Effects of crowding and host plant quality on morph determination in the soybean aphid,Aphis glycines

Abstract

AbstractPolyphenism constitutes a key example of the adaptability of a genotype to a shifting environment. In aphids, the relative production of apterous and alate individuals is influenced by cues resulting from increases in population density, changes in plant quality, and natural enemies. The ability to respond to multiple environmental cues offers aphids an effective way to optimize their fitness. Understanding the mechanisms behind alate morph production is particularly important in aphid species that are agricultural pests. We tested the effects of crowding and host plant quality cues on morph determination inAphis glycinesMatsumura (Hemiptera: Aphididae). We exposedA. glycinesto pre‐natal and post‐natal crowding under laboratory conditions. In the field, we reared nymphs ofA. glycinesat high‐density vs. low‐density conditions in clip cages set on the top vs. bottom nodes of infested and uninfested soybean plants,Glycine max(L.) Merrill (Fabaceae). Apterous adult body size was used as an estimate of aphid performance, and the proportion of morphs developing under each condition was determined. Both pre‐ and post‐natal crowding induced the production of asexual alate individuals byA. glycines. In the field, we observed additive negative effects of lower overall plant quality (infested vs. uninfested plants), within‐plant differences in nutritional quality (bottom vs. top nodes), and high vs. low levels of crowding on aphid size. These three factors also interacted to increase the proportionA. glycinesnymphs that developed wings. Our results suggest two levels in the production of asexual alate individuals byA. glycines: (1) a moderate level in response to aggregation, triggered by crowding cues, and (2) a high level in response to plant quality deterioration, triggered by both plant quality and crowding cues. These two processes ensure a flexible production of asexual alate individuals, allowing aphids to adapt to changing environmental conditions.