Hessian Fly

Adults are small, delicate, dark midges that are rarely seen and live only briefly. Larvae are small, legless maggots found behind the leaf sheath at the base of the tiller. A well-known immature stage is the "flaxseed": a small, hard, brown puparium resembling a flax seed, tucked at the stem base, which is often the easiest life stage to find.

Infested plants are often stunted with unusually dark, thickened leaves in the vegetative stage, and later develop weakened stems that break and lodge. Finding maggots or flaxseed-stage puparia behind the sheath confirms the pest.

Females lay eggs on the leaves, and the hatching larvae move down to feed at the base of the tiller before forming the flaxseed puparium. Several generations can occur per year depending on climate, commonly with distinct autumn and spring generations in winter-cereal regions, and the flaxseed stage carries the insect through unfavourable periods.

The insect survives between crops in stubble and volunteer cereals as the flaxseed stage, so residue and volunteer management, along with the timing of planting relative to adult activity, strongly influence infestation.

Larval feeding at the tiller base stunts and weakens plants; early infestation of seedlings can kill tillers, while later feeding weakens the stem so that it breaks and lodges before or during harvest, complicating harvest and reducing yield. The extent of loss depends on generation timing relative to crop stage and on varietal resistance.

Wheat is the principal host, with barley and other small grains and grasses also affected. Resistant wheat varieties are a cornerstone of management where the pest is important.

Monitoring relies on inspecting plants for stunting and, at the stem base, for maggots and flaxseed-stage puparia, and on tracking adult activity where degree-day models or regional advisories exist to indicate safe planting windows. Field history and the regional prevalence of resistance-breaking biotypes are important context.

Because larvae are concealed, management is preventive and cultural, centred on host resistance and timing, within integrated pest management.

• Grow resistant wheat varieties suited to the local biotype situation
• Adjust planting date to avoid the main period of adult activity where advisories exist
• Control volunteer cereals and manage stubble that harbour the overwintering stage
• Use crop rotation and field sanitation to reduce carry-over
• Conserve natural enemies and base decisions on field history and monitoring