Flies may seem indestructible at times, but they are easy fodder for a creature commonly known as the parasitic wasp. These parasitic wasps are tiny insects that destroy fly larvae before it hatches, eliminating flies before they develop into biting pests. Harmless to horses and humans, parasitic wasps are so small, they are barely noticeable. 

Companies that sell parasitic wasps deliver them still in the pupa. The pupas are scattered in areas where flies breed, such as paddocks, pastures and near manure piles.

“Fly parasites stop the pest flies from being able to reproduce by laying eggs in the pupa stage of the fly,” said Tom Spalding, president of Spalding Labs in Reno, Nevada, which sells the Fly Predator. “The wasp pupa is sprinkled once a month in manure areas. The existing flies take 30 days to die off, so you will see the full benefits in roughly a month unless you use traps to get the adults.”

Manure management is important, although not 100% effective in eliminating flies because the eggs fall off the manure when it’s being picked up, according to Spalding. “So you can still have flies even after cleaning the stall,” he said.

Parasitic wasps will travel 150 feet on their own, so placing the wasps on the far end of the corral is effective for controlling flies throughout the enclosure.

Because parasitic wasps are susceptible to the same pesticides that kill flies, fly spray should not be used in the area where they are released.

“You can spray your horse, but do it away from the manure area,” said Spalding. “And don’t use a barn spray system—it will kill the parasitic wasps.”

Spaulding notes that parasitic wasps are not affected by bait or sticky paper, so it is safe to use these methods of fly control when wasps are present.

It is important to control fly populations to reduce disease spread by these insects and reduce the irritation they can cause to horses and humans.

Parasitic wasps can be combined with other methods of fly control listed in this article to reduce fly populations. Other methods include manure management, traps and management of water and air around your horses. We will look at those other methods of fly control during June in other articles.