Direct and indirect effects of host plant selection on larval performance in the cabbage looper, Trichoplusia ni

Generalist insects should possess the ability to rank potential host plants for the suitability of their offspring. The decision to oviposit on a host plant that is inferior for larval development could have significant consequences for their offspring. Host plant quality can affect larval development and survival directly via nutrients and/or defensive chemicals, as well as indirectly by influencing larval condition and consequently their susceptibility to pathogens. In this thesis I examine the relationship between host preference and plant quality for larval performance in the cabbage looper, Trichoplusia ni (Hubner). I also examine the relationship between host plant quality, insect condition, immune responsiveness and resistance to pathogens. No-choice, two-choice, and multiple-choice experiments were performed to measure adult oviposition preference and neonate larval preference for six plant species. Two baseline and induced immune parameters, haemocyte numbers and haemolymph phenoloxidase (PO) activity, were estimated for larvae on two host plants, broccoli and cucumber. Haemolymph protein concentration was assessed as an indication of insect condition, and the susceptibility of larvae to T. ni single nucleopolyhedrovirus (SNPV) was used as a measure of disease resistance. Trichoplusia ni adults and larvae both preferred the same two plant species out of six that maximised larval performance. Larvae however, also correctly identified anise hyssop as a suitable host, whereas adults did not, indicating that larval diet breadth can be wider than adult host range. Larval development, survival and condition were much higher when larvae were reared on broccoli than on cucumber. Haemocyte numbers were significantly higher in broccoli-reared larvae, whereas PO activity was not. An immune challenge induced significantly elevated numbers of haemocytes for larvae reared on both hosts, but did not affect PO activity or protein concentrations. Susceptibility to T. ni SNPV was significantly greater in larvae reared on cucumber than on broccoli. These results clearly indicate that T. ni are capable of ranking the suitability of plants for larval performance. Larval development on an inferior host plant can affect both immune response and disease resistance indicating that bottom-up effects could be important in interactions between insects and entomopathogens.Land and Food Systems, Faculty ofGraduat