Bark beetles attacking conifer seedlings: picking on the weakest or feasting upon the fittest?
Plant stress from causes such as drought is thought to increase the susceptibility of plants to herbivore attack and damage. Although this forms the basis of the widely cited plant stress hypothesis, there are few cases where this has been tested thoroughly. We compared the frequency of attack and damage of an invasive pine bark beetle Hylates ater, on seedlings of Pinus radiata, using a 2×2 factorial design with artificially induced stress and insecticide application to distinguish effects of beetle attack and direct effects of stress. Contrary to expectations based on the plant stress hypothesis, twice as many unstressed seedlings (22%) were attacked than stressed seedlings (11%). However, stressed seedlings were twice as likely to experience sustained bark beetle feeding resulting in girdling, than unstressed seedlings (31% and 15%, respectively). In response to beetle attack, unstressed seedlings showed much stronger resin production than stressed seedlings, which is likely to explain the greater survival of unstressed seedlings after attack. Experimental drought stress caused a 50% reduction in shoot growth. Synthesis and applications. Maximizing seedling vigour and minimizing (pre-)planting stress are key for the resilience of planted forests. Managers should emphasize a stress-free transition of seedlings from nursery to field as well as correct planting techniques, in order to safeguard this vulnerable stage. This is particularly crucial since drought stress and insect invasions are likely to increase as a result of global change.