APRIL
1989

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

04-28-89

Plant Lice

Springtime. That time of the year when plant fanciers' thoughts turn to green growing things—and to insects! Indeed, each spring, growers of ornamental and food plants prepare to assist their leafy-green wards in the "Battle of the Bugs."

About half of all insects feed on plants. It's a natural ecological process; the plant is the producer, and the insect, a consumer. Most plant owners, however, take a dim view of such insect activity.

Almost all plants are associated with one or more species of aphids. Sometimes called plant lice, aphids are small insects about the size of the head of a pin. Some have wings and some are wingless, even within the same species.

Aphids are one of the suckers of the insect world. They get their food by sucking sap from the leaves and stems of plants. In doing so, they can weaken the plant, cause abnormal tissue growth, and even transmit disease organisms. That's enough to send even the most seasoned plant grower looking for the spray can.

How can a little insect provoke such fear? It has to do with numbers. You see, aphid populations can grow very rapidly. As Erasmus Darwin, grandfather of Charles, once wrote:

The countless aphides, prolific tribe,
with greedy trunks the honey's sap imbibe;
swarm on each leaf with eggs or embryons big,
and pendant nations tenant every twig.  

Many aphids give birth to live young. Some of these young can themselves become mothers in about a week. To help speed the process, some species have cut out the middle man—they reproduce without mating. No need to slow down the process by taking time to find a mate.

Aphids aren't despised by all creatures. For example, ants use the aphids' waste material, called honeydew because it is sweet, as a food resource. So attached are some ants to honeydew that they protect the aphids that produce it. This has prompted some scientists to call aphids "ants' cows." Ants are even known to move aphids from one plant to another as they manage their herd.

A few insects see the aphids themselves as the meal. Ladybird beetles are famous as predators on aphids. So are lacewings. Because the predators eat so many aphids, gardeners love these beneficial insects. Some people even purchase ladybird beetles for release to help in aphid control. However, it is important to make sure that aphids are present before turning the ladybird beetles loose. If aphids aren't present at the time of release, the ladybird beetles won't have food to eat and will leave.

In addition to causing plant stress, aphids sometimes create other problems by their presence. As aphids feed on trees, their honeydew can drift downward and leave rather unsightly spots on automobiles. Honeydew on plants provides a great place for sooty mold to grow, which results in a black layer on the surface of the plant. It is not a pretty sight to the plant owner.

All of this from a little insect called an aphid. It's no wonder that they are called plant lice. The very thought of having them makes most gardeners get itchy!

 

Writer: Tom Turpin
Editor: Carol McGrew