Reproductive behaviour and fitness trade-offs in the aphid parasitoid Diaeretiella rapae (Hymenoptera: Aphidiidae) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph.D.) in Plant Science (Entomology) at Massey University, Palmerston North, New Zealand

Abstract

Parasitoids are fascinating insects that lay eggs in or on the body of their hosts where parasitic immatures grow and develop by exploiting the fixed resources available in a host. This study investigated host-parasitoid interaction between the cabbage aphid Brevicoryne brassicae and its parasitoid Diaeretiella rapae. The research explored the reproductive decisions made by D. rapae, and how these decisions affect its fitness and pest suppression ability. The haplodiploid nature of reproduction in D. rapae imposes strongly contrasting outcomes of mating and oviposition decisions that directly affect population sex ratio. This study found that parasitoid fitness is the integral outcome of lifetime mating and oviposition behaviours. Poor host-parasitoid synchronisation was found in an uncontrolled/open system in spring in New Zealand; a low female/male ratio and a significant number of erroneous male-male mating pairs were detected in this D. rapae population. Adult emergence occurred only during the light period, with males emerging before females (protandry). Light triggered both mating and oviposition in D. rapae. Female D. rapae preferred to mate before oviposition, which allowed them to produce female-biased offspring. Females were found to allocate more time for choosing their mates whereas males were more active during mating and selected their mates quickly. Females mated once (monandrous), while males mated multiple times (polygamous) and became sperm depleted after their third mating. The monandrous and polygamous nature of D. rapae changed the female-biased sex ratio to a highly male-biased operational sex ratio, resulting in mating interference. Several factors including age, body size, mating status and previous mating experience affected mate selection behaviour in males and females. Female D. rapae emerged with developed eggs and did not require additional food to mature their eggs (autogenous), however, it took about two days for all their eggs to mature (weakly synovigenic). The nutrients acquired during the larval stage (by feeding on host resources) and during adult stage (by feeding on 10% honey solution) both affected individual fitness. Parasitoids lived longer after feeding on honey solution and this effect was more pronounced in females than in males. Female D. rapae fed on honey also carried their eggs longer without resorbing them. Females preferred to oviposit in larger hosts than in smaller ones, despite stronger defensive behaviour of the larger hosts. Females also preferred the larger hosts for ovipositing fertilised eggs that resulted in larger female offspring; the females that emerged from larger hosts lived longer and produced more offspring than those emegered from smaller hosts. Female oviposited multiple eggs per host (superparasitism) after repeatedly attacking their hosts. This resulted in two to eight parasitoid larvae developing in a host, but only one adult emerged from each (solitary parasitoid). Female D. rapae produced more female offspring when hosts were limited, and the number of males only increased when host density was higher. Females oviposited more unfertilised eggs when competing with conspecifics, which allowed them to conserve their fertilised eggs for future oviposition. Thus, the study suggests that strong intrasexual competition and intersexual selection exist during mating and oviposition in D. rapae. This study provides comprehensive information on interactions between cabbage aphid and D. rapae which can be used to develop effective biological control programmes for cabbage aphid and other aphid species using D. rapae or other parasitoids. Release of honeyfed, mated and 1-day old females in early morning and on sunny days would be most effective and result in quicker suppression of aphid populations. Raising females in low competition situations with large size hosts (5-7 day old) could help in producing efficient and female-biased broods in insectaries.