Filth fly pupal parasitoids - Hymenoptera: Pteromalidae (AKA Fly Predators)
A friend asked me what the biological benefit/implication of fly predators was. I had no idea. So, I decided to look into it. Here are some of the key bits of information summarized, and one of the articles I found if you have the time to read it.
The first question should be, "Do we want to eliminate flies in the first place?" While I am a lover of nature and all living things (or I strive to be anyway), house and stable flies are substantial disease vectors - they are called Filth Flies for a reason, and we should work to limit their range on our farms for the health and safety of our herds.
In summary: "With careful planning, horse owners can use pupal parasitoids as an environmentally-sound method to reduce pest house fly and stable fly numbers and reduce the risks to horses and their owners."
If you have hopes of fly-free summers, I'm sad to tell you that flies are just one of natures miracles that we need to learn to live with. However, using several methods, including biological ones, we should be able to keep flies at a manageble level. Weekly or Bi-weekly releases of predators at a rate of 2,000 per horse would increase success rate, which could increase the cost significantly.
Biological control using pupal parasitoids has been used in livestock facilities for many years. Unsuprisingly, most research is done on cattle and other livestock production facilities. There is less data on horse properties, and there are few guidelines on the best execution of parasitoid release in these facilities. While some information can be applied across the board, we can't ignore the differences in horse properties to livestock, for example, many fly species (and their parasites) are host specific (preferring the dung of cows to horses etc).
As with all things, success rates will be increased with an integrated approach.
Who are the fly predators? Meet: Hymenoptera Pteromalidae
There are several naturally occurring pupal parasitoid wasps found throughout the United States, though there is limited research on population dynamics specifically on horse farms. In North Carolina, S. cameroni was the primary species found. These are small wasps who are incapable of stinging animals or humans. The female wasps parasitize the pupal form of house and stable flies to raise their young. It's of note to say that only the females are predators. So, roughly half of the wasps aren't directly impacting the fly population.
Naturally occurring populations are rarely adequate to impact the large populations of flies produced by the abundant bedding (ie manure, rotting hay, etc) on a horse property. However, it's possible to support natural populations with commercially produced parasitoids, such as those from Spalding Labs. These additional predators may be effective in suppressing fly populations (coupled with other management methods).
While the complete life cyle of the Spalagia cameroni is 21-28 days, they are only actively parasitizing the pupal flies for 14 of those days. This is important when you're releasing several thousand at once- presumably at roughly the same age. It indicates that monthly application of the wasps wouldn't allow them to keep up with the fly population.
Species of wasp may have an impact on efficacy per a 2011 study "[These results] suggest that Spalangia spp. [to include the native S. cameroni] are more suited to successfully locate and attack hosts in habitats created by equine husbandry in Florida. Therefore, commercially available parasitoid mixtures containing Muscidifurax spp. may be ineffective if used as a control measure at Florida equine facilities." https://academic.oup.com/ee/article/40/1/88/404708?utm_source=TrendMD&utm_medium=cpc&utm_campaign=Environmental_Entomology_TrendMD_0
It's also worth noting that these little wasps aren't the only parasitic predator out there. Hister Beetles as an example, also do this job (many while also eating dung - so working overtime and controlling the flies on two fronts). Being mindful when using anti-parasitic medications (ie wormers) can help build populations of beneficial beetles.
Safety in numbers
As survival rates of the wasps, the percentage of females in the group, and success rate in hunting can vary, releasing enough predators will be a key component to success. The recommended release rate was 2,000 wasps per stabled horse every 2 weeks. At the time of writing, Spalding Labs was suggesting 10,000 predators for my 8 horses. So, based on the research, I'd need to increase that by almost 50% and double the shipment rate. It would put my annual cost at around $950. So, it's not a low cost solution.
While I type this, it keeps rattling around in my head that many birds, frogs, and spiders eat adult flies for free, at all times of the month.
Know thy enemy.
Ensure that you're targeting the correct species of fly - for these purposes we are looking at house flies and stable flies-(Muscidae) as these predators will not assist in horse fly (Tabanidae) or biting midges (Ceratopogonidae) management. So take some time to identify the flies on your property, and develop a plan to combat those specific species. This isn't a one size fits all solution, and again it's not lost on me that many insects, birds, spiders and frogs are far less picky about their meals.
Take a moment to understand the life cycle of these flies- as the predatory wasps are only impacting the pupal stage. Female flies lay their eggs in a fresh manure pile. The eggs can hatch in 8-20 hours and can mature out of the larval stage in as few as four days. They only spend two to six days in the pupal stage (when the wasps are active). With wasps being released every four weeks, we are missing several rounds of breeding.
Understand the environment
These soon-to-be fly moms are on the hunt. They're looking for manure piles, bedding in stalls, soiled hay, and fresh manure - interestingly they preferred fresh horse manure and manure mixed with pine shavings (Machtinger et al. 2014) to raise their babies.
It also helps to know where your flies are coming from. As fly predators only target the pupal stage of the lifecycle, if adult flies are coming in from neighboring properties pupal parasitoids will be of little help.
Take proactive steps
Covering manure piles with burlap or tarps to increase the temperature or spreading it out on fields (via spreaders or draggging) has shown to reduce fly breeding areas.
Making the property less attractive to adult female flies, in order to break up their breeding grounds should be a primary action.
Correctly composting/managing manure increases should be a focus for any horse property, as it also allows us to replace those nutrients back onto our grounds, to increase soil health.
When it comes to stable flies (the biting ones), the areas around spent round bales are prime breeding grounds. Keeping old hay cleaned up and properly composted can go a long way in reducing the fly population.
Unsurprisingly, chemical control isn't a good solution. Like anthelmintics (wormers), we are starting to see resistance to chemical fly treatments (oh good). This, along with the increasing cost, toxicity to nontarget species, environmental pollution, it seems to preclude their extended sole use. Further, "Automatic [chemical]
insect control systems found in many horse barns likely contribute to selection pressure leading to high levels of resistance in filth flies."
It appears as though there aren't major biological/environmental concerns related to releasing thousands of tiny wasps on our properties. Can commercially available parasitic wasps assist horse owners in our quest to reduce house and stable fly populations? Yes. Is it a silver bullet? No. I'd like to see more studies on farms using practical applications (ie not perfect) of fly predators compared those farms just taking all available breeding ground mitigation steps. What impact do the actual predators have on horse properties (which are far different environments than cattle feed lots).
What am I going to do? I'm going to repurpose the money I was spending on the fly predators, I think, and invest in creating an environment more attractive to more of the natural fly predators and less attractive to flies. What are your plans?
Main article referenced:
Kristen and the Ardani Herd