ABSTRACT
Insect pests, particularly specialists, are dependent on a host’s unique chemical and physical characteristics for location and stimulation of egglaying. In the Brassicaceae, volatile glucosinolate fi ssion products often serve as distant attractants, whereas glucosinolates on the leaf surface provide cues for oviposition. Glucobrassicin (3-indolylmethyl glucosinolate), the main glucosinolate in cultivated B. oleracea, and to a lesser extent sinigrin stimulated oviposition by P. rapae (Renwick et al. 1992) and P. brassicae (van Loon et al. 1992), though their fi ssion products had no effect (Trayner and Truscott 1991). As noted above, P. rapae possess a gut epithiospecifi er protein that redirects glucosinolate fi ssion toward the formation of nitriles. In the presence of epithiospecifi er protein, glucobrassicin fi ssion yields indole3-acetonitrile, which is a strong oviposition deterrent. This response may prevent females from ovipositing on plants that are already infested (De Vos et al. 2008). The ability to discriminate glucosinolate profi les is precise, for example, P. napi subspecies napi responded to aromatic and aliphatic glucosinolates, whereas P. napi subspecies oleracea responded mainly to long chain aliphatic forms (Du et al. 1995). The chemoreceptive cells responding to glucosinolates reside within the tarsal sensilla. Interestingly, these same cells are highly responsive to cardenolides which are strong oviposition deterrents (Du et al. 1995; Städler et al. 1995). Oviposition by P. xylostella, which feeds on a wide range of crucifers, is stimulated by all classes of glucosinolates (Reed et al. 1989) as well as isothiocyanate fi ssion products containing sulfur in the side chain (Renwick et al. 2006). Leaf wax components, such as n-alkanes, synergize the effects of glucosinolates (Spencer 1996), while as yet unidentified non-polar compounds are active alone (Hughes et al. 1997); however, intact wax bloom suppresses oviposition (Uematsu and Sakanoshita 1989; Justus et al. 2000), presumably by limiting perception of stimulatory compounds. Other physical features, such as trichomes, also negatively impact oviposition by P. xylostella (Adamson et al. 2008).
