By Ashley Dean and Dr. Erin Hodgson
Grasshopper activity in field crops has picked up recently, and these jumpy insects and their defoliation have caught the attention of crop scouts and agronomists across the state. Grasshoppers tend to thrive under dry conditions, and although most areas have received adequate moisture in 2024, consecutive drought years have likely led to higher than usual populations. This article discusses identification, scouting and management of grasshoppers in corn and soybeans.
Identification and Life Cycle
There are several species of grasshoppers in Iowa, but two common species found in field crops are the redlegged grasshopper and the differential grasshopper. Redlegged grasshoppers are the smaller of the two, growing to 1 ¼ inches long. They have yellow-green bodies and bright red on the hindlegs (Photo 1). Differential grasshoppers are up to 2 inches long and dark yellow or olive green, with black chevrons on the upper part of the hindlegs (Photo 2). Nymphs of each species look similar to the adult except they do not have fully developed wings. All grasshoppers have large hindlegs for jumping.
Grasshoppers overwinter as eggs in the soil, usually in grassy or weedy areas that are the primary feeding sites for nymphs and adults. Nymphs emerge in May or June and go through several instars before reaching the adult stage. Toward the end of the season, females lay eggs in the soil with a secretion that hardens and protects the eggs. Most grasshoppers have one generation each year.
Injury and Scouting
Grasshoppers feed and cause injury on many plants, including corn, soybeans, and alfalfa. They can attack most growth stages of these crops, so the onset of injury is dependent on environmental conditions each year that impact insect development, planting activities, and crop growth.
Grasshoppers generally consume leaf tissue of older corn plants, leaving the midrib (Photo 3), and they may feed on ears of corn by chewing through the husk. They may also clip silks. On soybean, nymphs chew irregular holes in tender leaf tissue. Older nymphs and adults can consume all leaf tissue except the tougher veins. Grasshoppers may also chew through soybean pods and destroy seeds. Nymphs and adults chew irregular holes in alfalfa leaves, and they are typically found feeding on the newer alfalfa growth.
Grasshopper injury is typically worse at field edges, especially near grassy or weedy areas like fencerows, waterways, and ditches. Grasshoppers often move into fields when grasses or weeds are terminated. Injury is typically worse during warm, dry summers, and outbreaks are more likely after consecutive dry years.
Scouting for grasshoppers involves looking for defoliation or estimating the number of grasshoppers per square yard. Estimating numbers is difficult because they are highly mobile insects. A sweep net can be useful to capture grasshoppers, but there is no established threshold based on sweep samples. See the Management section for recommended thresholds to use.
Management
Grasshoppers usually do not need to be managed in field crops, but agronomists have been concerned about the large number of small nymphs that are present, because this likely means feeding will continue for quite some time. Here are some management considerations for grasshoppers:
Natural enemies – There are several species of pathogens that normally keep grasshopper populations in check during wet years. If you start to see dead or sickly-looking grasshoppers (Photo 5), that likely indicates activity of these pathogens in the field that may contribute to control of the population. This disease is often called “summit disease” because it changes the behavior of grasshoppers to find a high point prior to death. Spores are spread once grasshoppers begin to decay.
If the field is corn, what is the growth stage? What is being injured? Is pollination complete? An insecticide might be worthwhile to protect pollination if grasshoppers are present in the field, silks are being clipped to within ½ inch of the ear tip, and pollination is not yet complete. To minimize the impact on ear development, an insecticide might be worthwhile if grasshoppers are present and a considerable amount of defoliation is occurring above the ear or if there is injury to ears from grasshoppers.
If the field is soybeans, what is the growth stage? What is being injured? The threshold for insect defoliation is 20% after bloom (R1). Thresholds based on insect counts are eight or more adult grasshoppers per square yard or 15 or more nymphs per square yard. If pods are being injured, consider an insecticide if more than 5% of pods are injured.
If the field is alfalfa, where is the feeding happening? The threshold in alfalfa is 20 grasshoppers per square yard near the field edge, and eight grasshoppers per square yard in the interior.
Location of infestation – In all crops, a border or localized treatment might be a good option if grasshopper populations and injury are restricted to a particular area or the border rows. This could save on application costs and unnecessary insecticide use on the entire field.
Timing of application – Keep in mind that some insecticides have a lengthy pre-harvest interval that needs to be considered.
Protecting grain quality – Injury by grasshoppers and other pests that feed on ears and pods can often be entry points for pathogens that can reduce grain quality.
Making management decisions about grasshoppers is complex, considering how mobile the insects are and that there are no defoliation thresholds in corn. Consider the severity of injury to the crop, the life stage of the insect (nymphs will feed for a long time), and the cost of an insecticide application before deciding to manage grasshoppers. If the crop is stressed due to drought or other issues, tolerance for grasshopper feeding should be lower.
Source : iastate.edu