Key to killing webworm is getting to them early
How and when do I treat for webworm? I have had them in my ash trees the past two years.
Webworms that feed on trees like your ash which appear in the late summer into fall are called fall webworms.
These caterpillars hatch out in midsummer and begin to spin a loose silken web. The larvae congregate within the nest, feeding on the foliage. As they grow and require more food, they expand their nest.
Fall webworm often leaves a brown skeletonized remnant of the leaf, making the plant look sick and ragged. Their feeding rarely hurts the plant and though repeated infestations can cause some minor branch die-back, the damage is usually purely aesthetic.
Fall webworm appears in late summer into the fall and feeds on a wide variety of different plants from your ash tree to peach, plum, birch, mulberry and more.
There’s a similar insect called the western tent caterpillar. Tent caterpillar shows up in the spring and spin a silken nest within which they stay during the day, venturing out at night to feed. Tent caterpillar is predominantly a problem on members of the genus Populus that includes cottonwood and aspen.
The key to controlling either one is to get to them early. Keep a close eye on those trees and start spraying as soon as you see them. Fall webworm and tent caterpillar are much easier to kill when small plus their damage is minimal at that point.
There are a number of different insecticides you could use.
Bacillus thuringiensis is an organic spray that’s completely safe to use. Honestly, I’ve noticed some resistance to it developing so it’s not working quite as well as it used to, but still is an effective control if you hit these insects while they’re young. Once they’ve reached mature size, bacillus thuringiensis is pretty ineffective.
There are a number of chemical insecticides that will do a good job for you. Permethrin, Malathion, Cyfluthrin or Bifenthrin all work great. You may need to mix a spreader-sticker with the insecticide so it penetrates the silken web to get at the insect.
Systemic products usually don’t work on caterpillars such as these, I’m afraid you’re stuck with spraying to control them.
Your best plan is to wait until late summer and keep a close eye on the tree. The moment you see small webs cropping up, spray the tree thoroughly.
It is possible that you may not see any of them later on this year at all. Fall webworm tends to be a sporadic issue and you may have them or you may not. Just monitor them and react as soon as you can.