Kitsap County has been hit hard by tent caterpillars in recent years, and the infestation is increasing. If these pesky critters ravaged your garden, eradicating them in the moth stage will reduce their presence next year.
Tent caterpillar moths emerge several weeks after the caterpillars cocoon. They are large (about 2 inches across) moths with brownish, cinnamon or rusty red wings. Sprayed on host foliage, Bt (Bacillus thuringiensis) is the treatment of choice for caterpillars. However, since tent caterpillars don’t eat during their brief moth stage, Bt is not an effective treatment for the moths. Though the moths live for about three days, they often hatch out in flushes. Where tiny young caterpillars were still actively feeding while husky 2-inchers were looking for nesting spots, moths may appear for several weeks.
To lure the moths to their doom, replace porch and outdoor lights with yellow bulbs. (Yellow light is especially attractive to moths, though ordinary lights work fairly well, too.) Place wide bowls of water under the lights, adding a little vegetable oil. This makes a film that coats moth wings and smothers moths quickly when they follow the light’s reflections into the water. Where the moth invasion is heavy, fill large buckets (or a kid’s wading pool) with water and a little oil, then float tea light candles at dusk as the moths arrive.
For future protection, encourage bats and swallows (which eat night flying moths in large quantities) to move in by installing swallow houses and bat houses. Russell Link’s wonderful book, “Landscaping With Wildlife In The Pacific Northwest,” offers placement tips and do-it-yourself patterns for both.
Tent caterpillar moths begin laying eggs immediately. To take advantage of them in that inert stage, we can call on almost-invisible parasitic wasps to defend our plants. Many nurseries sell Trichogramma wasps, tiny, pin-sized creatures that don’t sting people. Instead, they lay their own eggs in the host eggs of the tent caterpillars. When the wasp eggs hatch, the tent caterpillar eggs are destroyed as the baby wasps eat their way out. To get the most bang for your buck, place wasp egg cards in trees that were heavily infested by the caterpillars, since these will generally have the most egg cases. Each card contains about 12,000 wasps, enough to cover an area of about 2,500 square feet.
Another line of protection comes from horticultural oils (including dormant oils, sun oils, and superfine oils), which can smother tent caterpillar egg cases. Spray the oil on defoliated trees and shrubs as soon as you notice fresh egg sacs this summer, and repeat next winter when trees are naturally bare. This summer, hand pick any visible egg cases as soon as you notice them (they look like Styrofoam). Egg cases can be crushed, burned, or added to active worm bins or municipal garden waste containers.
Don’t just toss egg cases down on the ground, or the caterpillars may find plenty to eat when they hatch next spring. Although they prefer to eat foliage of fruit trees, roses, alders and cottonwoods, emerging tent caterpillars can subsist on grass and perennial foliage just fine. Don’t put egg cases in your own compost either, unless it gets hot enough to bake those little eggs. Tent caterpillar moth eggs are tough: one year, I put some in a plastic zip bag for show-and-tell garden talks and by spring, voracious baby caterpillars were eating their way out of the bag.