Beneficial Insects and Other Arthropods

Introduction

Most insects and other arthropods found in the yard and garden do not feed on or harm plants. Many of these are just passing through or have innocuous habits. Others feed on and destroy pest species. In many cases, the activities of these beneficial species can completely prevent or greatly limit pest problems. It is important to recognize these beneficial arthropods so they may be appreciated and conserved. 

Beneficial arthropods are categorized broadly as either predators or parasites. Adult or immature stages of insect predators, or both, actively search out and eat prey insects. Predators include lady beetles, lacewings, spiders, and more. Insect parasites develop in or on a single host from eggs or larvae deposited by the adult parasite. Common insect parasites are tachinid flies and the braconid and ichneumonid wasps. 

Insect Predators

Lady Beetles 

Often called ladybugs, lady beetles (Figure 1) are the most familiar insect predator. Most adult lady beetles are round to oval, brightly colored and often spotted. The immature or larvae stages, however, look very different and often are overlooked or misidentified. Lady beetle larvae are elongated, usually dark colored, and flecked with orange or yellow (Figure 2). 

Adult and larvae feed on large numbers of small, soft-bodied insects such as aphids. One group of small, black lady beetles (Stethorus) is important in controlling spider mites and others specialize in scale insects or mealybugs. Lady beetles can rapidly control many developing insect problems, particularly if temperatures are warm enough. 

One species of lady beetle, however, the Mexican bean beetle, is a plant pest. This common Colorado insect is found feeding on bean leaves. It is distinguished from other lady beetles by spotting and color in the adult stage. Larvae of the Mexican bean beetle are yellow and spiny. 

Green Lacewings 

Several green lacewing species (Figure 4) are commonly found in gardens. The adult stage is familiar to most gardeners: a pale green insect with large, clear, highly-veined wings that are held over the body like a tent when at rest. Adult green lacewings primarily feed on nectar and other fluids, but some species also consume a few small insects. 

Green lacewings lay a distinctive stalked egg meant to prevent predation or parasitism from other arthropods. Lacewing larvae emerge in four to 10 days. These larvae, sometimes called aphid lions, are voracious predators capable of feeding on small caterpillars and beetles, as well as aphids and other insects. In general shape and size, lacewing larvae superficially resemble lady beetle larvae. However, immature lacewings usually are light brown and have a large pair of hooked jaws sticking out from the front of the head (Figure 5). 

Syrphid Flies 

These flies are called by several names, such as flower flies or hover flies. Most are brightly colored, yellow or orange and black, and may resemble bees or yellowjacket wasps. However, syrphid flies are harmless to people. Usually they can be seen feeding on flowers. 

It is the larval stage of the syrphid fly that preys on insects. Variously colored, the tapered maggots crawl over foliage and can eat dozens of small, soft-bodied insects each day. Syrphid flies are particularly important in controlling aphid infestations early in the season, when cooler temperatures may inhibit other predators. Similar in appearance to syrphid fly larvae is a small, bright orange predatory midge (Aphidoletes). These insects often can be seen feeding within aphid colonies late in the season. 

Predatory Bugs 

True bugs (Order: Hemiptera) are predators of insects and mites. All feed by piercing the prey with their narrow mouthparts and sucking out body fluids. More common, but less frequently observed, are the various light brown damsel bugs, also called nabid bugs. Damsel bugs are found on the foliage of all crops, where they seek out aphids, insect eggs and small insect larvae.

Most common of all the predatory true bugs are the small (less than 1/8 inch) minute pirate bugs. Minute pirate bugs are most frequently seen in flowers or in crevices of a green plant, where they feed on thrips, spider mites and insect eggs. Other predatory bugs common in yards and gardens include ambush bugs and assassin bugs. 

Ground Beetles 

Various species of ground beetles are found under debris, in soil cracks or moving along the ground. Immature stages are distinctly different from adults and more often are found within the top few inches of soil. Ground beetles are general feeders with powerful jaws. Almost any garden pest that spends part or all of its life on the soil surface may be prey for these insects. 

Mantids 

Mantids are uncommon in most of Colorado but are familiar insects to most gardeners. Mantids are general predators that feed on almost any insects of the right size. They have one generation per year with winter spent as eggs within a pod (ootheca). An invasive species of mantid, the Chinese mantid, is sometimes available for sale.  

Hunting Wasps 

A large number of wasps from several families prey on insect pests. Many take their prey, whole or in pieces, back to their mud, soil, or paper nests to feed to the wasp larvae. These hunting wasps can be important in controlling garden insect pests. For example, the common Polistes paper wasps, when hunting, may thoroughly search plants and feed on caterpillars, often providing substantial control of these insects. 

Predatory Mites 

Several mite species are predators of plant-feeding spider mites. Typically, these predatory mites are a little larger than spider mites but are more rounded in shape and faster moving than their prey. Predatory mites often can provide good control of spider mites. Low humidity can restrict their activity. They are also more susceptible to insecticides than are plant-feeding species. 

Spiders 

All spiders feed on insects or other small arthropods. Most people are familiar with many common web-making species. However, there are many other spiders — wolf spiders, crab spiders, jumping spiders — that do not build webs but instead move about and hunt their prey on soil or plants. These less conspicuous spiders can be important in controlling insect pests such as beetles, caterpillars, leafhoppers, and aphids.