It`s Insect vs. Insect

The parasitic wasp on the fly pupae. The parasite attacks the fly pupae.

You’ve likely seen the ads for fly parasites, those tiny wasps that you release near your manure pile so they ’eat the flies.’ The idea is that these wasps are a natural fly enemy and reduce populations.

Some companies call these little wasps fly ’predators,’ which sounds more daunting. However, it’s not strictly correct, since the wasps don’t eat flies, said Dr. John Campbell, a University of Nebraska entomologist.

Fly parasites are gnat-sized wasps that sting the pupal stage of the fly, then lay an egg in it. The egg then hatches into a wasp larva, which then eats the fly pupa. The wasp larva pupates inside the fly pupal case, then emerges as an adult wasp.

Some companies refer to the small wasps by names other than ’wasps’ because some potential customers are concerned about releasing wasps on their premises, thinking of the larger stinging wasps we’re all more familiar with.

Of course, these small wasps are unable to sting people, pets, or horses and – unless you really looked – you’d never know they were around, said Dr. Jerry Hogsette, a USDA research entomologist who has studied fly parasites in horse facilities. In fact, most of the wasps sold are native to North America. They just aren’t found in large-enough numbers to control a fly population.

You’ll Need Manure
Parasitic wasps feed on fly pupae. Therefore, for them to be successful, there has to be a source of flies breeding on site, i.e. a manure pile.

Immature stages of flies develop in hay/manure mixtures found trampled in paddock feeding areas and the areas where manure and used stall litter is dumped. If flies breed in these areas, and the areas can’t be cleaned, parasitic wasps may help. The flight distance of these wasps is limited, so if the neighbors are ’growing’ flies and these flies, as adults, fly from their place to yours, then the parasitic wasps probably won’t help you.

Delivery Service
The parasite pupae arrive at your farm in a small package. Johnnie Rutledge, who owns Rutledge Enterprises, says that his new customers are usually amazed at the ’about two handfuls’ size of the packages. He said that, although the parasite pupae are tiny, they do give rise to thousands of fly parasites. You release the contents onto a manure pile or other breeding area, and allow the wasps to infiltrate the fly population.


Release recommendations – how often you should order and release fly parasites – vary by insectary, but most are right around 1,000 parasites per horse per month. This typically means that you’ll get on a delivery schedule. Hogsette says you can release parasites up to every one to three weeks throughout the fly breeding season for maximum effectiveness.

One delivery won’t do it. The fly parasites need continuous reinforcement to maintain a high level of fly control. The flies have a distinct advantage in actual numbers produced. A single fly produces up to 900 eggs, but a single parasite will attack less than 50 fly pupae. The life cycle of a fly is also much shorter than that of the parasite, also contributing to the fly’s advantage.

Last, the size and strength of the fly are important advantages that enable it to travel greater distances. So a high level of parasites must be maintained in the breeding site. Too many people expect a fly-free environment as soon as they release the parasite pupae. That’s not the case.

In Campbell’s research trials at feedlots and dairies, he released the parasites at a rate considerably above the level recommended. The overall cost was high, although the rate of parasitism was increased. The biological control approach is to rear these parasites in commercial laboratories and deliver them weekly to feedlots and dairies. The concept of this system is that if you increase the number of parasites, you’ll see a corresponding increase in the degree of parasitism.

But in research conducted by the APS Livestock Insect Laboratory, University of Nebraska, Lincoln, and at North Platte, Campbell and his team failed to achieve reductions of either stable flies or houseflies that approach control status despite wasp releases four times the recommended rates. They didn’t have confidence in them because they have not proven to be effective for cattle feedlots and in dairy-cow barns.

Campbell says that parasites can play a role if enough are released, but it could be cost prohibitive. If good sanitation is practiced along with judicial use of insecticide, he says, the fly problem can be handled without parasites.

This doesn’t mean they won’t work for a horse barn, which would be a more restricted environment. However, Campbell’s research does show that parasites might not be the best choice for someone with a large turned-out herd of horses, or a sprawling network of outbuildings with many small manure piles.

Since fly parasites work best in conjunction with other methods, one way to get the most out of them is to keep things clean. This means keeping the area dry, since flies love to breed on wet manure. Keep feed droppings, like sweet feed, cleaned up, too, so that flies don’t congregate and breed on spills. Work on your barn ventilation, because airflow helps keep things dry and less appealing for flies.

Anywhere that manure stays relatively dry, there will be natural parasites to flies. But because we overload areas with manure when we keep horses, we need to supplement nature with new parasites to keep populations down in the extreme situations we create. (One caveat: If you haul manure off-site often, you may want to consider an alternate site for your parasites.)

A single method of fly control usually doesn’t produce good results, so you need to use parasites along with baits, strips and traps, which will kill adult flies but not harm the parasite-destroying-larvae process. Remember: If it kills flies, it will kill the parasites as well. Spray your horses away from the manure pile, and skip overhead misters.

Bottom Line
If you have a lot of horses on a small farm – and don’t have your manure hauled away regularly – fly parasites might be for you.

However, we don’t think you can expect fly parasites to do it all. Proper manure handling, the use of traps and strips, and applying fly masks, fly repellents and the like to your horses, are all still going to be necessary. You’ll also need to be careful about your use of other pesticides around the wasps, as equine fly repellents will can them.

Shopping for parasites is easy in the sense that most companies selling them specialize in helping horse and livestock owners. Prices are competitive, but shipping can become an issue, especially if you receive shipments monthly, and you may want to select someone closer to you to keep these charges down.

Also With This Article
Click here to view ”Fly Wasps.”
Click here to view ”Insectary Comparisons.”

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