Conifer phenolic resistance markers are bark beetle antifeedant semiochemicals

1 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. 2 By a new bioassay, we tested the effect on host acceptance and tunnelling of male and female I. typographus in an artificial gallery (13 mm long) for 4 h (nochoice 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. 3 All tested chemicals decreased the tunnelling activity of I. 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. 4 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.