Tiny fungi living inside white spruce trees help protect them from destructive budworm insects in two ways: the fungi directly produce toxic substances that kill or harm the budworms, and they also trigger the trees to produce more of their own natural chemical defenses called terpenes. Scientists found that spruce trees with more of these helpful fungi in their leaves were better protected against budworm damage. This discovery shows that trees don’t fight pests alone but partner with beneficial fungi as part of their defense system.