Conifer phenolic resistance markers are bark beetle antifeedant semiochemicals

Phenols are important in conifer resistance to fungi associated with bark beetles and as markers for resistance to beetle mass-attacks. However, the mechanism of phenolic activity in conifer resistance to Ips typographus bark beetle remains unclear. By a new bioassay, we tested the effect on host acceptance and tunnelling of male and female L typographus in an artificial gallery (13 mm long) for 4 h (no-choice test). To the artificial diet, an aliquot of host (catechin, taxifolin, or resveratrol) or nonhost (E-conophthorin) compounds was added, singly tested at doses close to those of tree bark. Host acceptance and tunnelling was measured by the amount of diet removed by the insects. All tested chemicals decreased the tunnelling activity of L typographus, with an antifeedant effect stronger in males and increasing with dose. No mortality was recorded. The nonhost volatile spiroketal, E-conophthorin, had the highest antifeedant activity both in males and females. Among host compounds, effects and dose-response were weak in females. Both catechin and E-conophthorin gave a 50% reduction of tunnelling at a concentration of 0.1% for males, the pioneering (host selecting) sex in Ips. The threshold of activity for host compounds to males was at concentrations of 0.03-0.1%, which corresponds to, or is less than, the concentrations reported from spruce host bark. The results allow us to support the hypothesis of a direct behavioural antifeedant mechanism for resistance from those phenols that are particularly active for the pioneering males during tree attack.