Green cloverworm larvae are light green with three pairs of stripes running the length of the body. There are three pairs of ‘true’ legs behind the head separated by three legless segments from three fleshy ‘prolegs’ near the middle of the body, with another pair of prolegs on the terminal segment. Larvae wiggle vigorously when disturbed. Smaller stages may drop from the leaf when disturbed and hang from a silken thread. Larvae chew irregularly shaped holes in the leaves from July through September. To determine if treatment is justified, sample a minimum of 10 locations in the field. Use a cloth and bend over one rowfoot of soybeans on either side, shaking insects onto the cloth. Then calculate the average number of larvae per row-foot.
Refer to Table below to find what response, if any, is necessary. Note that treatment thresholds vary with stage of soybean development, density of larval population, control costs, and expected soybean market value. This is an easy insect to kill, with most insecticides listing green cloverworm on the label.
|Guidelines for Determining the Need for Insecticides to Suppress Green Cloverworm Larvae on Soybeans|
|Resample in 7 to 10 days if: (do not treat)||Insecticides are probably justified (treat) if:|
|(Expected Market Value in $/bu)||(Expected Market Value in $/bu)|
|The Average Number of Larvae per Foot of Row is a,b,c|
|Soybean developmental stage||Less than or equal to:||Greater than or equal to:|
|Full bloom through early pod set||9||8||7||19||16||15|
|Full pod through beginning seed||7||6||5||15||13||11|
|a Resample in 3 to 4 days if the average larval density is between these values.|
|b Based on control costs of $6.50/acre. Other factors including the prevalence of natural control agents, previous defoliation, moisture availability,|
|larval age distribution, and yield potential may influence the “no treat, resample, treat” decision.|
|c Severe, extended weed competition may lower these treatment thresholds by pre-stressing the soybean plants before the insect defoliation develops.|
|However, research in Iowa has shown that yield losses from velvetleaf competition and simulated green cloverworm defoliation are additive|
|for practical purposes (velvetleaf densities below one weed per 5 feet of soybean row and defoliation during full bloom).|
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