Cattlemen are, by and large, aware of the economic losses associated with internal parasites. With regards to external parasites, grubs, lice and ticks often get our attention but it is not uncommon for us to ignore or diminish the problems of biting flies. Nuisance flies, such as house flies and face flies, can affect cattle to a lesser degree, but horn flies in sufficient populations can significantly reduce animal performance and economic thresholds may vary greatly depending on the body condition of the cattle and other stresses they may be exposed to at any give time.
Sound management practices are paramount to good parasite control as there are fewer pesticide products available and resistance issues have compromised the effectiveness of certain classes of pesticides.
Resistance to certain pesticides by insects that have multiple generations each year (like horn flies), creates challenges in developing an effective control program. “Cutting corners” or trying to cheapen a control program by using one ear tag rather than two, not replacing ear tags as suggested, or letting the back rubbers run out of pesticide, does nothing but help develop resistance in the fly population and makes control in subsequent years ineffective.
Resistance is also more likely to build in a population of flies if a producer does not frequently rotate between the classes of available pesticides.
The two common classes of pesticides for horn fly control on beef cattle are the pyrethroids and organophosphates. Rotation between these two classes of pesticides will slow and reduce the development of resistance. A new class of pesticide, spinosyn, has become available for use as a spray or pour-on, and could be included in a rotation. It should be noted, however, that control of resistant genes in a fly population, once it is developed, seldom occurs.
Horn flies are the most abundant biting flies found on beef cattle during the spring and summer months. Both male and female take blood from the host and feed 20-30 times per day. They stay on the animal continually, leaving only to lay eggs. They feed mostly on the backs, sides, bellies and legs of cattle. Populations begin building in the spring and last until first frost.
As few as 14 days are required for a complete generation, The adult female fly must lay her eggs in fresh cow manure. The eggs hatch within 48 hours into first instar larvae which feed in the manure pat and progressively grow into second and third instar larvae.
Horn fly larvae develop only in fresh cattle manure. Third instar larvae crawl from the manure to a drier area and pupate. The adult fly will emerge from the pupal case and seek an adult host, typically cattle. During mid-fall, the adults do not emerge from the pupal case and the horn fly spends the winter in the pupal stage.
Horn fly control is typically recommended when the number of flies per animal is estimated to be two hundred. This population is generally reached in late spring, but will depend on weather conditions. Some of the more popular control products include insecticide impregnated ear tags and ready-to-use pouron formulations. Other methods for applying pesticides would include sprays, backrubbers, dust bags and feed-through additives that contain an Insect Growth Regulator (IGR).
Some application methods are obviously more labor intensive than others. For instance, pesticide sprays must be applied thoroughly and cattle must be gathered to achieve good coverage. Pesticide sprays generally provide only about three weeks of control. Materials applied by backrubbers and dust bags give good control as long as the devices are properly maintained. Cattle must use them frequently to be effective. Regardless of the method of control, rotation between insecticidal classes is important to manage the buildup of resistance in horn fly populations.
The suggestion is to rotate every year or every other year between a pyrethroid or an organophosphate control product.
Never use the same class for more than 2 years in a row, and use label rates.
Endectocides, which have become popular for use in controlling internal parasites, will provide 4-5 weeks of horn fly control, but would be fairly expensive as the primary means of horn fly control.
Often mineral or feeds contain a feed-through product (IGR) that can be effective for horn fly control by interrupting the life cycle in the manure pat. IGR products inhibit the larvae from developing into the pupal stage.
Feed-through additives are only effective when cattle are consuming the required amount of the IGR product. They work best when non-treated cattle are not within close range. If untreated cattle are in close proximity (across the fence or in a neighbor’s pasture), the fly population will exchange from one herd to another. It has been suggested that horn flies may move up to one mile to seek untreated hosts.
It could be that for effective control, you and your neighbors may want to coordinate horn fly control measures to ensure that resistance does not become a problem.
Food for Thought:
Does the fountain of youth and the fountain of knowledge come from the same fictitious underground stream?
