tent caterpillars in our walnut tree?
August 22, 2018 1:41 PM   Subscribe

I looked up the other day and discovered what's probably some sort of tent caterpillar infestation in our walnut tree. What do I do next?

This massive webby cloud thing with hundreds of caterpillars inside has defoliated an entire branch of our 5-10 year old walnut tree. The caterpillars are maybe 1.5-2" long? I wanted to just leave the caterpillars, because they will presumably turn into butterflies and go away, but my partner pointed out that they might be back next year in greater numbers.

It seems like would be pretty easy right now to cut the branch off and cram the whole thing, along with all the caterpillars, into our big ole' yardwaste bin, but I'd rather keep the branch. I like that branch! I'd also rather not kill the caterpillars.

I will remove the branch if I need to, but I'm hoping to hear that they'll go away in a few weeks and never return. Not worried about losing a walnut crop; we usually lose walnuts to squirrels anyway.

Sorry, no pictures!
posted by aniola to Home & Garden (13 answers total)
The best way to remove them is with the branch. I think they're a pest. I see the Wikipedia article is kinder than I.

I remove them, branch and all, and put them in a plastic bag. Then I toss in a rag with a few drops of gasoline and throw it in the trash.
posted by humboldt32 at 2:10 PM on August 22, 2018 [1 favorite]

These pests (and, yes, they are PESTS) are known as tent caterpillars or bagworms. They do not come out of a cocoon as butterflies. They are harmful to your tree and you really have no choice but to treat them.

Your local nursery or garden center will have good advice. Pesticides are an option, though the bags that are woven by the caterpillars can sometimes be impervious to liquids.

I used to wire cloth rags to the end of a pole or pipe that will reach the affected branhes, soak the rags in kerosene and set them alight. The fire will quickly burn the bag and destroy the caterpillars. You will lose some leaves in the process, but nothing in comparison to the damage if you don't treat them.

I know this may sound like a strong response, but these little suckers will easily spread from branch to branch and tree to tree. Deal with them now before they become a worse problem.
posted by John Borrowman at 2:14 PM on August 22, 2018 [7 favorites]

These guys? Gosh, that picture brings back memories... specifically memories of the woods behind my elementary school being overrun by thousands upon thousands of them one year. Kids would gather them up in bags by the hundreds and it didn't seem to make a dent.

Wikipedia suggests that once the caterpillars mature (into moths, btw, not butterflies), they won't go very far. "When fully grown, the caterpillars leave the natal tree and seek protected places on the ground or under the eaves of buildings to spin their cocoons. About two weeks later, they emerge as adults. Shortly after eclosing from the cocoon, the female moth secretes a pheromone which draws males to her. Mating typically occurs in the early evening and the mated female, already fully laden with eggs, typically oviposits the full complement later that same evening." I think that means it's extermination time, sadly.
posted by showbiz_liz at 2:50 PM on August 22, 2018

Think of the squirrels. And all the other trees. I'm pro-bug (I moved a wasp nest last year), but... tent caterpillars are not endangered. And they will spread to neighboring trees.
posted by amtho at 3:01 PM on August 22, 2018

We had these in a tree when I was a kid. My father was advised to cut the bag open in multiple places so that they would no longer be protected from birds and other predators. That did the job.
posted by theperfectcrime at 3:48 PM on August 22, 2018

Traumatized by tent worms since I was a kid, so not clicking on any links, thanks.
They tented in our plum trees. My dad would take a blowtorch to their hellhomes. I watched from the safety of our house, from behind my hands. The sheer sound of the blowtorch was gratifying. Days later, the weather would have destroyed what was left of the hellhomes.

To this day, I will not look at one.
posted by BostonTerrier at 4:35 PM on August 22, 2018

The town I live in is literally named after cottonwood trees, which flourished here when I lived here the first time, in the 70’s. They would get bagworms, and the treatment was to burn them. However, not everyone bothered, and now my town that’s named after cottonwoods doesn’t have any cottonwood trees left at all.

Kill it with fire.
posted by MexicanYenta at 5:14 PM on August 22, 2018 [3 favorites]

My dad used the same method as John Borrowman describes, the flaming rag on a stick. I've read since that the "cut the bag open" can clue in wasps and birds that there is a food source in the bag for the intrepid critter, so they will break into the tests/bags themselves in the future.
posted by rudd135 at 5:38 PM on August 22, 2018

This branch is super-close to our house and our neighbor's house, so I'm going to give the flame on a stick a pass.

Sounds like the worms won't try migrating to a less reachable spot on the tree if I cut the bag? Or if they try migrating someone will eat most of them first?
posted by aniola at 6:58 PM on August 22, 2018

They would get bagworms, and the treatment was to burn them. However, not everyone bothered, and now my town that’s named after cottonwoods doesn’t have any cottonwood trees left at all.

Sorry if I seem like I am singling you out Mexican Yenta, this is addressed to everyone.

First there is a big difference between bag worms and tent catpillars. Tent catepillars are social, they live in big bunches in large silk "tents." From their Wikipedia entry, "Defoliated trees typically refoliate after caterpillar attacks and experience no lasting damage." They are eaten by birds and other insects. There are sometimes outbreaks of tent catapillars, but one tent in your yard is no big deal. You could open it up as suggested, and the birds will have it gone in no time, no pesticides needed. Bag worms build individual bags of leaves to live in and are not normally a problem either. The cottonwood trees that were described as disappeared in the comment above, were far more likely to have aged out and not been able to regrow new young trees because of changed/lessend flow in a nearby river. Many cottonwood trees need periods of flooding in order to produce young trees. People changing the water courses and using up all of the water is absolutely devestating to the cottonwoods of the American west.

I commend the OP for being cautious before needlessly destroying wildlife, it is a tactic we should all employ. Nature rarely creates beings who completely decimate their own food source, and even when She does there are checks and balances. Humans are possibly the exception, we're really good at jumping to conclusions, repeating annectdotal information as if its fact, and just generally f***ing s*** up.
posted by WalkerWestridge at 7:26 PM on August 22, 2018 [2 favorites]

If you in a drought zone, for the love of god wait until it rains before burning catapillar tents!
posted by monotreme at 10:40 PM on August 22, 2018

There are several different caterpillars that build nests in trees that are often confused or misidentified. My feelings about removal depend on a combination of how destructive the insect is and whether or not it's native. Invasives that are also very destructive are obviously on the kill list, screw those guys. I'm a little softer toward natives, even if they're a little destructive.

Fall webworms are actually native to North America, and they like any kind of deciduous trees including walnut. Their web looks kind of like messy white hair, with a sort of dirty-looking clump where the webworms gather. They're not very destructive. Some cosmetic damage but nothing a tree can't handle.

Eastern tent caterpillars mostly like apple and cherry trees and build nests in the spring. Their nest is much more opaque than that of the webworms. They can make ornamental trees look pretty ugly, but they don't typically cause permanent damage. If you're in California, you're not in range for this guy. The Western tent caterpillar also prefers fruit trees (also willow, cottonwood, birch, but not walnut) and, again, is typically only a minor pest.

Gypsy moths are the evil pest that haunted my childhood in the 80s. They're non-native, invasive, and very destructive. They also don't really like walnut trees that much.

A lot of things are popularly called bagworms that are not, it's usually a mis-attribution. Actual bagworms are an entire family of moths including many different types, but their bags are camouflaged with sticks or lichen or bark.
posted by desuetude at 1:06 PM on August 23, 2018 [2 favorites]

The cottonwood trees that were described as disappeared in the comment above, were far more likely to have aged out and not been able to regrow new young trees because of changed/lessend flow in a nearby river.

No rivers here. No flooding, either. This is the desert. Doesn’t seem like there should have been trees here at all, but there were, and are. Just not cottonwoods anymore. But then, very little about this town makes sense.
posted by MexicanYenta at 2:27 PM on August 23, 2018

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