Zusammenfassung
1. Olfactory cues can determine the host preferences of herbivorous insects, but their role in host shifting is unclear. Host specificity and the potential for host shifts are important criteria
for screening and post-release evaluation of biological control agents for invasive plants.
However, the role of olfactory cues in mediating host shifts in biological control agents is not
well understood.
2. To investigate the role of olfactory cues in host selection of a reportedly monophagous
flea beetle (Agasicles hygrophila), an important biocontrol agent for invasive alligator weed
(Alternanthera philoxeroides), we extracted and analyzed the volatiles produced by the host
plant A. philoxeroides and the non-host plants A. sessilis, Beta vulgaris, and Amaranthus
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mangostanus. Moreover, we used electrophysiologcial techniques, behavioural bioassays and
field trials to test the antennal responses and behavioural preferences of A. hygrophila to
combinations of different plant volatiles and treatments, and pure compounds in different
dosages and combinations.
3. We show that A. hygrophila female beetles indeed use olfactory cues to select plants for
feeding and oviposition and that the survivorship of larvae on the second preferred non-host
plant A. sessilis, a close relative of the first preferred host plant A. philoxeroides, was over
75% in a field trail. Although female beetles responded to many volatile compounds from
host and non-host plants, (E)-4,8-dimethyl-1,3,7- nonatriene (DMNT) positively encouraged
the beetle’s feeding and oviposition preferences, whereas (Z)-3-hexenol displayed repellent
effect. Remarkably, complementation assays with (Z)-3-hexenol on host plant or DMNT on
non-host plants significantly shifted A. hygrophila host preferences to non-host plants and
resulted in oviposition and egg hatching on the non-host plant A. sessilis in field trials.
4. Synthesis. We demonstrate an olfactory mechanism by which a specialized herbivorous
beetle uses the ratio of two common plant volatiles, DMNT and (Z)-3-hexenol, to
discriminate between its host and non-host plants in nature. This study highlights an
important mechanism by which olfactory cues could lead to undesired host range expansion
in biocontrol agent, thus representing an important warning of the potential for a host shift
and development of invasiveness in a common biocontrol agent, the flea beetle.
