The potential of Metarhizium anisopliae as a biological control for Sweet potato weevil in Papua New Guinea

Cylas formicarius is a serious insect pest in any region where sweet potato is grown. Larval and adult feeding renders tubers inedible as a result of the production of furanoterpenoids, as well as feeding injuries on tubers that facilitate the entry of other soil-borne pathogens. In Papua New Guinea, small-holder farmers relied solely on cultural control practices with little success. This had led to the search for other nonchemicalalternatives of control. In this thesis, the potential of using theentomopathogenic fungus, Metarhizium anisopliae, as a biological control agent against C. formicarius is discussed. The aims of this research were to (1) identify and screen the isolates of Metarhizium for their virulence on adult C. formicarius, (2) determine the prelethal effects of M. anisopliae on adult C. formicarius and possible conidia transmission, (3) investigate the behavioural modification of C. formicarius and the efficacy of differentconidial formulations on the control of SPW. Soil samples were obtained from abandoned properties or fields with a history of sweetpotato cultivation in Queensland, Australia. Using an insect baiting technique, fungi were isolated from the samples. All ten fungal isolates isolated from tropical soils were identified morphologically as Metarhizium sp. The use of molecular methods using polymerase chain reaction (PCR) to amplify r-DNA ITS gene regions and sequence analysis identified these isolates as M. anisopliae. Laboratory screening bioassaysresulted in the selection of three of most virulent isolates (viz. DAR 82478, DAR 82479 and DAR 82480) causing high mortality on adult C. formicarius; however, these isolates also showed variation in certain growth characteristics such as germination and mycelial growth.Based on its virulence on C. formicarius and desired growth characteristics, DAR 82480 was used to further study the pre-lethal effects on the adult insects in dose-mortality experiments. At a low LC50, M. anisopliae negatively affected female fecundity. Infected females oviposited fewer eggs, with a higher proportion of these eggs not buried but exposed to predation and desiccation. Furthermore, it was evident that infective conidiawere horizontally transmitted between sexes, possibly through mating and gregarious feeding. It is likely that the low dose of M. anisopliae will not cause direct mortality of C. formicarius but will have indirect effects in regulating the weevil population. The results of horizontal transmission is the first to be noted in C. formicarius and this has important implications for managing C. formicarius, in that the infected insects can be ansecondary inoculum source. While behavioural modification has never been reported in C. formicarius, 'clean' adults avoided arenas with M. anisopliae-inoculated sweet potato. In contrast to this, the response of adult weevils differed when weevils were exposed to isolates of M. anisopliaewith contrasting virulence. Weevils showed repellent behaviour to the virulent isolate, migrating to the sweet potato inoculated with less virulent isolate. Although the variation in behaviour towards these isolates has not been investigated further, the emission of volatile compounds may play an important role. Further tests on the effects of gender did not alter the response of weevils choosing uninoculated sweet potato or sweet potatoinoculated with the less virulent isolate. With the availability of a C. formicarius-based sex pheromone, high dose of this synthetic pheromone could be employed as 'lure-andinfect' technique to efficiently control this weevil. While microbial agents such as M. anisopliae can be formulated to achieve effective and efficient pest control, the accessibility and availability of these materials for formulation xv is another issue. Cheap formulations of isolate DAR 82480 for the control of C. formicarius were compared under glasshouse conditions.