Natural predators, such as spiders and particularly the European paper wasp, also cause butterfly numbers to dwindle, says Whitney Crenshaw, an entomology professor at Colorado State University. These wasps migrated to the state five years ago and now infest backyards along the Front Range. One of their favorite foods is caterpillars, Crenshaw says. The wasps, which look like yellow jackets, build open, paperlike hives and mature in mid-July.

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