Q: A few branches of a beech tree on the edge of our woods looks like it’s been flocked with fake snow, and there’s piles of black ash or something underneath that area. What on Earth is going on?
A: Boogie-woogie aphids! That’s a colloquial name for beech blight aphids, a native insect that feeds in aggregations on beech. Despite “blight” in their name, they are not a risk to the tree’s overall health, and plenty of other insects and birds feast on aphids, so there is no need to take any action.
This is a fun observation – when disturbed, the group wiggles their posteriors in a somewhat uncoordinated fashion, making the cottony mass dance excitedly. The juveniles (nymphs) are the only life stage reactive in this way, and they’ll even defend themselves from a predator (or a finger) with a jab from their stylet, which is like a straw they use to ingest plant sap. It won’t hurt you, but could dissuade other insect predators from messing with the colony.
They are cousins to other types of woolly aphids, named for their coating of white waxy fluff. North America’s only carnivorous butterfly caterpillar, the Harvester, feeds on beech blight aphids (plus other aphid species), and becomes covered in their wool in the process, which makes them hard to spot amid the population.
Like other sap-sucking insects, aphids produce honeydew, a sticky waste liquid that contains sugars the insect didn’t need from the sap it ingested. The black material is growth of a sooty mold, a type of fungus which feeds on this residue. It doesn’t infect the plant, but the conspicuous mounds of fungal growth (uncommonly three-dimensional compared to the types of sooty mold we usually see in gardens) can become quite spectacular on beeches supporting this particular aphid. It will eventually weather off as the honeydew production wanes.
Q: The garlic crop I wintered-over was attacked by a type of fly, I’m told, and I wasn’t able to get a full harvest. I want to plant a new batch of garlic and maybe some chives this fall. Anything I should do to prevent a recurrence?
A: Sounds like you had an experience with allium leafminer, a small fly whose larvae chew into the foliage and tunnel their way down into the bulb. While the feeding damage itself might be fairly limited, the injured tissues that result can become infected with bacteria or fungi, which cause more drastic plant losses from decay. Allium leafminer is fairly new to Maryland, having been detected in North America for the first time less than a decade ago.
This insect has two generations per year. Adults that arose from the first generation of the year that ruined your earlier crop will be hatching about now, and will continue to be active for about the next month or so. They’ll seek-out alliums (the onion family, which includes leeks, chives, scallions, shallots, and ornamental and edible onions) and lay eggs into the foliage, creating a distinctive row of pale dots. When those larvae are finished feeding, they’ll pupate and wait out the winter in either the soil or the crop’s tissues itself.
There are two chemical-free approaches to protecting your plants. You can exclude the fly adults by using fine insect netting or floating row cover to block access for egg laying, which should be deployed now if you’re already planted. You could also plant late, so the adults are mostly gone or dead by the time the plants go into the ground; garlic, for instance, can be planted in late October.
When pesticides are warranted, try the active ingredient spinosad, which is absorbed into leaf tissues (only where applied, though) and kills the larvae which ingest treated foliage. Look for the egg-laying spots first and then treat, following label directions, including regarding re-application. Make sure the product is labeled for use on edible plants since there are many different formulations of spinosad and they are not necessarily interchangeable.
You can find more information on our allium leafminer web page.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.