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Bagworms: The Creepiest (or Sneakiest) Caterpillars?

Q. Tim in Cream Ridge New Jersey writes: "I am a longtime fan; I even won a blue ribbon from PHS (the Pennsylvania Horticultural Society) this past summer for my 2100 sq ft raised bed vegetable garden because of your help and guidance over the 12 years I have been listening.

"My question is about bagworms, which have infested large areas of my property in the beautiful horse country of western Monmouth County. We have a large hedge of very mature Arbor Vitae that are pretty much being stripped by these things. They are also on blue spruces around the property, but I am more concerned about the Arbor Vitae, as these trees separate our property from our neighbors, and I'd hate to see this big hedge turn into a stand of dead trees.

"I had an expert from what I believe to be a reputable tree care company tell me that only chemical sprays could solve the problem of these horrible little "worms." While I have taken hours to pick and burn the "pinecones'', I cannot reach all of them and he advised using an insecticide spray when they are eating during the growing season.

"As you can imagine, I am a steadfast organic grower with bird baths, pollinator gardens, bird houses, fruit trees, and wildlife all around my little homestead. I picture a chemical being sprayed into the trees and birds falling out dead. YIKES!!

"I have never heard of these worms until this past year; but the tree care rep said that this area of NJ in particular is known for these infestations. Of course, now I notice them on stripped trees all around! I've also noticed that out here on Route 195 (the road to the beaches) there are signs that state-planted evergreens "have been sprayed with noxious spray."

"PLEASE let me know how to get in front of these bagworms in an organic way. They are even on my skip laurels now!!"

A. First, those signs are not about insecticides; they warn of nasty foul-smelling agents that are sprayed to deter people looking for free Christmas trees. If you ignore the signs and cut one of these trees and bring it inside, your living room will soon smell like seventeen skunks held a steel cage match in your house. (We don't know who won that match, but we can be sure about who lost.)

I wish bagworms were that easy to explain. But let's start with the fact they are not 'worms'; and neither are the many other caterpillars whose common name includes the word worm; like the corn earworm, the cabbage worm, tomato hornworm, etc. For some unknown reason, farmers called these caterpillars 'worms' and the name stuck. So if something {quote} 'wormy' is attacking your plants, they are caterpillars, which is an important distinction, as we shall soon see.

Bagworm nests look much like pinecones, so they are often ignored when they first show up on coniferous plants like arborvitae and juniper. Those are their favorite hosts, but they'll also breed on many other plants, including, pines, spruce, apple, boxelder, elms, black locust, maple, oaks, persimmon, poplars, and willows. (That's per the USDA; I've personally only ever seen them on arborvitae and juniper.)

Now we get to the weird part. The adult males are hairy moths that emerge from their bags to mate in the Fall. But the adult females are grub-like creatures that have no wings and spend their entire life inside "bags" constructed of locally available materials that again, look like the host plant produced them naturally. The adult males mate with these weird grub-like things through an opening in the bag. (I am NOT making this up.) And then, as is often true in nature, the adults die, but not before the female lays fifty to 100 eggs inside her bag. (Which, BTW, also has an opening for the deposit of moth poop.) Ahem.

The eggs survive winter, and then pupae from the hatched eggs will emerge in early Spring: April through June depending on your location and the weather. They drop down on a thread of silk, and if the wind is right, it can carry them to another host plant. If it doesn't, they'll feed on the same plant they were born on. Either way, this is when they begin actively feeding, so this is the time to spray the plant with the old original form of Bt. This organic caterpillar killer has been around for many decades and ONLY harms caterpillars that are actively feeding on the sprayed plant parts. It will not affect birds, bees, butterflies, frogs, toads, pets, people, naked mole rats or mechaGodzilla.


While feeding, the little worms actively collect material from the host plant with which they will quickly make their own bags, emerging only to feed. And because the bag is composed of material from the plant they're eating, its easy to miss. But you will eventually see the damage. If you've had bagworms on a plant in the past, keep a close eye on it for the next couple of months and have your Bt at the ready. You can (and should) also prune off any bags you can reach at any time of the year and do what an Ohio State University Extension Bulletin calls the 'Bagworm Stomp', which should be self-explanatory.

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