Hot, dry weather has reduced fly populations in some areas but for others, peak fly season is yet to come, said a University of Nebraska entomologist.

Cattle producers should take action as early as possible to ease overall fly control, said Jack Campbell, Institute of Agriculture and Natural Resources entomologist at NU's West Central Research and Extension Center at North Platte.

Flies reduce grazing, depress weight gain and increase disease susceptibility, he said. Cattle are commonly bothered by stable flies, horn flies and face flies, all of which emerge in May and rapidly reproduce to peak mid-July through September, he said.

Insecticide sprays, dust bags, oilers and ear tags help control adult flies. Feed additives and parasitic wasps destroy developing flies in manure.

"We've found the most effective method is the forced use of dust bags, where the cattle have to go under them to get to water," Campbell said.

Experienced herds quickly learn to use dust bags, but younger animals learn more slowly, so bags should be placed in gateways where cattle can't avoid them, he said.

Ear tags are efficient up to 60 days after application, but often run out when fly problems are worst, Campbell said. Producers must re-tag or use other control methods, he said. Insecticide sprays and pour-on treatments last about three weeks, but frequent applications may stress cattle too much to justify benefits, he said.

Cattle often will stand in water or seek shade to avoid flies and neglect normal grazing. This can depress weight gain by nearly a half pound per day, he said. Cattle also tightly bunch together for protection, which can amplify heat stress, Campbell said.

"Nebraska studies show a 10- to 15-pound advantage in calf weaning weights where cows have had good fly control," Campbell said.

Stable flies are most threatening for feeder operations, he said. Horn flies usually infest pasture and range cattle with as many as several thousand flies feeding 20 to 30 times each day on one animal.

All flies can weaken the immune system. "Because of the reduction in energy, cattle aren't as resistant to other diseases," Campbell said. "If they've spent most of their energy fighting flies, they don't have as much immunity."

Until cooler weather sets in, producers can only treat livestock and wait for fly numbers to decline, Campbell said. ©