Premature Leaf Abscission: An Induced Plant Defense Against Gall Aphids

We hypothesized that premature leaf abscission represents an adaptive plant response to herbivore attack. To the extent that this host response reduces the pest population and is cost effective, it should be considered an induced plant defense. To test this hypothesis we examined the patterns of abscission by two species of cottonwoods (Populus), quantified the impacts of abscission on the survival of two Pemphigus gall aphid species, and examined aphid behavioral and life history countermeasures to this host response. These studies revealed that premature leaf abscission is induced by gall aphid attack. This host response reduced the aphid population by 25% on narrowleaf cottonwood and by 53% on Fremont cottonwood. Galled leaves rapidly declined in quality; only 1 wk after colonization ungalled leaves contained 10.9% more chlorophyll than galled leaves, and differences continued to increase with time. This induce host response is selective and dosage dependent; leaves with 3 or more galls were four times as likely to be dropped as leaves with 1 gall, and 20 times as likely to be dropped as ungalled leaves. Selective abscission is effective even at low aphid densities. At least 98% of the aphids in the galls of abscised leaves died, 90% within 48 h of abscission. Mature aphids on senescing leaves had lower body masses than those on green leaves, demonstrating that the process of leaf abscission is detrimental to aphids before leaves actually fall. A few aphids (≤1%) escaped leaf drop by migrating from deteriorating galls to nearby galls in less danger of imminent abscission. Since the effect of premature leaf fall force gall aphids to emigrate, shifting to new hosts may have evolved as a counteradaptation to escape this induced plant defense. Sessile plant pests should be particularly susceptible to induced leaf abscission, and this may explain why galling aphid species are 3.5 times as likely to host—shift as nongalling aphids.