No abstract

The host plants of arthropod pests may affect parasitoids and predators directly or indirectly, through multitrophic interactions. Direct plant effects may involve simple mechanisms such as reduced parasitoid searching efficiency caused by trichomes. Multitrophic effects often involve complex interactions that are not well understood, and their impact on natural enemies and biological control are difficult to predict. Knowledge of the direct and multitrophic effects creates opportunities to increase the effectiveness of natural enemies by incorporating natural enemy-enhancing traits into crop plants. The strategy may have potential for both generalist and specialist natural enemies, but the enemies' behavior and other factors will affect the results. Although combining natural enemies and plant resistance may slow the adaptation of some insect pests, it may speed up adaptations of others. A better understanding of plant/pest/natural enemy evolution is necessary to predict how to combine natural enemies and plant resistance for the best long-term results.

Augmenting generalist predator populations in new apple (Malus domestica Bork) plantings could potentially aid in the establishment of balanced orchard ecosystems that are less susceptible to pest outbreaks. Habitat can be an important factor in retaining predators in a system and can affect predator efficiency. We investigated the potential of increasing a complex of generalist ground-dwelling predators and enhancing biological control in a young ÔGolden DeliciousÕ apple orchard through ground habitat manipulation. We modified the orchard floor with four comparative habitat treatments: (1) detritus-rich compost mulch layer, (2) detritus-free synthetic mulch layer, (3) herbicide-treated vegetation thatch, and (4) bare soil with vegetation hand-removed. Relative abundances of predators and alternative prey in the habitats were measured with pitfall trapping throughout the growing season. Predation was measured using sentinel Cydia pomonella larvae and directly observed in night experiments. Throughout the season, the compost mulch treatment consistently supported significantly greater densities of alternative prey resources for predators, and generalist predators were more abundant in the compost mulch than the other habitats. Predator complex abundance was positively correlated with increasing alternative prey availability in the compost mulch habitat. However, predation of C. pomonella was significantly lower in the compost mulch than in the herbicide-treated thatch habitat. Our study revealed that a prey-rich organic mulch can enhance ground-foraging generalist predators on the orchard floor, but habitat structure may be more important than alternative prey for predation of C. pomonella.

No abstract

Vegetation texture (i.e., plant density, species diversity, and structural complexity) may influence the abundance of a herbivore by affecting its movement, altering the suitability of host plants, and by changing the herbivore's vulnerability to attack by natural enemies. We investigated these effects on the Mexican bean beetle (Epilachna varivestis) on snap bean (Phaseolus vulgaris) growing in high—and low—density monocultures and intercropped with short (0.5 m) or tall (1.2 m) corn plants (Zea mays). Beetle populations consistently reached higher densities on beans in monocultures than on beans intercropped with tall corn. Neither bean plant density nor plant species diversity alone significantly affected beetle abundance. Instead, the presence of tall corn plants contributed most to lower bean beetle density in the tall corn—bean intercrop, primarily by reducing adult colonization. However, the tall corn also increased the suitability of intercropped bean plants to the beetles because adults preferred to feed on and larvae developed faster on beans in the tall corn—bean habitat. Host plant effects were independent of the influence of microclimate and risk of predation. Abundance of natural enemies of the bean beetle was low and overall, predator density and bean beetle larval survival did not differ significantly among habitats. However, it is difficult to predict the effect or mortality factors, such as natural enemies, on bean beetle populations because larval mortality over time was affected by habitat type (i.e., there was a significant interaction between habitat type and time). Our results were partially consistent with the resource concentration hypothesis. Whereas Mexican bean beetles colonized bean monocultures more readily than they did tall corn—bean diculture, the presence of more suitable host plants in the diculture acted to increase, rather than decrease, bean beetle density in the tall corn—bean habitat. However, higher adult feeding preference and faster larval development on beans in the tall corn—bean habitat did not offset lower adult colonization in this habitat.