**Zigzag Sawflies** (*Nematus oligospilus*) are an invasive defoliating pest of willows and certain types of poplars. The conflict is **severe defoliation and ornamental damage**: the larvae feed rapidly on the leaf tissue, leaving behind a distinctive **zigzagging pattern** of feeding along the leaf margin and midrib. With multiple generations per season, populations can quickly build up, causing complete defoliation of host trees, especially in nurseries and urban plantings.
Taxonomy and Classification
Zigzag Sawflies belong to the Order Hymenoptera (Wasps, Bees, Ants) and the Family Tenthredinidae (Common Sawflies). They undergo complete metamorphosis. They are widespread invaders, having established rapidly across Europe and North America.
Physical Description
Larvae are $10\text{ mm}$ to $12\text{ mm}$ long.
- **Adult (Key ID):** Small, black body with a yellowish abdomen; harmless and non-stinging (Sawflies are related to wasps but lack a narrow “wasp waist”).
- **Larva (Key ID):** Greenish-yellow larva with a dark head; distinguished by their habit of raising the abdomen when resting.
- **Damage ID (Key):** Highly distinctive **zigzag feeding pattern** on the leaf edge; rapid and complete defoliation of host trees, often beginning in the lower canopy.
- **Conflict:** Ornamental, Forestry.
Distribution and Habitat
Native to East Asia; invasive in Europe and North America. Habitat is urban parks, floodplains, and nurseries where willow and poplar trees are common.
Behavior and Conflict
The conflict is their invasive status and high reproductive rate.
- **Rapid Reproduction:** They are capable of producing multiple, overlapping generations annually, allowing infestations to escalate quickly.
- **Host Range:** While primarily a willow pest, their impact on urban plantings is significant due to the popularity of the host trees.
Management and Prevention
Management is **Monitoring and Biological Control**.
- Inspecting host trees frequently for the characteristic zigzag feeding damage, especially early in the season.
- Targeted application of insecticides to susceptible larval stages for heavy infestations; *Bacillus thuringiensis* (Bt) is generally ineffective against sawfly larvae.
Conservation and Research
Research focuses heavily on identifying and introducing specific natural enemies (parasitic wasps) from their native range to develop effective long-term biological control programs in invaded territories.