When Aphids are the Root of the Problem

Q. Aletha in Rochester, Michigan writes: "Root aphids were unknowingly introduced into my garden last season from lettuce plants I purchased at my local farmer's market. When I discovered the problem, I placed the affected plants in the garbage. As I did more reading, I discovered that root aphids can overwinter. I removed and trashed all of the remaining plants and soil from that particular 2x6 elevated bed, with the intent on replacing it with new soil this spring. I left the shovel I used in the bed because I wasn't sure if it would spread the problem to my other beds. With the new planting season just around the corner, I'd like to know what I can do to prevent the root aphids from reappearing in that bed or any of my seven adjacent beds. Can you advise?"

Q. Sue in South Arkansas adds: "One of my flower beds is infested with root aphids. How can I eradicate them? How often should I retreat the bed to be sure they are gone??

"The flower beds were nice and healthy, then my mom bought some ornamental cabbages from a store and planted them in the flower bed. Within a few days, her lettuce was dying and the ornamental cabbage was withering. Normally, everything she plants thrives so this was concerning. She dug down underneath and found that the plants' roots weren't even attached to the dirt any longer. Then she found the little creatures.

"We looked up pictures and narrowed it down to root aphids. She began treating the bed with anything she could find: diluted dawn dish soap, neem oil, etc.. When that didn't work she tried something with pyrethrin in it, but the critters just won't die! She currently has three Ziplock bags containing some of the aphids and has been trying different things to see what will kill them, as she is afraid that they're going to venture into other beds. She is careful to sanitize her tools so that she doesn't unknowingly spread their eggs. She is at her wits end!"

A. Her?! Imagine how the aphids feel! Good thing she didn't have any napalm laying around.

Anyway, you have to be careful with products containing any type of pyrethrum, as one form (collected from the dried leaves of a specific variety of daisy) is natural and organically approved, but the others are sound-alike man-made products. Their spellings are frustratingly alike--just a few different letters here and there--and all forms are toxic to bees. Perhaps more importantly, they are contact pesticides that kill insects that munch on the sprayed plant parts; they aren't labeled to affect anything unground. However, it occurs to me that a liquid solution of pyrethrum (that's the old original organically approved form of this compound) could be very helpful when used as a soil drench.

To try this, water the area well, wait until sundown and then pour the drench around the root zones of affected plants. When used out in the open, sunlight and air quickly degrade the effectiveness of pyrethrum (which is why the more stable chemical versions were developed). But once it's underground and protected from the elements, it should remain effective MUCH longer. Just to be safe, lightly mulch the soil surface with some compost or screened topsoil after the application to make sure the drench isn't compromised by sunlight.

A solution of Dawn (or other brand name) dishwashing soap, or even professionally made insecticidal soap, should be avoided. Soaps and oils only kill insects that are visible and become covered by the spray. They have no residual effect, and using any kind of soap solution as a soil drench could kill your plants.

Neem oil is mostly used as a contact spray, but it does have some residual action. You may want to try making a drench with it and testing it on one plant to make sure there are no adverse effects.

As to the aphids themselves, they are VERY small--about the size of most mites. They can enter your garden through purchased plants, in infected soil or compost you purchased, and/or by flying from plant to plant when they reach the end of their life cycle and come above ground to lay eggs. Ants love to eat their {quote} 'honeydew' and have been observed moving them around from plant to plant. So be suspicious of any unusual ant activity.

All sources agree that any infested plants you discover should be trashed immediately, along with some of the soil around their roots. And yes, wash shovels and trowels with soapy water to avoid hitchhikers.

I really like the idea of using beneficial nematodes against them. These microscopic creatures are fierce predators of soft-bodied underground pests like lawn grubs, and I expect they would go after root aphids with great zeal. They're shipped and/or sold in a sponge-like container containing millions of the underground heroes. Wait until dark and then water them into the affected areas; you should see results fairly quickly. Note: They pose no threat to earthworms, pets, people, etc.

And finally, be careful that your online search results are specific to ROOT aphids. Despite using the word 'root' every time, more than half of my search results were about above-ground aphids.