Watch out for surge of headflies from mid-July

Farmers are warned to look out for the annual emergence of head lies (Hydrotaea irritans) from around mid-July, as these are a major source of annoyance for pastured cattle. These flies hang around until late September, mating and depositing eggs in pasture soil and leaf litter, until colder weather kills them off.

Headflies, with their characteristic yellow abdomen and orange wing base are nuisance flies, as opposed to blood suckling types, such as the stable fly and horn fly. They cause considerable irritation as they feed on proteins from secretions such as saliva, tears, sweat and milk, and use this to mature their eggs.

“Headflies hang around in woodland and travel to and from the cattle several times a day,” explains Dr John McGarry, a fly specialist working at the University of Liverpool Institute of Veterinary Science and contributor to the Control of Worms Sustainably (COWS) Guide to insect pests of cattle.

“The males emerge first, followed by the females, usually in the last two weeks of July.

“The males die a week after mating and the females lay batches of eggs in the soil during August and September. These turn into carnivorous larvae which feed on insects in the ground. They grow slowly for ten months before emerging the following summer as adult flies.”

Cattle affected by headflies move to the shade, are restless and may stamp and swish their tails. This leads to less time spent grazing and decreased performance.

Close-up of headflies clustered around an animal’s eye

Research has also linked headflies with the transmission of summer mastitis in dairy cows and heifers as they feed on secretions from the teats.

“Treating headfly infestations is usually through applying synthetic pyrethroid chemicals,” says Dr McGarry. “But avoiding grazing on wet, low-lying fields near woods from July to September may reduce headfly numbers. Stock should be checked at least once a day. Head flies are more commonly a problem in the north of the UK than the south.

“Chemical control may include insecticide-impregnated ear tags which give five months cover and should really have been applied at turn-out, and ‘Pour-on’ treatments, which need re-applying every three weeks, depending on the fly challenge and if there has been heavy rainfall.

“Sprays applied via a knapsack are also available, which have a repellent action, but again, do need re-applications.”

Where insecticides are not used, applying teat sealant to pregnant heifers and dry cows can help stop the spread of summer mastitis bacteria by flies. Garlic licks, fly trapping and the use of fly predators, may offer some form of fly control, but there is little evidence to support this in grazing animals.

The cost of each treatment varies on who is selling it and on the longevity of its effect, but ranges from £3 per ear tag to 50p/dose for a pour-on – which may need doing several times in a season.

“Farmers will need to gauge how much performance is being lost and how much the treatment is, not forgetting the cost of handling the animals and the labour required to do that.

“For example, pedigree dairy animals in high risk fields might be given two ear tags inserted at turnout, whereas beef youngstock in low risk fields might receive one pour-on application. The level of use will depend on the fly challenge in any one season and the history of fly problems on the farm.”

There is more information on head flies and many other flies that affect cattle in the Control of Worms Sustainably (COWS) manual, which can be found at www.cattleparasites.org.uk