Bagworms, Webworms and Other Caterpillar Pests
Question. Help! A beautiful blue spruce in my front yard is being eaten by bagworms. We have been pulling the bags off as we find them, but every day there is another new crop. What can I do to get rid of these 'worms' and save my blue spruce?
- ---Mary in Winchester, KY
- ---A.J. in Montgomery County, PA
- ---David in Southern Maryland (near Washington, DC)
- ---Ruth in Medford, NJ
- ---Elaine in Doylestown, PA
- ---Jay in Phoenixville, Pa
- a.   a tent-pitching tree-eater like the Fall webworms we're currently seeing (hairy, inch-long caterpillars that become large pure white moths);
b.   the tent caterpillars who built their big ugly communal nests in trees back in the Spring (big, two-inch long, mostly black caterpillars that become reddish-yellow moths);
c.   this year's big nuisance, bag worms, whose small, individual nests look a lot like pine cones (a great camouflage, as these pests mostly attack evergreens, where the natural-looking 'bags' often go undetected; here's a link to a Univ. of Delaware bulletin showing the nests: (http://ag.udel.edu/extension/horticulture/pdf/ent/ent-04.pdf); or
d.   any other pest caterpillar—including ones that DON'T make nests or attack trees.
The safe, sane—and highly effective—solution is to prune off any 'bags' and/or tent-like nests you can reach, and then spray the old original form of Bacillus thuringiensis on the rest of the tree while the caterpillars in question are actively eating (which is right now in most locales for bagworms and Fall webworms). Don't worry about that panic-inducing name—most people just call this organic pest killer by its initials: "Bt" , with a letter added at the end to indicate what strain or variety it is. There are currently three different Bts, each affecting a different type of insect pest. The one that stops caterpillars—ALL caterpillars, by the way, not just ones that build little homes in your trees—is the kurstaki strain, "BTK".
One of the oldest, most effective, and easiest to find tools in the organic arsenal (its been in use for close to 40 years), BTK is a naturally occurring soil bacteria that just happens to be deadly to caterpillars. It's available from Gardens Alive and other natural gardening product suppliers, and at most garden centers (retail brand names include Dipel, Green Step, and Thuricide; ingredient labels should say 'Bt' or BTK). Don't be surprised if a store keeper thinks it IS a chemical; BTK is used by lots of farmers to protect their sweet corn from tip-eating earworms, often without their knowing its one of those high-faluttin' organic things!
When pest caterpillars chew on Bt sprayed leaves, their stomachs stop working and they soon die. Bt has NO effect if you spray it while they're not eating, like in the winter—they must be actively feeding. And Bt is one of the safest things you can use. It will not harm you, your pets, earthworms, or beneficial insects. Birds could eat caterpillars that just ingested Bt and the birds would not be harmed. You won't even hurt the caterpillars of pretty butterflies when you use Bt, because those caterpillars are feeding on weedy looking plants in the wild, not your tree leaves. (The only butterfly caterpillars that do appear in any kind of garden situation feed on carrots and parsley plants.)
And like we said earlier, Bt will work on ANY pest caterpillar that's actively feeding—cabbage lopers, tomato hornworms (what IS it with these 'worm' monikers for pest caterpillars? If I were an earthworm I'd sue!), those nasty little ones that eat rose buds—ANY caterpillar; just spray it on the leaves they're eating and they will soon be daid.
Back to the ones causing tree troubles: As AJ notes in his email, you can rent an industrial sprayer or use a pressure washer to get the Bt up into tall trees. Flush the sprayer several times first to make sure there aren't any soap or herbicide resides in there. And remember—you have to spray when your specific caterpillar is out and feeding. If you can't tell for sure, contact your local county extension office; they'll know when the pests are actively feasting in your locale.
And finally, insect pests prefer weakened plants; you can greatly lessen the chance of infestation by treating your trees right. Don't use chemical fertilizers. Don't prune without being sure of what you're doing (pruning in the Fall, for instance, is ALWAYS wrong). Don't mulch with wood chips or shredded bark. Never pile any mulch up against the trunk, and never mulch more than an inch or two thick; those 'mounded' trees are doomed.
In addition, always try and plant new trees and shrubs in the Fall or very early Spring; summer plantings have very little chance of ever being healthy. And no matter when you plant, keep young trees well watered until they are very well established.