They are incredibly crafty and know exactly what they’re looking for. It may be that some of your neighbors have them and thus they were all ready around. But, these techniques will guide you in the right direction! Good luck with those allium.

We’ve definitely seen that as a possible remedy, it’s good to know about clear vs. black plastic. I have my doubts about it for onion flies because they seem to be able to handle extreme cold extremes like we get. (And thus can probably tolerate rather high temperatures, too. Plus, the larvae can go quite deep into the soil!) I’d agree that time is required for it to be effective, so it does slightly limit this as a solution to specific geographies as well. I’ve not seen any successful reports of people using this method “for real,” it’s always just a “theoretical” solution. I’d be very interested to see if it does actually work, at least if the rest of the cards align.

Something I learned years back is black plastic will heat the soil at most only a couple of inches down. If you want to really want the soil to heat a lot further down, you need to use clear plastic. It allows the sun’s rays to penetrate unlike black plastic. Clear plastic is used for solarizing soil. I’ve had good experience using clear plastic.

BUT it takes several months of hot sunny weather to be effective. The plastic needs to be laid down tight against the soil and the edges need to be firmly fixed without the chance of any wind getting in and lifting the plastic. I dug a small shallow trench (just 3-4 inches in depth and width) around the perimeter, tucked the edges of the plastic in and covered it with the soil I dug up. Laying some rocks on top of the plastic helps too.

As I mentioned, it really needs a few months of consistently hot sunny weather to work. I used it to kill persistent weeds. Whether it will work on the maggots, I have no idea. It might be worth a try IF your summer weather is suitable.

We have not had issues with carrot flies and haven’t heard of anyone having issues at our latitude. They might not come this far north. But, the protection methods we laid out here would be almost identical, they have very similar behaviors and lifecycles to the onion fly. It’s good you pulled your onions, it’ll only get worse! You’ll have them for years at this point, so it’s a good idea to implement protections like we’ve discussed in this article. Might as well get a two-fer and protect your carrots too! Good luck and thanks for your comment.

Yes,I’m dealing with onion flies in a new patch of soil. As soon as I saw some of my onions affected, I knew it would only be a couple of weeks before all were dead. I wasn’t going to take a chance and pulled everyone up. Lots were still good so as green onions to eat, I didn’t lose everything. At the worst, there were only a few maggots to be seen but I know lots were too small to notice.

Thanks for the kind comment! One of the things we’ve observed about this topic is that there’s a lot of “armchair opinions” out there. As in, theoretical interventions (like black plastic, heat, steam, etc), but none of these are backed up by detailed reports of efficacy. It takes a LOT of heat to eradicate pupae and also long periods of that heat application. At least for us, at subarctic latitudes, that’s not in the cards. Having used black plastic before, we’d be really surprised if you could really reach “pupae killing” temperatures, much less maintain them for long enough, in all but the hottest of climates. Thanks for your comment, glad your garlic and leek survived!

Sorry to hear about your troubles, buy you found the right place! We’re still fighting onion flies 4+ seasons after our original infestation, but the protections we outline in this article are still working 100%! You’ll be thrilled that you can get back to successfully growing allium! Glad you found our article and that you’ve found it helpful. Good luck next season.

I’ve done some pretty extensive research on this topic and that’s never been something that I’ve come across. It’d be nice if it were that easy! There are some commercial insecticides that are largely selective for onion fly, but they’re expensive and difficult to acquire at the “home garden” scale.

That’s a good idea and one I didn’t really think about at the time. Thanks for the idea, maybe others will give it some thought!

I think what you have done is the simplest and obviously is proven to work. So that is the path I will take as well. Again, Appreciations for the time you have taken to help so many.

No worries. We read those same articles as well, which is exactly why we pioneered a path forward! I wish we could direct them with signs! Mine would simply say “Go Away!”

I haven’t encountered anything about heat being used and I’m not sure the temperature of lethality, particularly for the pupae. I do know that they are insanely hardy, though. I’d have to imagine that sustained temps >145F (or possibly less) would definitely do it, but as you point out, that’ll also wreck all the good biology in your soil. One possible issue that the pupae can burrow down at least a foot, so that temperature would have to permeate down a ways. (Meaning, your second idea has some merit.) I do think it’ll be challenging to ensure you get every pupae, it’s remarkable how many we had in our soil. (I’d easily guess into the thousands, possibly 10’s of thousands…and that was in <160 square feet of garden space.)

I can tell you that we did (and still have) pretty good success with just covering our crop these days. We do still get a few flies emerging each season, but it's magnitudes less (10-20 vs 1000's) than we got the first season after initial infection. Once you get over the initial hump, it's really down hill from there. (You do always have to be ever vigilant to prevent reinfection, though.) I am sure there's other paths forward, such as what you're exploring, but they do seem a bit more complicated and maybe not foolproof.

As I understand things we have 4 stages, flies, eggs, larvae, pupae.

I have read that heat over 95 degreeF for several days will kill the eggs. Well that is hard because the eggs are on the plant and so that would likely kill the onion as well. Can not kill the larvae that way either BUT have you read anything about heating the soil before they emerge from the pupae stage? My wife suggested using briquettes in a raised bed in the fall after the last batch has done its damage. But will damage the health of the soil? Yes, but soil can recover fast with introduction of good compost (assuming no onion maggots in that).

WE can trap the flies, and protect against the flies as in your method. Sounds most feasible.

One last thought was to remove the infected soil to an unused corner area of the farm, cover that with a tarp and get it superheated by compost methods. My compost internal heat is over 140 degrees. The area where the soil was removed could have landscape fabric placed in case there are residual pupae, then new fresh uninfected compost or mushroom manure then planted and covered.

thoughts?
Yours was the ONLY site I found that had positive ideas and results. Appreciations on that!! And sorry for all these questions.

Glad you found us! That sounds like a rough infection, for sure, we completely sympathize. A lot of commercial onion farms use exactly the method you propose. Perhaps not sacrificial, but they operate on the idea that 100% of their crop will not be hit in any given season. They also try to reduce impact by capturing (via sticky traps) as many flies as they can. It’s obviously difficult to control fly behavior, they’re not going to follow a sign saying “Go here, please.” But, you will definitely see some benefits from diversifying locations. On a larger scale, this might look like different allium gardens across the property. On a smaller scale, it’d be distributing your onion crops throughout your (smaller) garden. Hope that helps, thanks for the comment!

The symptoms of the two will be very different. With leaf miners, you’ll see the evidence on the onion stalks. They’ll look like little white spots, streaking up and down the stalk, where the fly has eaten the plant. (They feed on the onion sap.) If they lay eggs, they’ll be on the stalks as little black spots and possibly small holes in the stalk. A serious infestation can result in the onion stalk growing in a wavy pattern as opposed to straight.

Onion maggots, however, will show virtually no evidence until your plants die since the entire infestation happens pretty much under the soil. There will be no evidence on the stalks and it’s difficult to see the eggs once they’ll laid into the soil. There are differences between the flies, too, but often the gardener doesn’t get a good, close look at the flies. Hope that helps, thanks for the comment!

I’d also add that we advocate for the same protections – row covers! They are effective against most pests, when it comes right down to it!

Thanks for your comment, glad you found our post! I honestly wouldn’t expect any cover crop to help much and in my extensive research, never found any suggestions to that end. The larvae burrow deep into the soil and will simply emerge the next season, nothing short of covering every bit of the soil would stop it. (Which, if you could retire the bed for a few years, a plastic wrap or something might do the trick.) The only further advice I’d give (and I’ve meant to update this article) is to truly remain vigilant with your protections and the overarching process of releasing flies over several years. We got a bit lazy in the 3rd year of protections and had a few flies pop up inside the netting. I ignored them, figuring they wouldn’t be too much of a problem. Well, they didn’t do a ton of damage since it was only a few flies, but we did get some more attacks on our allium and yet more fresh larvae in our soil. They are super, duper annoying and you absolutely have to continue disrupting the lifecycle for years to come, once you have them.

We tried onions at FCG our first couple years there. They did OK, but not amazing like we get in our raised beds these days. Could chalk that up to experience, too, though. I think we got hit because we had a ton of garlic and onion growing, our garden was an ideal target. Nonetheless, we’ve been free of them for years now, so the prevention technique works!

Thanks for the comment! Glad we could help you, hopefully you have the same luck we did. We just harvested our second year of allium after recovery, still smooth sailing here. May your bug nets be tight and garlic safe!