Using semiochemicals to predict biotic resistance and facilitation of invading phytophagous insects

Invasive species are leading causes of biodiversity loss and economic damage. Allocating limited resources towards prevention and management of invasions requires risk assessments based on ecological knowledge for species of potential concern. Interactions of potentially invasive species with heterospecifics in a novel community will contribute to biotic resistance and facilitation experienced by the invader, and thus the likelihood of establishment. To experimentally predict heterospecific interactions, we conducted field experiments using synthetic aggregation-pheromone lures to measure the response of ecologically relevant species (possible predators, competitors, and facilitators) to the simulated presence of two potentially invasive spruce bark beetles, the North American Dendroctonus rufipennis (tested in Norway) and the European Ips typographus (tested in eastern Canada). The Canadian beetle community responded strongly to I. typographus lures, suggesting potential for considerable biotic resistance, whereas D. rufipennis lures prompted little response by the Norwegian beetle community. D. rufipennis was attracted to I. typographus lures, suggesting potential facilitation between these species through cooperative mass attack on trees. Our findings will inform invasive-species risk assessments for I. typographus and D. rufipennis and highlight useful methods for assessing interactions among other taxa that rely heavily on semiochemical communication.

Methods
Multiple-funnel traps in experimental sites in Canada and Norway were baited with a variety of semiochemical dispensers including controls, with the goal of determining if relevant heterospecifics (bark beetles and predators thereof) responded to the aggregation pheromone blends of two potentially invasive bark beetle species, Dendroctonus rufipennis (tested in Norway) and Ips typographus (tested in Canada). Both experiments were set up as randomized block designs. The experiment in Canada had seven replicate blocks, and the experiment in Norway had ten. Beetles captured in the traps throughout the spring and summer were IDed to species in most cases, though only to genus for some, and counted.

The Host treatments consisted of α-pinene and ethanol (Synergy Semiochemicals Corp., Burnaby, BC, Canada).

The Ipslure treatments consisted of the commercialy available Ips typographus lure (methylbutenol, cis-verbenol, and ipsdienol; Ipslure®) plus the host volatiles (α-pinene and ethanol).

The DRlure treatments consisted of the commercially available Atlantic Dendroctonus rufipennis lure (frontalin and seudenol; Synergy Semiochemicals Corp., Burnaby, BC, Canada), further amended with MCOL, and host volatiles (α-pinene and ethanol).

The control treatments did not have lures or host volatiles.