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Controlling Leaf Miners


Q. I have leafminers in my boxwood shrubs. Is there a natural product I can use to control them instead of a commercial pesticide? Thanks,
    ---Ray in Martinsville, NJ
Is there a safe, natural way to kill boxwood leafminers? I have 40 boxwoods that are five feet tall. So far they have not been sprayed or treated with anything. Thank you!
    ---Teresa in Linden, Virginia
I have a leafminer problem I want to remedy in a client's boxwoods. I'm proud to say that I've taken care of scale infestations by simply thinning out and removing overgrown plants in the yard, and want to resist using chemicals here as well. I tried spraying water on the adults last year and will feed the plants some worm casting tea this Spring. Do you have any other suggestions?
    ---Hal in Wilmington, DE
A. "Leafminer" is a generic term for a wide variety of insects whose larvae tunnel deep inside plant leaves (where traditional chemical pesticides can't reach them), creating distinctive and unsightly little roadmap/river-like trails. The first step in controlling any leafminer is to identify the TYPE of insect responsible, so you can look up its life cycle and use that information to your advantage.

In boxwoods, the pest is a very small (one-eighth of an inch long) yellowish-orange mosquito-like fly whose larvae spend winter inside the infested leaf, emerging as adults in early Spring. Their flights should be visible just when the shrubs are putting on their first flush of new green growth. Adult females lay their eggs inside that lush new growth.

At that point, a good pruning of the new growth will eliminate many of the little beasties. (It's also the best time of year to prune boxwoods.) Burn the prunings or seal them inside a trash bag. After that, removal of infested leaves offers great long-term control.

In addition, the low-toxicity pesticide spinosad (a fermented version of a naturally occurring soil bacterium) is often effective against leafminers. Spinosads must be ingested to affect a pest insect, so wait until you see the very first trail to spray it. Approved for use in organic agriculture (and by the EPA for use against these pests), spinosads should never be used near any kind of water or when bees are visiting the plant in question. Just spray once or twice when the little roadmaps first appear and let native beneficials (especially predatory mini-wasps) take care of things after that.

Q. We have leafminers in both our orange and lemon tree; are there any beneficial insects that will take care of them? I really don't want to spray because we have bees that visit year round. Thank you,
    ---DeAnn in Rowland Hts, CA
I have leaf miners on my citrus plants. They are also damaging some of the fruit. What can I do that won't damage the fruit? Many thanks,
    ---Lanier in Miami
A. This leafminer is a hungry, hungry caterpillar whose eggs are deposited on the leaves of citrus trees by a night-flying moth. They typically only attack the actual fruits if there are no young leaves left on the tree, so Lanier in Miami may have misidentified his pest, have multiple pest problems (which is not uncommon), or is under such severe attack that his trees are being defoliated.

And yes, these leaf-tunnellers are that voracious, sometimes causing so much damage that they impact the harvest by destroying so many leaves that there aren't enough left for photosynthesis. To make matters worse, citrus only grows in super-warm places like Florida and California, where a pest like this will have multiple generations a year.

Luckily, pheromone traps are commercially available for the adult moth. Here's some info on them from the website of John Panzarella, a very gifted citrus enthusiast who seems to have created (accidentally, he admits) some exciting new citrus varieties. Anyway, John shows photos of the traps and provides a link to ISCA Technologies, a specialty company that sells sticky traps and pheromone lures for a variety of pests common in commercial agriculture. The traps and lures are sold separately; John's link takes you to the page for the basic un-baited traps, then you'll need to search 'lures' under the creature's scientific name, Phyllocnistis citrella for the bait that will attract the citrus-miner 'parents' to the sticky trap.

The pheromone lures are the specialty items here; you can buy the basic sticky traps anywhere (or make them yourself), but you can't duplicate the essential lure, which is pest-specific. (Just like the ones we recently discussed for pantry and clothes moths inside the home. As John warns, these kinds of traps aren't used to control the pest as traps; their value is in early detection, which is how you'll be using them to control your own backyard miners.

Set up a few traps as directed and inspect them every morning. When you see adult moths stuck to the traps, wait a week and then begin weekly spraying with Bt—the organic caterpillar killer sold under brand names like Dipel, Green Step and Thuricide. Also approved by the EPA and for use in organic agriculture, Bt ONLY affects caterpillars that eat the sprayed leaves, so it's perfectly safe to spray when bees are visiting. It also doesn't harm water, fish, people, pets, toads, birds, wombats, etc. It doesn't even affect pretty butterflies, as their caterpillars aren't eating citrus leaves. Bt is the safe and effective answer for any leafminer that wants to become a moth. (And any other kind of pest caterpillar.)

Now: What about all the other kinds of leafminers out there? First, find out what they are: Caterpillar, fly or beetle larva. If it's a caterpillar, spray Bt when the nasty little things are feeding. If it's a non-caterpillar on a non-flowering crop (like spinach; a favorite target), try spinosads. Otherwise, destroy any obviously infected leaves ASAP, and encourage the many kinds of beneficial insects that can and do reach the pests inside those leaves to chow down on them. Here's a link to a Previous Question of the Week with lots of tips on attracting native beneficials to your garden.

Under NO circumstances should you EVER use standard chemical insecticides to try and control any kind of leafminer. Those poisons will not harm the pests, but will kill bees and beneficial insects (and birds, and toads, and frogs, and you, and fido...).

In many cases, leafminers get out of control BECAUSE homeowners sprayed the leaves with poisons that killed off the many beneficials about to take care of the problem. It galled him to say it, but a chemical-loving entomologist once acknowledged to me at a conference that, "the best way to control leafminers is to not spray and let Nature go to work."

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