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
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.