Oklahoma cattlemen are constantly aware of the potential of external parasites to cause economic loss and transmit disease. External parasites of primary concern from an economic perspective are biting flies, grubs, lice and ticks. Nuisance flies such as house flies and face flies, can affect cattle to a lesser degree. Generally it takes a large population of external parasites to have an economic impact, but that economic threshold may vary greatly depending on the body condition of the cattle and other stresses they may be exposed to at a given time.

Producers must use sound management practices in regard to 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, such as horn flies, creates challenges in developing a control program. Resistance is more likely to build in a population if producers do 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 the development of resistance in the fly population. A new class of insecticide, 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 “removal” of resistant genes in a population, once they are expressed, does not normally occur.

Horn files are the most abundant biting flies on beef cattle that are present during the spring and summer months. Both male and female flies take blood from the host and feed 20-30 times per day. They stay on the animal continually, only leaving to lay eggs. They are found most often on the back, side, belly and legs of cattle. Populations begin building in the spring (as early as April) 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 a suitable host, typically cattle. During the 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 to start on cattle when the number per animal is estimated to be two hundred. This population is generally reached in late spring, but will depend upon weather conditions. Some of the more popular control products include insecticide impregnated ear tags, and ready to use pour-on formulations. Other methods for applying pesticides would include sprays, backrubbers, dust bags and feed-through additives.

Some application methods are obviously more labor intensive than others. For instance pesticide sprays must be applied thoroughly and cattle must be gathered to expect good coverage. Pesticide sprays generally provide only about three weeks of control Materials applied by backrubbers and dust bags give good control as long at the devices are properly maintained. Cattle must use them frequently to be effective. Regardless of the method of application, rotation between insecticidal classes is important to manage the buildup of resistance in horn fly populations.

Endectocides which have become popular for use in controlling internal parasites will provide four to five weeks of horn fly control, but would be fairly expensive as the primary means of horn fly control. Often mineral or other such products contain a feed through product which is an insect growth regulator (IGR). These can be effective for horn fly control by interrupting the fly life cycle in the manure pat. Larvae are not able to complete their development to the pupal stage. Feed through additives are effective only when cattle are consuming the required amount of the product. They work best when non-treated cattle are not nearby. If untreated cattle are close, the fly population will exchange from one herd to another.

Other biting flies that can become problems on cattle are Stable flies, Horse and Deer flies. Stable flies are generally more of an issue in feedlots or other confined areas, but can sometimes become problems in pastured cattle in May and June. There are several species of horse flies in Oklahoma. The females are vicious biters and populations usually peak from midsummer through September. Since they do not remain on the cattle for any length of time, it is very difficult to get enough pesticide on them to be very effective in controlling the population.

Source: Bob LeValley