Grasshopper Management in Corn

  • There are several grasshopper species that can attack corn.
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  • Grasshopper populations fluctuate widely from year to year and may cause serious damage during periodic outbreaks.
  • 13;10;Early scouting and proper management practices can help keep grasshopper populations and damage to a minimum.
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Biology

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Grasshoppers feed on grasses and weeds and often move to cultivated crops when the vegetation is consumed or reduced by drought conditions. Grasshoppers usually cause minor damage every year; however, they may become very destructive during periodic outbreaks.

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Grasshopper populations are affected by weather conditions. Several years of hot, dry summers and warm autumns contribute to the development of large populations.1 Survival of nymphs and adults may be higher during dry weather and long warm autumns, which can provide grasshoppers more time to feed and lay eggs.

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Identification

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Although there are over 100 species of grasshoppers in the United States, only four species are likely to become an economically damaging pests to crops, including corn.

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Figure 1. Differential grasshopper. Photo courtesy of David Riley, University of Georgia, Bugwood.org Figure 2. Migratory grasshopper. Photo courtesy of Joseph Berger, Bugwood.org – UGA. Figure 3. Redlegged grasshopper Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org. Figure 4. Twostriped grasshopper Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.

 

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Differential Grasshopper (Melanoplus differentialis) is a relatively large grasshopper. Adults are 1.5" – 1.75" long and yellowish or olive-green in color. The femur of the jumping leg is distinctly marked with black chevrons (Figure 1).4

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Migratory Grasshopper (Melanoplus sanguinipes). The adult migratory grasshopper is about 1" long, brown to gray with a distinctive black mark behind its eye. It is a strong flier and may swarm over long distances (Figure 2).

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Redlegged Grasshopper (Melanoplus femurrubrum). Adults are about 0.75" – 1" long, brownish-red in color, and have a pinkish-red tibia on the hind leg (Figure 3 - left).

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Twostriped Grasshopper (Melanoplus bivittatus) is another relatively large species. Adults are 1.25" – 2" long and are grayish or brownish-green with two distinct light yellow stripes extending from the eyes to the wing tips (Figure 3 - right).1

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Life Cycle

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In late summer or early fall, female grasshoppers lay clusters of eggs 0.5" to 2" below the soil surface in an elongated pod. They prefer firm, uncultivated soil such as Conservation Reserve Program (CRP) fields, roadsides, and pastures.

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Each egg pod consists of 20 to over 100 eggs, which are cemented together. A female can lay up to 25 egg pods.

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The eggs will overwinter in the soil and small, wingless nymphs will emerge in May or June. Grasshopper nymphs go through five to six stages and require 40 to 60 days to become adults.6 There is usually only one generation per year.

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Damage

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Grasshoppers can feed on aboveground tissues of a corn plant, including leaves, tassels, green silks, and ears. They feed from the outer edge of the leaves inward and the damage appears as round to ragged holes on the leaves.

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Very large grasshopper populations can consume an entire leaf leaving only the midrib.

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Grasshopper feeding on fresh silks may result in reduced pollination and often causes corn ears to be blank or only partly filled.7 In severe infestations, the entire corn stand is stripped of leaves leaving only bare stalks. Damage can be severe in dry years when natural vegetation is limited and grasshoppers migrate to corn fields.

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Scouting and Thresholds

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Because grasshoppers often lay eggs in field margins, these areas should be monitored closely in the spring and early summer. Scout deeper into the filed for grasshopper activity as the season progresses.

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Grasshopper density can be estimated by using the square-yard method. Count the nymphs and adults in field borders or infested field areas at five random locations to determine an average number of grasshoppers per square yard.6 Action thresholds for field borders and infested areas in the field that warrant insecticide treatment are listed in Table 1.

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Management

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Chemical Control. Grasshopper control is most effective before they become adults, as adults are highly mobile and more difficult to control.

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Targeting insecticide applications, during the nymph stages, in egg laying sites, or before they move into the field can greatly reduce the area that must be sprayed and the amount of insecticide that should be used.3 Please contact your local agronomist for insecticide recommendations and rates.

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Cultural Practices. As grasshoppers lays eggs in undisturbed areas, tilling these areas will discourage females from laying eggs. In CRP fields, where tillage is not an option, plant tissue can be shredded to reduce grasshoppers’ food supply. Controlling weeds on a fallow ground can also prevent adult females from laying eggs because there will be no food source. If the eggs are already laid, the nymphs will not have anything to feed on after emergence.2

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Biological Control. Several biological products are available to manage grasshoppers. The microsporidium Nosema locustae occurs naturally and is also formulated as microbial insecticide that causes disease in grasshoppers and reduces grasshopper populations. Spores are mixed with a bait and then placed in areas with grasshoppers. When the grasshoppers eat the bait, the spores infect and kill the grasshoppers.

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The fungus Entomophaga grylli causes epidemics in grasshopper populations when the weather is warm and humid. It kills late season grasshopper populations when sprayed on plants and the spores are eaten.8 Unfortunately, the mortality caused by these microbes is slow and may have little effect on grasshopper outbreaks.

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Generally, best management strategies depend on field and weather conditions and grasshopper population. Growers should determine the location of most of the nymphs, estimate the intensity of the infestation, and manage accordingly.

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Table 1. Action Thresholds of Grasshoppers
Nymphs per Square YardAdults per Square Yard
Rating Field BordersWithin BordersField BordersWithin BordersTreatment
Light25-3515-2510-20 3-7 No
Threatening50-75 30-4521-40 8-14 Probably
Severe100-150 60-90 41 or more 15 or more Yes
Source: University of Nebraska-Lincoln.5