Factors influencing the efficacy of biological control of Gonipterus sp. n. 2 by Anaphes nitens

Abstract

The Eucalyptus snout beetle, Gonipterus sp. n. 2 (Coleoptera: Curculionidae), native to Australia, is a major pest in South African Eucalyptus plantations. In 1926 the egg parasitoid Anaphes nitens Girault (Hymenoptera: Mymaridae) was imported from Australia into South Africa, and after its release it became an example of the success of classical biological control. However, recent outbreaks of the beetle have raised questions about the efficacy of biocontrol by this natural enemy. In this thesis the prevailing hypotheses in the literature for the failure of biocontrol of Gonipterus sp. n. 2 by A. nitens were summarized in a review. Elevation, climate, host plant susceptibility, genetic variation, and population dynamics were found to be involved in biocontrol success. The effects of elevation and climate were further studied in a data analysis with field data collected in KwaZulu-Natal between 2010 and 2015. Severity of beetle attacks was found to increase with elevation as expected from previous studies, but unexpectedly this was not accompanied by a decrease in parasitism. Few clear relationships with weather variables were found, the most direct relationship being the negative relationship of parasitism with potential evapotranspiration (PET). To test whether this biocontrol system could be enhanced by addition of intraspecific variation to the parasitoid population, a new A. nitens population was imported from Queensland, Australia, from an area climatically better matched to the current Eucalyptus growing areas in South Africa. Reproductive compatibility of the newly imported Australian and existing South African A. nitens populations was ensured by performing controlled crosses. Subsequently a population genetics study was performed using 15 newly developed microsatellite markers with populations from the native range in Australia and introduced populations from South Africa, Portugal, Brazil, and West Australia. Overall, genetic diversity was high, and population structure was present. The introduced populations formed separate genetic clusters, while the native Australian population was composed of a mix of these clusters. Interestingly, the A. nitens population from the Western Cape province in South Africa formed a separate genetic cluster from the other South African populations and grouped more closely with the sampled native Australian population. Next, a field release was performed in two Eucalyptus plantations in KwaZulu-Natal. The microsatellite marker set was further used to determine establishment of the newly released A. nitens population by comparing pre- and post-release samples collected at the release sites. Signs of establishment were found in the form of unique Australian alleles being present in two post-release samples, and by post-release individuals being assigned to the Australian population by a DAPC clustering method. Ultimately more studies such as whole genome sequencing are needed to unambiguously establish whether those individuals were true descendants of the newly released A. nitens population. In conclusion, this study provides an overview of the factors involved in biocontrol of Gonipterus sp. n. 2 by A. nitens and gives new insights into how they might be responsible for failing biocontrol. Furthermore, a first attempt was made to use population genetic techniques to enhance biocontrol of an important Eucalyptus pest.