The importance of phenology in studies of plant-herbivore-parasitoid interactions

Thesis title: The importance of phenology in studies of plant-herbivore-parasitoid interactions

Author: Minghui Fei

Abstract

As food resources of herbivorous insects, the quality and quantity of plants can directly affect the performance of herbivorous insects and indirectly affect the performance of natural enemies of the herbivorous insects. In nature, plant quality and quantity are dynamic and can change in individual plants over the course of a single growing season. Many multivoltine insects are known to attack short-lived annual plants that are present for only 2 or 3 months in the field. These short-lived plants may germinate and grow at different times and locations during the growing season. In this situation, each generation of insects is obligated to search for potentially new species of food plants across the growing season, which may differ in qualitative and quantitative traits. The aim of this thesis was to explore how seasonal phenology of potential food plants effects a multivoltine herbivore-parasitoid interaction. In particular, I examined potential qualitative and quantitative constraints imposed by the seasonal phenology of several food-plant species on the development and survival as well as on oviposition decisions of a gregarious specialist herbivorous insect and its natural enemy that both have multiple generations per year. As a model system, I used a multivoltine specialist herbivorous insect associated with different plant species, the large cabbage white butterfly, Pieris brassicae L., and its specialized multivoltine endoparasitoid, Cotesia glomerata L.. Pieris brassicae primary feed on plants in the large family Brassicaceae. I used three annual brassicaceous plants, Brassica rapa L., Sinapis arvensis L., and Brassica nigra L., which grow rapidly and exhibit differing phenologies, each growing within a short period of time and with little temporal overlap amongst them. These plants are known to serve as food plants for successive generations of P. brassicae and related species.

In bioassay experiments under controlled greenhouse and semi-field conditions, I found that P. brassicae and C. glomerata were marginally affected by seasonal-related and plant species-specific differences in food-plant quality. Pieris brassicae was also marginally affected by the ontogenetic variations in food-plant quality. In addition, food-plant shifts in different generations had small effects (both positive and negative depending on plant species) on the performance of P. brassica and C. glomerata. Survival and performance of P. brassicae was much more constrained by quantitative than qualitative aspects of the food plant. The survival and performance of C. glomerata was also affected by similar quantitative constraints as that of its host.

In behavioural experiments under controlled greenhouse and semi-field conditions, I found that female P. brassicae oviposition preference order for food plants declined with plant age of different plant species (S. arvensis and B. nigra). Female P. brassicae butterflies may ‘anticipate’ future quantity or quality potential of the food plants when choosing oviposition sites. Pre-adult experience had minor effects on P. brassicae butterfly oviposition preference and had no effect on C. glomerata landing preference. Pieris brassicae also did not exhibit consistent preference for any of the plant species, whereas C. glomerata had a clear preference on B. rapa. Further studies on trophic interactions need to incorporate more spatial and temporal realism, i.e. plant species shifts (temporally dynamic interactions) as well as to ‘track’ insect foraging behaviour in the field (spatially dynamic interactions). Thus far virtually nothing is known about these areas or as to the success of naïve insects in locating new patches of food plants or hosts in different habitats.