Sweetpotato Volatile Chemistry in Relation to Sweetpotato Weevil (Cylas formicarius) Behavior

The sweetpotato weevil (SPW) (Cylas formicarius elegantulus (Summers) (Coleoptera: Curculionidae)) is the single most devastating pest of the sweetpotato (Ipomoea batatas (L.) Lam. ) worldwide. Attempts to develop host-plant resistance have been only moderately successful due in part to deficiencies in parent and progeny selection methods. Host- plant phytochemicals play critical roles in insect behavior, modulating a cross-section of key behavioral decisions. Thus, identification of the phytochemicals the female weevil uses in decision making could greatly facilitate development of host- plant resistance. The volatile chemistry of the sweetpotato was studied in relation to the host-finding behavior of the female weevil. Critical biologically active volatiles were determined via isolation (Tenax trapping), fractionation (gas chromatography-thermal conductivity detector), identification (gas chromatography and gas chromatography-mass spectroscopy), and bioassay (olfactometry). Differences in volatile chemistry among sweetpotato clones that may relate to differences in resistance or susceptibility to the female SPW were assessed. Volatile extracts from storage roots (site of oviposition) and aerial plant parts were attractive to female SPW, the former being substantially greater. In total, 33 compounds were identified from storage roots and aerial plant parts, including 23 terpenes. Three oxygenated monoterpenes (nerol, Z-citral, and methyl geranate), found in storage roots but not aerial plant parts, were identified as attractants. The sesquiterpene volatile fraction was repellent to female SPW with α-gurjunene, α-humulene, and ylangene active in the concentration range emanating from storage roots. The aerial plant parts emanated a higher composite concentration of sesquiterpenes than storage roots. Differences in the relative attraction among four sweetpotato cultivars to female SPW was inversely correlated with the composite concentration of headspace sesquiter- penes. Selection of clones with decreased volatile attractants and/or increased deterrents using an analytical means of quantification may significantly facilitate developing resistance to the SPW. The sweetpotato weevil (SPW) (Cylas formicarius elegantulus) is the single most devastating insect pest of the sweetpotato (Ipomoea batatas) worldwide, causing damage both in the field and during storage (Sutherland, 1986). The oligophagous SPW consumes most parts of the sweetpotato, while eggs are laid mainly in the basal stem and storage roots (Cockerham et al., 1954). Even very low pre- or postharvest infestations reduce quality and marketable yield (Proshold, 1983). In addition, sev- eral extremely bitter tasting and toxic furanoterpenoids are syn- thesized by the storage root in response to feeding by weevil larvae which render the commodity unfit for consumption (Uritani et al., 1975). Initial work on volatile phytochemicals emanating from the sweetpotato stemmed from the finding that the SPW could readily orient in the dark to sweetpotato plant parts placed in their vicinity (Nottingham et al., 1989a; Starr et al., 1991). The females are attracted by the volatiles emanating from the leaves and storage roots. In contrast, males are attracted by leaf volatiles but little attracted by those emanating from the storage roots (Nottingham et al., 1989a). This suggested that there are qualitative and/or quantitative differences in the volatile compounds between aerial plant parts and the storage roots. In addition to within plant differences, the degree of attraction is known to vary among cultivars (Nottingham et al., 1989a) and species of Ipomoea L. (Starr et al., 1991). Existing management strategies for the SPW do not provide adequate control (Talekar, 1994) and attempts to develop host- plant resistance have not been successful. To further our under- standing of the phytochemicals the female SPW uses in decision making, we studied the volatile phytochemistry of the sweetpo- tato, the role these compounds play in the behavior of the female SPW, and whether differences in these compounds among clones might account in part for differences in resistance/susceptibility to the SPW.