An odd soybean pest: Imported longhorned weevil

Each summer since 1994, a few soybean fields throughout Iowa have been attacked by imported longhorned weevils. This insect is typically a minor pest, but serious defoliation can occur when the beetles concentrate in border rows or spots in a field. Problems typically appear in mid-June and early July. The weevils are almost always associated with grass, either following set-aside ground or dispersing out of adjacent pasture. The weevils can strip all the leaves from V5 or smaller soybean plants, but older plants apparently produce leaves faster than the weevils can defoliate them. The weevils cannot fly, so the injury is often confined to border rows or very small areas of a field. I have never seen, or heard, of an infestation defoliating an entire field. There are no economic thresholds for this insect.

Soybean rows defoliated by weevils (row 3 foreground sprayed; row 3 background unsprayed).

Imported longhorned weevil on young soybean plants.

Imported longhorned weevil.

Chemical control of imported longhorned weevils has proved difficult, based on my experience and that of others. In 1994, I evaluated three insecticides against this pest and found that Lorsban 4E (1 pint per acre) and Sevin XLR Plus (1 pint per acre) gave better control than Pounce 3.2EC (4 ounces per acre). Even in the Lorsban and Sevin plots, not all weevils were killed at these low-end manufacturer's label rates. If a field is sprayed, I would recommend confining the insecticide to only those areas where the weevil occurs, and using the high label rate of either Lorsban (2 pints per acre) or Sevin (2 pints per acre) for controlling this insect.

This article originally appeared on pages 105-106 of the IC-484(14) -- June 19, 2000 issue.

Updated 06/18/2000 - 1:00pm