From the Mailbag: How do I control aphids?

August 10 - a complete stranger asks the following:

"Hey Trav - It looks like we've got aphids all over a couple of our plants. How do we control them? A friend told us to just spray dish soap on them - will that work? "

While our plants have been enjoying the rather nice weather we've experienced overall this past spring and summer, the aphids have also found it to their liking. I keep having to deal with them on the Brussels sprouts and cabbages in my garden.

To answer the question - soap can be an effective control agent with smaller insects. Soap acts by coating the insect, which blocks the spiracles (openings in its body through which it breathes) and also dries the insect's body out. The problem is, some soaps (depending on the length of the fatty acid chains found in the soap) can also damage or even kill plants. In all likelihood your dish soap is going to be safe for the plant, but it's a good idea to first test it on just a few of the plant's leaves. If it's going to hurt the plant, it'll be apparent within a few hours.

A better idea is to purchase a soap made specifically for this purpose, such as Safer's Insecticidal Soap ("Safer" is the name of a company that specializes in natural and/or organic products for your garden). Whatever type of soap you decide to use, be sure to spray so the entire leaf surface is coated. Since aphid infestations tend to cause plants' leaves to roll up, you may have to hold some leaves open while you spray.

Personally, I have found it effective to simply squish the aphids using my thumb. If there are a lot of them, I'll dip my thumb in soap before I begin the massacre.

For more information on pest control, read A Pest Management Primer.