KMG: Horn Flies Meet Resistance of Their Own As Field Trial Tests Major Insecticide Ear Tags

Horn flies are a perennial source of discomfort and annoyance for pastured cattle, and a constant challenge for producers trying to control them. One expert thinks cattlemen can be part of the problem, sometimes handicapping their own efforts by “doing dumb things” that enable horn flies to exercise their inherent knack for building up resistance to insecticides – like tagging every second or third animal in a herd or leaving tags on too long so dosage levels drop.

However, entomologist Elmer H. Ahrens, who has just finished a comprehensive competitive insecticide cattle ear tag field trial involving nine ear tag products, is quick not to lay the blame squarely on producers.

Ahrens admits beef and dairy cattlemen are facing off against hardier breeds of horn flies that have survived just about every chemical cattle and dairy producers have thrown at them. Even with the highly effective pyrethroids and organophosphate products, cattlemen are definitely seeing increased fly resistance.

During a 40-year career, including stints with the USDA’s Animal and Plant Health Inspection Services and the Agricultural Research Services, Ahrens has studied and tested numerous chemical compounds to control insects, starting with the first cattle ear tags back in 1970 originally designed to control screw worms and ear ticks. “At the end of the day,” he concludes, “you have to realize we’re never going to conquer insects.

“We’re killing the susceptible ones while the stronger ones survive and pass the survival genes along to their offspring,” adds Ahrens, alluding to the idea the fittest do indeed survive. The result is the continual emergence of new generations of super flies that cost the cattle industry upwards of $700 million a year in reduced weight gain, slower growing rates, higher feed outlays and a myriad of health problems.

Change in Momentum

However, new chemistries, along with creative rotation strategies and new application techniques appear to be loosening the grip on the upper hand flies have held in the continuing struggle between producers and parasites. In particular, Ahrens acknowledges the positive influence veterinarians have had lately urging dairy and beef producers to switch tags and rotate chemistries. Additionally, he has been encouraged by the performance of endosulfan, a chemical class of pesticide never before used in animal health.

In his latest ear tag trial, one of the most comprehensive tests ever undertaken (See Horn Fly Trial Chart), Ahrens assembled ten herds, the largest being 40 head and the smallest 26, with two tags per animal (per label directions), except in one case where the herd received only one tag. He also has an untagged control herd. He used nine different insecticide cattle ear tags whose primary pesticide ingredients included either pyrethroid or organophosphate compounds or combinations and a new compound for controlling horn flies on cattle – endosulfan, first introduced by KMG Animal Health under the brand, Avenger™ last spring.

“The weather conditions overall were excellent,” states Ahrens, meaning lousy if you were raising cattle or planning a vacation in the Southwest Texas area, but ideal for conducting a fly control field test. Lots of rain left the region moist enough to propagate huge fly populations; but a dry spell in the middle of the summer mitigated the chance of the manure pats, where the horn flies breed, from being washed completely away. “If it gets too wet, the pats could be destroyed and the fly numbers would drop,” says Ahrens.

Texas is generally a good place for such trials because climate extremes come with the territory and heavy fly pressure is virtually a given. The combination of moist and dry weather swings is one reason for the large fly populations, but entomologists John Riner, PhD, technical manager for KMG Animal Health, offers another: “The weather conditions and horn fly propagation situation in the southwest Texas region has attracted many other insecticide fly tag manufacturers to set up trials and work with Ahrens because his flies seem to be tougher.”

The numerous trials have exposed horn flies in southwestern Texas area to a variety of pesticide products, and while many flies have been eliminated during the process, many others have survived and passed along their resistance traits to their progeny – paving the way for an inevitable sequel of tests, only against fly generations that seem to get hardier each year. In other words, fly control manufacturers know that if their products work in Texas, they’ll probably work better someplace else where conditions are not so harsh.

“We run a competitive trial here annually,” Dr. Riner confirms. “However, the trial this past summer – focusing on the effects of tagging the animals with two devices – was the most wide-ranging, exacting test to date, and designed with strict guidelines and methodology.”

Trial Protocols

The cattle breeds involved in the study were highly similar and the herd size was at least 20 head. All animals were tagged at the same time according to manufacturer’s guidelines (See Chart). Additionally, the geography was consistent with the topography stretching from lowlands to highlands so that all the test cattle were tested under the same conditions under the same pressure. The trial also called for herd spacing of at least half-a-mile with the pastures and property separated by a road. There were no shared fence lines that could lead to cross contamination.

“In our trials,” explains Ahrens, “we like to create a realistic scene of what a typical rancher with his 20 to 40 cattle might encounter. Each herd came under heavy pressure from at least 1,000 flies per animal. At some points during the course of the trial, especially because of the extraordinarily wet conditions followed by a dry spell, the fly pressure was actually topping 2,000 per herd,” Ahrens stresses.

Insecticide Fly Tag Trial Conclusions

·After nine weeks, Saber Extra was at 0% control.

·After 10 weeks, Double Barrel was at 23% control.

·After 11 weeks, CyLence Ultra (40% control); Double Barrel (16%); C0-Ral Plus (46%) were under 50% control.

·After 13 weeks, all insecticide ear tags except Avenger (2 tags per animal or 1 tag per animal) and Stocker had dropped to 0% control or were under 50% control.

·After 17 weeks, only Avenger with 2 tags per animal provided 96% control and Avenger with 1 tag per animal provided 70% control. All other tags in the trial were at 0% control or less than 2% control.

“After 17 weeks, only Avenger was still controlling flies at a rate of 96%,” Ahrens reports. “Even the herd with just a single Avenger tag per animal was warding off flies at a 70% pace. By contrast, the effectiveness of the other tags fell off precipitously by only the 13th week. These results showcase the importance of developing new classes of chemistry to fight flies because the older chemistries are quickly losing effectiveness, especially where resistant horn flies are a problem.”

Underscoring his concern about resistance, Ahrens says cattlemen used to get five months of fly control when the synthetic pyrethroids and organophosphates products were first introduced. “Now the control lasts only two or three months, making it all the more imperative that the industry discover and develop new chemistries that beef and dairy cattle producers can incorporate into rotation strategies that are so important to fly control.”