It's raining inch worms! Well at least it seems that way sometimes, some years they're worse than others and this year seems to be one of those years. There are many different worms known as inch worms the ones we are seeing a lot of this time of year are the spring cankerworm (Paleacrita vernata), the oak leaf roller (Archips semiferana), and the linden looper (Erannis tiliaria). Both spring and fall cankerworms feed on a variety of trees including apple, ash, beech, elm, hickory, linden, maples, and oaks. The linden worm also has a varied diet and is commonly found on basswood, linden, apple, maple, oak and other trees.
The larvae chew small, irregular holes in young leaves, skeletonizing them. As the larvae mature they begin chewing larger holes until finally consuming the entire leaf, leaving only the main vein of the leaf. Usually this process is not a danger to a healthy tree but with severe populations the entire tree may be defoliated using considerable resources for the tree to refoliate weakening it. These worms then spin down from the trees on a strand of silk to pupate becoming a nuisance when they land on people, pets, cars, and yard furniture on the way down, although they cause no harm to mammals.
Cankerworms move by arching the middle part of their bodies to poll the hind prologs up to meet the anterior true legs (see pictures at http://www.forestryimages.org/browse/detail.cfm?imgnum=1394029 ). Many caterpillars move in the same fashion. Spring cankerworms are about 19 to 25 mm (1/4 to I inch) in length and are green to reddish brown in color with a single yellowish strip on each side. They have only two fleshy prologs at the end of the abdomen. Linden loopers have a bright yellow band of color down each side with brown to black lines running down the back.
Trapping the adults is the best strategy. The trunks can be banded with sticky adhesives, such as tanglefoot, in order to trap the females as they crawl up the trunk to mate and lay eggs. The trapped females then attract the males who also become trapped in the sticky band.
The 2-3% dormant horticultural oil spray is most effective against the egg stage. Be sure to thoroughly wet the trunk bark if spring cankerworms are present. Reduce the oil rate, especially on maples if the trees seem to be active in the spring.
Spraying with Bacillus thuringiensis (Bt), a biological control, is quite effective against the young cankerworms and looper larvae. For best results wait for all eggs to hatch but spray before the larvae grow to more than 25 mm (1 inch) in length.
For more information see the attached website.