A Convincingly Crappy Crepe Calamity

Q. Nancy in Milton Delaware writes: "A fungus has infected crepe myrtles throughout our area. I have had my trees treated. I don't know what the tree service used by name; just that it's a fungicide. But the black "mold" or "soot" or whatever it is, has been deposited all over the shrubs underneath. Is there anything I can apply to my shrubs to get the black off without damaging the plants?"

A: I knew right away from the 'black soot' description that Nancy's problem was not caused by a fungus, but clusters of aphids sucking sap from the leaves and then depositing their "Honeydew" down below. But I did not know (and soon learned) that these aphids are a species specific to crepe myrtle! Commonly known as the crepe myrtle aphid, its scientific name is Tinocallis kahawaluokalani.

Although native to Southeast Asia, the species name is clearly Hawaiian, which makes sense as the pest was first discovered and named in Hawaii. Interestingly (to me anyway) these specific species of little bugs feed only on crepe myrtle and no other plants. And while they do make plants look nasty, they do no long-term damage to the crepe myrtles they're sap-sucking, which means that they would be considered only a cosmetic problem to insect experts.

But that is small comfort to gardeners left with poop covered plants.

Although these specifically named pests are specific to crepe myrtle specifically, the honeydew problem occurs with all species of aphids and is not strictly ornamental if you park a car underneath the affected plants and the frass (aka bug poop) eats the paint away, which by the number of websites we found devoted to this problem, occurs all too frequently.

One easy remedy suggested for pooped-on plants is to clean the honeydew off with a soft cloth wet with warm water. You can also spray the frass with a light horticultural oil (a version designed for warm weather use; NOT 'dormant oil'), which is said to cause the poop to flake off the affected plants. Or a very dilute solution of vinegar (two tablespoons of vinegar per gallon of water) can be used instead of plain water; but keep the water warm.

However, as with our own health, prevention is a lot easier than remediation.

Like most insect pests (only more so) aphids are attracted to plants under stress. Excessive Nitrogen fertilization is a major cause of aphid infestations as it forces excessive new growth full of tasty aphid-attracting sugars. If you have a 'conventional' lawn service that uses chemical fertilizers, you're going to have excessive numbers of aphids on nearby plants.

Same with "pest control". If the crepes in question were sprayed with a fungicide (because someone yelled "fungus!" in a crowded theatre), the only harm done was to the health of the homeowner, the pesticide applicator, dogs, cats, children, etc. It would not have affected the problem, because the problem is clearly not a fungus.

But if the tree service recognized the problem for what it really was, they would have used an insecticide, which would limit the life spans of all the above and kill the beneficial insects that are excellent at controlling aphid populations in an environment that isn't overly contaminated with chemicals.

But that's not likely if a 'tree service' is involved. The more visits a conventional 'service' makes, the more they can charge, so overuse of chemical treatments and treacherous wood mulch is the norm. And if that is the case you're going to be saying "Hello aphids!" every season.

But if you forego useless, unnecessary, and dangerous 'treatments' your landscape is likely to take care of itself. Beautiful diaphanous green lacewings are the fiercest predator of these pests. You can buy them professionally raised in the egg stage; place the eggs or egg container around the plants you wish to protect and "aphid lions" will emerge. These larval forms of lacewings will hatch out and immediately seek out any nearby aphids. Note that the early stages of this highly beneficial insect look like little lobster dragons before they become adults.

The same is largely true of ladybugs, whose larval stage have a very similar appearance to green lacewing babies before they transmogrify into the ladybird beetle we all know and love. You can purchase live adults from many sources. Release them at sunset into a thoroughly wet garden to avoid their flying away. They'll lay their eggs near any aphid infestations they can find, and the larval mini-monsters that emerge will chow down on as many as 50 pests a day.

Note: the multi-colored Asian ladybugs that many people dislike because they seek shelter in homes over the winter are the alpha predators here, which makes sense as they're both from the same part of the world. If you are lucky enough to have them decide to stay with you this winter, fill a clean bag for a canister vacuum with shredded straw, shredded leaves, or raffia. Moisten the material with a mister and then suck the ladies into the bag and store it until Spring in the fridge. This is easy to do as the ladies almost always cluster in one corner of an upstairs room.