Kairomone traps: a tool for monitoring the invasive spruce bark beetle Dendroctonus micans (Coleoptera: Scolytinae) and its specific predator, Rhizophagus grandis (Coleoptera: Monotomidae).
The Eurasian spruce bark beetle Dendroctonus micans is a major pest of spruce which is expanding its range in France, Turkey, England and Wales. Its monospecific predator Rhizophagus grandis has followed its prey naturally into most areas and since the 1960s has also been mass-produced and released within newly infested locations. Assessing bark-beetle presence at previously uninfested sites, or predator establishment after release or natural spread, currently depends on direct observation of D. micans brood chambers. This is a slow and unreliable process because bark-beetle attacks are solitary, cryptic and occur on living trees. An effective trapping method would thus be most useful. A synthetic lure for R. grandis was first field-tested in release-recapture experiments. Release points were surrounded by one or two concentric rings of traps baited with either D. micans larval frass or with a blend of synthetic chemicals. Kairomone-trapping using the synthetic lure was then tested in the field, monitoring R. grandis in 58 spruce stands in Belgium and France. In the release-recapture experiments, up to 65% of the released predators were caught within 48 h. Catches with the synthetic lure and with prey frass were not significantly different. Field-trapping assays showed R. grandis establishment in 30 stands, including areas where the predator had been introduced artificially. Trapping was unsuccessful at the southern and western limits of D. micans' range. Combined visual and trapping approaches identified 32 stands containing the predators, with two false negatives with the trapping method, compared to 19 for the visual surveys. Unexpectedly, male and female D. micans adults were trapped in some cases. The combined approach identified 43 attacked stands, among which there were four false negatives with the trapping method and 11 with the visual surveys. Synthesis and applications. To our knowledge, this is one of the first cases of using kairomone traps to monitor a predator after releases for biocontrol. Furthermore, because this predator is strictly prey-specific, this technique also identifies stands colonized by D. micans bark beetles. Following inoculative predator releases, kairomone trapping could be used to survey for D. micans in areas under colonization or in European Union 'Protected Zones'.