It may be best to let webworms weave away

We’ve been inundated by bag worms on our cottonwoods, apple trees and aspens. (They apparently don’t care for the Russian olives that are prevalent out here.) I’ve sprayed but with little result, and frankly, I don’t think there’s enough spray in Grand Junction to contend with this. Or, if there is, I’ll be bankrupt. I’ve noticed that the bags are all over the place in the neighborhood, not confined to our property, but seemingly we’ve been hardest hit. I may have 40-50 trees with bags, some completely covered, some less so.

Are my trees going to make it after the worms become moths (will the damage be permanent)? I seem to remember last year (it wasn’t this bad) the trees bounced back.

Secondly, is there a product available that I can apply in the spring when the trees awaken from their dormancy that will protect them (and not allow this ghastly sight for us)? I remember a product that we could apply in the ground to be absorbed by the roots, but if I did the math right, for the number and size of our trees, I’m looking at thousands of dollars in product.

— Ralph

Bagworms that appear in the fall are actually called fall webworms. These little 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 webworms often leave a brown skeletonized remnant of the leaf, making the plant look sick and ragged. Their feeding rarely hurts the plant and repeated infestations can cause some branch dieback,but usually the damage is purely aesthetic.

As an aside, there’s a similar insect called the western tent caterpillar which is sometimes confused with fall webworm. Tent caterpillar shows up in the spring and is predominantly a problem on members of the genus populus which includes cottonwood and aspen. Fall webworm appears in late summer into the fall and feeds on a wide variety of different plants, from peach and plum to birch and ash. Tent caterpillars spin a silken nest within which they stay inside during the day, venturing out at night to feed, while fall webworms stay within their nest while feeding.

The key to controlling either one of these little buggers is to get to them early. The best way is to keep a close eye on those trees and start spraying as soon as you see them. They’re 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 it’s still an effective control if you hit them while they’re young. Once they’ve reached mature size, B.T. is pretty ineffective.

There are a number of chemical insecticides that will do a good job for you. Permethrin, malathion, 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.

The common systemic product available on store shelves contains the insecticide imidacloprid and doesn’t work on caterpillars like these little devils. There is another systemic called dinotefuran that will do the job, but I think you’ll have a very hard time finding it. The systemic should be applied about the first of July.

You could simply let them go and allow them to run their course. Your trees may look a bit ragged, but as I mentioned before, their damage is rarely of any consequence. Besides, pests like these tend to come and go. You’ll have a bad year or two and then they’ll kind of disappear for a couple more.

Dennis Hill is the nursery manager at Bookcliff Gardens, Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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