Cutworm Activity in Sweet Corn and Tomatoes

By Elaine Roddy and Janice LeBoeuf, Vegetable Crop Specialists, OMAFRA

We have had reports of cutworm activity in both sweet corn and tomatoes this week.  In tomatoes, the early arrival of the variegated cutworm is definitely worth noting.

As corn plants start to emerge, keep an eye out for cutworm feeding damage.  Early–season cutworm injury appears as plants cut off at, or just below the soil-line. In older seedlings, the cutworms may deeply notch the base of the plants, causing them to wilt.  Cutworms are often most active on sandy knolls and in areas that had significant green cover during the early spring.

Most species of cutworm do not overwinter in Ontario.  In the early spring, adult moths are transported on the trade-winds from the overwintering sites.  Given the mild winter conditions of 2012, it is possible that some species of cutworm moths have overwintered in Ontario this year.

Female moths are attracted to dense, green cover to lay their eggs.  Often, when they arrive in Ontario in early spring, the main source of habitat for the females are winter annual or perennial weeds and cover crops. Egg hatch and larval feeding often coincides with planting and crop emergence.

During daylight hours, most cutworms remain buried in the soil.   They emerge to feed at night.  As a result, Insecticides are most effective if they are sprayed in the evening as the cutworms emerge from the soil to feed.

In sweet corn, the control threshold is 10% damage.  Keep the crop stage and worm size in mind when making spray decisions.  Sixth and Seventh instar larvae (1.5-2” long) are close to pupation and unlikely to cause additional damage to the plants.  Once corn plants reach the 4-6 leaf stage, they can sustain larger amounts of feeding without economic injury.

Early-season cutworm can also attack tomatoes.  However, this spring, tomato growers should also scout their fields for variegated cutworm, a climbing cutworm.  Although there is typically a small overwintering population in Ontario, damage is not usually noticed until July, when moth populations move in from more southern areas.  However, with the mild winter there may have been more winter survival in Ontario (variegated cutworm overwinter as pupae) giving potential for an unusual May outbreak.

Fields that had living weeds or cover crops (egg-laying sites) until very close to transplanting could be at risk of variegated cutworm and early-season cutworm feeding this spring.

Unlike early-season cutworm species, the variegated cutworm does feed during the day.  Figures 1 and 2 show very small larvae feeding on tomato transplants on a 30°C afternoon in full sun.  This field had both clipped plants and plants defoliated by foliar feeding.

There is no established threshold for climbing cutworm feeding on tomato seedlings, but the general threshold for early-season cutworm is 5% damaged plants.  In this case, damage could be counted as clipped or severely defoliated plants.

Variegated Cutworm Larvae
Figure 1: Five early-instar variegated cutworm larvae feeding on a newly transplanted tomato plant.
Variegated Cutworm Larvae on Tomato Transplant
Figure 2: Newly-transplanted tomato plant defoliated by variegated cutworm larvae.

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