LAUSR

Abstract Since the mid-1980s, five species of seed-destroying Melanterius weevils have been used in South Africa as biological control agents against invasive Australian Acacia trees. Recorded levels of seed-destruction by the weevils are highly variable. To gain an understanding of these relationships, four years of observations on three chosen Melanterius/Acacia associations are reported that provide measures of: seed production; larval and pupal survival; adult weevil emergence-patterns and longevity; and levels of seed-destruction. Annual and geographical fluctuations in seed-set are exaggerated by the actions of other competing biological control agents which prevent seeding by destroying buds and flowers. Highly unreliable seed sources, coupled with low levels of larval and pupal survival, probably explain why the weevils are not more prolific. Most adult weevils emerge within six months of their prepupae settling in the soil, but a proportion of the population remains dormant and emerges as adults only in subsequent years. Our data indicates that some adults live for more than a year after emergence and therefore have access to seeds for two or more seasons. These characteristics enable the weevils to persist through years of low seed production and cause higher levels of seed damage in subsequent years than would otherwise be possible. Acacia-seed destruction by the weevils cannot cause measurable declines in the densities or distributions of mature populations of the target plants and thus the weevils could be assessed as unsuccessful biological control agents. However, Melanterius weevils are well adapted and efficient ‘back-up’ agents whose impacts accrue over time and reduce the reproductive fitness of the invasive Australian acacias.