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Responses of the seed-feeding beetle Acanthoscelides macrophthalmus and its recruited parasitoids to resource availability – Implications for the biological control of Leucaena leucocephala in South Africa

Sharratt, Morag, Olckers, Terence
Biological control 2019 v.135 pp. 102-109
Acanthoscelides, Chrysomelidae, Leucaena leucocephala, agroforestry, biological control, insects, larvae, parasitism, parasitoids, phenology, pods, pupae, seed damage, seeds, species recruitment, subtropics, Central America, South Africa
Introduced from Central America for agroforestry, Leucaena leucocephala (Lam.) De Wit (Fabaceae) has become invasive in several tropical and subtropical regions worldwide. The seed beetle Acanthoscelides macrophthalmus (Schaeffer) (Coleoptera: Chrysomelidae: Bruchinae) was released in South Africa in 1999 to curb the plant’s high seed production. Monthly sampling was conducted during 2011/2012 at four sites in KwaZulu-Natal Province to determine the reproductive phenology of L. leucocephala populations, the levels of seed damage inflicted by A. macrophthalmus, and the extent to which the beetle has recruited native parasitoids. There were typically two podding cycles annually, with variations in peak pod production between sites and periods of very low seed availability. The levels of pre-dispersal seed damage on canopy-held pods were erratic, fluctuating greatly between months and sites, and seldom exceeding 50%. Levels of post-dispersal damage on soil-surface seeds were substantially lower, seldom exceeding 5%. Parasitism of the larval/pupal stages of A. macrophthalmus also varied between months and sites, reaching a maximum of 45%. There was a moderate but significant negative correlation between percentage seed damage and seed availability, indicating higher damage during periods of seed scarcity. In contrast, there was no significant correlation between percentage parasitism and seed infestation by A. macrophthalmus. These trends suggest that neither A. macrophthalmus nor its recruited parasitoids are tracking resource availability, ensuring a negligible impact on seedling recruitment. Additional seed-reducing agents, particularly insects that exploit green pods, are required to regulate L. leucocephala populations in South Africa.