Conservation of insect natural enemies in heterogeneous vegetable landscapes
The position taken is that populations of natural enemies occupying both soil and foliar habitats are regulated by lower trophic levels. A series of hypotheses were tested on the influence of lower trophic levels on population persistence of natural enemies with divergent life history traits; the entomopathogenic nematode, Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae) and the larval parasitoid, Diadegma insulare Cresson (Hymenoptera: Ichneumonidae). Initial studies on the entomopathogenic nematode, were aimed at understanding their ecology in vegetable landscapes and thereafter investigations were conducted to determine the influences of insect host availability and plant diversity on population densities and the mechanisms underlying their effects. Investigations for D. insulare, focused on its responses to densities of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae) and flowering coriander. Several strains of entomopathogenic nematodes that were able to infect and reproduce in common vegetable insect pests were recovered along grassy banks adjacent to cultivated areas of a vegetable production system. Soil moisture was the principal factor associated with the presence of these endemic nematode populations. Persistence of H. bacteriophora populations differed in plots with different insect host availability, but persistence differed among strains and was related, in part, to the abilities of the nematodes to survive under test site conditions. For those strains with high survival rates, populations increased with increasing host availability, whereas no such effects were observed for populations with low survival rates. Higher nematode population densities were observed in plots with more diverse plants and insect herbivores. A correlation was found between nematode population density and chrysomelid beetle abundance, suggesting that persistence was at least partially as a result of nematodes recycling within these insects. D. insulare aggregated twice as much in areas with high compared with low populations of diamondback moth. Parasitism was greater when flowering coriander was present, but this effect was only observed at low population densities.In summary, bottom-up effects of plant and herbivore communities on natural enemies above and below-ground were similar. Population persistence for the two natural enemies was higher with increasing insect hosts and plant diversity; but the effects occurred at different spatial and temporal scales.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:conservation biological control heterogeneous vegetable landscapes entomopathogenic nematodes diadegma insulare hymenoptera ichneumonidae
Date of Publication:01/01/2004