cutworms? #142968 - Ask Extension


cutworms? #142968

Asked July 24, 2013, 5:17 PM EDT

I live is South Fayette Twn and there appear to be cutworms in some type of cocoon attached to arborvitae. You can see there heads popping out on occasion. They are dark with stripes of lighter color, less than 1 in long. When killed, their is a bright green inner body. Cocoon are brown with some green resembling the bush itself. They appear to kill the arborvitae. How to kill the insect?

Allegheny County Pennsylvania

Expert Response

It appears that your arborvitae are infected with bag worms.

There are a couple of products you can use to try and manage the bagworm population. The ideal time to manage their populations however is around mid-June to late June with an application of Bacillus thuringiensis (B.t.). This allows enough time for all the eggs to hatch and receive treatment at the same time. However, watching the plants until this time is important, just in case the population is extremely high and warrants treatment sooner. At this time, a B.t. application is possible, but it does not work well against larger instars (older insects) as they are more difficult to control. Another product possible to use now, has the active ingredient spinosad. This product also works better against smaller larvae but can provide some control against larger caterpillars. Both of these are bio-rational products. Unfortunately, larger caterpillars frequently need broad spectrum products to achieve the best control this late into the summer. Products such as carbaryl (Sevin), acephate, or one of the pyrethroids (active ingredient ends in "-thrin") are going to be the better options. Applications must cover foliage well as the caterpillars only get their dose from feeding since they are protected in their bags. Applications should be applied before mid-August because the caterpillars begin to form cocoons at this time and stop feeding; thus treatments would do little. During August we have populations of parasitic wasps that attack bagworm cocoons which helps keep their populations low most years. In September, females release pheromones into the air, males find them and they mate. Females lay eggs in the bags and then die. So, hand-removal from September through May is the cheapest method of keeping their numbers low.

The link below includes a fact sheet on bagworms that will provide additional insight into their development and control.

God luck with your efforts

Rick Bentz Replied July 25, 2013, 7:27 AM EDT

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