For the Birds Radio Program: Forest Tent Caterpillars

Original Air Date: May 16, 2001

Forest Tent Caterpillars are covering the north woods right now. The ways we control them can be worse than the caterpillars.

Duration: 5′18″


I was at KUMD recently when the station’s development director discovered an army worm­ more correctly called a forest tent caterpillar–crawling on him. He didn’t particularly want to kill it, but he did want it off his shirt and out of the station, so I took it with me as I left. The first tree I passed on the university grounds was an elm tree, and no way was I going to put a pest in an elm tree. When I was a little girl, I loved the elm trees in my front yard, and these beautiful trees that provide homes for Baltimore Orioles are just too precious to be chomped on by anything, in my opinion.

The next tree I came to was an ornamental that I’ve never seen birds in so I gently set the army worm there on a leaf. These caterpillars are sort of pretty individually, with a line of golden keys along their backs and their bland, rather stupid faces. They’re like people I suppose–individually a bit blind and selfish but rather innocuous for the most part, with most of the ecological problems they cause coming from their sheer numbers. I set this army worm in the tree before I noticed any others. They were on at least half the leaves, quietly munching away. Then I looked down to the sidewalk I had been walking on. They were crawling from one side to the other by the dozens, and the sidewalk was splotched with hundreds of greasy spots where army worms had been squashed.

Forest tent caterpillars do enormous damage to individual leaves, but overall don’t do irreparable harm to trees under most circumstances. Deciduous trees are designed to regenerate leaves if they’re destroyed, and unless a tree is very stressed, usually in severe drought conditions, it recovers within weeks from the temporary setback that caterpillars cause. But the caterpillars do indirectly kill nestling birds. It’s creepy to watch hordes of caterpillars crawling over tiny baby warblers, sometimes damaging delicate eye tissue but overall probably not hurting the babies too often. But parent birds build their nests under leaves to protect their babies from downpours and sunburn, and defoliation above their nests can be lethal. Most neotropical migrants, like vireos and warblers, don’t have time to re-nest if their first brood is lost.

Forest tent caterpillars may cause problems for warblers, but Black-billed Cuckoos thrive on them. Most birds don’t eat these hairy caterpillars, but cuckoos feed them to their babies and eat them themselves, and their population surges during army worm highs. Army worm populations are cyclical, and within a couple of years a fly that eats them will become so numerous that the caterpillar population will crash. At that point, cuckoos will also become more scarce.

During years with army worm outbreaks like this, a lot of people understandably want to get them out of their yards. To see hundreds or even thousands dropping to the ground like raindrops and weaving their way across country roads is bad enough–to see hordes smearing up a newly painted house or defoliating a fragile ornamental tree you’ve been nurturing for several years is extremely trying.

Limited use of pesticides is sometimes warranted, but these bugs are surprisingly resilient, in part due to their sheer numbers, and most of the time you end up poisoning a lot more beneficial insects than army worms. The Minnesota DNR’s webpage has a forest tent caterpillar info page that says Bacillus thuringerensis is the best solution because it spares beneficial insects, but that’s only true if you don’t consider Cecropia and Sphinx moths and Monarch butterflies beneficial. BT kills ALL lepidopteran larvae-every butterfly and moth caterpillar that eats it.

Widespread use of BT and crops genetically altered to produce BT in their tissues will ultimately cause army worms and other pest species to grow resistant, but chemical companies bank on the hope that by the time one pesticide’s danger is understood or its target species grow resistant, they’ll have a new poison waiting in the wings. As long as people keep thinking carcinogens and toxins that kill lovely butterflies are more palatable than annoying but fairly innocuous pests, we’ll live in a world that has both toxins AND army worms-while beautiful but fragile species continue to decline and disappear.