ultimately cause measurable yield loss. In addition to direct feeding damage, soybean aphids can transmit plant viruses, although this has not been a major yield factor in Maryland.
Scouting and Thresholds
University research across the Midwest has established an economic threshold of 250 aphids per plant, averaged across multiple sampling points, with populations actively increasing and plants still in the <R6 stages. Scouting should involve checking 20–30 plants per field, spread across several locations, including edges and interiors. The undersides of leaves and growing points are key inspection sites, as aphids prefer sheltered feeding areas.About 80 aphids are on the underside of this leaf. Image: H. Schug, Univ. of Maryland.
Natural enemies, including lady beetle adults and larvae, green lacewing larvae, minute pirate bugs, and various parasitoid wasps, play an important role in regulating aphid populations. Their activity should be noted during scouting. In some cases, robust predator populations can keep aphid numbers below threshold, especially if environmental conditions are unfavorable for aphid reproduction. Rain events can also play a key role in knocking down large aphid population levels, so this must be taken into account when making management decisions.
Management Considerations
If thresholds are reached and plants are still younger than R6, an insecticide application may be warranted. Although you should also take into consideration the weather and the 1.3-2.5% yield loss from wheel tracks with applications made during R1-R5. Pyrethroid products have been the primary tool for soybean aphid control in the Mid-Atlantic, but resistance has been confirmed in parts of the upper Midwest. Although Maryland populations have not shown resistance that I am aware of, it is important to rotate insecticide modes of action where possible to delay resistance development. Follow label directions closely, including restrictions on application timing and rates, and be aware of preharvest intervals.
Hot, dry conditions can favor both aphids and spider mites. If using a pyrethroid during these conditions, monitor for mite flare-ups in the weeks following treatment. Avoid unnecessary applications to preserve beneficial insects, which are critical for season-long pest suppression.
In summary, soybean aphids are present but only a few fields show damaging levels in most Southern Maryland. Continued monitoring is key, and management should be based on established thresholds to protect yield while minimizing unnecessary insecticide use.Soybean leaf covered in molted aphid skins. Image: H. Schug, Univ. of Maryland.
As we move into July and August, insect pressure tends to ramp up across Maryland row crops, particularly in soybeans and corn. These months are critical for monitoring pest activity and making timely, threshold-based decisions. Several pests are commonly active during this period, including corn earworm, stink bugs, Japanese beetles, other defoliators, and spider mites.
Corn earworm moths typically migrate into Maryland in larger numbers in mid to late July, although some can overwinter here, and early flights have already been reported moving up the coast. In soybeans, larvae can cause significant injury during the R1 to R5 growth stages by feeding on pods and developing seeds. When I ran the numbers, the threshold was approximately 2.5 larvae per 15 sweeps, but thresholds can vary depending on market value and treatment cost. You can use the NCSU dynamic threshold calculator to run your own field-specific estimates: https://go.umd.edu/CEW. It’s important to sample evenly across the field, as female moths may lay eggs in clusters, leading to localized infestations. Many corn earworm populations show resistance to pyrethroids, so growers should consider using alternative insecticide chemistries when treatment is needed.Figure 1. Common soybean pest caterpillars. Graphic by Hayden Schug, University of Maryland.
Defoliating insects, such as Japanese beetles, bean leaf beetles, grasshoppers, green cloverworms, and soybean loopers, can all be active during July and August. Last year in Southern Maryland, there were soybean fields that reached defoliation thresholds from grasshopper feeding alone. In many cases, damage was more severe in the center of the field, so it’s important to walk the entire field and not rely solely on edge scouting. Defoliation thresholds are based on cumulative injury from all feeding insects. Economic thresholds are 30 percent defoliation before bloom (V1–V6), 15 percent from bloom through pod fill (R1–R6), and 25 percent from full seed to maturity (R7–R8). A helpful visual guide and estimator tool is available through the University of Nebraska (Figure 2).Figure 2. Sampling soybean defoliation. Infographic by Justin McMehan, University of Nebraska.
Stink bugs, including brown and brown marmorated species, are another priority pest in soybeans and corn during mid-summer. They damage pods by piercing and feeding on developing seeds, often resulting in aborted or shriveled beans. Scouting is most important from R3 to R5. The general treatment threshold is five stink bugs per 15 sweeps, but this drops to three per 15 sweeps in fields grown for seed or planted early.
Finally, spider mites may become a problem during periods of hot, dry weather, especially along field edges. These pests cause stippling and bronzing of leaves and can spread quickly under drought stress. Treatment should be considered when active mites are present on the undersides of leaves and bronzing or stippling begins to move into the upper canopy. Threshold estimates follow the 15 percent defoliation threshold, and plants should be treated when that level of injury is reached.
In corn, Japanese beetles can clip silks and interfere with pollination. The treatment threshold is 3 or more beetles per ear, and pollination is still ongoing. Almost all pollination occurs within the first 4-5 days of silking, so any damage done by silk clipping after that period should not affect yield. It is important to look at ears throughout the field as Japanese beetles tend to congregate on field edges.
Regular scouting during the next several weeks will be essential to managing pest pressure effectively. Making decisions based on economic thresholds helps protect yield while avoiding unnecessary insecticide applications. If you would like assistance with scouting, pest identification, or choosing control options, contact your local Extension office.
Hayden Schug, Agriculture Agent | University of Maryland Extension, Charles County | hschug@umd.edu
Ben Beale, Principal Agriculture Agent | University of Maryland Extension, St. Mary’s County | bbeale@umd.edu
Cereal leaf beetle (CLB) is a familiar pest for Maryland wheat growers, and this time of year is ideal for scouting. We often find cereal leaf beetles while scouting for foliar disease like powdery mildew and stripe rust or checking wheat for flowering stage in preparation for fusarium head scab fungicide applications. While both adults and larvae can be present, it’s the larvae (Fig. 1) that cause the most damage. Often mistaken for bird droppings, these larvae feed aggressively on leaf tissue, potentially leading to significant yield loss in wheat if populations are high enough. In Southern Maryland we have found low populations of cereal leaf beetle larvae feeding in wheat throughout most fields. However, only one of these fields met the threshold that would warrant an insecticide application, so it’s important to understand threshold levels.
Figure 1. Cereal Leaf Beetle larva on wheat. (Hayden Schug).
Adult CLBs (Fig. 2) overwinter in wooded areas and field edges, becoming active as temperatures rise in early spring. They move into small grains and lay distinctive orange or yellowish eggs on the upper surface of wheat leaves, typically singly or in short rows. After hatching, the larvae feed for two to three weeks, chewing narrow, elongated strips between the leaf veins. This results in a “windowpane” effect—thin, transparent patches where only the leaf epidermis remains (Fig. 3). Larvae appear dark and slimy due to a coating of fecal material, though they are actually yellow underneath. When walking through a field with CLB present, a tell tale sign is black stains that appear on pants legs.
Figure 2. Adult Cereal Leaf Beetle on wheat. (Hayden Schug).
Scouting should begin in late March or early April, especially in no-till fields, those near wooded areas, or where there is a history of CLB infestation. When scouting, check 10 tillers at 10 random locations per field and look for larvae, eggs, or adult beetles, which have shiny blue wing covers with reddish thoraxes and legs.
Treatment is recommended when there is one larva per flag leaf after boot stage. If treatment is necessary, insecticides containing pyrethroids such as lambda-cyhalothrin (Warrior) or zeta-cypermethrin (Mustang Max) are effective. Be mindful of insecticide rotation to avoid resistance, and always check pre-harvest intervals.
Scouting and timely management are key to minimizing the impact of cereal leaf beetle. Keep an eye on your fields while out scouting to determine if you need to add an insecticide to your fungicide treatments.
Farmers and crop consultants are encouraged to complete this survey
This survey is intended to estimate the impact of pests on soybeans and currently used management strategies. Your perspective is valuable in making these estimates as accurate as possible. Your responses will be kept confidential and only aggregated responses for the state will be published. You can find last year’s estimates at https://midsouthentomologist.org.msstate.edu/Volume17/Vol-17-1_TOC.html.
Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu Dwayne Joseph, Agriculture Agent; and Haley Sater, Agriculture Agent University of Maryland Extension
Figure 1. Photo of a stink bug on a sweep net. Photo by N. Krambeck.
There is emerging concern among growers on the Eastern Shore of Maryland that our warmer winters and longer cover crop growing season may allow several stink bug species to overwinter and utilize cover crops for shelter and food. These stink bugs could then move into soybean fields after cover crop termination, potentially causing feeding injury and damping off damage to soybean seedlings. Fall-planted cover crops offer many benefits to soil health and the environment, so a survey was conducted during the 2024 growing season to investigate whether cover crops provide a suitable overwintering habitat for stink bugs.
Species of phytophagous stink bugs that are known economic pests of soybean include the brown stink bug, Euschistus servus (Say), green stink bug, Acrosternum hilare (Say), and the brown marmorated stink bug (Halyomorpha halys). Stink bugs use their piercing-sucking mouthparts to feed on the foliage and pods of soybeans, causing discolored, shriveled beans, reducing both the yield and quality of the crop.
Stink bugs typically overwinter as adults in protected areas such as fence rows, grassy field borders, under stones, or tree bark. Most species have one generation per year. They become active during the first warm spring days, typically in April. Females usually start depositing eggs in June. Nymphs hatch from these eggs and pass through five instars before becoming adults, with approximately five weeks elapsing between hatching and adult emergence. Adult stink bugs generally reach their highest population levels in September, when they can become an economic problem for soybeans.
To determine if stink bugs use late-season cover crops as overwintering habitat, 37 cover crop fields were sampled on the Eastern Shore of Maryland between mid-April and mid-May. The majority of fields surveyed were planted in a wheat-only cover crop. Densities of adults and nymphs were determined by taking ten sweeps with a standard sweep net at five to ten areas across each field. Fourteen fields were sampled twice before the cover crop was terminated. The other fields were sampled once due to weather constraints before terminating the cover crop. Eight fields were sampled again in June when soybean plants were 6-12 inches high.
Ninety-nine stink bugs were counted across the 51 scouting times, averaging 1.94 stink bugs per field per scouting date. 94% of the species counted were adult native brown stink bugs. The majority of stink bugs (79%) were counted during the last week of April and the first week of May. Along with stink bugs, 225 ladybird beetle adults and larvae were counted. Fifteen stink bugs were found during the scouting of soybean seedlings in June. The low number of stink bugs found in 2024 in spring cover crops suggests they are not overwintered in cover crops, and adding an insecticide to cover crop burndown spray is unnecessary to control them.
We want to thank all the farmers who allowed us to sample their fields. If you are interested in participating in this study in 2025, please contact Emily Zobel at ezobel@umd.edu or (410) 228-8800. The Maryland Soybean Board provided financial support for this project (project # 80333).
Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu University of Maryland Extension, Dorchester County
The second generation of corn earworms/podworms/headworms (Helicoverpa zea Boddie, Figure 1) will soon emerge from corn fields and other crops. On soybeans, the podworms may feed on foliage, flowers, and fruit. Female moths will lay eggs over the entire plant, but caterpillars prefer to feed on tender vegetative terminals and/or flower clusters. Fields in bloom to pod-set stage with an open canopy and stressed soybeans during this second-generation flight typically reach the highest populations and are at the most risk. Due to our early summer weather this year, more fields might be at risk compared to last year. Podworm populations can be spotty, so one field may have high podworm populations while the field next to it may have low podworm populations. The only way to know what is happening in a particular field is to sample it. You can sample pod worms using either a sweep net or drop cloth.
Figure 1. Corn earworm/soybean podworm moths are robust, light brownish-tan with a 1” to 1 1/2” wingspan. They have a prominent dark spot on each forewing. They tend to be active during the evening and dark hours but can be commonly seen in fields during the day. Photo by Ronald Smith, Auburn University, Bugwood.org.
Thresholds are generally around 2-3 worms per 15 sweeps. NC State University (NCSU) has a nice podworm threshold calculator that helps you customize your threshold based on treatment cost, row width, and price of the bean into consideration: https://bit.ly/NCSUwormthershold. Davis Owen, Delaware Extension Entomologist, recommended applying Besiege or Elevest at their low rates last year to control soybean podworms. Pyrethroids are not guaranteed to provide good efficacy.
Even though fields in the bloom stage are at risk for high populations of podworms, it is generally recommended not to spray during this stage and instead wait for the end of flowering or early pod set to treat. Soybean plants produce more flowers than they can convert into pods. A study conducted by NCSU has found that soybean plants can compensate for this loss of flowers due to caterpillar feeding damage. No yield losses were recorded even with caterpillar levels three times higher than the economic threshold for podding-stage soybeans. Also, young caterpillars tend to have a high mortality rate. They are often difficult to reach with chemical controls while feeding within flower clusters.
Continue to scout during pod growth and fill, as serious yield loss can occur when large caterpillars coincide with soybean seeds that have achieved almost full size.
Reports are for crop conditions up to June 6, 2024.
Western Maryland
I have a good friend who says, “I don’t have forty years of experience farming; I have one year’s experience forty times.” This spring has been just like that. We have had more moisture than last year, but then came the hot days, cool weather, and more moisture. Corn is planted, and the first cutting of hay is in the barn. Soybean planting is winding down, and grain harvest will be here before you know it, starting with barley. With the moisture we will see how much disease took its toll. Fungicide applications weren’t always as timely as we would hope because of the frequent showers. Until next month when harvest is underway it is then the story will be told.—Jeff Semler, Washington Co.
Central Maryland
Planting is almost complete. Slugs have been an issue; some soybean fields will need replanting. It was getting a bit dry, which did provide a good hay-making window. However, a thunderstorm is rolling in as I write this, giving us some needed moisture. Small grains are looking good. Summer annual weeds like pigweed, crabgrass, and foxtail are starting to take off with this warmer weather.—Kelly Nichols, Montgomery Co.
Northern Maryland
Spring has been a little challenging in our area. A dry spell in mid-late April made herbicide efficacy less than ideal, followed by several weeks of a lot of rain, which activated herbicides but prevented the timely termination of other cover crops. Early planted corn and beans are up and out of the ground, with earliest planted corn around V6 and getting it’s second shot of nitrogen. Rainy weather in late April early May delayed planting the remainder of the crop by about two weeks. These later planted fields are much further behind than usual but are looking good. There has been some slug damage present, it gets worse the further west you go in the region. Barley nearing harvest and wheat is starting to turn. There are symptoms of head scab but it remains to be seen how severe DON levels are in the grain. The wet weather has made it very difficult to make timely dry hay.—Andy Kness, Harford Co.
Upper and Mid Shore
Corn acreage is looking good, with a nice color and uniform growth. Depending on when it was planted, corn is in the V2 to V5 stages right now. Most of our full-season soybeans are already in the ground. As usual, rain across the region has been hit or miss, with some areas getting a lot more than others. These downpours might not be ideal, but they’re certainly better than no rain at all. Barley harvest is just around the corner. Summer annual weeds are popping up and are ready to impact yield potential. If your residual herbicide program hasn’t kept them in check, now’s the time for a POST application. And remember, using multiple modes of action in your tank-mix slows down the evolution of herbicide resistance in weeds.—Dwayne Joseph, Kent Co.
Lower Shore
has been planted and currently in emergence stage to around V4 stage. Corn is looking good, and being sprayed to combat early season weeds. Soybean planting is currently underway. In a few instances, early planted soybean (early April) had to be replanted due to slug damage. Wheat is looking good and drying down. Farmers are gearing up to cut wheat, which is earlier than anticipated.—Sarah Hirsh, Somerset Co.
Southern Maryland
As I write this, a very welcome rain is falling outside my window. As we move into the hotter days of June, the old adage that we are only a week away from a drought at any time is holding true. Two weeks ago it was too wet to get into fields and this week we have corn fields starting to show curling leaves and drought stress. In general corn is off to a good start with most sidedress N applications completed. There have been a lot of catch up full season beans planted in the last 2 weeks. Slug feeding has been minimal this year. Barley is drying down with harvest expected any day. Wheat will not be far behind. Ryegrass continues to be a challenge for producers in both burndown situations in corn and beans, as well as small grains. Many fields may need a harvest aid to kill the ryegrass and allow for timely harvest of wheat. Given the wet conditions following flowering, we are encouraging producers to get the wheat crop off as quick as possible to maintain grain quality.—Ben Beale, St. Mary’s Co.
Emily Zobel, Senior Agriculture Agent Associate | ezobel@umd.edu
University of Maryland Extension, Dorchester County
Corn
Continue scouting for cutworms and stink bugs. The threshold for cutworms is 2-5% cut plants up to the V5 stage and with active larvae present. Stinkbugs will move into corn as surrounding small grains are harvested. When scouting for stinkbugs, pay close attention to the thorax of brown color stink bugs as the beneficial spined soldier bug is often mistaken for the invasive brown marmorated stink bug. The spined soldier bug has a prominent spine on each “shoulder.” NCSU’s suggested threshold is 13 stinkbugs per 100 plants for V1-V6, 10 stinkbugs per 100 plants for V14-VT, and 28 stinkbugs per 100 plants for R1-R2. The most critical time to treat if between V14 to VT is just before the primary ear is exposed to avoid banana ear. Stinkbugs will likely be around the primary ear at this point so that should be the target of the application.
Soybean
Early-season defoliator pests that are active right now include bean leaf beetles, green cloverworms, slugs, and grasshoppers. Soybeans can typically withstand a decent amount of defoliation before yield losses occur. If defoliation reaches 30%, and you are finding one grasshopper per sweep or 2-3 bean leaf beetles per plant treatment may be advisable.
Figure 1. Bean leaf beetle with feeding damage on a soybean leaf. Photo by Adam Sisson, Iowa State University, Bugwood.org.
Alfalfa
Begin scouting for potato leafhopper. Ten sets of 10-20 sweeps using a sweep net should be taken in random locations in the field. A detail threshold for alfalfa is based on the plant size and cost of the hay can be found online but a general guideline is 3” or less is 20 leafhoppers per 100 sweeps, 4-6” tall is 50 per 100 sweeps, 7-10” tall is 100 per 100 sweeps, and greater than 11” is 150 per 100 sweeps. If the field is more than 60 percent bud stage or if it has experienced “hopper burn,” the alfalfa should be cut instead of sprayed. Fresh-cut alfalfa should not be sprayed as leafhopper adults tend to move out of the field when it is being cut.
Figure 2. Potato leafhoppers in a sweep net. Photo by Bryan Jensen, University of Wisconsin, Bugwood.org.
Maria Cramer, PhD Candidate and Kelly Hamby, Entomology Extension Specialist Department of Entomology, University of Maryland
Background
Research Questions
Are the NST Poncho 250® and the in-furrow pyrethroid Capture LFR® effective at controlling pests and increasing yield in high-input (Bt) or low-input (non-Bt) field corn in Maryland?
Do Poncho and Capture hurt slug predators and flare up slug damage?
Study Design
In order to capture the range of pest pressures and growing conditions in Maryland, we replicated our study across 3 UMD research farms (Keedysville, Beltsville, and Queenstown) and over 3 years (2020-2022). At each location we planted one field of a Bt hybrid and one field of a similarly-yielding non-Bt hybrid as early as possible in the growing season (Table 1). In 2020 our Bt hybrid was LC1196 VT2P (Local Seed, Memphis, TN) which expresses Cry1A.105/Cry2Ab2 proteins. In 2021 and 2022 we planted P1197YHR (Pioneer Hi-bred International. Johnston, IA) which contains Cry1Ab and Cry1F proteins. We planted P1197LR (Pioneer Hi-bred International, Inc. Johnston, IA) for our non-Bt hybrid all three years. All hybrids had excellent yield potential and were grown with standard no-till practices. In each field we established 3 replicates of 3 treatments at planting: 1) an untreated control, with bare seed and no in-furrow product, 2) an in-furrow pyrethroid treatment using Capture LFR® (active ingredient: bifenthrin, rate: 13.6 fl oz/acre), and 3) an NST treatment using Poncho® (active ingredient: clothianidin, rate: 0.25 mg/seed). Each replicate consisted of 24 rows of corn at 30 inch row spacing, and was 200 feet long.
Year
Location
Planting date
Sampling dates
2020
Keedysville
May 18
June 8
Beltsville
May 21
June 10
Queenstown
May 13
June 3 and 4
2021
Keedysville
May 14
June 1 and 3
Beltsville
May 17
June 2
Queenstown
May 4
May 25 and 26
2022
Keedysville
May 26
June 10
Beltsville
June 2
June 21
Queenstown
May 12
May 31
Question 1: Are the NST Poncho 250® and the in-furrow pyrethroid Capture LFR® effective at controlling pests and increasing yield in high-input (Bt) or low-input (non-Bt) field corn in Maryland?
Data Collection
In order to evaluate how the treatments affected pest pressure, we visually sampled V2-V3 corn for types of pest damage (Figure 1), recording the number of plants and area damaged. We counted the number of healthy and stunted plants to determine if the treatments impacted stand. Because neonicotinoids can sometimes stimulate plant growth unrelated to pest damage7,8, we measured plant height to determine if plant growth was impacted by either treatment. At the end of the growing season, we measured stand again and harvested the corn to collect yield data.
Figure 1. Diagnostic seedling pest damage: a) soil pest, b) cutworm, c) armyworm, d) slug, e) stinkbug, f) miscellaneous feeding damage from a spotted cucumber beetle.
Results and Takeaways for Question 1
Poncho reduced insect damage more consistently than Capture LFR (in both Bt and non-Bt corn) and increased Bt corn stand. Capture LFR sometimes reduced insect damage (in non-Bt corn), but never improved stand.
We compared the number of seedlings with any type of pest damage between treatments and found that Poncho decreased damage about 62% in Bt corn and about 66% in non-Bt corn (Figure 2a and 2b). Compared to the control, Capture did not reduce damage in the Bt corn, but did reduce damage by about half in the non-Bt. Poncho increased stand about 8% compared to control in the Bt corn (25,731 ± 456 plants per acre and 23,623 ± 714 plants per acre, respectively), but did not improve it for non-Bt. Capture did not impact stand for either Bt or non-Bt corn.
Figure 2. Mean % ± SE of seedling A) Bt and B) non-Bt corn plants damaged by pests. Data were collected across three UMD research farms from 2020-2022. Within each graph, treatment bars with different letter above them are significantly different from each other.
There were no yield benefits from using either insecticide in either corn. This was likely due to a lack of economic pest pressure.
Non-Bt and Bt yields were the same across treatments (Figure 3A and 3B). This was probably because pest pressure was so low. Even though Poncho and Capture decreased pest damage, pests were below treatment thresholds—for example, armyworm damage in the control ranged from 0% to 5.4% of Bt plants, and 0% to 22.9% of non-Bt plants, in both cases below the treatment threshold of 35%9. Cutworm damage was similarly low ranging from 1% to 6.3% in Bt control and 0.5% to 3.8% in non-Bt control, also below the treatment threshold of 10% feeding damage9.
Figure 3. Mean yield ± standard error in bushels per acre corrected to 15.5% moisture of A) Bt corn and B) non-Bt corn. Yield data from 2020-2022 across three UMD research farms. Treatments did not significantly impact yield.
Takeaway: Pest pressure and yield were similar between the Bt and non-Bt varieties, and non-Bt yielded well without any insecticides. In general, without pre-existing pest problems in a given field, at-planting insecticides are unlikely to pay off in Maryland.
Question 2: Do Poncho and Capture hurt slug predators and flare up slug damage?
Data Collection
To assess the effect of treatments on slug biocontrol agents, we measured slug predatory ground beetles and predation. We measured predatory beetles with pitfall traps for three consecutive weeks. Because the predators that eat slugs also attack caterpillars, we used sentinel caterpillars to see how much predation was occurring (Figure 4). We placed sentinel caterpillars in the plots overnight, collected them the following morning, and assessed signs of damage from predators. To determine if slugs were flared up by the treatments, we measured slug abundance once a week for 6 weeks beginning between 14 to 21 days after planting and measured slug-damaged seedlings during V2-V3.
Figure 4. Sentinel caterpillars placed in field overnight and collected in the morning to determine predator activity.
Results and Takeaways for Question 2
Predation on sentinel caterpillars was not decreased by insecticides.
We measured the percent of sentinel prey that were damaged by predators overnight (Figure 5) and saw no relationship between treatment and predation rates (Figure 6). This suggests that the insecticides did not decrease predator activity in treated plots. We did generally see some level of predation all weeks at our locations, indicating that predators are usually present in seedling corn.
Figure 5. Top: predators feeding on sentinel prey. Bottom: examples of damaged prey proportions. Images: M. Cramer, University of Maryland.
Figure 6. Mean ± SE % sentinel prey caterpillars consumed across three UMD research farms from 2020-2022. Control, Capture, and Poncho did not significantly differ.
Predator abundance was not altered by insecticides.
When we measured the weekly counts of ground beetles, we found similar results between treatments. This was true when we looked at all ground beetles (predators, omnivores, and seed-eaters), as well as when we looked only at predatory beetles (Figure 7A and 7B).
Figure 7. Mean ± SE count of A) all ground beetles, and B) specifically predatory ground beetles, caught per week in pitfall traps across three UMD research farms from 2020-2022. No significant differences.
Slug natural enemies did occur throughout the study, suggesting that biocontrol could be more intentionally leveraged.
The two most abundant ground beetle species in our study were both predators. One of these species, Chlaenius tricolor (Figure 8) is a slug predator that consumes slugs in agricultural ecosystems5,10. Although its abundance was not affected by treatments, it was present at all locations in all years, suggesting that it is a particularly important slug natural enemy in Maryland crops.
Neither insecticide increased slug abundance or slug damage.
If treatments had negatively affected predators, we would expect to see more slugs and damage in the insecticide plots. However, when we compared slug counts between treatments, we found that the insecticide treatments were not different from the control (Figure 9). Slug damage to the seedling corn was also similar across the control and insecticide treatments (Figure 10).
Figure 9. Mean number of slugs per replicate plot ± SE the week closest to seedling sampling across three UMD research farms from 2020-2022. No significant differences.
Figure 10. Mean ± SE % of corn seedlings damaged by slugs across three UMD research farms from 2020-2022. Control. No significant differences.
While slugs can be damaging in many crops, the worst slug damage in our study did not affect corn stand or yield, suggesting that corn is generally tolerant of slug damage at the levels we observed in this study.
Slug damage was scarce across years and locations except in 2021 at Keedysville. Even in that case where a high proportion of seedlings (42% ± 4% on average) were damaged by slugs, we did not see an associated decrease in stand or yield. Corn seedlings were able to outgrow the slug damage as the weather warmed, even when they appeared severely defoliated. The seedling resilience we observed is supported by work on hail damage in corn which shows that as long as the growing point is intact, corn can regrow from complete defoliation11.
Even though we did not see non-target effects in this study, both pyrethroids and neonicotinoids can decrease natural enemies in crop fields6,12–14.
Acknowledgments
We would like to thank the farm managers and staff of WYEREC, WMREC, and CMREC Beltsville for their expertise and assistance. We would also like to thank the Hamby lab’s many undergraduate researchers for helping complete this project with all their hard work.
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Kelly Hamby, Associate Professor and Extension Specialist, University of Maryland and
David Owens, Extension Entomologist, University of Delaware
Figure 1. Barley Yellow Dwarf patch in a field of malting barley, March 2023. Photo: David Owens, Univ. of Delaware.
Last season, aphids transmitted an unusual amount of barley yellow dwarf virus (BYDV) to wheat and barley across the Delmarva Peninsula. BYDV is particularly important when it infects plants in the fall. Fall BYDV infections can stunt plants (noticed as early as green-up, Figure 1) and cause more serious yield loss than spring infections. Our most common small grain aphid species are bird cherry oat aphid (Figure 2) and English grain aphid, although bird cherry oat aphid are associated with greater and more severe incidence of BYDV.
Figure 2. Bird cherry-oat aphids.
Historically, planting after the Hessian fly-free date (Table 1) reduced the likelihood of fall BYDV infection. However, fly-free dates were calculated more than 100 years ago, and it is now not uncommon for our first killing frosts to occur in late October or even November. Long falls with milder weather allow more time for aphids to colonize fields and potentially transmit the virus. Small grains varieties vary in their susceptibility to BYDV, and planting varieties with at least some tolerance can help. Unfortunately, resistant varieties are not available in barley. Finally, monitoring and managing the aphid vectors may be necessary.
Identifying bird cherry-oat aphid: A magnifying hand lens is required to identify aphids. Bird cherry-oat aphid ranges from orange green to olive green to greenish black. Wingless individuals typically have a reddish orange patch around the base of the cornicles (tail pipes). Winged individuals tend to be very dark. Their legs, cornicles, and antennae are similar in color to their bodies and medium in size.
Monitoring and thresholds: Typically, monitoring aphids in the fall and at green-up provides the best chance of identifying and mitigating BYDV risk. Scout ten locations per field avoiding field margins and look at 1 ft of row in each, making sure to look at the crown (at or below ground level), at the stem, and on the undersides of leaves. English grain aphids tend to feed on the uppermost portions of the plants while bird cherry oat aphids tend to cluster on the lower portions, especially in barley.
University extension threshold recommendations vary by region. In southern states, 6 aphids/row-ft is considered justification for a treatment in the fall. North Carolina uses a threshold of 20 aphids/row-ft where BYDV has been a problem and cold weather is not in the 7 day forecast. For other small grains, consider increasing the threshold to 25-50 aphids per foot of row.
In 2022, one of the malting barley fields sampled averaged 17 aphids per row-ft in early November. Because of unusually warm winter weather in which average temperatures were greater than 38 degrees, aphid populations peaked in one field at 235 aphids per row-ft that had averaged 1.8 per row-ft in November. This highlights the need to regularly monitor aphid populations during periods of mild weather.
Natural enemies: A number of natural enemies feed upon or parasitize aphids and they often do a good job keeping aphid populations down. One natural enemy per 50-100 aphids should be sufficient to control aphid populations. In addition, they are good at finding aphids even when their populations are low. Small wasps that develop within aphids leaving behind “mummy” aphids (Figure 3A), lady beetles, lacewing larvae (Figure 3B), and flower fly larvae (Figure 3C) are especially common aphid natural enemies. Insecticides will also kill these natural enemies.
Figure 3. Aphid natural enemies A) parasitoid wasp and golden or tan colored “mummy” aphids, B) lacewing larva eating aphids, C) flower fly larva eating aphids. Images: David Cappaert, Bugwood.org.
Insecticides: Seed treatments (e.g., Cruiser, Gaucho) provide some protection from fall aphids, but do not continue to provide protection into the spring and are not economic in years where aphids do not occur. Due to the differences in economics and BYDV susceptibility of malting barley varieties, seed treatments may be more useful than in feed barley or wheat. We generally recommend a foliar insecticide when aphid populations reach threshold. Small grain aphids are generally quite susceptible to insecticides. Pyrethroid products (e.g., Warrior) or a pyrethroid-neonicotinoid mix (e.g., Endigo, labeled for barley only) work well for aphid control.
Table 1. Hessian fly-free dates for Maryland and Delaware counties
Flanders, K., Herbert, A., Buntin, D., Johnson, D., Bowen, K., Murphy, J. F., Chapin, J., Hagan, A. 2006. Barley Yellow Dwarf in Small Grains in the Southeast. https://entomology.ca.uky.edu/files/efpdf1/ef150.pdf.
