By Laura Jesse
Extension Specialist
Iowa State University
Many gardeners strive to grow beautiful, insect-free plants, but as an insect lover, my favorite plants are the ones that always get pest insects. One favorite in my yard is a yellow daisy that becomes infested annually with beautiful red-colored aphids. For good insect viewing it is hard to beat the aphids. Aphids come in a variety of sizes and colors but most have a soft, pear-shaped body up to one-eighth inch long. Common aphid species come in all shades of green as well as black, pink, yellow and the already-mentioned bright red. The one distinguishing characteristic present on all aphids is not always easy to see, but it is there: each aphid has a pair of tubes on the top side of the back end of the abdomen. The tubes are called cornicles and are used to secrete wax and other substances.
Aphids have one of the most fascinating life cycles of any insect. Females can clone themselves and give birth to live young. This is one of the reasons why aphid populations can increase so quickly. Females don’t need to pause for sexual reproduction and egg laying, they simply clone a good thing! The next time you have aphids, look for a larger aphid surrounded by smaller ones. This will probably be an adult female surrounded by her cloned daughters. There are male aphids. In the fall, females will produce male aphids, mating occurs and the females lay eggs capable of overwintering.
Aphid excrement is called honeydew because is contains a lot of sugars from the plants they feed on. The honeydew is usually the first thing that a gardener notices when there are aphids or other plant-sucking insects on a plant. The honeydew collects on leaves below the feeding aphids and makes them shiny. A fungus called black sooty mold is commonly associated with aphid honeydew and is often more damaging to the plants than the aphids. The sooty mold coats the leaves with the honeydew and prevents sunlight from reaching the leaves. I have noticed that the sooty mold, not the aphids, usually kills my daisy.
Aphids always seem to attract other insects. While lady beetle and lacewing larvae commonly feed on aphids, many species of ants will actually take care of them. These ants love to consume the sweet honeydew and will do their best to get rid of predators that attack their aphid flock. They have even been known to carefully move their aphids to a fresh plant if the one they are feeding on wilts.
My favorite part of aphid watching is the parasitic wasps. It is easy to tell if they have been attacking because the aphids become jumpy! Seriously, the aphids will twist around in circles when blown on. Aphids are nearly defenseless and apparently this jerky motion must help to prevent wasp attack. The parasitic wasps are tiny, not much bigger than the aphids themselves. The female wasp will approach an aphid and then rapidly bend her abdomen underneath her body and lay an egg in the aphid while still facing it. It is an acrobatic move that is definitely worth waiting for. The egg hatches inside the aphid where the wasp larva feeds, eventually turning the aphid into a mummy. Aphid mummies are immobile, usually a golden-yellow color and the aphid is dead by this time. After pupating, an adult wasp emerges from the aphid mummy by making a hole in the abdomen. If parasitic wasps have been present, aphid mummies can be found, usually near the edges of the aphid colonies.
Aphids are not the only interesting insects to inhabit our gardens. Be sure to stop and examine the insects feeding on your plants before you reach for an insecticide. I try to encourage all gardeners to think of themselves not as plant people, but rather as insect people who lovingly care for a garden full of delicious plants for feeding insects. So far, I have not had much luck with this campaign!
Contacts :
Laura Jesse, Plant Pathology, lrahnsen@iastate.edu, 515-294-0589
Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu