by Alan Eaton, UNH Extension
In 2012 & 13 we had several spots in New Hampshire where tiny striped caterpillars appeared and fed on tomato foliage. In each case, they turned out to be yellow-striped armyworm. This species is relatively uncommon here, but a pattern seems to be developing that it feeds on tomato foliage in high tunnels. Occasionally it chews into young, green fruit. This insect is a general feeder, with two (maybe three?) generations a year in NH. It overwinters in the pupal stage. The UNH insect collection has adults collected here as early as May 30 and as late as October 21. The adults are attracted to lights and look similar to male fall armyworms. The females lay their eggs in masses, so one female can scatter many eggs in one high tunnel.
The caterpillars are relatively distinctive, and retain the same general color pattern through most instars. Even the tiny ones show the dark spot on the side, close to the head, and the thin yellow stripe. They can get relatively large (an inch or so), but nowhere near as large as tomato or tobacco hornworms. This year they appeared in the UNH tomato tunnel June 18.
As with other caterpillars, this insect can be controlled with Bacillus thuringiensis insecticides, but they are more effective on the young caterpillars than the older ones. B.t. must be consumed by the caterpillars, for it to work. If you wish to knock them off faster, chemical insecticides are an option.