Good Bugs in the Field
Ladybugs are common predators, and there are many different species that are active in Northeastern vegetable fields. Some, like the pink spotted lady beetle, are native species, while others, like the seven-spotted ladybeetle, were introduced. Here, a pink spotted lady beetle feeds on Colorado potato beetle eggs.
Photo by Whitney Cranshaw, Colorado State University (Creative Commons license).
Not all insects are pests; some do no harm to crops, and many actually work on your behalf by attacking other insects that can damage crops. In fact, there are dozens, if not hundreds, of species of so-called beneficial insects hanging around your fields. The two major types of bug-eating bugs are predators and parasitoids.
Predators include lady beetles, ground beetles, lacewings, predatory mites and spiders (which are almost insects, but not quite). Predators tend to be larger than their prey, and they usually eat a lot of them. They're not fussy eaters in that many are generalists and will often eat more than one kind of prey.
Parasitoids include many families of tiny wasps, as well as tachinid flies. They specialize in one kind of insect host, which is usually bigger than they are. Only females search for the host, and then they deposit eggs or larvae in, on or near it. The immature stages of the parasitoid remain on or in the host, consuming it as they grow. Unlike parasites, such as fleas and ticks, parasitoids usually kill their host.
It can be a challenge to pronounce the names of many of these predators and parasitoids. Whether you pronounce them right or not, it's important to know they're out in your fields and to take steps to conserve their populations.
Lady beetles may be the most familiar group of beneficial insects. As adults, they are easily recognized by their oval-round shape and bright red and orange colors. However, lady beetle larvae don't look at all like their parents. In their youth, lady beetles are usually dark-colored with yellow or orange flecks and have an elongated body. Both the adults and larvae can eat a lot of small, soft-bodied insects like aphids. And there are many species of lady beetles, both native and introduced.
The pink spotted lady beetle (Coleomegilla maculata) is a relatively common native lady beetle. It's a medium-sized, pink to red, oval-shaped beetle with six spots on each forewing. Both the adults and larvae are important aphid predators. They also prey on mites, as well as the eggs of European corn borer, imported cabbage worm, fall webworm, corn earworm, asparagus beetle, Mexican bean beetle and Colorado potato beetle. This insect is a major pest control machine. However, pollen is an essential part of its diet, so maintaining areas of early flowering plants, including dandelions, in or near fields may help support dispersing adults in the spring.
The convergent lady beetle (Hippodamia convergens) is common from southern Canada down to South America. It has a slightly elongated shape, white lines that converge on a black area behind the head, and anywhere from a few to 13 black spots on red forewings. The adults and larvae feed mainly on aphids, and they are active in the spring and summer in the eastern U.S. when aphids are present.
There are many other species of lady beetles that feed on insect pests in vegetable crops. These include the two-spotted lady beetle, the nine-spotted lady beetle, and the transverse lady beetle. However, not every kind of lady beetle is helpful. One species, the Mexican bean beetle, is a well-known plant pest that can do severe damage to beans. (Another beneficial insect, Pediobius foveolatus, is a commercially available parasitoid wasp that lays its eggs in Mexican bean beetle larvae. Wasp larvae feed inside the Mexican bean beetle larva, kill it, and pupate inside it, forming a brownish case or "mummy.")
Ground beetles are often dark and shiny, and the many different species range in size from 1/8 inch to 1 inch. Adults are often found under stones and debris. Those species that are active at night are black, and those that are active during the day may be metallic or bright in color. Pests consumed include the eggs and larvae of Colorado potato beetle, root maggots and asparagus beetle, as well as the larvae of cabbage worms. Rove beetles, soldier beetles and flower beetles are also insect pest predators.
Lacewings are green or brown insects with net-like, delicate wings, long antennae and prominent eyes. The larvae are narrowly oval with two mouthparts that pierce their prey and suck out the fluids. Often the larvae are covered with the debris of the bodies of their prey.
Lacewings overwinter as larvae in cocoons, inside bark cracks or in leaves on the ground. In the spring, adult lacewings become active and lay eggs on tree trunks and branches. These whitish eggs are laid singly and can be seen connected to the leaf by a long, threadlike stem. Lacewings feed on aphids, leafhoppers, scales, mites, and the eggs of butterflies and moths.
Syrphid flies are also known as flower flies or hoverflies. Most are brightly colored, yellow or orange and black, and may resemble bees or yellow jacket wasps. However, syrphid flies are harmless to people. Usually they can be seen feeding on flowers. The larval stage of the syrphid fly 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 called Aphidoletes. These insects can often be seen feeding within aphid colonies late in the season.
True bugs can be plant pests, like tarnished plant bugs, but others are predators of insects and mites. True bug predators typically have front legs that grasp and hold prey while they pierce their bodies with narrow mouthparts in order to extract their body fluids. Most likely to be seen are damsel bugs, ambush bugs and assassin bugs. Minute pirate bugs are quite small, less than 1/8 inch, and are frequently seen in flowers or in crevices of a green plant, where they feed on thrips, spider mites and insect eggs.
Hunting wasps include several different kinds of wasps that prey on insects. Many take their prey, whole or in pieces, back to their mud, soil or paper nests to feed to immature wasps. The common Polistes paper wasps, when hunting, may thoroughly search plants and feed on caterpillars, helping to control these insects.
Predatory mites are predators of plant-feeding spider mites. These mites are usually a little larger than the spider mites they eat, but have a rounder shape and can move faster than their prey. Predatory mites often provide good control of spider mites. Low humidity can restrict their activity, and they are more susceptible to insecticides than are plant-feeding species.
Spiders feed on insects or other small arthropods. Everyone knows about the web-making kinds of spiders, but there are many other types of spiders that don't build webs, including wolf spiders, crab spiders and jumping spiders. Instead of waiting for insects to come to them, these spiders move around and hunt their prey. Although less easily observed, hunting spiders can be important in controlling beetles, caterpillars, leafhoppers and aphids.
There are many kinds of parasitoid wasps. The Braconidae wasp family alone has over 1,900 species, and many are important natural enemies of insect pests. Adult braconid wasps are small, typically less than 3/8 inch long. The female usually inserts eggs into a host, which may live for some time before the wasp larvae mature and emerge as adults. Here, a tomato hornworm continues to feed while hosting cocoons of the braconid wasp Cotesia congregata.
Photo by Vern Grubinger.
Tachinid flies are insect parasitoids that are typically gray or brown and covered with dark bristles. Most look like common flies, but they have very different eating habits. The adults lay eggs on various caterpillars, beetles and bugs, usually near the head. The eggs hatch almost immediately, and the young maggots tunnel into their host. After feeding internally for a week or more, the tachinid fly larvae eventually kill the host insect. The many kinds of tachinid flies are important natural controls of insect pests, particularly caterpillars. However, tachinid flies are rarely observed, and thus their beneficial activities are often overlooked.
Braconid and ichneumon wasps include a large and diverse group of insect parasitoids. Some are small and attack small insects such as aphids. Others live in the eggs of various pest insects. Larger wasps attack caterpillars or wood-boring beetles. External evidence of these parasitoids' activity is often more obvious than with the tachinid flies. For example, aphids that are parasitized by these wasps are typically small and discolored and called "mummies." When a diamondback moth larva is parasitized by the wasp called Diadegma insulare, it can often be seen as a white, fuzzy cocoon attached to the underside of a cabbage leaf. Many farmers have also seen the white cocoons of the parasitoid wasp called Cotesia congregata attached to the outside of tomato hornworms.
It's important to recognize that naturally occurring insect predators and parasitoids are on the job, whether you can see them, identify them and pronounce their names, or not. Conserving these populations includes two major tactics. One is to provide sources of pollen, which may mean leaving "weedy areas" undisturbed on the farm so that dandelions, wild carrot, goldenrod, etc., are allowed to flower. The other tactic is to try not to kill them, by avoiding the use of broad-spectrum insecticides of any kind, organic or conventional.
There is an excellent website that provides photographs and descriptions of biological control agents of insect, disease and weed pests in North America. It can be found at http://www.nysaes.cornell.edu/ent/biocontrol/.
The author is a vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at firstname.lastname@example.org.