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Caterpillars’ ‘Horror Show’ Center Stage

They’re back! Like a bad horror flick, tent caterpillars have returned to the North Country. Actually, they never left, having overwintered in egg masses the female moths (adult caterpillars) deposited on tree twigs last summer, and hatched in late April. But now that they’re out, woodlot owners and maple sugar producers are concerned, and the rest of us are cringing at the thought of having a moving carpet of caterpillars underfoot for another season.

Maple syrup producers especially dread their onslaught since the caterpillars have eaten into their production and income, says Steve VanderMark of Cornell Cooperative Extension. “Many producers have also paid for costly sprays from airplanes, with no government cost-sharing, to save at least some of their sugar maples’ leaves. Those leaves make the sugar for next year’s sap, and with reduced sugar due to defoliation, some sugar bushes were not worth tapping the last two or three years,” he explains. “Forest and sugar bush owners have even seen significant tree die-offs where the caterpillars have compounded long-term stresses such as drought.”

Variously known as tent worms, and a host of other names unsuitable to print, there are actually two species of tent caterpillars: The eastern tent caterpillar, whose tentlike silk nests are visible in cherry and apple trees right now, has a single whitish stripe down its back. The forest tent caterpillar, which despite its name doesn’t build silk nests, has a row of keyhole-shaped “footprints” down its back. Both species are native.

Eastern tent caterpillars hatch about a week earlier than their forest tent caterpillar cousins, and have a head start right now on defoliating trees. As mentioned, they prefer cherry and apple, but will turn to other host plants if they have to. Eastern tent caterpillars are of special concern to horse owners, as they are now known to cause pregnant mares to miscarry when the mares inadvertently ingest caterpillars that are in the pasture grass.

It’s the forest tent caterpillars, though, that will really start to make their presence known in a few weeks. They’re small right now, a half inch or so, and while recent cool rainy weather has put a damper on their activities, they’ll grow fast. Many areas of St. Lawrence County saw severe defoliation (60 percent or more leaf loss) last season, and some hot spots were nearly entirely stripped of foliage to the point where the damage was visible from satellite. The bad news is that this year the outbreak is likely to continue. The good news is that areas defoliated for two consecutive years may see a natural decline in their population.

That’s small consolation to maple producers who have seen their livelihoods affected. One defoliation can significantly lower sap sugar content for one or more years. And repeated defoliation will weaken trees, making them more susceptible to other stressors, like the recent outbreak of lecanium scale insects (which are responsible for the sticky droplets coating your windshield if you park under a maple these days). Timber managers as well are watching how the caterpillar outbreak progresses.

It may seem like needless worry over a phenomenon that has been occurring for a few thousand years. Indeed, under ideal conditions only 15 to 20 percent of overstory trees die as a result of two or even three consecutive defoliations, according to Dr. Douglas Allen of SUNY-ESF. But trees have never experienced water stress like they have in the past two decades. It takes two to three years for a tree to recover from prolonged dry spells such as we’re having about every other year. Other pests like the scale add stress. As if that wasn’t enough, a researcher at the University of Alberta discovered that caterpillar outbreaks in fragmented forests last much longer than those in large tracts of forest.

Many maple producers are having their sugar bushes aerially sprayed to control tent caterpillars. The best product for killing them is a called Bt (for Bacillus thurengiensis, the organism that makes the toxin) and is available under various trade names like Dipel or Thuricide. Bt is nontoxic to humans, wildlife and other insects. Timber managers may want to consider delaying thinning operations in heavily infested stands, as harvesting operations can further stress trees, and anyone with a pregnant mare would do well to try and limit the eastern variety around pastures. Such controls might include removing host trees or using an insecticide such as Bt on infected trees in pastures or near pasture edges.

Homeowners can use Bt on their yard trees, but you’ll have to hurry to control the eastern tent caterpillar, as they’re getting past the life stage that’s easy to control. Since they congregate in their nests in daylight, their nests can be thrown to the ground and squashed where they’re reachable. Forest tent caterpillars are still small enough that there’s time to spray Bt, and while they can sometimes be found on the same fruit trees as easterns, a professional spray rig would be needed to reach into the canopy of the mature trees they infest.

When these pests eat up one food source they’ll travel to seek another. Using sticky barriers like ‘Tanglefoot’ or other brands on the trunks can help protect small yard trees from getting climbed by hungry hordes on the march. Watering yard trees during dry periods will help them recover from defoliation.

There are many natural controls for tent caterpillars like viruses and fungi, but it takes a few years for these to build up to the point where they bring down the caterpillar population. Infestations typically last from two to four years in large tracts of forest, and up to six years in small blocks. One reason fragmented forests have longer infestation times is that sunlight inhibits a caterpillar-killing virus. A species of non-biting fly parasitizes the caterpillars, and though these flies can be somewhat of a nuisance themselves, keep in mind that they’re beneficial. Unfortunately, no birds native to the area are known to eat the tent worms in a big way.

Remember, caterpillar season won’t last forever, and the infestation cycle will run its course and we’ll all breathe a sigh of relief.

Paul Hetzler is Program Assistant at Cornell Cooperative Extension, Canton.



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