Webworms on Knock Out Roses

Insect Description

Fall webworms are distributed throughout the United States and Canada and have a wide range of host plants including almost all shrubs and fruit, shade and ornamental trees. Roses are among the preferred hosts. Adults are white moths with brown spots on wings. The black- or red-headed larvae or the worms are light green to yellow colored and hairy, measuring about an inch long. Worms with black heads have black spots on their backs while red-headed ones have orange to red spots.

The female moths that emerge between late May and July lay eggs in masses of 200 to 500 eggs on foliage undersides. The black-headed species lay eggs earlier than the red-headed ones. The larvae start to emerge in a week and feed on foliage, weaving a web over infested areas as they feed. It takes about six weeks for the larvae to fully mature, after which the pests drop to the ground and overwinter as pupae, emerging as adults the following summer.

The pests are most damaging to knockout roses and other infested plants while in their larval stage. The worms rapidly skeletonize the plants, leaving only the foliage veins. Pest presence is indicated by the presence of gauzy nests at the tips of the branches. These nests house the feeding pests. The damage caused by lighter pest presence is mostly aesthetic in nature. However, when fall webworms occur in large numbers, knockout rose plants are defoliated entirely and are often killed.

Cultural control methods include manually picking off the tents from the knockout roses and destroying them. The best time to do this is when the tents are small. Time this at dusk or early morning, as this is when the larvae are mostly inside the tents. Do not attempt to burn the tents, as this creates fire hazards in the landscape. Prune and remove areas containing egg masses. Chemical control options include the use of products containing Bacillus thuringiensis or Bt. Apply insecticide when larvae are small and before major damage has been done.