On the West coast and especially in
Western Washington, there are a few varieties of tent
caterpillars, the most common being the
Western Tent Caterpillar (Malacosoma Californicum) and the less common
Forest Tent Caterpillar (Malacosoma Disstria). They primarily feed on
Cherry,
Alder,
Apple,
Willow,
Birch,
Cottonwood,
Ash and
Rose. Infestation can be rather disgusting with the unsightly tents in the crooks of branches, the ugly brownish-orange wriggling mass that can be seen basking in the sunlight, the gut wrenching (yet oddly satisfying) *pop* as you squish them under your shoes and the massive defoliation that occurs in their presence. There have been extreme seasons in the past, namely in the early 1960s and in 1986 where they have become an
environmental hazard, causing the streets to become slick with the millions of migrating caterpillar
carcasses. All that aside, there are several
benefits to them.
One, the defoliation can allow more sunlight to reach the lower plant life that normally doesn't get as much sun during the spring and summer months. Generally the destruction caused by tent caterpillars isn't severe enough to kill any trees since they will just refoliate later, except if the trees are already weakened by other conditions like weather and prior diseases.
Two, the tent caterpillar is a valuable part of the natural food chain. Their poo is an excellent fertilizer for other plant life and caterpillars have many natural predators. Rodents and raccoons tend to like the caterpillar larvae while the birds looove the moths, since they all see a free meal. It is quite amusing to see the birds pick off the moths mid-flight. Predatory insects like ants, ground beetles and yellow jackets will eat them as well, or in other cases they will use them for their own ends. The Tachinid Fly for example, will attack and lay their eggs on their heads or other parts of the caterpillar bodies to serve as hosts for the parasitic larvae that will burrow into their bodies and kill them.
Controlling them is not really necessary unless you are seeking to have beautiful landscaping or are a farmer and they are destroying your crops. There are several methods that be used to control them, short of polluting with kerosene or possibly starting a wildfire (see above writeup). The best time to control the tent caterpillar population is during the winter season, where you can scrape or prune off the brownish egg casings that resemble styrofoam the adult moths have laid the previous summer on the branches and twigs of the host trees. Early in the season, you can just scrape off the tents as they form with your hands (wear gloves if you feel inclined) or prune infested branches and then stomp on them or burn the prunings (do NOT try burning them directly off the trees in your yard, you COULD burn your own house down if you are careless!). A biological agent, like Bt (Bacillus thuringiensis), is the most effective control for the ones out of reach and when properly applied to the foliage immediately surrounding the tents (figure a 2' radius) will selectively kill only caterpillars, since it is a bacteria that produces a neurotoxin that paralyzes their digestive tracts. Bt is only effective if the caterpillars eat the treated leaves and is quite safe to use as it will not harm plants, other insects, animals or humans. You can also resort to chemical sprays, but be warned, they could do MUCH more damage than the caterpillar, including killing all the helpful insects and their predators that you do want in your garden! Another thing you can do is to help out the natural predators by allowing them easier access in the morning and evening hours when the birds feed. You can simply punch holes in the otherwise impervious tents with a stick, or my favorite, shoot the tents with a paintball gun.
Yes, they are icky. Yes, they are disgusting. Yes, they do some damage. But let nature take its course and you will have a lot more plant and animal diversity seeing its way to your yard as the season goes on.
This info was compiled from just about every page I could Google™ on the subject. Just do a search for tent caterpillar if you want to see the whole bibliography or learn more than you probably ever wanted to know about entymology.