Summer Fly Control in Cow-Calf Herds.

 By Heather Smith Thomas.

The battle against flies is constant during warm weather, but there are several ways to reduce these pests.  Different flies have different habits and behaviors, so a combination of tactics is most effective.  Stable flies breed in rotting organic matter such as old hay, silage and other feeds, and bedding.  Horn flies breed in fresh cattle manure.  Horseflies/deerflies breed in wet areas or ponds, and black flies breed in flowing water, often many miles away, so it’s impossible to control them at their breeding sites.  In many beef operations, horn flies are the most irritating and costly parasite.

There are many ways to combat flies, including sanitation and physical removal of breeding sites, biological strategies like use of parasitic wasps and dung beetles, and chemical control with pesticides.  Often the best control is obtained with a combination of several tactics, according to Sonja L. Swiger, PhD (Veterinary/Medical Extension Entomologist, Texas A&M).
“In cow-calf herds the most problematic species is horn flies, though we also get a lot of calls about stable flies.  In Texas the stable flies emerge early if we have a mild winter,” she says.

“For horn flies we generally need to use integrated pest management tools.  These tactics include pasture rotation, leaving the fresh manure behind and going to a new site, so you are not overloading any one site with larvae.  You can decrease the flies even more if you also drag the pasture you just left, to break up the manure pats so they dry out faster.  Then there will be fewer horn fly larvae able to mature.  If the pats are still mounded up when you move the cattle, they develop a bit of crust over them and the larvae can continue to live inside them,” says Swiger.

The horn fly life cycle is short.  “It only takes about a week to go from egg to adult. Rotating pastures often, getting away from that manure, can make a difference, if a person has the space to do this.  Calving season may affect a rotation program, however, whether the herd is still calving and whether you can change locations.”

Some producers are trying to get away from using chemicals and pesticides, looking for other ways to help control these pests.  “One benefit of rotation, if you can leave the pasture long enough, is that dung beetles can get in there and tear down the manure.  They are our most beneficial insect, but you often don’t see them.  Many people are concerned that they don’t have any dung beetles—because they have decreased in numbers sometimes because of the use of pesticides.  There are still many there hiding under the manure.  We did some studies a few years ago, pulling up the manure and were surprised at the high number of dung beetles we were finding, but we had to actually remove the manure, to find them,” she says.

Some of the products used for parasite control (such as ivermectin) are harmful to dung beetles.  “New products are coming out that are more natural (and less harmful) and some have research to show they work, while some do not.  The impact on dung beetles is minimal unless you are over-using products.”

Chemicals
“We recommend waiting as long as possible in the spring to start using a chemical product; wait until after you can count 50 to 100 flies per side.  The reason to delay is because cattle can tolerate a few flies and we want to wait until control is more important.  In some areas (and with some types of cattle) the animals can handle higher loads than others.  If your animals start to look uncomfortable because of flies, it’s time to treat them.  This also makes sure there are more adult flies present at the time of treatment.  Most products applied to the animal will only kill the adults, and you want the biggest impact for your efforts,” she says.  You can knock down the population that would soon be laying eggs in the manure.

“The nice thing about control for horn flies is that they pretty much live on the animal (compared with other biting flies that are only there long enough to get a blood meal) so anything you put on the animal will get to them.  Just make sure you use a product that will kill them, because they quickly develop resistance to the chemicals we use.  Unfortunately we are limited in products to rotate.  We recommend a rotation (to minimize resistance), such as switching between an organophosphate and a pyrethroid, but the only way we can now use organophosphates is in ear tags,” says Swiger.

“Not everyone uses ear tags, however.  I’ve done a lot of research that shows they are effective, and one of the better options, but there are times they don’t work—if the population of horn flies is resistant to that product.  And when the population gets really high you can’t just rely on one product.  We usually recommend using several methods—like a pour-on as well as a feed-through product, or combine one of those with ear tags.  The tag companies recommend two tags per animal, and research results concur.  It helps to use multiple applications, often,” she says.

The feed-through products end up in manure and contain insect growth regulators which affect the larvae, and they don’t mature to become flies.   Swiger advises producers to use a feed-though as well as a topical product, to hit both stages of the life cycle.  Feed-through products are the only option for impacting the larvae.

Some producers also use back-rubbers, oilers, dusters, etc. for cattle to self-treat.  “The cattle can use these in a walk-through to water, for instance, and do a daily treatment,” says Swiger.  This can be a good addition to any control program but may not work as a stand-alone strategy if some of the animals don’t use it or the product runs out.

“Read labels before applying products to calves.  Most are not labeled for use on calves.  Young calves get their protection from the cow.  If the cow is treated, and has fewer flies, her calf won’t have as many flies either,” she explains.  Calves may also get some benefit from topical products on the cow, since they are always rubbing up against mom while nursing.  Some of the fly tags can be used on calves, if they are over a certain age/weight.

Fly Traps
There are several kinds of fly traps and some work best for different types of biting flies.  “There are no effective traps for horn flies.  Most of the traps work best for horse flies and deer flies, and there are also some traps for stable flies (sticky traps they land on).  Those flies take a blood meal and leave, whereas horn flies never leave the host animal.  Traps for house flies don’t work on any of the biting flies because they are attraction baits; they don’t use the same food source,” she says.

The only trap that removes horn flies is a cow vacuum.  “It’s an enclosed area that the cows walk through and it blows the flies off the animals and sucks them up into a bag.  It’s very expensive, and used mainly in dairies, since it has to be set up somewhere that the animals have to go through it.  It also utilizes electricity and you’d have to find a way to power it.  It might work in a feed yard but rarely in a pasture situation.”

This trap has a bag that collects the flies.  “After 6 hours with no blood, a horn fly will die, so if they end up in the bag they die off.”

Resistant Cattle
Some cattle are more attractive to flies than others; there are usually a few in the herd that have fewer flies.  Some producers are selecting and breeding for fly-resistant animals.  “One of the issues is that often the cattle that have the most fly resistance (Zebu-based breeds) don’t have the best traits for meat production.  Angus seem to be the breed most people want and they are not very resistant to flies.”

There are usually some animals in a herd that attract more flies than others, and bulls attract the most flies.  “This seems to be due to testosterone levels, but we don’t really know why,” says Swiger.  There are individual differences; some animals have no resistance and need more help to control their flies.  Sometimes color is a factor (more flies on dark-colored cattle) but not always.  

A few cattle breeders have added horn fly resistance to their criteria for seedstock selection.  One of the first to do this was Kit Pharo (Pharo Cattle Company, Cheyenne Wells, Colorado).  He has been working on genetic resistance to parasites for a long time, and has been selecting for horn fly resistant cattle for 20 years. Within any breed, some individuals are much more resistant to horn flies than others.   Pharo now offers “low fly” bulls that pass this genetic resistance to their offspring.  

He says that evaluating and scoring fly resistance is not difficult.  “We simply assess how many flies each animal has.  There is often a big difference between animals within the same herd.  While a few animals are black with flies, others will have very few flies.  In the cool morning hours, flies will be on the animal’s back and easy to count.   As the day warms up, flies tend to move to shaded areas of the belly.”

The use of the fly resistant genetics works by keeping the resistant heifers as herd replacements and at the same time culling the mature high-fly cows.  Pharo recommends mating the fly-resistant replacement heifers to a different fly-resistant bull.  

Horse Flies, Deer Flies And Black Flies
These pests are very location-based.  “Many eastern states are more likely to have large black fly populations in late spring and summer months.  Black flies are associated with moving streams and are hard to control.  Many states that experience high numbers of black flies treat the known water ways with Bti (a natural insecticide often used to control mosquito larvae) to lower their populations. In other states, like Texas, black fly populations are only present in small areas and typically just in the same areas from year to year when there is rain.  Control measures for adult flies are not usually an option for most people, and difficult to come by,” says Swiger.

Horse fly and deer fly populations are similar; when there is more water/moisture we tend to see higher numbers.  “Their larval habitat is typically associated with the margins of water bodies (their larval stage is spent in mud). Traps are very effective at controlling large populations of horse and deer flies and sometimes the animals can gain relief with the use of pour on products,” she says.