Toxicity and repellency of Tephrosia Candida to larval and adult Diaprepes root weevil (Coleoptera: Curculionidae)

Authors: Lapointe, Stephen; McKenzie, Cindy; Hunter, Wayne

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2003
Publication Date: 6/1/2003
Citation: Lapointe, S.L., McKenzie, C.L., Hunter, W.B. 2003. Toxicity and repellency of Tephrosia candida to larval and adult Diaprepes root weevil (Coleoptera: Curculionidae). Journal of Economic Entomology. 96(3): 811-816.

Interpretive Summary: The tropical legume Tephrosia candida may be an ideal cover crop for association with citrus. We have shown that the leaves of this plant repel feeding by the Diaprepes root weevil (DRW) and that the roots are toxic to larval DRW. Our studies showed that while adults prefer not to feed on the leaves, larvae placed in pots or on diet containing roots of T. candida feed normally but are intoxicated resulting in death or greatly reduced weight gain. We will now identify the active constituents of the leaves and roots and assess their possible use of botanical insecticides or repellents.

Technical Abstract: Leaves of the tropical legume Tephrosia candida DC deterred feeding by adults of the Diaprepes root weevil, Diaprepes abbreviatus (L.) compared with leaves of Citrus macrophylla Wester, a common citrus rootstock, or T. vogelii Hook. f. When larvae were placed in pots containing plants of the three species for 28 d in a greenhouse, larval survival and weight gain were significantly reduced in pots containing plants of T. candida compared with larvae in pots with C. macrophylla or T. vogelii. Diet incorporation of lyophilized roots of T. candida into an artificial diet increasingly inhibited the growth of larvae and increased larval mortality with increased concentration of roots, while roots from C. macrophylla or T. vogelii had no effect compared to the diet-only control. Roots of T. candida, but not of T. vogelii, contain at least one constituent that acts as an antifeedant towards adult D. abbreviatus and as a toxicant towards larvae. No antifeedant effect of roots of T. candida towards larvae was observed in no-choice pot tests or in a diet incorporation bioassay. In pots, larval feeding damage to roots of T. candida was evident. In the diet incorporation assay, 97% of larvae survived 29 days on a diet of cellulose powder (a nutritionally inert filler) despite losing weight. We conclude that decreased survival and weight gain of larvae fed fresh or lyophilized roots of T. candida were the result of ingestion of a toxicant and not deterrence from feeding.