Researchers discovered that a fungus living inside poplar trees helps protect them from harmful insects by boosting the tree’s own chemical defences and producing its own deterrent compound called stachydrine. In laboratory tests, the fungus successfully deterred leaf-eating insects from feeding on poplar leaves. However, in field studies, the endophyte-infected trees attracted more aphids while repelling beetles, suggesting the fungus affects different types of insects differently depending on how they feed.