New strategies for fly control

MADISON, Wis. — Flies are a common nuisance on dairy farms. This pest is more than pesky, however. Flies are vectors that, if left unchecked, can introduce disease to a herd and cause milk production to plummet by 10%-20%.

During his presentation, “New Strategies for Fly Control in Dairy Farms” Oct. 2 at World Dairy Expo in Madison, Dr. Mike Catangui, entomologist and parasitologist manager at MWI Animal Health, discussed the dangers of flies and shared tips for controlling them on farms.

The stable fly and the house fly are two fly species found on dairy farms. The stable fly feeds on blood, using its syringe-like mouth to penetrate an animal’s hide and draw out blood. The house fly’s soft, sponge-like mouth feeds on liquid and fermenting materials, such as something sweet on the feed, Catangui said.

Though the house fly does not bite, it can carry many diseases such as Cryptosporidium, E. coli, Salmonella, and highly pathogenic avian influenza. The stable fly carries the bovine leukemia virus and Mycoplasma wenyonii.

“We see a lot of house flies among newborn calves,” Catangui said. “We don’t think too much about it; they’re just flies. But there’s a potential that those flies are carrying Cryptosporidium, for example, and anything that will cause scours in calves. They also might be accidentally ingested.”

The lifecycle for both flies is similar. They go through metamorphosis, starting as eggs that are laid on decaying or organic matter. Within 48 hours, they turn into maggots and remain at this stage for three weeks. From there, they become pupa and then adult pupa or the actual fly.

Catangui said a stable fly’s bite is painful and can cause direct blood loss. Each stable fly can consume 11-15 microliters of blood per feeding and can feed up to two times per day. Catangui said 233 flies on an animal would extract 7 milliliters of blood, resulting in a blood loss of approximately one-half tablespoon.

“These flies bite newborn calves, and the dark spots that you see on pens in nurseries is blood,” Catangui said. “It’s digested blood they excreted while waiting to feed again. This is a sign they’ve been feeding on those calves. Flies breed on the bedding and have everything they need to sustain their lifecycle.”

Biting flies also cause cows to stomp and switch tails continuously, thereby wasting energy. Painful bites also cause bunching in cattle and aggravate heat stress. In addition, Catangui said biting flies can trigger irregular feeding patterns, leading to bloat and digestive issues.

Catangui said flies are cheap to control, and the return on investment is significant.

Fly control products for dairy cattle come in various forms and include on-animal sprays, empty-barn residual sprays, house fly baits, aerosols, automatic misting systems, parasitic wasps, fly traps and larvicides. An effective program requires a combination of products that are determined through trial and error.

“Larvicides for the breeding site work for about six months or until calves are moved,” Catangui said. “They are very long lasting.”

Insecticide resistance can be a problem on dairy farms, however, Catangui said.

“In farms that have been using insecticides for many years, there might be some resistance, and we’re forced to change the way we control house flies,” he said.

A method for preventing flies from outwitting the program is real-time testing for resistance or susceptibility to the insecticides being considered.

“Test before you treat,” Catangui said. “Resistance is not permanent. Give the product a break, and within three years it’s good again.”

MWI Animal Health’s lab in Sioux Falls, South Dakota, offers free fly testing to MWI customers.

“We expose your flies to several products,” Catangui said. “Resistant flies look the same as susceptible flies. You wouldn’t know what kills them if you didn’t test.”

Catangui recommends combining what works and confusing the flies by promoting a more diverse line of active ingredients, formulations and application equipment. He said 50% of a spray’s efficacy is related to the application equipment.

The use of more natural insecticides is another method for combating resistance. Catangui said a product made from flowers — specifically pyrethrum plants — has proven to be an ideal insecticide. It kills flies quickly and efficiently and has no resistance issues even after many years of use. It can be applied anytime directly on live animals and the surrounding environment as often as necessary without causing harm to animals or leaving residues in milk or meat.

“It was the first insecticide known to man,” he said. “How come it still works? We think it’s because of the number of molecules in an extract of the flower. It contains six large molecules, and any one of them can kill the fly. The proportion of molecules is not the same, and flies cannot figure it out.”

Pyrethrins are the collective name of the six naturally occurring active insecticidal ingredients or esters found in the pyrethrum plant. Pyrethrin sprays come in oil-based, water-based and organic forms.

“This is an old concept, but we’re coming back to it because it’s a natural product,” Catangui said.

When discussing the economics of fly control, Catangui shared an example of controlling stable flies in milking cows using a mist sprayer and an oil-based pyrethrin insecticide. Using a U.S. average of 61.8 pounds of milk produced per cow per day multiplied by 10% — the minimum expected milk loss caused by flies — equals 6.2 pounds of lost milk. When that milk is multiplied by a milk price of $0.18 per pound, it equates to a loss of $1.12 per cow per day. The cost to treat in his example was $0.04 per cow per day, which factors in spraying twice per month for six months from May through October for a total of 12 sprays.

“That is a return on investment of $27.00 for every dollar you spend on fly control,” Catangui said.

Catangui also talked about the screwworm fly and the case of New World Screwworm that was confirmed in Nuevo Leon, Mexico, Sept. 21 — less than 70 miles from the U.S. border.

The screwworm fly has a metallic blue or green body and orange eyes and is larger than a house fly. It was eradicated from the U.S. in the 1960s.

“We know this insect well,” Catangui said. “It’s a very damaging insect that lays hundreds of eggs, and the maggots feed on live tissues.”

This fly lays eggs on any active wounds in an animal, and the maggots penetrate and feed on the animal from the inside out.

“If these flies lay eggs on a calf’s navel and you don’t do anything, this calf will die,” Catangui said. “The maggots will start feeding into whatever is in that area, which is pretty much the digestive system of the animal. It is a nasty, nasty insect.”

Catangui said the government has a plan in place to eradicate the screwworm fly again if it were to re-enter the U.S. The U.S. Department of Agriculture encourages immediate reporting of any suspicious wounds, maggots or infestations to a local accredited veterinarian, state animal health official or USDA veterinarian.