Beneficial Insects

Many people believe that the only “good bug is a dead bug”, but in the garden, this old axiom is definitely not true.

There are a great many “good” insects that fight on our side against destructive insect pests. These allies and partners of the insect wars are called “beneficial insects”. Many times it is only through their assistance that the tide of battle turns our way.

For example, aphid populations can often be heavy in gardens, especially when conditions are right for their growth. Many want to minimize the damage caused by these insects without using large amounts of pesticide. If we are good stewards and protect the beneficial insects, they will help us maintain the overall health of our vegetable and flower gardens.

Beneficial insects work in two different ways. Predators feed outright on the bodies of insects that eat the tissue or suck the juice of plants. These helpful insects are generally much larger than their prey which gives them an edge as they attack. Lady beetles, lacewing larvae, and assassin bugs are examples of predator insects.

Other beneficial insects lay their eggs on or within the bodies of their prey. Once the egg hatches, it is the larva, or young of the beneficial insect that uses the host insect for food. Insects that complete their life cycle in this manner are called parasites. Adult parasite insects are often much smaller and weaker than their prey and rely on their agility to provide the edge needed for success. Parasitic wasps and flies are included in this group of beneficial insects.

One of the best known predators is the ladybeetle. These rounded beetles come in many sizes and colors. The most common species found in Arizona is the convergent ladybeetle, named for the two converging white stripes behind the head. The beetles are brightly colored with red front wings speckled with black markings. The adults lay orange egg clusters on plants near groups of aphids. The eggs hatch into tiny black and orange larvae which feed on aphids in great numbers. As the larvae grow, they resemble tiny beaded dragons. Once they reach maturity, they form a rounded black and orange-marked pupa attached to the plant. The pupae is often mistaken for bird droppings.

During the last week in May, up on the Mogollon Rim above Strawberry, I found a wild rose with a heavy aphid population feeding away. Sure enough, right there among them was the familiar black, yellow and red larvae of a lady beetle happily enjoying breakfast. I have no doubt that the wild rose is now aphid free.

The green lacewing is another outstanding example of a predator insect. Adult lacewings are delicate, pale green insects about one-half to three-fourths of an inch long. Their wings have many veins, which gives them the netlike or “lace” appearance. They are attracted to lights at night and may be mistaken for moths except they have a characteristic fluttering flight when disturbed. Lacewings lay their pale green eggs on the tips of threadlike stalks on the underside of leaves. The immature lacewings hatch within a few days. They are no longer than one-eighth inch and are light brown in color. Their shape resembles that of an alligator and have large, sickle-shaped mandibles with which they suck the juices from insect eggs and small prey. They are ferocious feeders, and consume large numbers of aphids and other insect pests. When the larvae mature they form a yellow silken cocoon in which to pupate.

There are also two species of lacewings that are brown as adults. They also feed on small insects and insect eggs in the larval stage

The praying mantis is among the best known of the predator insects. It sits and waits on plants until another insect crosses its path, and then it captures its victim with its spiny front legs. Female praying mantises lay their eggs in one to two inch long “cases” made of a dark brownish-gray papery material with numerous compartments. The egg cases are glued to twigs or branches, and are commonly found attached to the underside of boards. Praying mantis young emerge from the cases in the spring. They look like miniature adults.

Other predators include the descriptively named assassin bugs and ambush bugs. There are also damsel bugs, big-eyed bugs, minute pirate bugs, syrphid flies, wasps, and dragonflies. Altogether, they make a formidable array of defense working to help maintain the balance of nature.

Beneficial insects that act as parasites include some wasps, flies and beetles. The adult form lives outside of the host insect but lay their eggs on or within a living host. After the eggs hatch, it is the young which feed on host tissues until the host is killed. Immature parasites complete their development in only one host. Because they are extremely specialized, they often only attack one or a few closely related species of insect. Parasites of insects do not in any way harm humans or their pets.

A fascinating example of a parasite is the eucharitid wasp which attacks ants. This particular wasp lays her eggs on the leaves of trees. The eggs hatch into mobile immature larvae that are able to crawl about on the leaf surface. In the spring, worker ants climb into the trees in search of aphids and other insects for food. The parasite larva attaches itself to any worker ant that comes close and hitches a ride back to the nest when the worker ant goes home. Once in the nest, the parasite drops off and attaches itself to a larval ant. The wasp larva feeds on the ant larva, eventually killing the ant. After emergence from the pupa, the adult wasp flies out of the ant nest to lay her eggs on leaves once more.

Other types of parasitic insects control aphids, whiteflies, grasshoppers, beetles, moths, bees and insects. Even though they are often not seen by the average person, they are definitely there and doing their job.

In the fight for control of the garden, the predator and parasitic beneficial insects are the little known heros of the garden. Both types destroy many insects every day that would otherwise damage or kill our tender garden plants. Some work quickly and produce dramatic results; others work so slow that their efforts are rarely recognized in the garden. Both, however, are critical to maintaining the balance of nature in the plant’s favor. They deserve our respect and appreciation.

If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221, extension 204. For more information, contact Rick Gibson at gibsonrd@ag.arizona.edu.