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Filth fly parasitoids

Pupal parasitoid wasps, chiefly Muscidifurax raptor, M. raptorellus and Spalangia species[9]

Filth fly parasitoids are tiny wasps that lay their eggs in the pupae of house flies and stable flies, killing the developing fly. They occur naturally on most farms where flies breed, and several species are sold for release around barns, dairies, horse farms, feedlots and poultry houses as part of a fly control program.[1][2][3]

Group
Insect
Order
Hymenoptera
Family
Pteromalidae
Sources
9 cited
Updated
2026-11-11

No licensed photo of this species is available yet; the identification notes below describe what to look for.

On this page
  1. Identification
  2. Where it lives
  3. Biology
  4. Damage and signs
  5. Health risk
  6. Control
  7. Related profiles
  8. Common questions
  9. Sources

How to identify it

  • The wasps are tiny: 1 to 3 mm long by Cornell's measure, 1/16 to 1/8 inch by Penn State's, and purple-black. Penn State notes they are almost unnoticeable.[2][3]
  • Their work is easier to spot than the wasps themselves: fly pupal cases with a small exit hole where an adult wasp chewed its way out.[2]
  • They are stingless as far as people and animals are concerned: they sting only fly pupae and neither sting nor bite people, livestock or other insects.[2][1][3]

Where it lives

Cornell lists Muscidifurax raptor and M. raptorellus as native and naturally present in the northeastern United States, and Penn State says parasitoid wasps turn up naturally on most farms with developing flies. Farm surveys find different mixes by region and farm type: Spalangia made up 99.9 percent of the parasitoids found at Florida horse farms, and Spalangia cameroni was the most common species in Pennsylvania poultry layer houses. Kentucky Extension discusses the species sold to Midwest farms.[2][3][4][5][1]

Biology and life cycle

A female wasp searches manure and moist, rotting organic matter for fly pupae, pierces one with her ovipositor and lays her eggs inside: M. raptor lays one egg per pupa, M. raptorellus up to eight.[2]

The wasp larva feeds on the paralyzed fly inside its pupal case and kills it, then pupates, and the new adult chews its way out to search for more hosts.[2][1]

Each wasp can attack about 100 fly pupae over an average life of about 26 days, searching both inside and outside barns.[2]

Because M. raptorellus lays several eggs per host, it averaged about four wasps per fly pupa in a New York dairy-calf trial, so fewer parasitized pupae have to be bought to put the same number of adult wasps on a farm.[6]

Species differ in the flies they favor: Kentucky notes that Spalangia nigroaenea is the commercially sold species most likely to attack both house fly and stable fly pupae in feedlots, that Muscidifurax raptor and M. zaraptor parasitize some house flies, and that Spalangia nigra, S. cameroni and S. endius parasitize some stable flies.[1]

Surveys show which species actually do the work. At four Florida horse farms the parasitoids recovered were S. cameroni (56.5 percent), S. nigroaenea (34.0 percent), S. endius (5.8 percent) and S. nigra (3.7 percent). In Pennsylvania layer houses the order was S. cameroni, S. nigroaenea, Trichomalopsis species and S. endius, and parasitism varied by farm and month. Penn State names S. endius and S. cameroni as the main species found naturally on horse farms.[4][5][3]

Damage and signs

  • They attack only fly pupae, mainly those of house flies and stable flies.[2][1][3]

Health risk

These wasps do not sting or bite people, animals or other insects; they attack only flies.[1][2][3]

Control

Prevention

Avoid residual insecticides in and around barns, which kill the parasitoids. When a chemical is needed, use fly baits or insect growth regulators and keep sprays off the manure where the wasps work.[2][1]

In lab tests of residual sprays on plywood, permethrin was the most toxic to the parasitoids, though the five wasp species responded so differently that the authors could draw no general rule by chemical class. The growth regulator pyriproxyfen, by contrast, was judged compatible with the main house fly parasitoids below 600 mg per kg in treated hosts; above that, fewer wasps emerged.[7][8]

Sanitation

Manure and water management decide success. Remove most manure, spilled feed and manure-soaked straw regularly, but not every bit, so the wasps have fly pupae to build up on; fix broken or overflowing waterers, and control weeds that hide manure and keep it wet.[1]

Biological control

Parasitoids are sold for release around barns and animal confinement areas, usually shipped and released repeatedly before and through the fly season; Cornell's program releases them weekly from May through September. Release numbers vary by livestock type and supplier: Cornell recommends 200 per cow and 1,000 per calf each week for dairies, and Kentucky notes that suppliers usually provide a formula based on animal numbers.[1][2]

First confirm that house flies or stable flies are actually developing on the property; if they are not, Penn State warns that releasing wasps will not help. They will not control pasture flies such as horn flies and face flies, and face fly pupae have a hardened case the wasps cannot get through.[3][1]

Buy species matched to your flies and region. Cornell recommends mixes of M. raptor and M. raptorellus in the Northeast, Kentucky advises against unspecified blends and against Nasonia vitripennis, which proved ineffective in Midwest feedlot trials, and Penn State cautions that suppliers vary in their knowledge and colony quality.[2][1][3]

Results vary. Cornell reports these wasps, combined with other IPM tactics, controlling 35 to 80 percent of house flies and stable flies on farms, while Kentucky's 1994 review found the evidence for confined livestock unclear and treated wasps as a supplement to sanitation. In a three-year New York dairy-calf trial, weekly releases of M. raptor alone killed 31 to 38 percent of sentinel fly pupae and M. raptorellus alone 59 to 80 percent, with mixed releases slightly below M. raptorellus alone.[2][1][6]

Releases must be repeated every year, and parasitism dropped once releases stopped in the New York trial. You can check the wasps' work by holding fly pupae from breeding sites in a jar to see whether flies or wasps emerge.[1][6]

Common questions

Do fly parasite wasps sting horses or people?
No. They sting only fly pupae, and they neither sting nor bite people, animals or other insects.[2][1]
Will fly parasites control horn flies or face flies?
No. The wasps sold for barns attack house fly and stable fly pupae. Kentucky says such programs should not be expected to control pasture flies like face flies and horn flies, and Penn State notes that face fly pupae are hardened so the wasps cannot get in.[1][3]
Why aren't my fly parasites keeping up?
Too many fly breeding sites can overwhelm them. They work best alongside regular removal of manure and spilled feed, good water management and weed control, and residual insecticides around the barn kill them.[1][2]
Which fly parasite species should I buy?
Match them to your region and flies. Cornell recommends M. raptor and M. raptorellus in the Northeast, and a New York calf-barn trial found M. raptorellus killed more fly pupae than M. raptor. Surveys of Florida horse farms and Pennsylvania layer houses found mostly Spalangia species, chiefly S. cameroni. Kentucky advises against unnamed blends and Nasonia vitripennis.[2][6][4][5][1]

Sources

  1. University of Kentucky Entomology, “Biological Control of Flies, ENTFACT-502 (L. Townsend).” entomology.ca.uky.edu/ef502 (accessed 2026-11-11)
  2. Cornell University, New York State Integrated Pest Management, “Muscidifurax raptor and M. raptorellus, Biocontrol Agent Fact Sheet (K. Wise and P. Kaufman, May 2023).” cals.cornell.edu/integrated-pest-management/outreach-education/fact-sheets/muscidifurax-raptor-and-m-raptorellus-biocontrol-agent-fact-sheet (accessed 2026-11-11)
  3. Penn State Extension, “Filth Fly Control on Horse Farms.” extension.psu.edu/filth-fly-control-on-horse-farms (accessed 2026-11-11)
  4. Journal of Economic Entomology, via PubMed, “Seasonal abundance of stable flies and filth fly pupal parasitoids at Florida equine facilities (Pitzer et al. 2011).” pubmed.ncbi.nlm.nih.gov/21735936/ (accessed 2026-11-11)
  5. Journal of Medical Entomology, via PubMed, “Filth fly parasitoid monitoring techniques and species composition in poultry layer facilities (Pagac et al. 2022).” pubmed.ncbi.nlm.nih.gov/36130177/ (accessed 2026-11-11)
  6. Journal of Medical Entomology, via PubMed, “Individual and combined releases of Muscidifurax raptor and M. raptorellus as a biological control tactic targeting house flies in dairy calf facilities (Kaufman et al. 2012).” pubmed.ncbi.nlm.nih.gov/23025187/ (accessed 2026-11-11)
  7. Journal of Economic Entomology, via PubMed, “Susceptibility of house flies and five pupal parasitoids to abamectin and seven commercial insecticides (Geden et al. 1992).” pubmed.ncbi.nlm.nih.gov/1593014/ (accessed 2026-11-11)
  8. Pest Management Science, via PubMed, “Effects of pyriproxyfen on wild populations of the housefly, Musca domestica, and compatibility with its principal parasitoids (Biale et al. 2017).” pubmed.ncbi.nlm.nih.gov/28692174/ (accessed 2026-11-11)
  9. iNaturalist, “Muscidifurax raptor taxon page (order and family).” www.inaturalist.org/taxa/298673 (accessed 2026-11-08)

Updated 2026-11-11. Built from sourced records. Spot an error? Tell us.