Feeding location of aphid prey affects life history traits of a native predator

The green peach aphid (GPA), Myzus persicae S., and the cabbage aphid (CA), Brevicoryne brassicae L., exhibit a feeding preference when exposed to different canola (Brassica napus L.) plant structures. Preference may be associated with the concentration and/or composition of allelochemicals; specifically, glucosinolates (GLS). Furthermore, each aphid species employs a different strategy for tolerating plant defensive chemistry; GPA excretes glucosinolates while CA sequesters these toxic compounds. Given these different detoxification mechanisms, it is possible that both feeding location and aphid species may affect prey suitability for Hippodamia convergens larvae. We conducted a factorial laboratory experiment to evaluate whether one or both factors impacted predator fitness. We hypothesized that plant structures with higher GLS concentrations will negatively affect the development and survival of immature predators, which will also vary based on prey detoxification strategies. Results confirm that when predators fed on either aphid species reared on canola structures having lower GLS concentrations, development was faster than when they fed on aphids reared on structures having higher GLS. Although predators consumed more GPA than CA, consumption rates did not differ between rearing location for GPA, but fewer CA were consumed when reared on reproductive canola parts. These findings suggest that: 1) plant-mediated differences in prey quality exist for canola aphids; 2) the type of adaptation used by aphids to overcome plant defenses have important consequences for prey suitability; and 3) reduced feeding by H. convergens larvae on unsuitable prey may offset deleterious effects of plant defenses against aphid pests. By evaluating source-sink dynamics at the plant level, we can further understand trophic interactions involving plant defenses and how these interactions may influence community dynamics and structure.

EFFECTS OF PREDATOR DIET BREADTH ON STABILITY OF SIZE SPECTRA

Many marine ecosystems have the remarkable property that the abundance of organisms of a given body size is approximately proportional to the inverse square of that size. Size-structured models have been developed for which this “invariance-of-biomass” state is an equilibrium solution. These models are built on the coupling of predator growth to prey abundance, where prey suitability is determined by a size-based function referred to as a feeding kernel. In this paper, the local stability of the equilibrium state is investigated in a limiting case where predators only consume prey of a preferred size. In this special case, it is shown analytically that the equilibrium state is always unstable. It is concluded that some degree of diet breadth, in terms of the range of prey sizes consumed by a predator, is an essential prerequisite for the invariance-of-biomass state to be stable, as widely observed in the field.