The convergent lady beetle, Hippodamia convergens Guerin-Meneville, is a specialized predator of cereal aphids on the High Plains, completing its first generation each year in winter wheat, the resulting adults dispersing into summer crops and producing additional generations, contingent on the availability of aphids. In the present study, we tested the collective value of supplementary plant resources (sugars, pollen, and seedling wheat leaves), and small amounts of alternative prey, eggs of Ephestia kuehniella (Zeller) (Lepidoptera: Pyralidae), all provided together, for improving beetle life history and reproductive success even when suitable prey, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), were provided ad libitum. Although a monotypic diet of S. graminum yielded slightly faster larval development and heavier adults than the omnivorous diet, preoviposition periods were extended and 21-d fecundities greatly reduced when this diet was continued through adult life, largely due to fewer oviposition days, although egg fertility was unaffected. The results highlight the critical importance of plant-derived resources even when suitable prey are not limiting. However, monotypic diet beetles that diapaused for 21 d in the presence of supplementary plant resources, plus moth eggs, achieved the same reproductive success as those reared on the omnivorous diet, with or without diapause, demonstrating that access to these resources post-emergence was sufficient to compensate for their absence during development. The diapause treatment itself had no impact on the fitness of beetles reared on the omnivorous diet, likely because neither its duration, nor the caloric restriction imposed, were sufficient to diminish reproductive effort.
Disturbances have long been recognized as important forces for structuring natural communities but their effects on trophic structure are not well understood, particularly in terrestrial systems. This is in part because quantifying trophic linkages is a challenge, especially for small organisms with cryptic feeding behaviors such as insects, and often relies on conducting labor‐intensive feeding trials or extensive observations in the field. In this study, we used stable isotopes of carbon and nitrogen to examine how disturbance (annual biomass harvesting) in tallgrass prairies affected the trophic position, trophic range, and niche space of ants, a widespread grassland consumer. We hypothesized that biomass harvest would remove important food and nesting resources of insects thus affecting ant feeding relationships and trophic structure. We found shifts in the feeding relationships inferred by isotopic signatures with harvest. In particular, these shifts suggest that ants within harvest sites utilized resources at lower trophic levels (possibly plant‐based resources or herbivores), expanded trophic breadth, and occupied different niche spaces. Shifts in resource use following harvest could be due to harvest‐mediated changes in both the plant and arthropod communities that might affect the strength of competition or alter plant nitrogen availability. Because shifts in resource use alter the flow of nutrients across the food web, disturbance effects on ants could have ecosystem‐level consequences through nutrient cycling.
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