Effects of jasmonate-induced resistance in conifer plants on the feeding behaviour of a bark-chewing insect, Hylobius abietis

Conifer defences can be induced by exogenous chemical selicitors, thereby reducing damage caused by bark-feeding insects. However, the insect behavioural mechanisms involved are poorly understood. Thus, effects of artificially induced plant defences on feeding behaviour of the pine weevil (Hylobius abietis), a serious forest pest, were examined to explore mechanisms involved in decision-making processes connected to feeding. To induce plant defences, we applied methyl jasmonate (MeJA), a naturally occurring plant hormone, to young Norway spruce (Picea abies) plants. The weevils' feeding behaviour on plants with and without MeJA treatment was studied in both a no-choice and a choice laboratory experiment. MeJA treatment did not affect the initiation of feeding, but it affected the weevils' subsequent feeding patterns. In the no-choice experiment, the only observed effect of its treatment was that it reduced the size of the initial feeding scars. In the choice experiment, it reduced both the numbers and sizes of the feeding scars and hence the total debarked area. Thus, the MeJA-induced resistance did not deter the pine weevils from attacking the spruce plants, but reduced the amounts they consumed at one place, which would reduce risks of girdling and plant death. This may be the behavioural mechanism behind the previously recorded increases in survival rates of MeJA-treated plants in the field.