The great outdoors can produce baffling mysteries. MVTimes Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it to onisland@mvtimes.com.
Dear Matt:
What did this to my columbine plant?
– D.Z.
Dear D.Z.:
These damaged leaves most probably show the work of a columbine leaf miner, the larva of a tiny fly in the genus Phytomyza. This pest (if you want to view it that way) appears to be quite widespread and common, and it’s hard to grow columbine (Aquilegia) without attracting at least a few of these insects. Severe infestations can weaken or, conceivably, even kill a columbine plant, but in most cases no permanent damage is done and the only negative impact is seen on the leaves, such this one, which might have pale, winding tracks on them. Depending on how bad the infestation is, and your tolerance for partially eaten leaves, you can remove leaves that you see affected by leaf miners, or you can do nothing, taking some satisfaction in the fact that your garden is supporting an interesting insect. Because they are enclosed within the leaf, using chemicals to control leaf miners is hard to do (and I recommend against using insecticides at all unless you have a really compelling reason).
More generally, leaf mining is an interesting strategy used by an astonishing array of insects. The majority of leaf mining insects are probably flies (there is one whole family of flies that specializes in leaf mining). But certain types of moths, beetles, and wasps have developed the same lifestyle for their larvae. If you know what you’re doing (and unfortunately, I don’t) you can often identify a leaf miner by the shape of its tunnels. Also, there is a very close association between a particular leaf-mining insect and a particular species of plant. So identifying the plant that is being mined is a good way to start on identifying the leaf miner itself.
The advantages of being a leaf miner are clear: you’re totally immersed in your food supply, as you crawl your way between the top and bottom cuticles of the leaf. And the tough, outermost layer of cells on the leaf offer some protection against predators, desiccation, and pathogens. The adult form of a leaf miner, though, is as vulnerable as any other free-living insect, and naturally occurring predators and parasites generally keep leaf miner populations controlled to a reasonable level.
Southern New England is lucky to have an entomologist who is one of the most knowledgeable people in the world about leaf miners. Charlie Eiseman, though based in western Massachusetts, has done extensive work on leaf miners and other obscure insects on Nantucket. Charlie has written a book, Tracks and Signs of Insects and Other Invertebrates, that touches on leaf miners, and he’s working on a book entirely devoted to these highly specialized insects. His website, charlieeiseman.com, is worth checking out if you have an interest in the more bizarre branches of the insect world.