Warble flies that cause cattle grubs belong to the fly family of the Oestridae. They are also called bomb flies, heel flies, gad flies, etc. They infest cattle and related bovids (e.g. bisons).

They are found only in the Northern Hemisphere (typically between 25° and 60° latitude): their southern limit in America is North Mexico. They can be found in the whole of Europe and down to North Africa. Incidence, prevalence and seasonal abundance are regionally different depending on ecological and climatic conditions. Warble flies can occasionally infest sheep, goats, horses and humans as well.

Warble flies cause so-called parasitic myiases, i.e. not the adult flies but the larvae are parasitic. Warble flies are obligate parasites, i.e. they cannot complete their life cycle without parasitizing their hosts.

There are two major warble fly species that attack cattle:

• Hypoderma bovis, the northern cattle grub.
• Hypoderma lineatum, the common cattle grub, attacks also horses.

There are other warble fly species specific of deer. The disease caused by cattle grub infestation is called hypodermosis.

These warble fly species do not affect dogs or cats.

Biology and life cycle of warble flies

Adults are hairy flies that look like bees and are 12 to 15 mm length. The adult flies are not directly harmful for cattle: they neither bite, nor sting, nor cause any immediate harm. But they persistently deposit their eggs on cattle. The larvae get inside the host and accomplish a remarkable migration eating themselves through the host tissues.

The life cycle lasts about 1 year. Adult flies have no functional mouthparts and do not feed, but survive with the nutritional reserves stored before birth. Their only task is to reproduce. Female flies deposit eggs on single hairs of the host in rows of about 5 to 8 eggs, often on the legs, but also elsewhere. The eggs are firmly attached to the hairs. A female fly can deposit up to 800 eggs on a host during her lifetime, which is usually less than a week.

Egg hatching occurs 3 to 7 days after oviposition. The larvae crawl down to the skin, which they actively penetrate. This is very irritating for cattle. Once below the skin, they migrate upwards, often along the connective tissue between the muscles, but also along nerve pathways. They produce enzymes that dissolve the host's proteins, which is also what they feed. After several months migration Hypoderma lineatum larvae reach the wall of the esophagus (occasionally the lungs), and Hypoderma bovis larvae the spinal canal, where they remain for 2 to 4 months till early winter. Afterwards they complete their migration to the back of their hosts where they perforate the host's skin to open a breathing hole. They remain there for about 1 to 2 months and molt twice.

The host's body reacts to these strange organisms forming the typical cysts or warbles around each larva. These are swellings of 2-3 cm in diameter. The larvae inside the warbles are white to yellow at their arrival, become progressively darker and are almost black when they leave their shelter. Mature larvae are up to 3 cm long.

In spring, mature larvae crawl out through the respiratory hole, fall to the ground, bury themselves, and pupate within a few hours. Humid or marshy soils are unfavorable for their development. Adult flies emerge 10 to 90 days later.

Cattle are often highly afraid of Hypoderma bovis flies approaching them for oviposition, and often react stampeding, a behavior called also "gadding". In contrast with this, Hypoderma lineatum, which is is slightly smaller (13 mm) causes usually no stampeding because its flight is less noisy and it often completes its approach to the host not flying, but crawling. After oviposition, hatching larvae not always penetrate the skin, but are often ingested by the host through licking. Once in the host's mouth they reach the esophagus and penetrate its wall where they remain for months before completing their migration to the host's back.

Click here to learn more about the general biology of insects.

Harm and economic loss due to warble flies

Warble flies are quite harmful for cattle. Flies approaching for oviposition frighten cattle that run away stampeding. This is highly stressing and the animals do not feed properly, which reduces weight gains (up 20 kg less) and milk production (up to 1 kg daily). Animals may hurt themselves, and abortions in pregnant cows can also happen. 

Infected cattle in endemic regions may develop several hundred warbles. Migrating larvae are usually not a serious problem for cattle, but occasional complications are possible. Inflammation of the esophagus wall by Hypoderma lineatum larvae can hinder swallowing. Hypoderma bovis larvae can damage the spinal chord causing various neurological disorders, up to paralysis in rare severe cases.

The cysts usually heal spontaneously once mature larvae abandon the host for pupation, but occasionally they can become infected and produce painful abscesses.

The numerous holes in the hides can strongly reduced their value and the carcasses may be also downgraded at slaughter.

Prevention and control of cattle grubs

It is known that cattle that have been infected with warble flies acquire certain natural immunity that reduces the disease incidence in the following years. This is why calves and yearlings are often more affected by warble flies than older cattle. Investigations with experimental vaccines in the 1990's were quite promising: protection of up to 90% was achieved in field trials. However, no commercial vaccine has been introduced so far.

In the 1950-70's the introduction of systemic larvicides containing mainly organophosphates (e.g. eprinomectin, fenthion, phosmet, trichlorfon, etc.) led to national eradication campaigns in many European countries. They were available for spraying or as pour-on and spot-ons. After topical administration to cattle migrating 1st stage larvae were successfully killed, but not those larvae already in the warbles. Later on other formulations were used as additives for food, drinking water or lick-stones.

Such eradication campaigns were quite successful and substantially reduced the incidence of hypodermosis. However, complete eradication was very difficult to achieve and disease outbreaks have been numerous since than. In the meantime such products have been withdrawn in many countries. In the discussion around the mad cow disease outbreaks in the early 2000's some authors speculated that they could be related with the previous use of such systemic organophosphates in cattle. However, this hypothesis has not been substantiated so far.

Large-scale control using the sterile male-release technology developed against screwworms was also attempted in some regions against warble flies, e.g. in the USA and Canada in the 1980's. However, it proofed to be unsustainable, mainly because it was not possible to ensure the industrial production of millions of sterile male flies that were needed for for such an approach. Learn more about biological control of flies and other insects.

Nowadays, newer and more modern systemic larvicides containing macrocyclic lactones (e.g. doramectin, eprinomectin, ivermectin, moxidectin, etc.), either as injectables or as pour-ons, allow very effective control of both migrating larvae as well as grubs in the warbles, and protect cattle against re-infestation for several weeks. Only eprinomectin is approved for use on dairy cows.

A sudden death of migrating larvae releases large amounts of antigens in the host's organism, which can cause severe allergic adverse reactions. To prevent this from happening animals, especially calves, they must be treated immediately after the end of the warble fly season, but not later than 2 to 3 months before the expected appearance of warbles in the backs.

For a small number of animals with a few warbles, individual grubs can be squeezed out. However care must be taken not to crush the grub inside the cyst, because this can cause a severe anaphylactic reaction. For a large number of animals the warbles can be directly treated with a larvicidal dust (often containing organophosphates).

So far there are no effective repellents against warble flies of cattle.

Resistance of warble flies to parasiticides

So far there are no reports on warble fly resistance to parasiticides.

This means that if a particular product has not achieved the expected control, it is most likely because the product is not adequate or it was not used correctly, not because warble flies have become resistant.

Learn more about parasite resistance and how it develops.