WHY IS INSECTICIDE RESISTANCE AN ISSUE IN HORN FLIES?

by: Dr. Nancy C. Hinkle and Annie Rich, Department of Entomology, University of Georgia and Joe Burnsed, UGA Extension Walton County

What is insecticide resistance? It's the inherited ability of flies to survive an insecticide dose that would kill the majority of flies in a normal population.

How does insecticide resistance differ from immunity? We all know immunity: We get shots to protect us from diseases (such as the flu and measles). These injections contain small amounts of dead virus, enough to fool our immune system into generating antibodies to protect us if we are ever confronted by the real virus. Immunity works on an individual basis: Throughout its life, an animal develops immunity when it is exposed to disease agents, and this immunity is not passed on to future generations.

By comparison, resistance is a whole-population phenomenon. Individuals in any population differ in their susceptibility to a chemical. For flies, this variation in susceptibility to an insecticide means that when the population is treated with a particular insecticide, most of them are killed. However, the survivors' genes get passed on to their offspring so that they are able to withstand the insecticide. In each following generation, the flies least able to endure insecticide exposure get killed, leaving only the 'tougher' survivors to reproduce, meaning that each generation grows more resistant to the insecticide they are exposed to. The weak die off, leaving stronger, less susceptible flies for producers to try to kill.

Here in the Southeast, the horn fly season starts earlier and runs later in the year than almost anywhere else in the country; only our neighbors in Florida and herds in south Texas have longer horn fly seasons. Horn fly numbers on herds are also much higher here than elsewhere. Georgia's heat and humidity are ideal for horn flies, allowing their populations to develop early, grow to high numbers, and persist late in the fall before being killed off by hard frosts. Typically, most Georgia horn fly populations are gone by Halloween, but in 2016 most of the state was still dealing with horn flies at Thanksgiving.

These long fly seasons mean that horn fly populations in Georgia have greater opportunity to develop resistance.

Because Southeastern horn flies have more generations per year and more flies per generation, the insecticide resistance process operates faster. While Michigan may have only five horn fly generations a summer, here in Georgia we have 15 generations every year. At the end of the fly season - November in Georgia, but August in Michigan Georgia flies are more resistant to the insecticide than their Michigan counterparts.

So it's a numbers game: Higher numbers of flies exposed to insecticide increase the chance that a few will be able to tolerate the exposure.

Georgia cattlemen lose more than $14 million every year to horn flies. Since horn flies negatively impact cattle weight gain, we still need to control flies on the herd. But how can we minimize development of insecticide resistance to ensure that we will have effective fly control products in the future? The strategy here is rotation - using chemicals with different modes of action to prevent flies from being repeatedly exposed to pesticides that work the same way.

For instance, there are only three types of insecticides in ear tags -- pyrethroids, organophosphates and abamectin -- so rotation among insecticidal ear tags depends on only these three groups. Sprays, dusts and pour-ons also include pyrethroids and organophosphates, so they don't contribute any options for rotation.

Fortunately, there are other materials that are formulated as feed-throughs, including diflubenzuron and methoprene, which are insect growth regulators that have completely different modes of action from one another and from the previously mentioned groups found in tags, dusts and sprays. Feed-throughs are formulated to be ingested by the cattle, either as blocks or mixed in the feed or loose minerals. These insect growth regulators pass through the cow's digestive system without being digested (never passing into the blood or muscle) and leave the body in the feces. Because horn flies develop only in fresh cow pats, this allows the insect growth regulator to kill any fly larvae in the manure. Note that because these products do not get into the blood, they have no impact on adult flies, so these products are for control of maggots that would grow into future fly populations.

To manage adult horn flies, we can use pyrethroids, organophosphates, or abamectin tags. But because ear tags work for only four or five months, we recommend delaying ear tag installation until May or June. Early in the season, before horn fly numbers have gotten high, we can suppress them using a feed-through. At the end of the season, we may need to supplement ear tags with a spray, dust, pour-on or other method until cooling weather suppresses horn fly numbers.

Pyrethroids and organophosphates are available in various formulations and constitute the majority of available products, so it is important not only to rotate by product name, but also to ensure that the way the active ingredients work is different. Insect growth regulators fit into some rotational schemes, although they will not prevent adult horn flies from flying in from surrounding properties; IGRs only prevent horn flies from developing in treated manure. The macrocyclic lactone, abamectin, is available in ear tags and offers another mode of action to rotate to from pyrethroids and organophosphates.

There is no easy solution for horn flies, and Georgia cattlemen bear the brunt of this challenge with the state's long growing season. However, with rotational use of products and proper timing of applications, horn flies may be effectively killed for multiple generations. The University of Georgia's Veterinary Entomology Program appreciates research and Extension support provided by the Georgia Agricultural Commodity Commission for Beef Additional information about horn fly control is available through the Georgia Pest Management Handbook and your county Cooperative Extension office.







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