Fight off pesky spirea aphids

Q:I was gathering some flowers for a vase, and I wanted some greenery, so I cut off some spirea branches. I laid them all out on the counter to make the arrangement when I noticed millions — I really mean millions — of green insects crawling all over the spirea stems and the counter. I destroyed them all, and when I looked outside I saw that the spireas were covered with these insects. How can I get rid of them and prevent this infestation from happening again?

A:Most likely your spireas are infested with an insect called the spirea aphid. Many kinds of plants have their own aphid insect pest. Spirea aphid populations grow incredibly fast. They all hatch as females, and those females begin giving birth to nymphs after just 20 days. Each female can have between 50 and 100 offspring.

They are sucking insects that prefer to suck fluids from the tender tips of plant stems or the bottom of leaves.

You can use a garden hose (at least twice a week) to knock many of them off the ends of the branches and out into the lawn where they will die. Pruning will remove the colonies on the ends of the branches.

However, neither method will kill all of them, and they will reproduce fast enough to rebuild the population in a couple of weeks.

These two methods are safe for predator-insect populations that grow much slower.

Leave the branch cuttings in a pile to let aphid predators (ladybugs and green lacewings are two common ones) escape. Ladybugs look wrinkled during the larval stage (not like a ladybug at all), so look out for that.

If you don’t see any predators, you could try using insecticidal soap or a summer-grade horticultural oil. Both of these are contact insecticides that leave very little residual, which could harm predators.

Even if you don’t have spireas there may be other aphids on other plants in your landscape. Aphid populations expand rapidly if there isn’t a lot of rain to knock them off the ends of branches. They secrete honeydew, a sticky sugar solution, to coat the surface of the plant they’re on so they can stick to it.

Honeydew may eventually be covered by a fungal growth called sooty mold, which turns all the sticky surfaces black. Both honeydew and sooty mold can be washed off with soap and water.

However, the solutions mentioned above will work on other types of aphids, too.