Parasitic wasps can be used with insecticides to help keep fly numbers down in feedlots and other confined livestock facilities.

There are three ways to control stable flies and house flies in a confined livestock facility: sanitation, sanitation and sanitation.

But since we know that the best sanitation practices won’t totally control pest problems, there are several other things that we can do to improve the efficacy of individual products and procedures.

First of all, before any program will be effective, the person doing the fly control must understand the problem and how to manage it. It is a known fact that 90% of house and stable flies develop in less than 10% of the area occupied by a livestock operation. If those areas can be identified and eliminated, fly numbers will be much easier to manage.

For example, studies have shown that more than one million stable flies can develop in one round bale feeding site that wasn’t cleaned up after the winter feeding season ended.

We will always have flies in livestock facilities, but they can be managed through an integrated pest management (IPM) program.

Education about the pests and where they develop, coupled with timely releases of parasitic wasps (parasitoids) and carefully managed insecticide treatment applications, should provide satisfactory suppression of the pest flies.

Though parasitic wasps are very sensitive to insecticides, their “lifestyle” lends itself to the use of pesticides in the same facility with proper scheduling and management.

Generally, parasitoids are not placed where populations of adult flies are located. Rather, they should be placed near areas where fly larvae (maggots) develop because the larvae crawl only a short distance to pupate. And it’s these fly pupae that the parasitic wasps attack and kill. First, the wasp stabs a fly pupa and lays an egg inside the case. Then the developing wasp larva eats the pupa.

Each feedlot or dairy is different, but a good rule of thumb is about 300 to 500 parasitic wasps per animal. Weekly releases during the fly season are recommended for best results.

If heavily infested fly larval sites can be identified and cleaned up via sanitation, the frequency and number of parasitoid releases and insecticide applications can be decreased.

Instead of mass spraying facilities, spot treatments of insecticides can be used successfully where adult flies tend to congregate.

Insecticide resistance, especially in the house fly, has become a major issue. Most insecticides used today are pyrethroids, a class of chemicals that have low toxicity to humans and other mammals and, therefore, are safer to use around livestock.

In studies conducted at four north-central Texas feedlots with herd sizes ranging from 30,000 to 100,000 cattle, house flies were collected and screened for resistance to some common pyrethroids approved for feedlot use.

Two of the yards practiced some IPM, while the other two did not. In the non-IPM yards, some of the commonly used older products showed less than 20% control at the labeled rate. All the pesticides tested performed significantly better at the feedlots using IPM methods, with a top control rate of about 75%.

But 90% is the normal accepted rate for effective control; and you’ll likely only achieve that with a well-developed and carried out IPM plan. Such a plan will help keep resistance down and allow us to keep some of our pesticides available for use as needed.

Taking into account the increased level of resistance to many of our effective pesticides coupled with the inability of beneficial insects to suppress heavy fly populations, both methods of fly reduction should be incorporated into a fly management program.

At the same time, improving your facility’s sanitation methods and practices will help keep parasitic wasp releases and chemical treatments to a minimum, which, in turn, will improve your bottom line and make your employees and neighbors happier.

(Photo courtesy Tom Spalding)