If Drought Persists, Grasshoppers in Soybeans Might be a Pest to Cause Worry

Description of Problem

It seems that some insect pests have taken advantage of the dry conditions maintained during the last month in western Kentucky. Since insects depend on certain thermal conditions to activate their metabolism, it is not surprising that the persistent warm temperatures are triggering the populations of different insects in the field. Immature individuals (known as nymphs) of short-horned grasshoppers have been observed feeding on both full-season and double crop soybeans in Lyon and Caldwell counties (Figure 1). Populations of these insects seem to be increasing and damage is already present (Figure 2).

Figure 1. Aggregation of short-horned grasshoppers on V2 soybean plant. (Photo: Armando Falcon, UK)

Figure 2. Damage on soybean leaf produced by grasshoppers. (Photo: Armando Falcon, UK)

Most grasshoppers found during the last week of June in western Kentucky are still in the immature (1st to 3rd instar nymphs) stage and the damage on soybean leaves (irregular holes in the leaves) did not exceed 30%, which is considered the threshold for the vegetative stages. Usually in the case of soybeans, if an average of 30 to 45 nymphs or 8 to 14 adults per square yard are found (average of  20-sample sites), insecticides may be used. Before doing any spraying, it is important to keep tracking the number of grasshoppers in the field as grasshoppers are insects that move easily and prefer field edges. Visual scouting is recommended to determine whether treatment is needed.

Additionally, due to the rainfall during the last weekend of June in the western counties, chances are that soybeans may be visited by other pests, such as defoliating caterpillars, Japanese beetles, and bean leaf beetles.

IPM Techniques & Scouting

The information below was taken from a University of Kentucky publication (Grasshoppers in Soybeans) and it provides valuable information for scouting grasshoppers.

  • In no-till fields it is important to scout for grasshoppers from cotyledon stage to first trifoliate. Under no-till conditions, grasshoppers may occur very early in the season and be distributed across the field. For conventional tilled fields, grasshoppers usually are not a problem until mid-summer but will be present all season. Grasshoppers tend to be more of a problem during dry weather.
  • Grasshopper populations are very hard to estimate. When counting grasshoppers in a field do not walk in a straight line.
  • Change the direction of your walk as you count so that the grasshoppers do not continually jump in front of you causing an overestimation of the population.
  • Watch for large numbers of hoppers as you move through the field and watch for defoliation. The number of sites you need to examine in a field is based on the size of the field.
  • Field borders should also be checked since grasshoppers tend to move into soybeans fields from the edges and spread across the field.
  • Naturally occurring fungal diseases can drastically reduce grasshopper numbers. Dead and or dying grasshoppers may be found clinging to foliage.

By Armando Falcon-Brindis, Entomology Research Associate, and Raul T. Villanueva, Entomology Extension Specialist

Posted in Grains
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