Diachasmimorpha kraussii (Hymenoptera: Braconidae: Opiinae) is a koinobiont larval parasitoid of dacine fruit flies of the genus Bactrocera (Diptera: Tephritidae) in its native range (Australia, Papua New Guinea, Solomon Islands). The wasp is a potentially important control agent for pest fruit flies, having been considered for both classical and inundative biological control releases. I investigated the host searching, selection and utilisation mechanisms of the wasp against native host flies within its native range (Australia). Such studies are rare in opiine research where the majority of studies, because of the applied nature of the research, have been carried out using host flies and environments which are novel to the wasps. Diachasmimorpha kraussii oviposited equally into maggots of four fruit fly species, all of which coexist with the wasp in its native range (Australia), when tested in a choice trial using a uniform artificial diet media. While eggs laid into Bactrocera tryoni and B. jarvisi developed successfully through to adult wasps, eggs laid into B. cucumis and B. cacuminata were encapsulated. These results suggest that direct larval cues are not an important element in host selection by D. kraussii. Further exploring how D. kraussii locates suitable host larvae, I investigated the role of plant cues in host searching and selection. This was examined in a laboratory choice trial using uninfested fruit or fruit infested with either B. tryoni or B. jarvisi maggots. The results showed a consistent preference ranking among infested fruits by the wasp, with guava and peach most preferred, but with no response to uninfested fruits. Thus, it appears the wasp uses chemical cues emitted in response to fruit fly larval infestation for host location, but does not use cues from uninfested fruits. To further tease apart the role of (i) suitable and non-suitable maggots, (ii) infested and uninfested fruits of different plant species, and (iii) adult flies, in wasp host location and selection, I carried out a series of behavioural tests where I manipulated these attributes in a field cage. These trials confirmed that D. kraussii did not respond to cues in uninfested fruits, that there were consistent preferences by the wasps for different maggot infested fruits, that fruit preference did not vary depending on whether the maggots were physiologically suitable or not suitable for wasp offspring development, and finally, that adult flies appear to play a secondary role as indicators of larval infestation. To investigate wasp behaviour in an unrestrained environment, I concurrently observed diurnal foraging behaviours of both the wasp and one of its host fly in a small nectarine orchard. Wasp behaviour, both spatially and temporally, was not correlated with adult fruit fly behaviour or abundance. This study reinforced the point that infested fruit seems to be the primary cue used by foraging wasps. Wasp and fly feeding and mating was not observed in the orchard, implying these activities are occurring elsewhere. It is highly unlikely that these behaviours were happening within the orchard during the night as both insects are diurnal. As the final component of investigating host location, I carried out a habitat preference study for the wasp at the landscape scale. Using infested sentinel fruits, I tested the parasitism rate of B. tryoni in eucalyptus sclerophyll forest, rainforest and suburbia in South East Queensland. Although, rainforest is the likely endemic habitat of both B. tryoni and D. kraussii, B. tryoni abundance is significantly greater in suburban environments followed by eucalyptus sclerophyll forest. Parasitism rate was found to be higher in suburbia than in the eucalyptus sclerophyll forest, while no parasitism was recorded in the rainforest. This result suggests that wasps orient within the landscape towards areas of high host density and are not restricted by habitat types. Results from the different experiments suggest that host searching, selection and utilisation behaviour of D. kraussii are strongly influenced by cues associated with fruit fly larval feeding. Cues from uninfested fruits, the host larvae themselves, and the adult host flies play minimal roles. The discussion focuses on the fit of D. kraussii to Vinson’s classical parasitoid host location model and the implications of results for biological control, including recommendations for host and plant preference screening protocols and release regimes.

This thesis investigated the autecology of the dacine species, Bactrocera cacuminata (Hering) (Diptera: Tephritidae: Dacinae). I specifically focused on the adult phase of the life cycle and resources believed to be significant to this life stage. The prevailing paradigm in dacine ecology predicts that the larval host plant serves as the centre of dacine activity, a state mediated by mutualistic associations with fruit fly-type bacteria. Contrary to predictions, an explicit test of this hypothesis found that the host plant of B. cacuminata, Solanum mauritianum Scopoli, acted almost exclusively as a site for oviposition and larval development. Other key adult behaviours, most notably feeding and mating, were rare at the host plant. Even in disturbed habitats, the paucity of key adult behaviours such as mating was striking. Adult flies of this species were therefore hypothesized to be utilizing other components of their habitat, i.e. resources vital to their life history requirements. Some of the resources that B. cacuminata are known to respond to include sugar, protein, methyl eugenol and the host plant. The latter three resources are believed to be critical in the reproductive success of dacine flies in general. I assessed the physiological status of flies arriving at these resources to determine if flies of different status foraged for resources differently. In dacines, the internal reproductive structures of the male and female flies have been used as predictors of physiological status. I quantified expansion of the male ejaculatory apodeme in B. cacuminata with age of fly and found that there is a threshold apodeme size that is strongly correlated Abstract with sexual maturity. Maturity of female flies could be accurately predicted by ovarian development. Using these methods to assess the physiological and nutritional status of flies arriving at resources (larval host plant, protein and methyl eugenol) in the field, I discovered that only sexually mature and mated females were responding to the host plant, while the males at the host plant were sexually immature. This confirmed the hypothesis that the host plant primarily served as an oviposition site. Additionally, this study revealed that sexually mature males with high nutritional reserves were most commonly collected at methyl eugenol (a plant-derived chemical that elicits a strong response in males of many dacine species) at dusk, the time of peak sexual activity in this species. This indicated that methyl eugenol was perhaps a significant resource in the context of the reproductive behaviour of this species. Methyl eugenol (ME) is one of group of phenyl propanoids to which males of certain species of Dacinae respond. The current hypothesis of the role of these phenyl propanoids is that they function as pheromone precursor chemicals. Response to these chemicals is hypothesized to be a trait under sexual selection. In Bactrocera dorsalis (Hendel), this effect is so strong that a single feeding on ME results in a strong mating advantage up to a month after males feed on the chemical. Bactrocera cacuminata fed on multiple occasions on ME in a laboratory bioassay. After a single 24-hour exposure to ME, investigations of mating competitiveness did not reveal any obvious advantage for ME-fed males over unfed males. However, ME-fed males did enjoy a higher mating success 16 and 32 days after exposure to the chemical, suggesting that some physiological benefits unrelated to the pheromone synthesis was driving this delayed advantage. Investigation of the physiological consequences of feeding on ME revealed no enhancement of nutritional or energetic reserves, suggesting that the delayed mating advantage observed was more likely a chance event. An alternate hypothesis about the proximate function of ME, proposed by Robert Metcalf, is that it serves as a mate rendezvous site. As mating behaviour was notably absent at the host plant, I tested Metcalf’s hypothesis. A field-cage experiment, spatially separating adult resources (host plant, methyl eugenol, sugar and protein) clearly demonstrated that methyl eugenol was functioning as a mate rendezvous stimulus for B. cacuminata. This is the first direct support for Metcalf’s hypothesis. A synthesis of the literature revealed that significantly greater ecological and evolutionary information was required to understand the basis of dacine response to phenyl propanoids. Different dacine species may be utilizing these chemicals differently, even if their evolutionary origin may have been as a plant based kairomone. My studies show that generalizations on the ecology and behaviour of Dacinae, often extrapolated from research on a few pest species, do not hold up in the case of B. cacuminata. This suggests that a more autecological, species-specific approach is required in dacine research, before any predictive generalizations can be made.