Species like tent caterpillars can be a problem when they appear in masses.
Tent caterpillars can completely defoliate a tree. Gardener’s Path will help you spot and get rid of them, and explain why you might want to leave them be.
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But there's no need to use toxins. Often it just helps to settle their natural enemies like specific insects (wasps, green lacewings) and birds.
When your trees are full of tent caterpillars, there was already an ecological disbalance before. For example, if you remove wasp nests in your garden, who will help decimating pests? Some people don't want ants in their garden but it's a bad idea to get rid of them.
You kill spiders? Better don't do it, they help keeping the ecological balance. Less spiders = way more mosquitoes!