Host plant architecture affects the costs of dropping behaviour in Phaedon brassicae (Coleoptera: Chrysomelidae)

Phytophagous insects escape from predators by dropping. However, if they drop to the ground, they must then return to the host plants. Large oval leaves may serve as safety nets when insects drop from overlying leaves. To clarify the effects of leaf shape on the sites to which insects drop, we investigated the dropping behaviour of Phaedon brassicae (Coleoptera: Chrysomelidae) on three host plants (daikon, bok choy, and Chinese cabbage). Daikon plants have cleft leaves, whereas bok choy and Chinese cabbage plants have oval leaves. When poked with forceps, larvae dropped less frequently than adults. The proportions of individuals dropping to the ground also differed among host plants. Both larvae and adults on the ventral (abaxial) leaf surfaces of daikon frequently dropped to the ground via clefts in the underlying leaves. However, larvae and adults on the ventral leaf surfaces of bok choy and Chinese cabbage frequently dropped to underlying leaves. Most larvae and adults that dropped to the ground finally returned to host plants. However, the return times were longer for larvae than adults. Therefore, the cost of dropping from daikon leaves was higher than were the costs of dropping from leaves of other crop species.