Interactions of biopesticides with synthetic pesticides and their implications for management of the Macadamia seed weevil, Kuschelorhynchus macadamiae (Coleoptera: Curculionidae)

Interactions of biopesticides with synthetic pesticides and their implications for management of the Macadamia seed weevil, Kuschelorhynchus macadamiae (Coleoptera: Curculionidae)

In Australia, macadamias (Macadamia integrifolia and M. tetraphylla) are the only native nut trees to be commercially grown and generate annual revenue of more
than AU$ 850 million. However, production has faced many challenges, including those caused by a number of insect pests. Macadamia seed weevil (MSW), Kuschelorhynchus macadamiae, is one of the key pests causing serious issues for growers. To control MSW the industry has been using broad-spectrum insecticides such as acephate and indoxacarb. Although chemical control is the current recommendation, the industry has a vision to produce macadamia nuts in a 'clean and green' environment. To achieve this goal, the industry has committed to the development of an integrated pest and disease management (IPDM) program by reducing the use of broad-spectrum insecticides and integrating biological and cultural controls into the IPDM program. Entomopathogenic fungi (EPF) are recognised as among the important biological control agents for controlling many insect pests. Despite this, there is neither peer-reviewed information available on the use of EPF for controlling MSW nor on the integration of EPF with the IPDM program on macadamias. In this thesis, both the potential of EPF for controlling MSW and the possibilities to integrate EPF with the current IPDM program on macadamias are examined for the first time.

In the first study, six strains of Beauveria spp. and six strains of Metarhizium spp. were identified using molecular techniques. The DNA sequences of the 5' region of elongation factor-1 alpha (EFT1) and the B locus nuclear intergenic region (Bloc) of all strains confirmed that they belonged to the fungal species Beauveria bassiana and Metarhizium anisopliae. All twelve strains of the EPF and a commercial biopesticide (Velifer®) were used in laboratory assays on MSW and the results showed that B. bassiana strain B27 and M. anisopliae strain ECS1 were the best strains in bioassays, as they induced the highest mortality to MSW and had the lowest median lethal time (LT50) compared to other strains of their respective species. In the second study, these two strains were used to study horizontal transmission from fungusinfected adults and conidiated cadavers to healthy adults. The results showed that the mortality of healthy adults varied from < 50% to 100% depending on the ratios of fungus-infected adults or conidiated cadavers with the healthy adults and the experimental conditions, i.e. a confined environment or the larger insect cages.

Insecticides (acephate, indoxacarb, trichlorfon, sulfoxaflor, spinetoram, betacyfluthrin, methidathion, diazinon) and fungicides (pyraclostrobin, carbendazim) are
commonly used for controlling insect pests and plant diseases during the period when MSW is active. The in vitro study showed that acephate, indoxacarb and trichlorfon at their full field concentrations (FFCs) were compatible with both fungal species whereas sulfoxaflor and spinetoram at their FFCs were compatible to only B. bassiana. Beta-cyfluthrin, methidathion and diazinon at their FFCs were moderate to highly toxic to both fungal species whereas both fungicides were very toxic to the EPF even at 6.25% of their FFCs. The interactions of acephate and indoxacarb with EPF for controlling MSW were also investigated under laboratory and glasshouse conditions. Their synergistic and additive interactions were measured and they provided better control of MSW under both sets of experimental conditions than either insecticides or EPF alone.

In conclusion, this study demonstrates that EPF are potential biological control agents for managing MSW either directly or indirectly (via transmission). In addition,
they are able to integrate with some insecticides, whereas fungicides cannot be integrated with EPF. Future studies, such as on attractants for MSW, the potential of
applying compost/mulch with EPF to cover weevil infested nuts, and understanding the movement of MSW after emerging from the infested nuts may allow EPF to be used in more innovative ways.