Tomostethus nigritus

Ash sawfly

Description

The adult is generally black, 7–8 mm long. First instar larva, whitish or almost hyaline, during eclosion emerges from the swollen chamber on the leaf margin, and molts soon after into the second larval instar which has a greenish cast as the gut fills with leaf chloroplasts. The last larval instar stops feeding, descends the trunk and enters the soil. Larvae don’t go too deep into the soil, they are concentrated in the vicinity of the root collar and form shiny, black parchment-like cocoons covered on the outside with soil. Pupae are found by mid March and adult emergence begin in late March and early April. The period of massive swarming varies from year to year but usually peaks in mid April. Tomostethus nigritus is a widely distributed Eurasian species which occurs at very low densities and occasionally reaches higher population levels.