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Open-access Article Offers Horse Owners Advice on How to Control Flies with Parasitoid Wasps

A female parasitoid wasp, Muscidifurax raptorellus, inserting an egg into a fly puparium. When the egg hatches, the wasp larva will eat the fly pupa. Photo by Lyle Buss, University of Florida.

By Leslie Mertz

Horses need help when it comes to insect pests, particularly the house flies and stable flies that can be a near-constant source of annoyance and even disease. Unfortunately, horse farm managers and horse owners are in the dark about how best to tackle the issue because the research just hasn’t been done, according to a new overview of equine fly management in the latest issue of the Journal of Integrated Pest Management, an open-access journal that is written for farmers, ranchers, and extension professionals.

Leslie Mertz

Leslie Mertz

One reason is that the U. S. Department of Agriculture’s treatment of horses as livestock is a gray area. While care and regulation of horses and horse-related activities come under the purview of the USDA, horses are not raised for “meat or fiber,” so they don’t get as much attention as cattle, pigs, or poultry for research funding. The other is that the horse community doesn’t have a strong lobbying presence to get funding for pest-control research projects, according to lead author Erika Machtinger, who was a doctoral student at the University of Florida when the research was conducted, and is now a postdoctoral researcher at the USDA’s Beltsville Agricultural Research Center in Maryland.

Research is indeed essential, said University of Kentucky extension entomologist Lee Townsend, who was not involved with the study. Located in Lexington, the “Horse Capital of the World,” he fields many questions from horse owners and managers.

“The importance is high because people are looking for effective fly control,” he said. “But they’re also looking for sustainable ways to do that, particularly those that avoid excessive insecticide use.”

One fly-management method that is gaining ground is the use of biological control methods, notably pupal parasitoids, which are wasps that attack and kill the pupae of house flies and stable flies (collectively called filth flies). In part of her doctoral work, Machtinger and her colleagues conducted what she believes is the first study of parasitoids to assess their attraction to different animal manures. Specifically, the researchers tested two species of parasitoid from the genera Spalangia and Muscidifurax, which are found in typical mixes sold by biological control companies.

House fly pupae with exit holes made by emerging wasp larvae.

“They sell them in this mix because it’s very difficult to keep them from contaminating each other’s colonies,” she said. “In the lab, we found that the Muscidifurax species we tested preferred bovine manure, and the Spalangia species preferred equine manure, so there seems to be some sort of differentiation there, which could impact control on a farm.”

While fieldwork is necessary to verify those results, she said, “It could have some control applications if Muscidifurax turns out to not be an appropriate genus or does not give the control horse owners are looking for. If that’s the case, then it’s up to the community to say to the suppliers that they need to have more say over what species are in the mix so they can get the ones that work best.”

On the practical side, the JIPM study provided a variety of fly-control guidelines for the horse community:

  • Learn the fly species. A first step in fly control is knowing what the pests are, because different fly species require different management strategies, including the selection of appropriate pupal parasitoids.
  • Track the problem. Monitoring flies is critical to catching a problem early, which is the best time to begin taking control measures. “Monitoring can be as simple as counting the flies on one leg of your animal once a week for stable flies, or observing house fly numbers around feed or your horse’s eyes or nostrils, just so you can get an idea of the population fluctuations,” Machtinger said. “And when you’re dealing with parasitoids, you also need to have an idea of whether the flies are even actually breeding on the farm. They might be coming from a neighbor’s property, so your facility might not be suitable for parasitoid release.”
  • Use the right timing for parasitoids. Filth flies can breed several times a month, and parasitoids are only active predators for about two weeks, so supplier recommendations of monthly parasitoid applications aren’t enough, she said. “The parasitoids can’t keep up with the life cycle of the flies at once a month. At a minimum, they should be released every two weeks.”
  • Use enough parasitoids. She suggests 2,000 per horse every two weeks.
  • Get rid of the straw bedding. She described it as “probably the worst kind of bedding you can have” because it’s perfectly aerated and has an ideal pH for flies. Sawdust, sand, and gravel are better choices for fly control.
  • Manage the manure. Filth flies feed and lay eggs on manure, so management is key. Breaking up horse pats in fields, for instance, helps dry it faster and cuts down on fly reproduction, she said. She also recommended extension agencies as excellent resources for other approaches.

Biological control for the stable fly (Stomoxys calcitrans), shown here on the left, includes the use of parasitoids specific to certain groups of flies. To select the proper parasitoids, horse owners need to be able to identify the flies correctly. Characteristic features of the stable fly are the checkerboard pattern of dark spots on the dorsal surface of the abdomen, and a lighter tan or grey spot between the black stripes on the dorsal surface of the thorax. The house fly (Musca domestica), shown here on the right, has downward-projecting, sponging mouthparts that differ from the rigid, piercing proboscis that projects forward from the head of a stable fly. Photo by Stephen Ausmus, USDA-ARS.

Townsend appreciates these guidelines, especially those dealing with parasitoids.

“The biological approach to insect control, whether it’s in field crops or in stables and equine operations, [involves] not only trying to do the best in terms of releases and locations and management, but also in trying to assess what’s going on as well, so you get some kind of idea of what the impact of your program is,” he said. “This is just a small fraction of all the work that stable managers and equine people face in caring for their animals efficiently. This is a really good article, and very useful in pointing out some directions, and things that need to be addressed.”

Machtinger hopes two things result from the JIPM paper. First, she hopes that it serves as an impetus for others to take up similar projects on fly control for horses.

“It needs to be done, especially in places like Florida and Kentucky where a decent chunk of the economy comes from these horse facilities,” she said.

Second, she hopes that the horse community and extension agencies take up the gauntlet.

“They need to raise their voices and insist that this kind of research is something that’s needed.”

Read more at:

Use of Pupal Parasitoids as Biological Control Agents of Filth Flies on Equine Facilities

Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, She resides in northern Michigan.


  1. Dr. Mertz, may I add a critical component necessary for beneficial insect (parasitic wasp) success? You already know this, but I know that others do not.

    If you release these wasps without their adult food source nearby, they will not stay with you. Adult parasitic wasps ONLY EAT nectar and pollen, which usually comes from plants that have the tiniest flowers. It is only the LARVA of the wasps that feed on flies, etc.. Therefore, it behooves the horse owner to provide adult food sources so they don’t have to release adult wasps over and over again. AND, they must provide overwintering sites for these wasps in order to have a thriving colony. That can be as simple as planting a bed of greens and mustard and leaving it standing until spring.

    Carrie McLaughlin

    • Ms. McLaughlin, thank you for your comment. Although there are numerous parasitoid species that do need flowers and other sugar sources for survival, this particular set of wasps do not require a nectar or pollen source for survival (much of their lives are spent down in the manure or rotting organic matter substrate). When food is needed, adult female wasps will feed on the fly pupa after they sting the fly to obtain sustenance. They do this by inserting their ovipositor into the pupa, constructing a temporary straw-like tube, and pulling out a small portion of the fly haemolymph (kind of like fly blood), which they then eat. When they are done, they break the tube and seal the hole ensuring the fly pupa remains alive for its offspring to feed upon. For example, many of these same wasp species do exceptionally well inside poultry houses with no access to plants. Also, these wasps do not need plants in order to overwinter – these are manure-inhabiting parasitoids and survive winters in this substrate. The continuous release of the wasps is needed due to the very fast fly development time and high egg production and the comparatively slower development time and low reproductive rate of the wasps. It is important to remember that one must release the wasps near where you find the fly developmental sites to obtain the best results.

  2. Hi Carrie – In this case, that is incorrect. These particular wasps develop in their entirety in the fly puparium. There is no need to have additional “food” sources. Please email me (author of the JIPM article) at if you have any questions or would like clarification. Dr. Mertz graciously wrote this blog article, but she is not the author of the paper.

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