A fungus that lives inside poplar trees helps protect them from insects by changing the tree’s chemical makeup and producing its own insect-repelling compound. Scientists found that this endophytic fungus makes poplar leaves taste worse to leaf-eating insects like gypsy moth caterpillars. However, in field conditions, the fungus unexpectedly attracts more aphids while keeping beetles and ants away, showing that endophytes can have complex effects on insect communities depending on the type of insect.