**Hessian Flies** (*Mayetiola destructor*) are small, gnat-like insects and one of the most historically damaging pests of **wheat** (and to a lesser extent, barley and rye) worldwide. The larvae (maggots) are the destructive stage; they feed by sucking juices from the base of the plant stem, causing weakening, stunting, and reduced yield. Infestation is often characterized by the plant turning dark green and tillering excessively, but failing to produce a head of grain. Their appearance in the U.S. during the Revolutionary War gave them their common name.
Taxonomy and Classification
Hessian Flies belong to the order Diptera (true flies), family Cecidomyiidae (gall midges). They undergo complete metamorphosis. The flies are non-feeding and short-lived. The larval stage is known for its ability to inject a toxic saliva that modifies the plant’s growth, making it a highly specialized pest.
Physical Description
The adult fly is very small, about $1/8$ inch long, and resembles a mosquito or small gnat.
- **Adult Appearance:** Dusky brown to black, with fragile wings. They live only a few days and are seldom seen by farmers.
- **Larvae (Maggots):** Small, translucent white initially, turning a greenish-white later. They are legless and look like tiny fly maggots.
- **Puparia (Flaxseeds):** The pupal stage is the most recognizable form. It is a brown, tough, cylindrical casing, resembling a **flaxseed**, found wedged between the leaf sheath and the main stem of the plant.
- **Damage Sign:** Stems break easily (often lodging after rain), and the plant may appear excessively dark green and stunted, failing to produce kernels.
Distribution and Habitat
Hessian Flies are distributed globally wherever winter wheat is grown. Their habitat is restricted to cereal grain fields. They have two main generations per year, one in the fall and one in the spring, which correspond to the planting and active growth phases of the wheat.
Behavior and Conflict
Female flies lay eggs on the upper surface of wheat leaves. The larvae crawl down and position themselves behind the leaf sheath near the stem node.
- **Feeding and Damage:** The maggot injects salivary secretions that disrupt the plant’s vascular system, preventing nutrient flow to the head. Fall infestations stunt the plant; spring infestations cause the stem to weaken and often break (lodging).
- **”Fly-Free Date” (Key):** The survival of the Hessian Fly is tightly linked to the wheat growth cycle. Farmers often rely on the **fly-free date**—the latest safe date for planting wheat in the fall—to ensure the crop emerges only after the fall generation of flies has died out.
- **Resistance:** Like many pests, they have evolved different **biotypes**, necessitating continuous breeding for resistant wheat varieties.
Management and Prevention
Control is highly focused on cultural methods, particularly timing and resistant varieties.
- **Resistant Varieties:** Planting wheat varieties specifically bred with resistance to the local Hessian Fly biotypes is the most effective and sustainable long-term solution.
- **Delayed Planting:** Strictly adhering to the “fly-free date” for planting in the fall is the most effective cultural control method, starving the first fall generation.
- **Crop Rotation:** Rotating wheat with non-host crops (e.g., soybeans, corn) helps break the life cycle by eliminating the flies’ food source.
- **Residue Management:** Plowing under or destroying wheat stubble immediately after harvest can help kill the flaxseeds before the next generation emerges.
Conservation and Research
Hessian Flies are managed as a historical and high-impact crop pest. Research focuses on mapping the genetics of new resistant wheat lines and tracking the evolution and migration of new fly biotypes to stay ahead of the resistance cycle.