A review of the invasion history and global management strategies of Eucalyptus snout beetles in the Gonipterus scutellatus species complex by researchers in the Tree Protection Co-operative Programme (TPCP) appears in the Journal of Pest Science. In the article, the researchers warn against a simplistic blanket approach to the application of biological control programmes against the invasive pest species. The authors identify gaps in understanding the various factors that influence the success of a biological control agent outside of its native range.
Dr Michelle Schröder, a postdoctoral Fellow in the TPCP at the Forestry and Agricultural Biotechnology Institute, leads research on improving the biological control programme of the Eucalyptus snout beetle (Gonipterus scutellatus species complex).
The Eucalyptus snout beetle (ESB) is a major pest in Eucalyptus growing regions across five continents. The term ESB refers to three of the eight cryptic species present in the invaded range that belong to the G. scutellatus complex. Thought to originate from southeast Australia, the ESB was reported for the first time outside of its native range in New Zealand in 1890. Subsequently, it was detected in South Africa in 1916. The beetle was assigned five different names before DNA barcoding revealed that the species in SA was in fact Gonipterus sp. n. 2, an undescribed species in the species complex. The three invasive species, which include G. platensis and G. pulverulentus are present in African countries, South America, Europe, North America and Asia; and they continue to spread globally. As a result, there is “consequently a need to accurately identify known and new populations of these insects,” according to the authors.
Anaphes nitens, an egg parasitoid native to Australia, was introduced in South Africa as the biological control agent for Gonipterus sp. n. 2 in 1926. A. nitens larvae feed on the yolk of ESB eggs, killing them before they hatch. Following its introduction in SA, A. nitens has been introduced as a biological control agent where Gonipterus species have been reported in Eucalyptus plantations. The researchers acknowledge that the global release of A. nitens as a biological control agent for Gonipterus species “calls to question” its efficacy.
“Confusion regarding the taxonomy of ESB, and the recognition that most early reports referring to a single species actually represented numerous different taxa, has been one of the most important obstacles to research and management of these pests in Eucalyptus plantations,” wrote the authors.
DNA barcoding was instrumental in clarifying the confusion regarding the taxonomy of the ESB, with a study by Mapondera et al. 2012 examining the morphology of the male genitalia revealing “ten distinctly different species of which eight are part of the cryptic species complex.” Five of these species remain undescribed, including the species in South Africa.
The authors advocate more in-depth understanding of the interactions between the different Gonipterus species, their host plants, the biological control agent and factors such as climate as the bedrock for a more resilient biological control programme. They also identify gaps in the management of the species.
“Understanding how climate influences the distribution of different species and populations of these insects is therefore important in the development of successful biological control agents. This will be especially important in the case of the ESB, where differences between species have almost certainly been overlooked,” forecast the authors.
The authors identify host susceptibility as one of the factors where there is little understanding of its interaction with the different species in the cryptic complex, with Eucalyptus globulus as a “very susceptible host of all three [invasive] species”. Management practices in the future would also benefit from better understanding “possible mismatches between the herbivore host and the parasitoid,”
On the application of augmentative biological control, the authors express concern that “very little research has been published on the impact of augmentative releases of A. nitens on Gonipterus populations.” The researchers suggest that future work should therefore focus on “evaluating the impact of mass releases on ESB populations over time.”