The Hessian fly (''Mayetiola destructor''), or barley midge, is a species of fly that is a significant pest of cereal crops, including

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

and

rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is grown principally in an area from Eastern and Northern Europe into Russia. It is much more tolerant of cold weather and poor soil than o ...

. Though a native of Asia, upon its discovery it was believed to have been transported into North America in the straw bedding of Hessian troops during the

American Revolution The American Revolution (1765–1783) was a colonial rebellion and war of independence in which the Thirteen Colonies broke from British America, British rule to form the United States of America. The revolution culminated in the American ...

(1775–1783), thus the origin of its common name. However, the report of an inquiry made in 1788 by

Sir Joseph Banks Sir Joseph Banks, 1st Baronet, (19 June 1820) was an English naturalist, botanist, and patron of the natural sciences. Banks made his name on the 1766 natural-history expedition to Newfoundland and Labrador. He took part in Captain James Co ...

states that "no such insect could be found to exist in Germany or any other part of Europe". Nonetheless, it appears that this species, or one exactly like it in habits, had been known for at least a century prior to the American Revolution from a locality near Geneva, and also for a long time from some regions in France. The Hessian fly was described by

Thomas Say Thomas Say (June 27, 1787 – October 10, 1834) was an American entomologist, conchologist, and Herpetology, herpetologist. His studies of insects and shells, numerous contributions to scientific journals, and scientific expeditions to Florida, Ge ...

in 1817. It is a very harmful insect. It mainly attacks the stem, although if it is especially hungry it will eat any part of the plant it can find. In 1836, a severe infestation of Hessian flies resulted in a crop shortage aggravating the financial problems of farmers prior to the

Panic of 1837 The Panic of 1837 was a financial crisis in the United States that began a major depression (economics), depression which lasted until the mid-1840s. Profits, prices, and wages dropped, westward expansion was stalled, unemployment rose, and pes ...

Distribution

''M. destructor'' is believed to have originated in the

Fertile Crescent The Fertile Crescent () is a crescent-shaped region in the Middle East, spanning modern-day Iraq, Israel, Jordan, Lebanon, Palestine, and Syria, together with northern Kuwait, south-eastern Turkey, and western Iran. Some authors also include ...

of the

Middle East The Middle East (term originally coined in English language) is a geopolitical region encompassing the Arabian Peninsula, the Levant, Turkey, Egypt, Iran, and Iraq. The term came into widespread usage by the United Kingdom and western Eur ...

, parasitizing wild grass species of the tribe Triticeae. With the global development of agriculture, particularly of

, it is now present in throughout Europe, North Africa, North America, and New Zealand.

Appearance

Hessian fly eggs are tiny and elliptical, being most easily recognized by their distinctive orange color. Larvae resemble white cylindrical grubs similar to other members of the family

Cecidomyiidae Cecidomyiidae is a family of diptera, flies known as gall midges or gall gnats. As the name implies, the larvae of most gall midges feed within plant tissue, creating abnormal plant growths called galls. Cecidomyiidae are very fragile small in ...

. As they age, larvae also develop a translucent green stripe down the center of the back. There are three larval instars, each varying in size. Individuals between the first and second instars can widely differ in size, with larvae doubling in size from the first instar (0.56-1.70 mm) to the second (1.70-4.00 mm). Pupae develop within the

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

of the second instar, known as the puparium. This stage is also called the flax-seed stage, as the hard, sclerotized, dark-brown cuticle the larva develops within resembles a seed of

flax Flax, also known as common flax or linseed, is a flowering plant, ''Linum usitatissimum'', in the family Linaceae. It is cultivated as a food and fiber crop in regions of the world with temperate climates. In 2022, France produced 75% of t ...

. Adult flies are brown or black. Females can sometimes be distinguished more easily due to their occasional reddish-brown coloring, a product of the eggs developing within the abdomen.

Life Cycle

The Hessian fly is a short-lived

gall Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants. Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors or war ...

-forming insect, living for around 30 days. Typically, multiple generations of the fly are seen, although the exact number of generations depends largely on region and its associated climate. For example, in the Coastal Plains region of

Georgia Georgia most commonly refers to: * Georgia (country), a country in the South Caucasus * Georgia (U.S. state), a state in the southeastern United States Georgia may also refer to: People and fictional characters * Georgia (name), a list of pe ...

and

South Carolina South Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders North Carolina to the north and northeast, the Atlantic Ocean to the southeast, and Georgia (U.S. state), Georg ...

, 5-6 total generations can be observed in a year; two in fall, one in winter, and one-two in spring. Females lay up to 200 eggs on wheat, but will lay on other

cereals A cereal is a grass cultivated for its edible grain. Cereals are the world's largest crops, and are therefore staple foods. They include rice, wheat, rye, oats, barley, millet, and maize (Corn). Edible grains from other plant families, suc ...

, such as

oats The oat (''Avena sativa''), sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural). Oats appear to have been domesticated as a secondary crop, as their seed ...

and

, albeit in smaller clutches. Eggs are most commonly laid and found in grooves on the upper side of the leaf blade in a host plant. Oviposition sites are carefully chosen through chemical, visual, and tactile cues, taking into account intrinsic factors that could maximize survival of offspring, such as number and depth of vascular grooves on the leaf surface. After hatching, first-

instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...

larvae migrate down to the leaf base, where they feed by attacking the abaxial surface of the sheath on the plant's youngest leaves. Feeding lasts between 2-3 weeks over 2 instars, with five-day old larva consuming the greatest amount of plant fluid. The second instar is largely immobile, lacking the creeping pads of the first stage. Following feeding, larvae enter the third instar/ pupal stage, which can last between 7-35 days. Eclosure, or emergence, can be delayed if environmental conditions are not ideal, with pupae entering either

aestivation Aestivation ( (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a lowered m ...

diapause In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions.Tauber, M.J., Tauber, C.A., Masaki, S. (1986) ''Seasonal Adaptations of Insects''. Oxford University Press It ...

if conditions are too warm or too cold respectively. Adults are the short-lived stage of life, lasting only 1-4 days. During this time, females mate and oviposit on host plants.

Feeding

The larvae of ''M. destructor'' feed on the nutritive tissue of cereal plants. This tissue is created by the interaction of specialized enzymes in the insect's saliva with cells in the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

and mesophyll of the plant, causing organelles to break down and rupture. Larvae also have paired mandibles that act similarly to

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

stylets, puncturing tiny holes in the epidermal wall where food can be accessed. Like in other plant-feeding cecidomyiid species, larvae have a simplified digestive system that allows for extra-intestinal digestion. Lytic enzymes, present within the midgut via holocriny, act along with salivary secretions to break down complex cellulose-based plant materials into liquid sugars and proteins, which are then easily consumed by the larva. While it primarily serves the purpose of transforming food into a usable form for the larva, this special combination of chemicals also produces a wide range of effects in the host plant, the exact scope of which is unknown. One such effect is carbon/nitrogen shift. As the plant interacts with the saliva of the larva, avirulence (Avr) genes in the saliva match up with similarly structured genes in the host that coordinate plant defense. The resulting gene-to-gene compatibility causes changes in numerous metabolic processes, such as

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

and amino-acid synthesis, leading to free-containing carbon compounds being directed away from the plant and leached out as a liquid. These interactions can cause up to a 36% loss of free-carbon compounds and a 46% increase of free-containing nitrogen compounds.

Host defense

''M. destructor'' is one of the most destructive

pests worldwide, hence its name. This has brought it much attention from wheat breeders and genetics researchers. As a result, the highest number of mapped R genes for resistance to insects in wheat are R genes for this pest specifically, with the unrelated Russian wheat aphid (''Diuraphis noxia'') also being of serious interest.

References