Caterpillars have deadly little enemies in the form of tiny parasitic wasps (many only 3 to 5 mm/0.1 to 0.2 in long). These wasps search diligently, leaf by leaf, throughout plants, looking for cabbage loopers, cabbage worms, tent caterpillars and other hosts in which to lay their eggs. The wasp larvae develop inside the caterpillar’s body, eventually killing it. Some species then spin their small white cocoons outside the remains of their host, while others remain inside the host cocoon until emerging as adults. One female wasp can parasitize 200 to 300 caterpillars in her short lifespan, making these parasites very valuable allies in our yards.
Unlike the much bigger yellowjackets, parasitic wasps do not sting people. The female wasps dine only on pollen and nectar, so you can attract them by growing plants with tiny, nectar-rich flowers in the garden: thymes, lovage, savory and other herbs, sweet alyssum, dill, cilantro, parsley, yarrow, candytuft, verbena, goldenrod.
—Linda Gilkeson, PhD
author of West Coast Gardening: Natural Insect, Weed & Disease Control