Watch for Variegated Cutworms in Sugar Beets
Ian MacRae, Extension Entomologist
NWROC, Crookston

Several weeks ago, alerts went out about watching for armyworm and variegated cutworms due to large numbers of moths captured in the region.

The armyworm threat appears to have passed with only minor infestations found. Regarding the variegated cutworm, we now have received reports of infestations in sugar beet. We've seen this before, back in August 2001, when both variegated and black cutworm were a late season (July/August) problem in beets.

Variegated cutworms are climbing feeders and it is not uncommon to see them feeding in the daytime, unlike our other common cutworm species. At this point, damage is generally minor, but with large populations, feeding damage can be significant to the younger growth in the beet crown.

At least with this species, there's less likelihood you'll be digging in the dirt to find them - they're a climber, feed in the canopy and are more likely to be out in the daytime than other species (although they still prefer a late evening supper). Look in the canopy and in the crown; look for worms (figure 1)and for frass (variegated cutworms are, apparently, very regular and leave quite a messy dining area).

The caterpillars (figure 1) are medium to dark grey with a light stripe on the side. They also have several small, diamond-shaped yellow spots on top near the head. Younger larvae are about 1/4 inch long while the mature, ready to pupate worms can be 1 1/2 - 2 inches long. When scouting, pay close attention to land adjacent to grassy borders and to weedy patches neighboring or in the field. The easily seen feeding injury includes irregular shaped holes in leaves to complete skeletonizing of leaves (figure 2).

Because it feeds in the canopy, it is susceptible to foliar applications of insecticides. Some research indicates pyrethroid insecticides may be more effective but remember possible efficacy issues with these chemistries if applying at high temperatures (in the 90's F). Control is recommended when populations reach 3-5 larvae/sq.ft. in late season beets, post canopy closure.