Pine sawfly defence and variation in host plant resin acids: a trade‐off with growth

Abstract

Abstract. The defence capability of pine sawfly larvae reared on Scots pines with low or high concentrations of resin acids was determined in a field experiment, where larvae were exposed to predatory ants, and in a laboratory experiment in which the rate of defence droplet replenishment was measured. Larvae on low resin‐acid shoots disappeared faster than larvae on high resin‐acid shoots when exposed to predatory ants. Larvae fed high resin‐acid needles produced 50% larger defence droplets than larvae fed low resin‐acid needles. In an additional experiment high predation pressure was simulated by repeatedly removing defence droplets. Larvae responded by feeding more on tissue rich in resin acids, but at the cost of a reduction in growth rate. Based on these data and earlier findings that survival and development of sawfly larvae are negatively affected by high resin‐acid concentrations, we conclude that pine sawflies face a dilemma of conflicting demands, i.e. although high needle resin‐acid concentrations enhance defence capability, they also reduce growth and survival. Our data also suggest that it is not necessarily the ovipositing female that has to balance this trade‐off: plasticity in larval feeding behaviour provides larvae with a means to at least partly solve the dilemma.