We have had a busy few weeks fielding questions from farmers and agronomists about various insects that are feeding in crop fields. Temperatures have been warm and that has prompted many insects to resume activity – they are hungry! The biggest concern lately has been true armyworms, but other questions have been about bean leaf beetles, black cutworms, and thistle caterpillars. We encourage people to keep scouting fields and to reach out to us or your local field agronomist with any questions. Here, we will highlight some of the insects we have seen or heard about lately that you should keep an eye out for.
True armyworm has been found throughout the state, which is not surprising considering moths were found in nearly every county that participated in the trapping network this spring. They’ve been found in corn, soybean, and alfalfa fields, though the crop was not injured in most cases. Most of these fields had a rye cover crop that was actively growing this spring (ideal egg-laying sites for females!) and roller-crimped around the time of planting. The larvae did not appear to be feeding on the crop but were feeding on the green rye tissue. Scouting for larvae is easiest when it is darker (dawn, dusk, or cloudy days) or cooler since they tend to hide under residue or in the soil during hot, sunny days. If larvae are large (over an inch), they will likely wrap up feeding soon and may be less susceptible to insecticides, so treatment may not be economical in those situations. However, if small larvae are present and the crop is being injured, treatment may be warranted. Use this article to find more information on dynamic treatment thresholds for true armyworm.
Similar to true armyworm, we monitor for black cutworm flights into Iowa each spring and predict cutting dates for corn. We have only heard two reports of black cutworm, and one of those fields in central Iowa was treated after larvae were found cutting V3 corn plants (8-10% of plants in one area of the field). Most corn in Iowa is likely past the V5 stage and is no longer susceptible to cutting; however, any late-planted fields should be scouted until plants are no longer at risk.
Stalk borers began migrating to corn last week, and although we have not heard reports of injury to crops yet, it is a good idea to keep an eye out for this pest. They tend to reinfest the same fields annually, so fields with a history of stalk borer injury should be prioritized. Look for dead heads in perennial grasses nearby; this is an indicator that stalk borers are in the area. Once they outgrow a stem, they will leave in search of a bigger stem (i.e., corn). Stalk borers tend to only move into the nearest 6-8 rows of corn and cause injury. If dead heads are found in nearby grasses, there is a limited window to use insecticides to kill larvae; once they have bored into corn stalks, insecticides are not effective. Corn is unlikely to be killed by stalk borer after the V7 stage.
Bean leaf beetle
Bean leaf beetles are an early-season soybean pest nearly every year, but winter mortality was variable across the state. Over the past few weeks, we have seen feeding on early-planted soybeans in central Iowa. This feeding has been less severe than we’ve seen the past few years, but it is a good time to scout for this pest. The overwintering generation of bean leaf beetle may vector bean pod mottle virus, which may be a concern in fields that are growing food-grade soybeans or soybeans for seed.
Soybean gall midge
Two soybean gall midge adults were found at the Iowa State University Northwest Research and Demonstration Farm this week. Based on our observations, larvae can be found in soybean stems approximately two weeks after the first adult emergence. We encourage people to begin scouting for larvae once soybeans are at the V2 growth stage and continue scouting throughout the season. Visit https://soybeangallmidge.org/soybean-gall-midge-distribution to see counties with known soybean gall midge infestations, and let us know if you find soybean gall midge larvae in counties not represented on the map.
Potato leafhoppers are migratory pests and typically arrive in Iowa in early June. We heard our first reports of people finding potato leafhopper in alfalfa this week and also noticed potato leafhopper adults on soybeans near Ames. They use a piercing-sucking stylet to feed on the leaves and inject toxic saliva that causes discoloration and stunting of plants. Injury is commonly confused with herbicide injury or nutrient deficiencies in soybean and alfalfa, and the insect is commonly confused with aphids. Use a sweep net to look for potato leafhoppers, and refer to this encyclopedia article for dynamic treatment thresholds.
We have not found soybean aphid in Iowa yet this year, but our neighbors to the north have started seeing colonies on soybeans. Typically, early in the season we are alerted to small soybean colonies when we see a lot of beneficial insects (e.g., ladybugs) or ants on plants. The beneficial insect community in Iowa usually does a good job of keeping populations low early in the season, but it is worthwhile to scout for soybean aphids to keep track of how populations are changing throughout the season. Learn more about scouting for soybean aphid here.
There is a suction trap network to monitor for migratory aphids in the northcentral region. Currently, there are 31 traps (4 in Iowa) that are monitored for about 6 months of the year. We learned that last week, the “Ames” trap had really high numbers of greenbug. Typically, this aphid feeds on small grains, sorghum, bluegrass, and corn. Like all aphids, greenbugs have a piercing-sucking stylet that removes phloem. Prolonged feeding can result in discolored leaves and stunted plants. Although they can build up colonies in vegetative corn, most of the time they aren’t an economic concern. Usually, predators and parasitoid wasps suppress colonies before corn begins to tassel. Greenbugs do vector viruses in other crops and turf.
Thistle caterpillars are a sporadic pest of soybean in Iowa, but many may remember the “outbreak” we experienced in 2019. So far this year, we’ve only heard of someone finding a single thistle caterpillar in a soybean field but keep an eye out for this pest! They are typically easy to spot since they web up trifoliates to make a nest to feed in. Inside the nest, you can find a spiky-looking caterpillar and lots of frass pellets. Sometimes, you may find sickly-looking caterpillars that are infected with entomopathogenic fungi (insect-eating fungi), which means biological control is hard at work in the field. The adult is the painted lady butterfly!
Honorable mentions: white margined burrow bug and hackberry emperor
Almost every year, we get photos of hackberry emperor butterflies fluttering around soybean fields. These are not pests of field crops, rather their larvae feed on hackberry trees. People often confuse this butterfly with the painted lady. A new one for us this year is the white margined burrower bug, which was found inhabiting soybeans in Missouri. According to this article from Nebraska, they typically feed on several weed species and may spill over to soybean when those hosts are terminated. They would not be considered an economic pest of soybeans, so don’t be alarmed if you come across this insect.