Contrasting Responses to Ethenylbenzene (Styrene) and 2-Ethyl-1-hexanol Suggest their Role as Chemical Cues in Host Selection by the Seed Beetle Zabrotes subfasciatus (Coleoptera: Chrysomelidae: Bruchinae).

Abstract

Seed beetles spend most of their lives within the seeds of host plants belonging to the Fabaceae family. Evidence suggests the cues that mediate pre-oviposition behaviour in Zabrotes subfasciatus (Boheman) are volatile organic compounds (VOCs) emitted by the seeds and suggests differential abilities in environment sensing between sexes. Here, we tested whether VOCs from different legume species trigger different levels of attraction, whether females and males differ in their ability to respond to VOCs, and whether the seeds promoting different behaviours feature singular VOCs. Finally, we tested females' response to selected VOCs from legume seeds with different levels of attraction. Behavioural assays testing the beetles' response to four Fabaceae species allowing varied levels of fitness (Phaseolus vulgaris, Glycine max, Lens culinaris, and Cicer arietinum) and antenna removal assays confirmed volatiles emitted by the seeds act as chemical cues for females and showed the seeds of the tested legume species trigger different levels of attraction. GC-MS analysis revealed the seeds of two of the species at the extreme of the preference hierarchy feature singular VOCs profiles and abundance. One of the differentially expressed VOCs found in higher quantities in P. vulgaris (ethenylbenzene = styrene) and one in C. arietinum (2-ethyl-1-hexanol) mediate opposing behaviours in Z. subfasciatus females. These results suggest host choice in Z. subfasciatus relies upon at least a two-component kairomone-based system: styrene attracts females to P. vulgaris seeds and 2-ethyl-1-hexanol keeps them from choosing C. arietinum as a substrate for oviposition. The system has evolutionary and agricultural relevance because it may allow for "niche" specialisation in seed stores and natural environments and may help develop pest management strategies.