Larvicides Lend a Helping Hand
By Caitlin Richards
The middle of summer means longer days and higher temperatures. In most areas, it also means the peak of fly season. Cattle have more than likely been combating this annoying pest for several months now, but we hope they have had the help of their owners’ fly control methods.
Ear tags and pour-on insecticides are effective methods, but they primarily take care of adult flies. The best way ranchers can help their livestock is by attacking flies at different points in the lifecycle. Using a feed-through larvicide can help cattle herd owners do so.
“The name larvicide is descriptive,” says Larry Hawkins, senior technical service veterinarian with Bayer Animal Health. “We use insecticides. We use herbicides. The suffix ‘cide’ means something got killed. In this case, the target is the larval stage of the fly.”
Flies, Hawkins explains, are similar to butterflies when it comes to lifecycle stages. Adult flies lay eggs from which the larva hatch. The larvae live for a week or so before spinning themselves into a cocoon to go through maturation stages. After 7 to 10 days, an adult fly emerges from the cocoon.
Of the various fly species, the horn fly and face fly reproduce in fresh cow manure, making them the primary targets for a larvicide. Once their eggs hatch in the manure the larvicide kicks in and exterminates the larva. Hawkins explained that this disrupts the fly’s lifecycle and decreases the fly population in the cattle’s environment.
“Out in pastures, doing something on the cattle is the only place where we can control horn flies and face flies,” Hawkins says. “It [the larvicide] works in the manure pat. It’s best to do something on the cows like a pour-on or ear tag, and then feed them something to kill the fly larva in the manure.”
Larvicides are considered a feed-through product. Most come as a powder containing rice hulls. The product can be easily mixed with mineral or feed. Hawkins explains that once the cattle consume the mineral or feed containing the larvicide, it passes through the cattle’s digestive system and into the cattle’s manure without being absorbed. Hawkins says this process takes about 48 hours.
Larvicides are designed to stay in the manure for about 6 weeks, ensuring that any fly larva emerging during the manure’s freshness period will be exterminated.
Ranchers will see the most reward from using a larvicide if they start using it before the fly season even starts. For most of the continental U.S., Hawkins says, the ideal time to start feeding a larvicide is in late April to the middle of May.
“That way, it can be there for the whole summer and into the peak of fly season,” Hawkins says. “If a producer doesn’t start early in the season, larvicide can still be added. It will start killing larva in about 48 hours. The earlier, untreated manure will still support fly larva for a few weeks, but the benefits will be seen in new manure pats.
“Every time that producers put out mineral or feed (with larvicide mixed in), they are also treating the fly larva,” Hawkins says. “If they put ear tags on the cattle, they are killing the adults. In all, they are supporting a good fly control program by attacking the life cycle of the fly at two points.”
Producers can have suppliers mix the larvicide into their mineral or bulk feed or add it in themselves on a per bag basis. Hawkins suggests mixing the larvicide with the mineral or feed in a cement mixer or a trough.
When feeding a larvicide, Hawkins says, it is important to pay attention to the rate of consumption among the herd. Some cattle within the herd may not eat as much mineral or feed due to weather or preference. This creates an uneven dose across the herd and, therefore, uneven larvicide coverage in the manure.
“If producers notice their cattle are only eating about half the amount of mineral in 2 weeks that they should be, they may need to adjust the amount of larvicide,” Hawkins says. “They can supplement the amount of larvicide to get the right dose into the cattle.”
To help ranchers calculate their herd’s mineral consumption, Hawkins suggests using the Rabon Calculator smartphone application designed by Bayer Animal Health. The app, he explains, will calculate how much the cattle are consuming. Ranchers can then compare the app’s calculated amount to the dose to ensure they are meeting the recommended dose per animal.
When ranchers start to think about their fly control plan for the year, a larvicide may seem like an added cost, but the potential gain is worth it. Hawkins states that it is economical and there is firm research out there that ranchers can afford to put larvicide out to support ear tags and pour-on products.
Achieving optimal fly control on a nursing cow, Hawkins explains, can result in adding 12 to 15 pounds to the calf’s weaning weight. The same is true with stocker animals, with about 10 to 15 pounds of gain under optimal fly control, he says.
“With that in mind, we know that if we control the flies we will see a difference in gain,” Hawkins says. “Simple mathematics let us know we are going to make some money by doing some fly control.”
Compared to other fly-control methods, adding larvicide may be one of the easiest methods to incorporate into a fly control plan. By simply pouring a bag of mineral, the fly population surrounding their herd can be greatly reduced, even if ranchers have adult flies intruding from neighboring areas.
“You may still see adult flies, but the point is, you are doing something in your herd that will reduce the population where you are,” Hawkins says. “You can’t control what your neighbor does, but you have reduced the population within your own property.”
Larvicides may only attack one stage of the fly lifecycle, but the same is true of ear tags and pour-on products. Together, multiple fly control methods strengthen a producer’s overall fly control and can result in significant gains.
Larvicides Lend a Helping Hand is excerpted from the July 2018 issue of The Cattleman magazine.