Authors: Buyung Hadi, Jeffrey Bradshaw, Robert Wright, Bruce Potter, Ian MacRae, Robert J. Whitworth, J. P. Michaud and Phillip E. Sloderbeck
Two species of grasshoppers from genus Melanoplus are common on soybean: the redlegged grasshopper (Melanoplus femurrubrum) and the differential grasshopper (Melanoplus differentialis). Other species of grasshoper may also occur on soybean, albeit less commonly.
Eggs of redlegged and differential grasshoppers are laid in cemented clusters (egg pods) approximately 0.5-2 inches (1-5 cm) below the soil surface. A cluster of redlegged grasshopper eggs may contain 25-30 individual eggs while a cluster of differential grasshopper contains 50-150 individual eggs.
Grasshopper nymphs resemble the adults, but are smaller in size and have incompletely formed wings. The nymphs of redlegged grasshoppers feature a broad, pale-colored stripe running on the side of the head, beginning around the gena ("cheek" area) and continuously crossing the side of the thorax through the side of the first segments of the abdomen. The nymphs of differential grasshopper usually have the broad, pale-colored band running on the side of the thorax but not continuously crossing the side of the head.
Redlegged grasshopper adults are 0.86-1.25 inch (22-32 mm) long with a reddish brown upper surface, a bright yellow underside and characteristic red color on the lower part of the hind legs. The adults of differential grasshopper are 1.1-1.73 inch (28-44 mm) long with yellowish brown bodies and distinct v-shaped black markings on the upper part of the hind legs.
Life Cycle and Seasonal History
Female grasshoppers lay eggs in the soil during summer and early fall. Females of both redlegged grasshopper and differential grasshopper predominantly lay their eggs in undisturbed grassy sites including field borders, roadside, prairie and pasture. In outbreak years, eggs are also laid in cultivated sites such as small grain stubble, alfalfa, clover and soybean. Differential grasshoppers are particularly likely to oviposit in soybean. The eggs overwinter and wingless nymphs begin to emerge in the spring the next year. Redlegged nymphs typically appear 1-2 weeks earlier than differential nymphs. The nymphs molt 4-5 times, although additional molts may occur before they become adults. Adult redlegged grasshopper begin appearing during early summer to midsummer.
Weather is the most important factor determining the grasshopper population. Low rainfall is associated with higher nymph and adult survival. Warm, dry falls will maximize the number of eggs being laid in a given year. This will produce a high grasshopper population in the subsequent year. Early hard-freezes may kill adults and reduce the number of eggs laid, leading to lower populations the next year. Two or more consecutive years of drought may lead to grasshopper population outbreak in the third year. Populations usually occur first on the field margins and subsequently spread into the field.
Warm weather that coincides with extended period of high relative humidity may create an adverse situation for grasshopper populations. Warm and humid conditions favor the growth of grasshopper-attacking fungi. A case in hand is the fungal pathogen, Entomophaga grylli, the causal agent of summit disease on grasshoppers. Grasshoppers attacked by Entomophaga grylli are often found dead in a stiffened position on the top of a plant with one or both legs extended backward, thus earning the name "summit" disease.
Plant Injury and Damage
Redlegged and differential grasshoppers are sporadic, yet serious, pests of soybean. Nymph and adult grasshoppers feed on leaves and pods of soybean. Grasshopper feeding leaves jagged-edged holes on leaves. Although defoliation is usually most severe in late summer when the grasshoppers are mostly adults, large grasshopper populations in the spring may cause widespread defoliation on seedling soybean. Unlike the bean leaf beetle that feed only on the pod surface, grasshoppers can chew through the pod wall to feed on soybean seeds. Pod feeding may dispose remaining seeds to secondary fungal infection.
Scouting and Threshold
Scouting should start early in the growing season (late April, early May) to ascertain the incidence and extent of grasshopper problems. This time period will broadly coincide with the nymph emergence from the soil. Scouting should start on field margins, fence rows, dirt roads and ditches. Soybean fields adjacent to undisturbed weedy sites, alfalfa, and small grain fields are susceptible to grasshopper infestation, so fields with this characteristic should be monitored carefully. There are at least two ways to estimate the economic threshold of grasshopper infestation: by estimating the number of grasshopper in the field or by estimating the defoliation damage due to grasshopper feeding.
A consensus threshold of 30-45 nymphs or 8-14 adults per square yard within the field justifies treatment control. If the grasshopper population is found on field margins but not within the field, it may be justifiable to treat only the margins. If more than 60 nymphs per square yard were observed on adjacent non-crop areas, the areas and the portion of the field immediately bordering the areas may be treated. If both crop and non-crop borders are treated, be sure the product chosen is labelled for both sites.
Thresholds based on defoliation include treating when defoliation in the field exceeds 30% prebloom, or exceeds 20% between blooming and pod fill. Grasshopper injury should be combined with other defoliating insects (e.g. bean leaf beetle, green cloverworm).
Tillage of small grain stubble or fallow before the egg laying period may be an effective deterrent to oviposition. Tillage after eggs are laid is ineffective since it does not cause egg mortality.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com