Previous studies report both that plant morphology does and does not influence the reproductive rate of phytophageous insects. While some of these divergent results may be due to species-specific differences, contradictory results are even reported for the same species on the same host plant. When effects of morphology are reported, postulated mechanisms for significant effects include differences in: crowding, plant chemistry, and shelter from environmental conditions. We examined the effects and interactions between plant spatial structure, nutrient availability and environmental conditions on the reproductive rate of pea aphids, Acyrthosiphon pisum (Harris), using a single cultivar of pea (Pisum sativum L.). A simple, naturally occurring mutation produces four near-isogenic morphological types of peas. Manipulations of aphid density (1, 5 and 10 of adult females) were crossed with treatments of plant morphology and nitrogen addition to determine if there were any effects of crowding or nutrient status that interacted with effects of plant morphology.  In a second set of experiments, the effects of morphology were examined in greenhouse conditions to determine if morphology interacted with environmental conditions to affect pea aphid reproductive rates.

Results/Conclusions In all cases, no effects of pea morphology were found. Further, there were no interactions with morphology, nutrient availability, aphid density or environmental conditions, although these factors could independently alter reproductive rates.  We conclude that previously reported differences in aphid reproductive rates associated with pea morphology may have been driven by uncontrolled differences in plant chemistry between cultivars rather than plant shape.