Conditions are Ideal for Significant Fly Populations on Cattle

For the past several years, fly populations on cattle have been significant beginning in late April then increasing to peak populations in August or September. This year is slightly different with the cool temperatures from April through May that slowed the fly populations down. 

However, with the higher humidity and warmer temperatures setting in most of Oklahoma these conditions are prime for both horn fly and stable fly populations to explode over the next several weeks. 

Horn flies are the most significant external parasites of cattle causing an estimated $1.8 billion impact on the cattle industry as a whole annually.  Although this fly is small and feeds from the back down the side and onto the belly of cattle it is the sheer numbers of these flies on a per animal basis that cause stress to cattle. 

Over the past two weeks when observing cattle in pastures the average horn fly count per head was well over 400 flies per animal.  This is double the treatment threshold of 200 flies per animal. Considering the number of times an individual horn fly will feed on an animal throughout the day is 25 times per day then those numbers will certainly cause significant stress to cattle. In addition, the number of horn flies transferring from larger animals such as cows or bulls to calves is at a higher rate than in the previous months. 

Calves are averaging around 150 horn flies per animal in the herds observed over the last two weeks. In a cow – calf system the horn flies on the cow or heifer impact weaning weights in calves due to the impact of that stress causes decreased milk production but when combined with direct fly numbers on the calf then the impact can be greater. 

Stable flies are usually a problem in cattle from March through early May but with the cooler April and May temperatures then this fly is affecting cattle currently.  Although this fly usually decreases when temperatures increase from June through August it is the combination of a cooler spring with ideal humidity for this fly to be more prolific than in previous years. 

The stable fly preferably feeds on the front legs of cattle and occasionally on the belly of animals.  Their bite is very painful to cattle causing significant behavioral reactions such as leg stomping, bunching and standing in water for long periods to avoid the bites. 

One of the preferred breeding sites for stable flies are old hay feeding areas that never dry out or retain moisture for several months.  The significant winter temperatures caused cattle operators to feed more hay and these areas are supporting stable fly development.  In fact, a typical ring / round bale feeding area can provide sufficient habitat for approximately 60,000 stable flies per week.

Treatment thresholds for stable flies are 10 flies per animal when observing just the front legs of animals.  Last week the average stable fly count per cow was 15 with calves having approximately eight stable flies per calf.  These numbers will cause significant stress to both the cow and calf but the stable fly populations will decrease if humidity levels begin to decline with consistent temperatures above 90°F. In a typical year, there are two distinct peaks of stable fly populations with one occurring in early April and another one occurring in late September.  The cooler spring temperatures has delayed the spring peak into June.

Regardless whether cattle are dealing with horn flies or stable flies these are on cattle herds across Oklahoma. This means cattle are not only dealing with one fly pest but also both fly pests and they both rely on blood meals as their main food resource. Cattle will need some relief from the biting activity from these fly pests now and the quickest method are insecticides applied directly to the animals. 

For stable flies, it is best to find their breeding habitats (hay feeding areas) and clean them up or pull some type of implement through those areas so they can dry out.  Consider that a hay feeding area may not look conducive for fly development but if you walk on that area and there is moisture seen beneath your feet then it can support stable fly development.  For an insecticide application, the best method is to spray the legs, brisket and belly areas with a product that is labeled for on-animal use. 

Since cattle will get their legs wet from laying in pastures or walking through water then applying the insecticide with a diesel oil as the carrier will persist a little longer than one applied with water.  Cattle producers will have to use a product that can be mixed with diesel and utilize a sprayer with pumps designed for diesel not water.

For horn flies, there are many different options to control this pest.  One of the most popular applications for horn fly control are pour-on products.  If using a pyrethroid pour-on be sure that it is synergized with piperonyl butoxide (PBO).  The synergist inhibits enzymes that insecticide resistant flies can develop to detoxify the insecticidal compound.  Insecticide impregnated ear tags are still a viable option for longer horn fly control but operators will need to rotate the type of product used every year to limit insecticide resistance. Rotating products is not based on trade names but by the chemical class the product belongs in.  For instance, a product can be in three broad chemical classes based on its mode of action (the manner in how it kills the fly based on target site) and products labeled for on-animal applications are either pyrethroids, organophosphates, or macrocylic lactones. 

Veterinarians or country extension personnel can help identify which chemical class a product belongs to based on the active ingredient listed on the product label if assistance is needed to identify the chemical class.  Sprays are still a good option to provide some relief to cattle with heavy horn fly infestations and the spray should be applied as a course spray that is directed at the backs, sides and belly of the animal. 

Insect growth regulators (IGR) that are feed through mineral are a very good option when combined with other control applications whether it be ear tags, pour-ons or sprays but producers will need to monitor consumption to be sure that the herd is averaging ~4 oz. / hd/ day to prevent flies from developing from the manure pats. Self-application devices such as oilers are a good option but you will have to either make it a force use system or put multiple oilers out to ensure the whole herd is treated properly. 

A typical force use system is setting it up where cattle have to go under the oiler to get to water or a feed area. Regardless of the type of insecticide application chosen it important to get cattle treated to provide some relief from the biting flies so that cattle are not stressed especially when you add the additional heat stress that becomes a factor with high heat indices.