Submitted to: African Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2002
Publication Date: 1/1/2003
Citation: Schachter-Broide, J., Gandolfo, D., Gurtler, R. 2003. Life history parameters of the biocontrol agent Gratiana spadicea (Chrysomelidae), reared on the natural host plant Solanum sisymbriifolium and the non-target crop Solanum melongena (Solanaceae). Laboratory Publication. African Entomology Vol. 11, No. 1, 2003.
Interpretive Summary: A leaf-feeding tortoise beetle native to South America was released in South Africa for the biological control of sticky nightshade, despite its ability to develop on cultivated eggplant during laboratory host-specificity tests. Natural and laboratory studies in Argentina were conducted at determining whether a host range extension of adult beetles could be induced and thus whether eggplant crops may be under future threat in South Africa. Critical life history parameters such as larval fitness and adult oviposition behaviour were compared with beetle populations both reared on eggplant and sticky nightshade. The results of these studies supports the contention that eggplant is a poor host for this beetle and thus highly unlikely to be utilized as and alternative host to sticky nightshade.
Technical Abstract: Gratiana spadicea (Klug), a leaf-feeding tortoise beetle native to South America, was released in South Africa for the biological control of Solanum sisymbriifolium Lamarck (wild tomato), despite its ability to develop on cultivated eggplant (Solanum melongena L.) during laboratory host-specificity tests. Our studies in Argentina, during which beetle colonies were reared and sustained on both plant species, were aimed at determining whether a host range extension could be induced and thus whether eggplant crops may be under future threat in South Africa. During no-choice tests conducted under natural climatic conditions, larvae reared on potted eggplants suffered significatly higher mortality, longer development times and lower pupal masses than those reared on potted wild tomato. During choice tests under laboratory conditions, adult beetles fed significantly more on wild tomato than on eggplant, irrespective of whether the larvae were reared on eggplant or wild tomato although the larger wild tomato-reared beetles consumed more than the smaller adults reared on eggplant. Under natural climatic conditions, females oviposited exclusively on wild tomato plants regardless of larval feeding history. During no-choice fecundity tests, females reared as larvae and adults on wild tomato had a significantly shorter preoviposition period and higher fecundity than females reared as larvae on wild tomato and as adults on eggplant, or as larvae and adults on eggplant. A significant percentage of females that spent part or all of their life cycle on eggplant laid only anomalous eggs (without a structure for leaf adhesion) or no eggs at all, whereas all females reared exclusively on wild tomato laid only normal eggs. The artificial rearing of G. spadicea on eggplant thus did not induce changes in adult feeding and oviposition preferences within the same generation, suggesting that eggplant is highly unlikely to serve as a viable alternative host for G. spadicea. Our study thus provides additional support for the decision taken to release G. spadicea in South Africa for the biological control of S. sisymbriifolium.