Answer: Almost every landscape near the Olympic Peninsula is being attacked by tent caterpillars, and there really isn't much a gardener can do about them because they're everywhere! When caterpillar populations get really large, benefical wasps come to the rescue. They lay their eggs inside the caterpillars (as evidenced by a white spot on the top of a caterpillar's head). When the wasp's eggs hatch, the larvae consume the insides of the caterpillar. While this sounds gruesome, it's nature's way of achieving a balance. If you simply can't wait for the parasitic wasp population to take care of the caterpillars, you can apply B.t. (Bacillus thuringensis) to your favorite plants. B.t. is a bacterium that acts as a stomach poison and is effective against all caterpillars. Once you've sprayed your plants, expect the caterpillars to continue to feed - until the bacterium begins to work.
Don't worry too much about the damage the caterpillars have done to your plants. Healthy plants will produce new leaves later in the summer. Best wishes with your garden!
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