Enhancement of the efficacy of the parasitoid, Microctonus hyperodae Loan (Hymenoptera: Braconidae) by provision of floral resources to improve biological control of its host, the Argentine stem weevil (Listronotus bonariensis) (Kuschel) (Coleoptera: Curculionidae)

Abstract

In this study, conservation biological control principles were applied to increase the efficacy of Microctonus hyperodae Loan (Hymenoptera: Braconidae), a parasitoid of the Argentine stem weevil (Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae)). The Argentine stem weevil is a serious pest in New Zealand pasture.

Seven flowering plant species were selected for laboratory experiments to assess the best plant species to increase the longevity of M hyperodae. Longevity of the parasitoid was significantly increased with buckwheat (Fagopyrum esculentum Moench) compared with six other flowering plants. Floral architecture measurements and nectar quality analysis were used to understand the floral selectivity and consequent 'preferences' of the parasitoids. A wider corolla aperture and a shorter corolla depth, as well as the sucrose/(glucose+fructose) ratio of the nectar were positively correlated with the longevity of M hyperodae. These results were compared with those for other parasitoids.

Lifetime carbohydrate reserves were analysed in continuously-fed and unfed (water control) M. hyperodae in the laboratory. Total sugar levels (as a measure of nutritional state) decreased continuously throughout unfed parasitoids' lifespans, whereas in continuously-fed individuals, levels increased initially and then decreased. Glucose dominated in the unfed parasitoids while glucose and fructose dominated in the fed individuals. The common insect haemolymph sugar, trehalose, was not detected in unfed M. hyperodae, but was detected in fed parasitoids. Parasitoids that fed from buckwheat in the laboratory showed higher sucrose levels immediately after feeding, but these declined to the levels observed in honey-fed individuals after one hour.

The fructose/total sugars (f/t) ratio was clearly different between the fed and unfed M. hyperodae. This ratio was a good indicator for distinguishing fed from unfed parasitoids and was therefore a useful tool for understanding the feeding history of M. hyperodae.

Buckwheat was deployed in the field to assess parasitoid abundance at different distances from the floral resources. The highest number of M. hyperodae occurred near buckwheat plots. The number rapidly declined with increasing distance and was at its lowest at 7 to 8m, followed by increasing numbers. Parasitoid numbers significantly increased near buckwheat plots compared with the middle and furthest distances.

Field-collected parasitoids were analysed to determine their nutritional state and feeding history. A high proportion (79%) of parasitoids collected near buckwheat plots had fed and, therefore, had an increased total sugar level and fit ratio. Only 20% collected from grasses on road-side verges apparently had access to sugar sources. The fit ratio in combination with total sugar levels was a useful tool to understand the nutritional state and feeding history of field-collected parasitoids.

The effect of buckwheat deployment in the field on parasitism rates of L. bonariensis was measured. Parasitism rates of the first summer generation of L. bonariensis near buckwheat plots were increased by 250% compared with the controls. Parasitism rates of the second summer generation of L. bonariensis did not differ between those near buckwheat and the controls.