Hessian Fly

Mayetiola destructor — the Hessian fly — is one of the most economically important wheat pests in the world and a flagship species in modern coevolution research. The species is one of about 6,000 species in family Cecidomyiidae (the gall midges — tiny long-legged delicate flies that include some of the most damaging crop pests). The species was almost certainly introduced to North America in 1776-1778 during the American Revolutionary War, in straw bedding and forage brought by HESSIAN MERCENARY TROOPS fighting for the British (the source of the common name). Wherever the British occupied territory and brought European straw with them, Hessian fly larvae overwintering in the straw were released into NA agricultural fields. The species spread rapidly across NA and is now one of the major wheat pests on the continent, also widespread across Europe, North Africa, and central Asia (essentially everywhere wheat is grown). Adults are tiny (3-4 mm), delicate, mosquito-like dark midges that are rarely noticed. Females lay eggs on young wheat leaves; larvae crawl down to the wheat stem base and feed on plant juices, weakening the stem and causing characteristic 'Hessian fly damage': stunted, yellowed wheat plants with brittle stems that snap and lodge in wind. Severely infested wheat fields can lose 30-50% of yield to Hessian fly damage. The species is the FOUNDATIONAL CASE STUDY in plant-insect GENE-FOR-GENE COEVOLUTION research. Wheat breeders have identified and deployed over 35 RESISTANCE GENES (designated H1, H2, H3, etc.) in commercial wheat varieties — each resistance gene confers protection against a specific Hessian fly biotype by triggering plant defense responses (hypersensitive cell death) when the fly attempts to feed on the plant. Hessian fly populations have responded by evolving corresponding VIRULENCE ALLELES (vr1, vr2, vr3, etc.) — fly biotypes that have lost the avirulence (Avr) gene products that triggered plant defense responses, allowing the fly to feed on previously-resistant wheat varieties. The gene-for-gene coevolutionary arms race exactly mirrors the famous gene-for-gene system in plant-pathogen biology (rust fungi vs. wheat, downy mildew vs. lettuce) and is one of the most-studied examples of coevolution in modern agricultural biology. The species is the subject of major USDA Agricultural Research Service Hessian fly research programs and is featured in essentially every modern textbook discussion of insect-plant coevolution.