Welcoming The Winsome Fly, Japanese Beetles’ Bane 

A natural predator introduced to control destructive Japanese beetles, the winsome fly is starting to take off in our environment. Here's how to use this beneficial bug.

by Michelle Bruhn
PHOTO: bjphotographs/Adobe Stock

If you garden, you’ve likely heard of and dealt with Japanese beetles and those endless buckets of soapy water. But now there’s a new step: You need to let some captured beetles go free … if they’ve been parasitized by the winsome fly! 

Japanese beetles are an invasive species introduced to North America in the early 1900s. It was only in the late 1960s that the first Japanese beetle was discovered in some more northern climates. And they weren’t really an issue until after 2001.  

Japanese beetles are 1/3 to 1/2 inch long and have a metallic green thorax with copper-colored wing coverings. The larva are indistinct white grubs that attract moles and ruin lawns as they feed on the root systems.

The adult beetles feed on over 300 plant species, which is why they are such a big problem for gardeners. They eat a lot of everything! 

But there is a light at the end of the tunnel … in the form of another insect called the winsome fly (Istocheta aldrichi)! This is a type of parasitic tachinid fly, one of the gardener’s best bets for biological control in the garden. This type of fly was imported from Japan decades ago with the purpose of taking out the beetles.

And it is working—just on nature’s timeline, not ours.

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It has taken over 10 years of heavy Japanese beetle infiltration in my area for the winsome fly to follow in enough population to be a formidable enemy. This lag time is typical and necessary in the grand scheme of things. A predator won’t populate an area until it knows it can feed itself. 

Female winsome flies lay eggs (up to 100 per day) on the thorax of the adult Japanese beetle. The beetles then burrow underground where the fly maggots hatch, enter the beetles and feed on them.

This both kills the beetles and stops the cycle of reproduction.  

What to Watch For

Look for white dots that appear as tiny paint spots on the thorax of the Japanese beetles in your garden. These white dots are the winsome fly eggs.

Since they move fast, I usually flick all of the Japanese beetles off into a bucket of water first, then use a pair of tweezers to pick out only those with eggs. I place those with eggs on open soil in my garden to burrow, die and produce more winsome flies.

Those without the parasitic eggs I toss to my chickens. A shallow bucket will draw in other birds as well. 

There are other natural enemies of Japanese beetle larvae. The larvae stay in the ground (between 4 to 8 inches deep) for about 10 months of the year. Birds (hello, chickens!) and mammals love to scratch up the grubs, and certain nematodes attack the grubs underground.  

How Do We Entice More Winsome Flies Into Our Gardens? 

Planting a diverse array of flowers is key. Bringing in larger amounts of companion plants, especially sweet alyssum and marigolds, helps lure in the flies. Companion planting goes so much deeper than a few “plant this with that” equations. But the beauty of companion planting with flowers and building a biodiverse ecosystem in your garden is that nature will eventually balance itself out. You just have to give it time. 

Three cheers for winsome flies! Have you seen these white dots on your Japanese beetles yet? 


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