Areal distribution of a Neodiprion sertifer (Hym., Diprionidae) outbreak on Scots pine as related to stand condition

Analyses of a recent N. sertifer outbreak in southern Sweden were made during on‐going attack, comprising a regional and local mapping of defoliation severity, as well as a study of intensity of larval consumption, abundance of cocoons and parasitation of larvae in relation to fertilizer application. It was found that (1) the outbreak was preceded by summer and autumn droughts, (2) it was limited to infertile soils, (3) pine stands aged 20–40 yr suffered most, (4) it was less severe in trees benefiting from a reduced inter‐tree competition, as measured from tree‐ring growth, and (5) less severe in a fertilized stand. The findings support the hypothesis on a fundamental effect of stand nutritive state on changes in number of the sawfly.