Host-limited dynamics of autoparasitoids

Enhanced stability in host-parasitoid interactions with autoparasitism and parasitoid migration

Previous studies based on simple non-spatial model have suggested that autoparasitism, in which females develop as primary endoparasitoids of hosts while males develop at the expense of primary parasitoids, stabilizes host-parasitoid steady state. To date, however, how the stabilizing role of autoparasitism would be affected by more complex spatial factors has not been adequately investigated. To address the issue, here we analyzed a spatially extended two-patch host-parasitoid model and compared it with the corresponding non-spatial model. Results showed that in the non-spatial model and the case of autoparasitoid, the host-parasitoid steady states can be unstable if the host׳s intrinsic rate of growth and/or carrying capacity is sufficiently large. However, in the spatially extended two-patch model with parasitoid migration, the unstable host-parasitoid steady states in each local patch may become stable, provided there is certain spatial unevenness in host growth and/or carrying capacity. Therefore, the migration of parasitoid together with spatial unevenness in host growth and/or carrying capacity stabilizes the host-parasitoid interactions. The stabilizing effects are stronger with the host density-dependent migration of parasitoid than with the random migration of parasitoid. In the case of primary parasitoid, the model demonstrated similar stabilizing effects associated with the migration of parasitoid. However, the parameter conditions for stability are much more stringent than in the case of autoparasitoid. We concluded that the stabilizing effects of parasitoid migration and autoparasitism can add to each other, leading to more stable host-parasitoid interactions.

Consequences of stage-structured predators: cannibalism, behavioral effects, and trophic cascades

Cannibalistic and asymmetrical behavioral interactions between stages are common within stage-structured predator populations. Such direct interactions between predator stages can result in density- and trait-mediated indirect interactions between a predator and its prey. A set of structured predator-prey models is used to explore how such indirect interactions affect the dynamics and structure of communities. Analyses of the separate and combined effects of stage-structured cannibalism and behavior-mediated avoidance of cannibals under different ecological scenarios show that both cannibalism and behavioral avoidance of cannibalism can result in short- and long-term positive indirect connections between predator stages and the prey, including "apparent mutualism." These positive interactions alter the strength of trophic cascades such that the system's dynamics are determined by the interaction between bottom-up and top-down effects. Contrary to the expectation of simpler models, enrichment increases both predator and prey abundance in systems with cannibalism or behavioral avoidance of cannibalism. The effect of behavioral avoidance of cannibalism, however, depends on how strongly it affects the maturation rate of the predator. Behavioral interactions between predator stages reduce the short-term positive effect of cannibalism on the prey density, but can enhance its positive long-term effects. Both interaction types reduce the destabilizing effect of enrichment. These results suggest that inconsistencies between data and simple models can be resolved by accounting for stage-structured interactions within and among species.

Interspecific host discrimination and within-host competition between Encarsia formosa and E. pergandiella (Hymenoptera: Aphelinidae), two endoparasitoids of whiteflies (Hemiptera: Aleyrodidae)

Interspecific host discrimination and within-host competition between Encarsia formosa Gahan and Encarsia pergandiella (Howard), two endoparasitoids of whiteflies, were studied under laboratory conditions. Interspecific host discrimination was studied at two time intervals (0 h and 72 h after the first species had oviposited). Parasitized and unparasitized Trialeurodes vaporariorum (Westwood) hosts were accepted for oviposition at the same rate by the two parasitoid species. Host type did not affect the handling time of the two parasitoids. The outcome of within-host competition was investigated after females of the two species parasitized the hosts at various time intervals. In four treatments, E. pergandiella was allowed to oviposit 0, 24, 48 and 72 h after E. formosa while in the other two, E. formosa was allowed to oviposit 0 and 72 h after E. pergandiella. In four of these treatments: E. formosa following E. pergandiella at 0 and 72 h, and E. pergandiella following E. formosa at 0 and 24 h, E. pergandiella prevailed. In the host discrimination experiment (72 h interval), 20% of E. pergandiella eggs were killed by E. formosa females. Interspecific ovicide was also observed in the within-host competition experiment, in which 6% of 72-h-old E. pergandiella eggs were killed by E. formosa females.