Interpreting Ambiguous Results of Host-Specificity Tests in Biological Control of Weeds: Assessment of Two Leptinotarsa Species (Chrysomelidae) for the Control of Solanum elaeagnifolium (Solanaceae) in South Africa

Solanum elaeagnifolium Cavanilles (Solanaceae), an exotic weed of arable and pastoral lands in South Africa, has been difficult to control by mechanical and herbicidal methods. Biological control, within an integrated management framework, may provide a solution. Specificity tests on two defoliating beetles, Leptinotarsa texana (Schaeffer) and Leptinotarsa defecta (Stål) (Coleoptera: Chrysomelidae), showed that both species have narrow host ranges on Solanum species and are unable to survive on plants outside that genus. In cages, most solanaceous crops, including potato, tomato, green pepper, tobacco, and cape gooseberry (groundcherry), were not attacked. However, both beetle species developed on cultivated eggplant and on five native South African Solanum species. Starvation tests indicated that there was no consistent loss of fitness on these plants relative to S. elaeagnifolium. Although choice tests in small cages were inconclusive, tests in walk-in cages revealed that the beetles have strong oviposition preferences for S. elaeagnifolium. In spite of these ambiguous results, further investigation suggested that the results of the cage tests had been overemphasized. Both Leptinotarsa species were eventually cleared for release in South Africa in 1992, some 7 years after screening tests began.