Parasitism, population densities and short-term impact of Anthonomus santacruzi on the reproductive output of Solanum mauritianum in KwaZulu-Natal Province, South Africa

Abstract

ABSTRACT Anthonomus santacruzi Hustache (Coleoptera: Curculionidae) was released in South Africa in 2008 to curb the spread of the invasive alien weed Solanum mauritianum Scopoli (Solanaceae) and is widely established in warmer regions. In KwaZulu-Natal Province (KZN), where S. mauritianum populations are particularly abundant, weevil populations display seasonal peaks in autumn, thrive at coastal locations, but barely persist at colder inland locations. Besides substantial winter declines and poor compatibility with colder areas, native parasitoids may be constraining weevil populations. In this study, A. santacruzi populations were surveyed during late summer at six coastal and three inland sites in KZN to determine the levels of parasitism. Also, weevil population densities were compared with those recorded during post-release surveys conducted since 2015. Parasitoid recruitment was negligible, with <1% of weevil-infested buds parasitised. Significant differences in A. santacruzi numbers between 2016, 2018 and 2020 across the same nine sites were due to lower numbers in 2020, as a result of earlier sampling (i.e. two months before the autumn peak) than in preceding years. Four sites that were sampled during late summer in 2015 displayed significant increases in weevil abundance, but significant decreases in floral and fruit production, over the six years preceding 2020. The significant negative correlation between weevil densities and fruiting, across all sites surveyed between 2015 and 2020, suggests some impact, albeit moderate, on the weed’s reproductive output. Climatic limitations rather than recruited parasitoids are currently limiting the distribution and densities of A. santacruzi in KZN.