Redundancy, synergism, and active inhibitory range of non‐host volatiles in reducing pheromone attraction in European spruce bark beetle Ips typographus

Abstract

Host and habitat selection in flight by conifer bark beetles is governed by inhibition of attraction by non‐host volatiles (NHV), in addition to the well‐known attraction to kairomones from host plants and to aggregation pheromones from conspecifics. Antennally‐active NHV from angiosperm birch and aspen trees were field tested on the European spruce bark beetle, Ips typographus (L.), by pheromone baited traps in Sweden using release rates of NHV comparable to those from a non‐host tree. Trap catches were significantly reduced by 50‐70% by individual NHV: trans‐conophthorin, 3‐octanol and 1‐octen‐3‐ol from non‐host bark, 1‐hexanol and (Z)‐3‐hexen‐1‐ol from both leaves and bark and (E)‐2‐hexen‐1‐ol from leaves. Combined NHV signals in binary and ternary blends with the same overall release rates (ca 5 mg/day) showed both redundancy and synergism in their inhibitory effects. The redundancy occurred among individual green leaf alcohols (GLV‐alcohols or C6‐alcohols), C8‐alcohols, and between these two groups, while trans‐conophthorin and verbenone (Vn, from unsuitable host trees) showed significant synergistic effects between each other and with GLV‐alcohols or C8‐alcohols. The coexistence of redundancy and synergism in non‐host chemical signals may indicate different functional levels (non‐host habitats, species, and unsuitable host trees) of these negative volatile signals in the host selection process of conifer bark beetles. We showed an active inhibitory range (AIR) of a NHV‐blend plus Vn on a standard pheromone bait to be at least 2 m, by using a central pheromone trap and surrounding concentric barrier trap rings with radii of 1, 2, and 4 m. The influx of bark beetles flying across concentric rings within the AIR was reduced by 55–99%, depending on the distance to the central pipe trap. Our findings support the hypothesis that as mixed forests have greater semiochemical diversity than pure host stands; they disturb olfactory guided host choice, and may reduce the possibility of outbreaks of conifer‐infesting bark beetles. The synergistic effects as well as the active inhibitory range found in this study also suggest that optimal combinations of these NHV and verbenone may have potential in protecting forests against I. typographus by reducing or stopping attacks on suitable host trees. Mixed stands with higher semio‐chemical diversity disturb olfactory guided host choice and reduce the risk for outbreaks of specialist herbivores.. This ‘semiochemical‐diversity hypothesis’ provides new support to the general ‘stability‐diversity hypothesis’.