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Our boxwoods have a moderate infestation of boxwood leaf miners. Sources recommend pruning to help manage them, and mention that you should cut at a certain time in the pest's lifecycle. But they don't give detailed info on how much to remove or the timing outside Missouri.

  1. How do I know when I should prune in my area?
  2. How much should I remove?
  3. And what should I do with the debris after pruning?
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You can control them when the adults fly (occurs in early spring, the exact time is variable to location), which happens for about 2 weeks. The large, overwintered worms emerge as tiny flies (sometimes mistaken for gnats) which will fly about, landing all over the boxwood plants.

There are a ton of effective chemicals you could use to knock them down, or you can use a dilution of soap and water, alcohol, neem oil, or another natural approach (which I recommend). Sometimes, people will use a systemic insecticide (the plant takes it in and circulates it throughout) which will also kill the feeding larvae.

About pruning, in many infestations, the pruning will often damage the plant more than the leaf miners will. If you do this, pick and choose, cutting out the worst infested areas, but try not to leave big holes. You really don't want to remove much, as the plants' health is already struggling as it is. To get rid of the growth, usually the larvae are underdeveloped in the trimmings, so you can usually just dry the clipping out in the sun, and the worms will die with the clippings. If you trim in early spring (after the worms have overwintered), there's a chance they will emerge.

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  • I had planned to clip off much of last years growth with a hedge trimmer to remove all (or mostly all) of the parasitized leaves. There are maybe 40 boxwoods from 4' to 7' tall, which seems like too many to selectively prune. Last year I used Captain Jacks Deadbug Brew (bonide.com/products/product.php?category_id=252) on the adults, but I just wasn't able to spray them daily for 2 weeks. I figured I'd go nuclear this year and physically remove them from the plants before they emerge. Bad idea? – That Idiot Jan 15 '15 at 12:35
  • @ThatIdiotYou won't get them all that way, but it will get a lot. If you do this, try to shear them as soon as possible before new growth starts, to prevent dieback. However, you don't want to wait until the flies emerge. – J. Musser Jan 21 '15 at 21:38
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I didn't prune my boxwoods at all due to the harsh winter we had. I wanted to give them a chance to recover first, but I wanted to deal with miners as well. I got them at the perfect time I think. I used a solution of Dawn soap and water and soaked the snot out of them. The miners were flying like crazy and a coating of dead bodies and soap bubbles were all over the ground. The next season came and I hardly saw any of the miners at all. I think catching the adults right after the hatch and before they lay new eggs is the key.

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Also, bear in mind that pruning depends on how badly your plants are infested. You could easily snip off a leaf or two, but when it's your entire plant you definitely need to take other pest control measures. Neem oil is very effective in combating that problem. You can also try introducing predatory insects as these feed on the larvae when they're injected into the plants by the mother leafminer. Here's a reference you can use for more info on getting rid of leafminers. 7 ways to get rid of leafminers.

I also found this video online that I think should provide a glimpse of what is involved in pruning. The entire video is not about pruning but the guy did mention something to that effective. https://www.youtube.com/watch?v=IifXXkJzncs

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