Although birds, mammals, frogs, and other higher animals can be important as natural enemies, they can rarely be effectively managed for biological control. These animals lie outside the scope of this discussion, which deals primarily with the predatory or parasitic insects of pest insects and mites.
These are tiny insects, only 1–2 mm in size. Nymphs and adults feed on mites, small insects, and eggs. They are very common in many agricultural situations, especially where broad-spectrum insecticides are not routinely used, and are considered to be very beneficial general predators.
Orius insidiosus, the minute pirate bug, is probably the most important species in our region. It is an important predator of thrips, aphids, and spider mites on many crops. It is also an important predator of insect eggs. It is considered to be one of the more important natural enemies of corn earworm and can destroy 50% or more of the eggs of this pest. Both the young and adults of Orius can consume 30 or more spider mites per day.
Assassin bugs (Reduviidae)
Some assassin bugs are parasitic bloodsuckers of mammals and have been implicated with the transmission of serious human illnesses. However, most species are highly beneficial predators of many serious crop pests. These are medium-sized bugs (up to about 1 inch long) and can subdue and kill medium-sized caterpillars and similar insects. They are generalist predators frequently found in gardens and fields.
Damsel bugs (Nabidae)
This is a small family of general predators commonly found in many crop and garden situations. Adults are 1/3–1/2 inch in length and slender bodied. The tan-colored Nabis ferus is a common species in our region. It feeds on many types of insects, ranging from leafhoppers to small caterpillars. Some other species of damsel bugs are black in color.
Stink bugs (Pentatomidae)
Stinkbugs are medium-sized insects with a broad, shield-shaped body. They are usually green or brown but are sometimes brightly colored. Many discharge a disagreeable odor, especially when handled, hence their common name. Many are plant feeders and some of these are serious pests on a variety of crops. However, some species, especially in the genera Podisus and Perillus, are important predators. Podisus maculiventris and Perillus bioculatus both feed on caterpillars and larvae of leaf-feeding beetles such as Colorado potato beetle and Mexican bean beetle. These are highly efficient predators capable of consuming many prey during the course of their development. Podisus has two generations per year, and each female can have 1,000 or more offspring. The eggs are laid in clusters on leaves. The young are small and round. The youngest nymphs of some predatory pentatomids may feed to a limited extent on leaf sap, but such feeding is not damaging.
Ground beetles (Carabidae)
Ground beetles vary in size from a few millimeters to over an inch in length. Most species are brown or black, but a few are metallic blue or green. There is generally one generation per year, but the adults of larger species are known to live 2 to 4 years. The larvae of some species may require more than 1 year to complete development. Carabids can be found in most agricultural and garden settings. Most species that have been studied are predaceous as both larvae and adults, although some are scavengers and a few feed on plants. The predatory species feed on insects found in or on the soil, earthworms, and similar small invertebrate animals. Many insects, even leaf-feeding insects, spend part of their life cycle in the soil or under leaf litter, especially to pass the pupal stage or to overwinter. Such insects often suffer a high degree of natural mortality at such times, and several studies have shown that ground beetles are important in such mortality.
Lady beetles (Coccinellidae)
Although frequently called “ladybugs,” these insects are not true bugs and therefore the other common names are preferred.
The lady beetles are a large group containing many important natural enemies. Although most are predaceous as both larvae and adults, a few are fungus feeders and a few feed on plants, including a couple of important pest species such as the Mexican bean beetle.
Predaceous lady beetles feed primarily on aphids, scale insects, mealybugs, and whiteflies. There are specialized lady beetles that limit their feeding to other prey groups, such as small caterpillars and leaf beetle larvae. Although most common species feed primarily on aphids and similar insects, other types of prey will occasionally be taken. Spider mites can be an important supplemental prey of many species of aphid-feeding coccinellids. Adult lady beetles also tend to feed on nectar and pollen taken from flowers.
Lady beetles overwinter in the adult beetle stage. Some species, such as our native convergent lady beetle, Hippodamia convergens, are known to congregate in enormous clusters. Other species overwinter singly or in small clusters. In spring they seek out the aphids or other hosts that will be both adult and larval food. Eggs are laid adjacent to the prey. Many deposit spindle-shaped eggs, laid on end on the leaf surface. Some species scatter individual eggs while other species lay in compact clusters of 10 to 20 or more. The eggs of the aphid-feeding species are usually yellow to orange in color, and 1.0 to 1.5 mm long. Eggs usually hatch in 3 to 7 days.
Lady beetle larvae are not as frequently recognized as are the adults, an unfortunate circumstance because they are just as important as the adults in natural control. The larvae of the aphid-feeding species are somewhat slender, with the body tapering to a point at the rear. Depending on species and stage of development, they will be 1/8 to 1/2 inch in length. The color is usually black or dark gray, but there usually are conspicuous, red, yellow, orange, or blue markings. The prominent legs are held to the side. The predators of mealybugs and scale insects may not be as conspicuous, as they are often covered in a white waxy coating similar to that of the prey insects. Larval lady beetles normally consume 500 to 1,000 aphids or similar prey during their growth.
If prey are abundant and temperatures warm, most lady beetle larvae complete development 2 to 4 weeks after egg hatch. When done feeding, the larvae pupate in the same location. The pupal stage also is unrecognized by most people. The pupal period lasts about 1 week; the entire life cycle takes about 4 to 6 weeks. Generally, there are two to three generations per year, more in warmer areas with longer growing seasons.
There are many species of beneficial lady beetles in the North Central United States, and only a few brief examples can be discussed here. The convergent lady beetle is one of the most common throughout the United States and is a very important predator of aphids and other pests. Recently, a large lady beetle from Europe was introduced into the United States for aphid control. Coccinella septempunctata, sometimes called C-7 (derived from the scientific name), has rapidly become established and spread throughout much of the region. It is very noticeable because of its large size.
The multicolored Asian lady beetle, Harmonia axyridis, is another introduced species. It is a specialized predator of aphids infesting trees such as fruit trees. It tends to congregate near buildings in the fall of the year and can sometimes be a bit of a nuisance. This is another reddish orange lady beetle, but the pattern and number of black dots of this species are variable; indeed, some have no black dots at all.
Members of the genus Stethorus are only a few millimeters long and black in color, and therefore are not very conspicuous. As both larvae and adults, these are important predators of spider mites and are capable of consuming many mites during their lives.
The twice-stabbed lady beetle is small and shiny black, with a bright red spot on either side of its body, hence the name. It is an important predator of scale insects and other pests. It is frequently seen in association with cottony maple scale and is undoubtedly important in control of this pest of silver maples.
Rove beetles (Staphylinidae)
This is the largest family of North American beetles, with about 2,900 species. Most are quite small and of cryptic habits. Most are thought to be predaceous, although many are probably scavengers. Although these are small insects, usually less than 1/4 inch in length, they are quite recognizable because of their slender, usually black body, shortened front wings (elytra), and behavior of curling the tip of the abdomen upwards when disturbed or running.
Most rove beetles are found in association with soil or decaying organic matter. Many are predaceous or parasitic and undoubtedly help reduce populations of filth flies. Several occur in agricultural soils where they probably feed on a variety of types of prey. A few species can be found in vegetation where they feed on many types of small insects.
Green lacewings (Family: Chrysopidae) are common throughout the United States and are frequently found in fields, orchards, and gardens. Both adults and larvae are important predators of aphids and other small insects. The adults, which are often attracted to lights at night, have pale green bodies about 1 inch long, large, clear, membranous wings with green veins and margins, and long hairlike antennae; the eyes are often golden and the body is slender and soft. The most commonly seen species are in the genus Chrysoperla.
The oval, white or greenish eggs are readily recognized because each is attached to a slender, hairlike upright stalk, usually about 1/3 inch in length. Although most species lay their eggs singly, some lay their eggs in clusters. The eggs are usually laid on foliage near colonies of aphids or other prey. The eggs hatch into small, gray, slender larvae that are called aphidlions. These larvae have enlarged sickle-shaped mouthparts used to puncture the prey and suck out the internal fluids. The larva ultimately grows to about 1/2 inch in length and then spins a spherical silken cocoon, usually on the underside of a leaf, within which it pupates. The entire development period is about 1 month, and there can be from one to several generations per year, depending on species and location.
Green lacewings are highly beneficial insects found in many types of crop and garden situations. They are also raised in commercial insectaries and can be purchased for biological control. Usually it is the egg stage that is sold.
Brown lacewings (Family: Hemerobiidae) are similar to green lacewings in general appearance but are brown in color and are smaller. The eggs are not stalked as in green lacewings. Brown lacewings occur both in field and forest situations but are not as common in agriculture as are chrysopids. Both larvae and adults feed on aphids and other small, soft-bodied insects.
This is a large family of common insects. The adult flies are small to medium in size, with the body often striped yellow and black: some resemble bees or wasps. They are often seen on or hovering near flowers, and the adult flies feed exclusively on flower nectar and pollen. Although the biological habits of the larvae are quite diverse, many are predaceous on aphids, scale insects, and other insects. The aphid predators are quite common. These pale green to yellow maggots have a sluglike appearance, and the larger species become 1/2 inch long. Some studies indicate that larvae consume as many as 400 aphids during development. Some larvae pupate on the foliage near the feeding site while others leave the plant and enter the soil to pupate. The puparium is often teardrop shaped. The life cycle takes 2 to 4 weeks to complete.
Flesh flies (Sarcophagidae)
This is a large family of medium to large flies. They somewhat resemble houseflies but are often gray-and-black striped and distinctly bristly. When they occur in numbers, they can be a significant nuisance because of their persistent droning and inclination to land on food and people. The larval habits are diverse, with some species breeding in carrion and others being parasitic on higher animals. Many species, however, are specialized parasites of other insects. Of the parasitic species, the largest group of hosts are the grasshoppers, and both nymphs and adults can be parasitized. Hosts of other sarcophagids include larval and adult bees (including honeybees and bumblebees), beetles, and caterpillars. One of the most common insect-parasitic sarcophagids in the northern United States attacks the forest tent caterpillar. During outbreaks of the host insect, this large fly occurs in abundance and is considered a nuisance by local residents. However, it provides significant control of the pest.
Tachinid flies (Tachinidae)
This family is by far the largest and most important group of flies, with over 1,300 species in North America. All species are parasitic in the larval stage, and many are important natural enemies of major pests. Many species of tachinids have been introduced into North America from their native lands to suppress populations of alien pests. Tachinid flies are variable in color, size, and shape, but many resemble houseflies. They are usually housefly shaped and gray, black, or striped, often with many distinct abdominal bristles.
Tachinids are usually fairly host-specific, and, as a family, most frequently attack caterpillars and adult and larval beetles. Sawfly larvae, various types of true bugs, grasshoppers, and other types of insects are also attacked. Egg formation and oviposition varies considerably, with most species laying eggs on, in, or near the host. Many tachinids exhibit an unusual trait in which the eggs mature within the mother fly, which then lays eggs that immediately hatch. In some species, egg hatching actually occurs within the mother fly, and she gives birth to living young, a behavior called “larviposition.” Egg and larval development are rapid for most tachinids, and pupation often occurs within 4 to 14 days after oviposition. The pupal period generally lasts 1 to 2 weeks. Many species are capable of several generations per year, but others are restricted to only one generation, especially if their hosts have only a single generation. Most, if not all, tachinids are internal parasites within their hosts. Most species are solitary, but some have anywhere from two or three up to a dozen or more capable of developing from a single host.
Ichneumonid wasps (Ichneumonidae)
This is one of the largest families of insects. All species are parasitic on other insects. As a group ichneumonid wasps are larger in size than many other parasitic wasps. They parasitize a variety of insects in several insect orders, but the Lepidoptera (moths and butterflies) and Coleoptera (beetles) contain the largest numbers of hosts. Many ichneumonid females have elongate, very noticeable ovipositors.
Braconid wasps (Braconidae)
Another large group and closely related to the ichneumonids, the braconids also are exclusively parasitic. Many are very important parasites of major agricultural pests, such as caterpillars, various beetles, aphids, fly maggots and other insects.
Chalcid wasps (Chalcididae)
This is actually an assemblage of several specialized families. Most species are insect parasites, but a few, such as alfalfa seed chalcid, are plant pests. Most are quite tiny insects and attack fairly small hosts, including aphids, scale insects, fly larvae, leafminer larvae, small caterpillars, and many other types of insects. Some chalcids are so tiny they complete their entire life cycle in the egg stage of their host insect. This is true of all species of the families Mymaridae and Trichogrammatidae. Trichogramma species attack the eggs of many types of serious pests, especially caterpillars. Trichogramma are commercially mass produced and are widely used in biological control programs in Europe and Asia, and to a lesser degree in the United States.