Food Quality Induced Variance in Larval Performance: Comparison between Rare and Common Pine-Feeding Sawflies (Diprionidae)

Abstract

This paper is a contribution towards a better understanding of why some monophagous diprionid sawflies have outbreaks while others seem to be always rare. We reared larvae of four northern European species, Neodiprion sertifer, Diprion pini, Gilpinia verticalis, and G. socia, on pine twigs collected from five habitats: deciduous forest, field edge, rock, plantation, and bog. The sawflies span from the serious pest N. sertifer to an always rare species G. socia; the pine twigs differed in e.g. needle morphology and nitrogen content, from 1.6% of dry wt in deciduous forest to 0.9% in bog. Larval mortality was higher in common than rare species. In a between-species analysis, production efficiency (cocoon weight divided by consumption) was positively correlated with nitrogen assimilation efficiency and larval growth rate. Neodiprion sertifer had the highest nitrogen assimilation efficiency and the second highest production efficiency, but this may be as well a consequence than a cause of outbreaks. The two outbreak species, N. sertifer and D. pini, showed less variation between the pine types in cocoon weight, production efficiency and larval mortality than the two Gilpinia species, suggesting that the continuous rarity of the latter species is strengthened by their sensitivity to food quality. Cocoon predation may nonetheless be a more important factor preventing outbreaks than plant defences.