Petiole gall of poplar is caused by:

• A. An aphid
• B. A mite
• C. A fungus
• D. A beetle

Answer: (A)

Petiole galls on poplar are formed by aphids. These small sap-sucking insects feed on the phloem and, through their feeding and saliva, induce the plant tissue to grow abnormally, creating a gall around the developing aphids on the petiole. The gall serves as a protected habitat for the aphids and their progeny. While mites, fungi, or beetles can cause other kinds of galls or plant damage, the telltale tissue changes and the typical association with poplar petiole galls point to an aphid as the responsible organism.