Factors influencing abundance and predation of two coccinellid species in an agricultural system: The influence of vegetational diversity and species interactions.

摘要

This thesis reports on studies of how environmental factors and species interactions may influence behavior of two insect predators, Coleomegilla maculata De Geer and Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae). The first species is native to North America, and is known to prey on some insect pests including European corn borer, Ostrinia nubilalis (Walker) (Lepidoptera: Pyralidae). The second species has been introduced from Asia and has recently become abundant throughout North America. Chapter 2 reports on studies on how adding a covercrop to corn a monoculture may influence densities of coccinellids in cornfields and predation on egg masses of European corn borer. Although we found a slight increase in numbers of C. maculata in fields with covercrops compared to corn monocultures during one field season, this did not affect predation on European corn borer egg masses. Predation was studied by observing feeding marks on egg masses added to the experimental plots, and by analyzing the gut contents of coccinellids beetles using a PCR-based method with prey-specific primers to detect prey DNA in predator guts. In the laboratory we were able to predict time since feeding using this method (Chapter 3). We amplified prey DNA from some field-collected individuals and concluded that egg mass encounter rate per predator appeared to be low (Chapter 4).; In Chapters 5 and 6 of the thesis studies are described where interactions between C. maculata and H. axyridis . Indirect interactions are mediated by a parasitoid, Dinocampus coccinellae (Schrank) (Hymenoptera: Braconidae). The parasitoid is more successful in attacking C. maculata than H. axyridis, but we found no evidence that this difference in parasitism pressure leads to a decline in C. maculata (Chapter 5). Direct interactions may consist of intraguild predation or of competition for resources. From a cage study described in Chapter 6 it appears that C. maculata larvae may have lower food requirements than H. axyridis larvae, and that in our experiment the first species appeared to avoid interactions with the latter species. Low levels of cannibalism were detected, but we did not find an effect of H. axyridis on the survival of C. maculata.