using wasps to control flies

Fly parasites are a natural non-invasive way to help manage pesky fly populations.

Flies and horses (well, horse manure!) go hand-in-hand. Good management of your barn and pastures will go a long way towards keeping your facility fly free, but if you need a little extra help, fly parasites or wasps could be your answer!

These tiny wasps can play an important role in your fly control program. They use house fly and stable fly pupae as packaged meals for their developing young. The process results in more wasps and fewer flies. Almost certainly there are already some beneficial wasps around your stable – they are barely visible to the naked eye and hang around where flies develop, so they are typically unnoticed. Usually there are too few to have a major impact because of the tremendous reproductive rate of flies. Commercially purchased “fly predators” or “fly parasites” can even the numbers game, and the species available should be more effective than those that are naturally present.

Will Wasps Work for You?

While wasps can be a successful addition to fly control programs, there are some important questions and realistic expectations to consider:

What kinds of flies are causing problems?
Stable flies and house flies are good targets for these wasps because they breed around barns and stables. Face flies, greenheads and other horse flies do not, so the wasps are not effective against them. Are there other fly “sources” around? Even the best fly control programs are unlikely able to cope with large numbers of flies moving in from other areas.

Are neighboring sources of flies likely to cause problems?
If so, have an alternative plan ready.

How is your general sanitation and waste management program?
Fly control with natural enemies is a numbers game. Successful use of natural control relies on releasing enough wasps to overwhelm the fly population. The basic reproductive capacity of flies will outpace that of the wasps if breeding sites are abundant. Widespread fly breeding sites complicate the releases.

What level of fly control do you expect?
You need to have realistic expectations from any fly control program, especially one based on beneficial wasps. And it is difficult to determine the success of fly control programs because you don’t know how bad the fly population could have been if no control was applied.

Using natural enemies to control pests is not an exact science and will not eradicate flies. Many factors come into play. Some reasons for success and failure have not been determined, but a few things are clear. Weather patterns can play havoc with fly control. Drought, excess rainfall or unseasonable temperatures can have big impacts on fly productions and limit the success of natural enemies. Even stable bedding choices can have a big impact on potential fly problems.

Selecting Your Waspswasps

If release of beneficial wasps fits your situation, or you are interested in giving it a try, the next step is to select a supplier. A lot of research has gone into the use of wasps to manage house flies and stable flies, but there are still many questions; it is more an art than a science. Much of the work has been done around large feedlots or dairies in very different parts of the country. It is difficult to know how much is directly applicable to equine operations.

Websites of companies that provide wasps for fly control allow you to get some basic information. However, all are organized differently and vary in the amount of information provided. Generally, you can contact someone by email or telephone to get answers for specific questions. Here are some to consider:

• What species are available? Scientific names can seem intimidating, but are useful in evaluating products. Several species in the genera Spalangia and Muscidifurax have been used in successful studies. Species of Spalangia tend to attack fly pupae that are deeper in the breeding media and in moister conditions than those of Musidifurax. Having both in a mix helps cover both wet and dry weather conditions. Avoid unspecified species “mixes” that do not list at least a genus name. Nasonia is not a desirable species for fly control.

• Are adjustments made for region of the country? Research on fly control with parasitic wasps has been promising but results have been inconsistent. Some wasps may be poorly adapted for certain areas. Ask if regional differences are considered in the wasps you will be receiving. Give strong consideration to commercial programs that adjust their approach based on geographical area. A “one program fits all” solution may not be very effective. For example, wasps in the genus Muscidifurax seem to do well in the Midwest, while Spalangia species seem more suited for southern areas.

• What release program do they recommend? No specific release rate has been established through research, so you may see differences in recommendations between companies in terms of numbers of wasps needed and release intervals. Usually the release rate is based on numbers of animals present.

• Are there clear release instructions? Beneficial wasps generally move just a few yards from the release site. Do the instructions account for that by recommending multiple releases instead of one or two major ones? You will receive fly pupae containing the wasps – the directions should state that the pupae should be covered rather than left on the surface of the ground and should not be exposed to direct sunlight. Clear, precise directions for release indicate a well-thought out plan for you to follow.

• Is technical help readily available with suggestions if fly numbers seem to be much higher than desirable? It is good to have access to someone associated with the supplier who can listen to your questions and offer suggestions or adjustments.

Beneficial wasps can play an important role in managing stable flies and houseflies. It is not a precise strategy, but a good manager can watch and adjust the program for the best possible results. Best of all, it’s natural!control flies with wasps

Stay on Top of Fly Problems

1. Learn more about houseflies and stable flies, especially their breeding habits. This will help you become aware of situations that favor them so they can be corrected.

2. Have a fly monitoring system. Sticky fly strips (houseflies) or sticky biting fly traps (stable flies) are a great way to monitor fly numbers. Put out a few, check them weekly, and record numbers caught. This gives you a record of fly numbers over time. You may pick up indications of increasing numbers before they reach an outbreak.

3. Have a backup plan if fly numbers get above acceptable levels. Fly baits can be used against house flies with minimal chance of harming the wasps, but the baits do not attract stable flies. Baits will be most effective when sanitation is good so there are limited competing food sources.

4. Space sprays of pyrethrins (fogs) can give a good knockdown of houseflies and stable flies with no residue left to harm natural enemies, as long as you are careful with the drift of small spray particles. Residual sprays can be applied to fly resting sites on walls and fences if fly control is needed and the baits and space sprays are not providing satisfactory control. Keep sprays away from wasp release sites.

Lee Townsend is an extension entomologist in the UK College of Agriculture. He is responsible for extension programs with livestock, forage, and tobacco insects and is coordinator of the Kentucky Pesticide Safety Education Program.


Lee Townsend is an extension entomologist in the UK College of Agriculture. He is responsible for extension programs with livestock, forage, and tobacco insects and is coordinator of the Kentucky Pesticide Safety Education Program.