Dealing With Tent Caterpillars And Moths

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Safe & Sensible Caterpillar Controls

Over the past few years, those revolting little tent caterpillars have been making a comeback. Gardeners all over the region have been seeing tent caterpillars’ ragged white tents, which can be found in some surprising places, but are most apt to feed on alders, cascara, willows, Indian plum, fruit trees, and rose family trees and shrubs. Native to these parts, the Western Tent caterpillar population explodes on a roughly 3-7 year cycle. If the caterpillars aren’t ravaging your home and garden, WSU tree scientists recommend leaving them alone. Huh? Well, as tent caterpillars thin the leafy forest canopy, young conifers capture more sunlight and put on extra growth. When you hear the pattering of caterpillar droppings hitting the ground, perhaps it may relieve your feelings to know that they help fertilize the understory of the woodlands. Or not.

When saggy bags full of ravenous tent caterpillars rip into the garden, it’s pretty hard to watch. Fortunately, there are a number of safe and effective ways to cope with these wild rascals. For starters, when pruning trees and shrubs in winter, check for egg cases and strip them off before they hatch. They look like little strips of dirty plastic foam, and peel off quite easily. You may also release tiny trichogramma wasps. These minuscule parasites could hold a party on a pinhead (4-6 of them could cluster there). They lay their own eggs in the egg cases of tent caterpillars, destroying the incipient tent caterpillars to nourish their own tiny offspring.

Buy Some Beneficial Wasps

Trichogramma wasps are among the most commonly used beneficial insects world-wide. When you buy trichogramma wasps, the container you purchase is not full of wasps. In it, you’ll find a postcard that you fill out and mail. In about a week to 10 days, the wasps arrive. You probably won’t be able to see them,  because they are so small, but they’re in there, raring’ to go. One cup is enough to control pests in most greenhouses or in the average city lot. For larger properties, place beneficial wasp cups about 50 feet apart, starting about 25 feet inside your property line. If the problem lies outside your yard, you may want to donate wasps to the neighbors or go in with them on a larger shared purchase. Each packet contains about 12,000 live trichogramma wasps, enough for an area of approximately 2,500 square feet.

The usual packaging holds a little paper card inside a plastic cup which can be hung from a tree branch or tucked between the trunk and a branch. Remove the cap from the cup, and your wasps will start hunting right away. For the best results, release your trichogramma wasps in the evening after a warm day. Their preferred temperature range is between 70 and 80 degrees F, but they cope quite well with the vagaries of maritime weather. To make any beneficial insect release more successful, lightly spray the release area with water before letting the insects go. All beneficials need a drink after coming out of dormancy and/or captivity. If there is plenty of moisture available, they’ll stick around to do the job you have in mind, rather than heading out to find water.

But First Identify Your Target

When caterpillars arrive, take time to identify your caterpillars before taking action. Tent caterpillars are about 2 inches long, dark brown and very fuzzy, with a white stripe down their back and linear or blobby red or blue side markings. If your caterpillars look like this and are emerging from baggy tents, you can move on to the next step, which is examining the caterpillars for signs of yet another kind of parasitic wasp. Typically, these head-of-a-pin tiny wasps lay a single egg on each caterpillar’s head, though an egg may appear anywhere on the body. The white eggs are about the size of a pimple and they mean certain death for the involuntary host when the baby wasps eat their way free.

Thus, if you find plenty of white-dotted tent caterpillars, let them be. Each parasitized caterpillar contributes to the population of the beneficial wasps, which are an excellent natural control. If the caterpillars are feeding on precious plants, you may want to do some hand picking. However, instead of squishing the pimpled ones, consider tossing them into an area of weeds or long grass, so they remain available to their natural parasites.

When Spray Is the Answer

If the caterpillars are everywhere and no dotted heads are in sight, it may be spray time. It’s important to know that toxic caterpillar pesticides only work on the caterpillars, not the webs, which are waterproof and impermeable to toxins. If your yard holds unreachable webs you can’t live with, spray surrounding foliage with Bt (Bacillus thuringiensis). This naturally occurring bacteria comes in several forms; the kind used on caterpillars interrupts their normal digestion and maturation processes. When they eat leaves sprayed with Bt, they stop eating and die within a few days.

Like most botanical pesticides, Bt doesn’t last very long, so you may need to spray several times if you have a bad infestation. Carefully targeted and timed Bt use minimizes or eliminates accidental non-target caterpillar kill. Because Bt dissipates so quickly, it won’t persist to be a problem for the later-appearing caterpillars of Painted Ladies and other handsome butterflies. Keep your eyes open if fluctuating spring temperatures cause caterpillars to hatch out in flushes, since timing is critical to success. Until caterpillars emerge from the webs, nothing is going to kill them, however deadly. Once babies emerge and begin to feed, you can start your spray program.

Play That Trombone

Always use a clean sprayer that has never been used for toxic herbicides. (Right?) Hand or backpack sprayers work well for treating shrubs and smaller trees. For tall trees, you may successfully apply Bt with a trombone sprayer, which can reach up to 20 feet or more. You’ll get more height if you don’t overfill the sprayer, as each gallon of water weighs about 8 pounds. Make sure the Bt is fresh (the package should have an expiration date), and mix it as directed, since brands can vary. Usually, the shelf life is a year or two for unopened bottles and about 3 months for opened bottles. Wettable powders may be good for two years if unopened. Bt that has frozen and thawed is probably no good.

As always, spray any pesticide on a calm, windless day when bees and other pollinators are not present (early morning or late afternoon). With Bt, you only want to cover the foliage surrounding the webs of feeding caterpillars. For light infestations, a single dose might be enough. trees and shrubs that are hosting emerged and feeding caterpillars. For a severe caterpillar attack, you may need to spraying Bt every 2 or 3 days for a week.

If You Just Give Up

If you do nothing, the caterpillars will pupate, then emerge as moths. A month or so after the last crawlers are gone (right about now), watch for large (2-inch) cinnamon brown or tawny, rust colored moths. You’ll probably spot them fluttering around light fixtures at night. In the morning, you may find dead moths under each outdoor light, looking like heaps of tattered leaves on the ground. Each moth lives for about three days, existing only to lay eggs that will hatch out next spring as tent caterpillars.

The number of tawny moths you see will be a good guide to the state of your yard next year. There are several good strategies for moth control, starting with  placing yellow light bulbs in your outside light fixtures. Next, set tubs of water beneath them, with a drop or two of vegetable oil added to help trap the moths. Soapy water works too, but tends to get stinky quickly in warm weather. If you see a lot of moths, set large tubs or kiddie wading pools out in open areas and float candles in them each evening to draw the moths to their doom.

Bats To The Rescue

The best predators for moths are bats and swallows, both of which are evening feeders. Swallow houses and bat boxes help bring these excellent critters into your yard where they can help reduce the moth population. Swallows like their houses placed in protected spots such as under the eaves of a house or garage. They also like their privacy, so position each nesting box so that the entrance can’t be seen from any neighboring boxes.

Bat houses need to be placed where they get a lot of sunlight for most of the day. Lack of warmth is the most common reason that bat houses remain unused. In his excellent book, Landscaping For Wildlife In The Pacific Northwest (U of W Press, 2000, 320 pp., $29.99), Russell Link offers lots of good tips on placement for successful bat house use. He also suggests painting bat houses brown or black to increase the warmth inside.

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