Wednesday, August 18, 2010

Know The Difference Between Clover Worms And Soybean Loopers - Especially This Year

From Ames Herbert, Virginia Tech Extension Entomologist

Soybean looper is a common pest of our soybean crop and can be found in small numbers in most fields, most years. But it is very uncommon to have large infestations. The moths have to migrate into Virginia from the south, so when we do have big problems, they occur late in the season.

Our last bad soybean looper year was 2005. In 2005, they moved into several soybean fields in mid-September. Being unfamiliar with the pest, growers were not looking for them and not sampling fields.

In a few cases, fields were totally destroyed with loopers eating all the leaves leaving only leaf veins, stems and stalks (see the attached images, insecticide protected vs. unprotected soybean field).

This insect is capable of doing a lot of feeding in a short period of time. This is why I try to correct folks when they call green cloverworms--loopers.

Although green cloverworms do "loop" or "inch" when they crawl, they are distinctly different from soybean loopers both in appearance and in the rate they feed. Both are light green with white longitudinal strips down the tops of their backs and sides.

The differences are that green cloverworms are of equal girth from head to tail, have 3 pairs of abdominal prolegs, and wiggle vigorously if you put one in the palm of your hand (see the images below).

Loopers tend to be smaller at the head end and fatter towards the tail end, have only 2 pairs of abdominal prolegs, and they don't thrash around in your hand. And most importantly, loppers eat much more, worm-for-worm, compared with cloverworms.

A pattern that played out in 2005 was that all the fields hit hardest by soybean looper had been previously treated with a pyrethroid. The pyrethroids had reduced the beneficial population so when the invading moths laid their eggs, most survived. Large numbers of soybean looper moths have now been spotted in several fields in North Carolina.

Almost all of our soybean fields have been treated with a pyrethroid. We could be set up for a huge problem. Loopers are also fairly hard to control with pyrethroids which provide only about 50% control, at best. Products with pyrethroid/neonicotinoid mixes will not help.

Field trials in 2005 showed that only Lannate, Larvin, Steward and Tracer provided acceptable levels of control. Belt and Intrepid could likely be added to that list, but we have not evaluated those.

Bottom line, stay alert to these late season pests and keep a watch on your soybeans, especially any that have been treated with a pyrethroid.

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