Distribution and seasonal abundance of the flowerbud weevil anthonomus santacruzi hustache (coleoptera: curculionidae) in KwaZulu-Natal and its impact on the invasive weed solanum mauritianum scopoli (solanaceae).

Abstract

Solanum mauritianum Scopoli (Solanaceae), native to South America, is an invasive weed of tropical, subtropical and warm temperate regions in many countries including South Africa. The seed-packed fruits are highly palatable to native birds which feed on them throughout the year, vastly aiding in the weed’s dispersal. Research into the biological control of the weed began in the 1980s after chemical and mechanical control efforts proved insufficient and resulted in the release of Gargaphia decoris Drake (Hemiptera: Tingidae), a leaf-sucking lace bug, in 1999. Anthonomus santacruzi Hustache (Coleoptera: Curculionidae), a flowerbud weevil, was later released in 2008 to reduce the excessive levels of fruiting by S. mauritianum populations. Although several thousand weevils have recently been released in KwaZulu-Natal province, where infestations of S. mauritianum are particularly severe, to date there has been no post-release evaluation to determine the extent of the weevil’s establishment, seasonal abundance and impact on the weed’s reproductive output. Twenty four sites with healthy populations of S. mauritianum were initially sampled in the KwaZulu-Natal midlands and coastal regions from February to October 2014 to determine the presence and abundance of A. santacruzi. Populations of A. santacruzi were recovered at 14 sites, mainly along the coast, with poor establishment recorded in the inland region. A preliminary assessment of the role of climate in the weevil’s establishment suggested that low temperatures may be a constraint. Six sites (three inland and three coastal) with established populations of A. santacruzi were subsequently chosen for monitoring across seasons from October 2014 to September 2015. Although seasonally variable, the numbers of flowers and flowerbuds of S. mauritianum were high at all sites throughout the monitoring period, indicating no distinct periods of food scarcity. However, the numbers of weevils were relatively low in comparison resulting in low levels of floral damage (up to 26%) and no apparent impact on fruiting. Although higher weevil numbers were recorded at the coastal sites, there was a consistent trend of weevil numbers peaking during the autumn months (April/May), at all six sites. Despite the low population densities of A. santacruzi, there were indications of density-dependent relationships between food availability and weevil numbers. At the study sites (i.e. where A. santacruzi had established), climatic factors (e.g. monthly temperature) had no significant effect on the abundance of the weevils. Ants were frequently associated with S. mauritianum inflorescences at the study sites and displayed a significant positive relationship with the numbers of mature fruits, presumably because of their high sugar content. However, there was no relationship between weevil abundance and the numbers of ants, suggesting that ants were not interfering with the weevil populations. A preliminary survey for parasitoids failed to provide any evidence that the weevil’s immature stages had recruited native parasitoids. Only seven years has elapsed since A. santacruzi was first released in KwaZulu-Natal. Although the weevil’s establishment and population proliferation has been confirmed at several sites, its impact on S. mauritianum populations is currently negligible. Should higher population densities of A. santacruzi be realized over the medium to longer term, its impact could become significant. Further monitoring of A. santacruzi populations should thus be conducted in KwaZulu-Natal, but also in other provinces, to determine their potential for the biocontrol of S. mauritianum. Keywords: Agent establishment, bugweed, flowerbud-feeding agents, resource availability, seasonal abundance, weed biological control.