Willow sawfly

Systematic position: Nematus oligospilus is classified within the order Hymenoptera, family Tenthredinidae. This insect is a widespread defoliator known as an invasive species in various parts of the world, posing a significant threat to willow cultivation and forestry.

Host plants: The primary hosts for this sawfly are species of the genus Salix (willows), though they may occasionally infest Populus (poplars). The pest is particularly damaging in short-rotation coppice plantations, nurseries, and urban landscaping, where high infestation rates can cause severe canopy damage.

Biology and life cycle: The life cycle includes egg, larval, pupal, and adult stages. Females lay eggs directly into the leaf tissue. Upon hatching, larvae feed voraciously on the foliage. Pupation occurs in the soil or leaf litter beneath the host tree. Depending on local climate and temperature, the species can complete multiple generations per season, allowing for rapid population growth.

Nature of damage: Damage is caused by the larvae, which are external feeders. Initially, they create small holes in the leaves, but as they grow, they consume almost the entire leaf blade, leaving only the primary veins. Severe infestations lead to total defoliation, which causes significant stress to the host plants, reduces biomass production, and can lead to plant mortality, especially in young saplings.

Control measures: Managing Nematus oligospilus requires an integrated pest management (IPM) approach to maintain population levels below economic injury thresholds:

• Regular field scouting to detect early instar larvae and egg clusters.
• Application of biological insecticides such as Bacillus thuringiensis during early larval development stages.
• Use of selective systemic insecticides if monitoring indicates a population explosion.
• Enhancing habitat for natural enemies, including predatory wasps and parasitic flies.
• Soil cultivation around the base of the trees to disturb and destroy overwintering cocoons in the soil.