Stand edge effects on distribution and condition of Diprionid sawflies

1 We investigated stand edge effects on Diprion pini cocoon condition and distribution after an outbreak of unprecedented magnitude in Finland. We hypothesized that forest fragmentation and the resulting proportional increase in stand edge habitats may have led to the increase in outbreak area: D. pini may profit from edge habitats and so escape its control mechanisms more easily. This hypothesis was based on the observation that no outbreak occurred in neighbouring Russian Karelia, where the proportion of edge habitats between different stand types and/or successional stages is significantly less than in Finland. 2 To test the hypothesis, we determined the amount and condition of cocoons in an outbreak area, along transects going from stand centres through edge habitats, towards neighbouring stand centres. 3 Both the total number and number of cocoons in different conditions varied significantly between edges and centres. The effects were more pronounced when age difference between neighbouring stands increased. 4 Edge habitats did not benefit D. pini. Both the total number and number of hatched cocoons were consistently higher in centre than in edge habitats. 5 Defoliation was unrelated to distance from the edge, even on transects where cocoon density and condition was correlated to distance from the edge. 6 We conclude that edge habitats have an inhibitive effect on D. pini densities, and therefore our original hypothesis was rejected. The importance of small mammals to D. pini mortality was demonstrated, and we suggest further investigation of this predator–prey relationship because of its potential for limiting outbreak risk and/or damage through silvicultural measures.