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Beneficial Bugs? Or Filthy Flies?


You Bet Your Garden
Question of the Week © 2018 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Beneficial Bugs? Or Filthy Flies?

Q. Alison in Philadelphia (specifically south of Fitler Square; north of Devil's Pocket and near the Schuylkill River Trail) writes: "I have a patio garden with pots, railing planters and a raised bed filled with hydrangeas, petunias, roses, impatiens, gardenias, pumpkin vines, a hibiscus, butterfly bush, two tomato plants, and a few herbs.

"I'm trying to attract 'good bugs' and pollinators. I've seen a few bees in the front, one butterfly out back, and many birds. The plants are all doing well—I even had a red tomato before the 4th of July! But I have an abundance of flies! The residents of a nearby apartment building always put their trash out several days early, which I know can attract flies; but the flies in my back patio are relentless."

Alison continues: "I bought fly traps and have caught literally hundreds of flies within a few days. But now I'm wondering: is this paradoxically attracting even more flies to my garden? And if I want 'good bugs', do I have to buy them online to release in the spring? Full disclosure: I have a dog and dispose of some of his waste in our trashcan, but that's not where the flies are; they're around the plants."

A. Let's analyze the young lady's missive. What's missing in her communication that is ordinarily the prime topic in an email to the show? (Besides insincere praise to the host in the hopes of getting her question picked.)

Here's a clue: How does she say that her plants are doing? It sounds like her plants are just fine, right? Wait a minute! Am I now going to tell you that flies are good?!

Everything on this planet is good for something, even me. Now, there are two possibilities here. One is that these are just nasty flies that can often be a plague in urban areas. Our beloved originating station WHYY is only three blocks from where the Declaration of Independence was drafted, and history tells us that Philadelphia was so under siege by flies that summer that the Founders had to keep the windows closed despite the stifling heat that year.

And the other reason?

The other reason is more like two reasons. There are over 150,000 species of what are called 'flies' in the world of entomology; and pretty much all of them are or can be pollinators. We've talked in the past about how flies pollinate the native Paw-Paw fruit, and in the tropics, there would be no chocolate without pollination by flies! (My, that sounds good: Chocolate covered Paw-Paws. Yum!)

Anyway, all members of the fly family—including mosquitoes—can be effective pollinators. There's even a sub-group of flies called "Flower Flies"—members of the Syrphid Fly family—that are famed pollinators; second only to bees.

So: do I think that's what she has?

I'd love to say yes, but a lot of the beneficial flies don't look like houseflies—many of them are imitators of bees and wasps; mimicking their appearance to deter predators.

However, there are also a lot of fly-like beneficial insects in the very-large fly family. The Tachnid Fly is in everyone's beneficial Top Ten—as is the Robber Fly. Then there's the Soldier Fly, Hoverfly, and the Pollen Flies. And again, even the common house fly is a well-known pollinator.

Now—I mentioned Paw Paws and chocolate before; they NEED flies for pollination. What about the plants she has growing? A little research dashed that hope. Of her list, only roses have a large percentage of fly pollinators.

So what's the chance that these are just nuisance flies?

"Just"? Small; all flies are going to pollinate flowers (and she even notes that she has lots of blooms despite few bees.) But in general? Yes, especially in the City; especially where neighbors are careless about their trash; and especially near large bodies of water, as she is to the Schuylkill river.

So what's an urban gardener to do?

I hate to say it, but number one is to dime out her neighbors. You can't have trash sitting out close to a week in advance—especially in the summer. Second, keep up with the traps. As she noted, fly traps are highly effective. Third, use oscillating fans to keep the flies at bay when you eat and drink—or just plain sit—outside. And most importantly, focus on keeping the flies out of the house

…Because in the garden, they will help pollinate plants and won't directly harm any plants.

In addition, she should avoid using any kind of manure or stinky fertilizers, as they could attract flies. She should also be diligent about cleaning up damaged tomatoes and the like. And read some of the 'bee' and 'beneficial' articles in our archives here at Gardens Alive and adopt some of the recommended attraction techniques.

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