The fly management iceberg

Every farm has flies. The issue is what kind, how many, where and their impact on livestock.

Veterinarian David Prentice recently presented information on fly control for farms for the Dairy Calf & Heifer Association.

Prentice began with fly basics: Fly reproduction is consistent among species, but reproduction varies with environmental conditions. Factors that impact reproductive cycles include moisture, organic matter and vegetation.

Flies can produce a staggering 10 to 30 generations each season. Temperature and humidity influence fly infestations, but since neither can be controlled, the best control plan is a year-round effort that focuses on fly species and seasonal conditions.

Prentice explained the 80/20 rule of flies, likening it to an iceberg.

“The flies you see on the hood of your truck and the flies you see on cattle are only 20% of the problem,” Prentice said. “The army behind – the reserves – are the pupae, larvae and eggs. This 80% is what we need to consider when it comes to fly control.”

The four species of economic concern are the horn fly, face fly, stable fly and house fly. Face and house flies have spongy mouthparts and are non-biting but can transmit disease. Horn and stable flies are biters and can result in uncomfortable animals and lower production.

The horn fly isn’t a significant problem on confinement dairies but it impacts pastured cattle. “They are bloodsucking flies,” Prentice said. “They feed 20 to 40 times per day. They are obligate parasites of the host – they live their life on the host and fly off the animal to lay eggs. Their bite is painful.”

Horn flies are a good example of how species’ habits influence the degree of the problem. This fly requires a fresh fecal pile to lay eggs, but most manure deposited indoors is trampled quickly. On pasture, a manure pat may remain undisturbed for a long time. Most economic losses due to flies are blamed on horn flies because they directly affect animal welfare.

The face fly is primarily a nuisance to beef animals. It is non-biting but feeds on saliva, tears and nasal secretions. Face flies spend most of their time off the animal in the grass and lay their eggs in fresh, undisturbed manure. Observed face flies are only a small representation of the total population, unlike horn flies which manifest in close to their actual numbers. Face flies are a contributing factor in spreading pinkeye.

Stable flies are the biggest problem in dairy operations. They prefer animals’ lower extremities including on and around the hocks. This fly spends very little time on the animal – after taking a blood meal, it travels to a resting area. Stable flies require several blood meals to reproduce.

It can be difficult to control stable flies because they spend most of their time around animals but not on them. The trick to managing this fly is understanding their reproductive habitats and identifying where they spend time.

Stable flies lay eggs in decomposing vegetative material including rotting hay, silage, used bedding, piles of grass clippings and fermented feed. Hay rings are notorious for harboring stable flies with a combination of rotting organic material and moisture. Refusal feed and moist conditions in barns and outdoors make it easy for this fly to reproduce.

“We can’t control the weather, but we can control rotting organic material, vegetation and leftover feed,” Prentice said. “Sometimes TMR material gets stuck in a bracket, and that’s an ideal place for rotting TMR which is loaded with goodies for stable flies. They lay eggs in there, so once a week, knock it out, and if that requires too much labor, treat it. The more we attack the 80%, the less we have to worry about the 20%.”

Calf hutches can be a source of fly breeding. Calves do a good job trampling bedding, which keeps it dry, but bedding around the hutch perimeter is subject to moisture from humidity or weather. If that area doesn’t dry out, it becomes stable fly heaven.

House flies are mostly a nuisance, but they can be a factor in spreading Staphylococcus aureus and E. coli. These flies are dubbed “filth flies” because they don’t care what they eat. They are not obligated to livestock, but farms provide the environment they prefer. House flies are active in most areas of the Northeast until the end of October.

Prentice compared the diameter of mouthparts among flies. A mosquito’s measures 16 microns; the horn fly’s is 92 microns. The stable fly bite is the worst at 230 microns, and its painful bite is the reason cattle toss their heads to try to escape the pain.

“Stomping and waving the tail take energy,” Prentice said. “It causes irritation and time that the animal isn’t eating or resting because they are spending time trying to get rid of flies. It’s small amounts, but small amounts over a large number of cattle over many days ends up being a lot.”

This is the rule of thumb for estimating fly numbers: 200 flies fit on the surface of the palm. If one biting fly takes in approximately 0.01 ml of blood, thousands of flies biting an average of 30 times a day adds up to 2.4 liters of blood being lost.

Because flies follow the 80/20 rule (with only 20% of a fly population being adults), it’s critical to have year-round fly control focusing on different aspects throughout each season. In most cases, spraying cures the immediate problem but not the underlying issue.

Fly management with chemical products is a matter of IPM, with the right product at the right time with rotation. Farmers should select products based on the difference between adulticide and larvicide treatments. Prentice suggested focusing on larvicides because they help manage the 80%.

Insect growth regulators (IGRs) are a good option for many farms. Timing is critical with IGRs and larvicide treatments, which should begin early in the season. Since it’s now past optimal time to employ such treatments, Prentice suggested a more aggressive plan with premises spraying and animal dusting.

“We’re never going to get all the flies,” Prentice said. “Get the majority and knock them down to where there isn’t an economic loss, but realize you aren’t going to get them all. Work toward an economic threshold.”

by Sally Colby