Green Cloverworm On Dry Beans And Soybeans

Prepared by John Gavloski, MAFRI Field Crops Entomologist (FPE)

Green cloverworm are being found in dry beans, soybeans, and alfalfa in Manitoba. They make holes in the leaves, and upon quick examination the feeding would be easy to misdiagnose as grasshopper feeding or hail damage. Shaking the plants over a tarp, tray, net, or something to catch the larvae would reveal the caterpillars if they are present. Green cloverworm will loop when they walk, so it is easy to mistake them for alfalfa looper. One difference though is that green cloverworm will rapidly wiggle when disturbed.

Green cloverworm overwinters south of 41° N latitude, and migrate north in spring. There are 6 larval instars. The green cloverworm typically produces two generations in Manitoba; what we have been seeing this August is the second generation. Fully grown larvae are approximately 1 inch long, pale green with two horizontal stripes along each side of the body. The larvae have three pairs of legs in the middle of the body, three pairs near the head, and one pair at the hind end of the body. Below is a photo of a green cloverworm larva and adult moth.

Figure 1. Green cloverworm larva

Figure 2. Green cloverworm moth

Two Management Threshold Models – One For Row Crop And Another For Solid Seeded Production Systems

For 30 inch rows, an economic threshold for green cloverworm in soybeans based on larvae/foot of row has been developed for plants in the late flowering to early pod development stage, R2-3 stage (Ostlie and Pedigo, 1985). Place a sweep net or drop cloth between the rows. Vigorously shake the plants from one row on to the cloth and count larvae/foot. Sample 4 spots/20 acres but do not sample field edges. Based on the current value of soybeans and control costs the economic threshold would be about 9 or 10 larvae/foot.

A second threshold model for solid seeded soybean crops is based on whole plant (not individual leaf) defoliation estimates. This method is suitable for a wider range of soybean development – from pre-bloom all the way to late pod fill. The defoliation due to green cloverworm will be considered together with the damage inflicted by other defoliating insects, wind damage, etc. to make a management decision. Management is recommended if defoliation reaches 40% in pre-bloom, 20% during bloom and pod-fill and 35% from pod-fill to harvest.

When assessing feeding by green cloverworm, consider the amount of defoliation to the whole plant, and whether pods are being fed on. In soybeans, green cloverworm will preferentially feed at the top of the plant, so defoliation will be worse there and can make it appear as though there is more defoliation than there really is. So before making your decision to spray for green cloverworms this fall, ensure that the whole plant defoliation is truly at the 35% level.

Green clover worm field populations are usually well regulated by predators, parasitoids, and pathogens. In years with heavy green cloverworm populations, a fungal pathogen can reach levels within the green cloverworm population and induce a population collapse.

From the September 2010 edition of CROPS E-NEWS