5 Beneficial Agents On the Farm
Biological control is the use of natural enemies to manage pests. The natural enemy might be a predator, parasite, or dis- ease that will attack the insect pest. Biological control is a form of enhancing natural defenses to achieve a desired effect. It usually involves raising and releasing one insect to have it attack another, almost like a “liv- ing insecticide.” You can facilitate a biological control program by timing pesticide applications or choosing pesticides that will be least harmful to beneficial insects.
A durable biological control program depends on two main strategies:
1) Using ecological farm design to make your farm more attractive to biological control “agents.”
2) Introducing beneficial agents onto your farm.
When plant pathogens are not inhibited by naturally occurring enemies, you can improve biocontrol by adding more effective beneficials. Such “di- rected biocontrol” operates in several ways. As naturally occurring enemies would do, introduced beneficials may:
Predators
Predators occur in most orders of insects but primarily in the beetle, dragon- fly, lacewing, wasp and true bug families ( Coleoptera, Odonata, Neuroptera, Hemiptera and Diptera, respectively). Their impacts have been highlighted worldwide by eruptions of spider mite pests where chemical insecticides have eliminated the mites’ predators. Tetranychid mites, for example, are usually very abundant in apple orchards where pesticides have destroyed natural predator populations.
The diversity of predator species in agroecosystems can be impressive. Researchers have reported more than 600 species — from 45 families — of predaceous arthropods in Arkansas cotton fields and about 1,000 species in Florida soybean fields. Such diversity can apply major regulatory pres- sures on pests. Indeed, many entomologists consider native, or indigenous, predators a sort of balance wheel in the “pest-natural enemy complex” because they tend to feed on whatever pest is over-abundant. Even where predators can’t force pest populations below economically damaging lev- els, they can and do slow down the rate at which potential pests increase. In spray-free apple orchards in Canada, five species of predaceous true bugs were responsible for 44 to 68 percent of the mortality of codling moth eggs.
Biodiverse farms are rich in predatory insects, spiders and mites. These beneficial arthropods prey on other insects and spider mites and are critical to natural biological control (Table 3). They can feed on any or all stages of their prey, destroying or disabling eggs, larvae, nymphs, pupae or adults. Some predators — like lady beetles and ground beetles — use chewing mouthparts to grind up and bolt down their prey. Others — like assassin bugs, lacewing larvae and syrphid fly larvae — have piercing mouthparts; they often inject powerful toxins into their prey, quickly immobilizing them before sucking their juices.
Many predatory arthropods — including lady beetles, lacewing larvae and mites — are agile and ferocious hunters. They actively stalk their prey on the ground or in vegetation. Other hunters — such as dragonflies and robber flies — catch their prey in flight. In contrast, ambushers patiently sit and wait for mobile prey; praying mantids, for example, are usually well camouflaged and use the element of surprise to nab their unsuspect- ing victims.
Most predators are “generalist” feeders, attacking a wide variety of in- sect species and life stages. They may have preferences — lady beetles and lacewings, for instance, favor aphids — but most will attack many other prey that are smaller than themselves. Some important predator species are cannibalistic; green lacewings and praying mantids are notorious for preying on younger and weaker members of their own species. The diet of most predators also includes other beneficial insects, with larger predators frequently making meals of smaller predators and parasites.
As a rule, predators are predaceous regardless of their age and gender and consume pollen, nectar and other food as well as their prey. However, some species are predaceous only as larvae; as adults, they feed innocently on nectar and honeydew or aid and abet the predatory behavior of their young by laying their eggs among the prey. Lacewings are predaceous only during their immature stages. Other species are lifelong predators but change targets as they mature.
Principal Insect Predators
Spiders. Spiders are among the most neglected and least understood of predators. They rely on a complex diet of prey and can have a strong stabilizing influence on them. Because spiders are generalists and tend to kill more prey than they actually consume, they limit their preys’ initial bursts of growth.
Many spiders live in crop canopies but most inhabit the soil surface and climb plants. Fields with either living plants or residue as soil cover tend to harbor diverse and abundant spider populations. Up to 23 spider families have been documented in cotton and 18 species have been tallied in apples. Because such diverse populations of spiders remain relatively constant, they maintain tolerable levels of their associated prey without extinguishing them.
Lady beetles (Coccinellidae , also called ladybugs or ladybird beetles). With their shiny, half-dome bodies and active searching behavior, lady beetles are among the most visible and best known beneficial insects. More than 450 native or introduced species have been found in North America. They are easily recognized by their red or orange color with black mark- ings, although some are black with red markings and others have no mark- ings at all.
Lady beetles have been used in biological control programs for more than a century and are beneficial both as adults and larvae. Most larvae are blue- black and orange and shaped like little alligators. Young larvae pierce their prey and suck out their contents. Older larvae and adults chew entire aphids.
Any crop prone to aphid infestation will benefit from lady beetles, even though this predator’s vision is so poor that it almost has to touch an aphid to detect it. Growers of vegetables, grains, legumes, strawberries and or- chard crops have all found lady beetles helpful in managing aphids. In its lifetime, a single beetle can eat more than 5,000 aphids. In the Great Plains, studies of greenbug pests in grain sorghum have shown that each lady beetle adult can consume almost one of these aphids per minute and dis- lodge three to five times that many from the plant, exposing the dislodged greenbugs to ground-dwelling predators.
While their primary diet is aphids, lady beetles can make do with pol- len, nectar and many other types of prey, including young ladybugs. In- deed, their extensive prey range — which includes moth eggs, beetle eggs, mites, thrips and other small insects — makes lady beetles particularly valuable as natural enemies.
Ground beetles (Carabidae). Predaceous ground beetles, or carabids, belong to a large family of beneficial beetles called the Carabidae whose adults live as long as two to four years. Several thousand species reside in North America alone.
Generally nocturnal, most predaceous ground beetles hide under plant litter, in soil crevices or under logs or rocks during the day. At night, their long, prominent legs allow them to sprint across the ground in pursuit of prey. Some species even climb up trees, shrubs or crops.
Most adult ground beetles range in length from 0.1 to 1.3 inches (3.2– 32 mm). Their antennae are fairly threadlike and their bodies — although quite variable — are often heavy, somewhat flattened and either slightly or distinctly tapered at the head end. Some species are a brilliant or metallic purple, blue or green, but most are dark brown to black.
Armed with large, sharp jaws, adult predaceous ground beetles are fero- cious. They can consume their body weight in food each day. Some cara- bids grind and eat such annual weed seeds as foxtail and velvetleaf. Larval carabids are not always predatory. In the Lebia genus, for example, adults are predators but first-instar larvae are parasites of chrysomelid beetles. (Instars are stages between successive molts.) Normally colorful, Lebia adults are just 0.1 to 0.6 inches (2.5–14 mm) long. Lebia grandis is a native and specialist predator of all immature stages of the Colorado potato beetle in cultivated potatoes in the eastern and mideastern U.S.
Lacewings (Chrysoperla spp.). Green lacewings — with their slender, pale-green bodies, large gauze-like wings and long antennae — are very common in aphid-infested crops, including cotton, sweet corn, potatoes, tomatoes, peppers, eggplants, asparagus, leafy greens, apples, strawberries and cole crops.
The delicate, fluttering adults feed only on nectar, pollen and aphid honeydew. About 0.5 to 0.8 inches (12–20 mm) long, they are active fliers — particularly during the evening and night, when their jewel-like golden eyes often reveal their presence around lights.
The larvae — tiny gray or brown “alligators” whose mouthparts resem- ble ice tongs — are active predators and can be cannibalistic. Indeed, green lacewing females suspend their oval eggs singly at the ends of long silken stalks to protect them from hatching siblings. Commonly called aphid li- ons, lacewing larvae have well-developed legs with which to lunge at their prey and long, sickle-shaped jaws they use to puncture them and inject a paralyzing venom. They grow from less than 0.04 inch to between 0.2 and 0.3 inches (from <1 mm to 6–8 mm), thriving on several species of aphids as well as on thrips, whiteflies and spider mites — especially red mites. They will journey up to 100 feet in search of food and can destroy as many as 200 aphids or other prey per week. They also suck down the eggs of leafhoppers, moths and leafminers and reportedly attack small caterpillars, beetle larvae and the tobacco budworm.
Minute pirate bugs (Orius spp.). These often-underestimated “true bugs” are very small — a little over 0.1 inch (3 mm) long. The adults’ white-patched wings extend beyond the tips of their black, somewhat oval bodies. The briskly moving nymphs are wingless, teardrop-shaped and yel- low-orange to brown.
Minute pirate bugs are common on pasture, in orchards and on many agricultural crops, including cotton, peanuts, alfalfa, strawberries, peas, corn and potatoes. They feed greedily on thrips, insect eggs, aphids and small cat- erpillars and can devour 30 or more spider mites a day. Clasping their as- sorted small prey with their front legs, they repeatedly insert their needle-like beaks until they have drained their victims dry. They are prodigious consum- ers of corn earworm eggs in corn silks and also attack corn leaf aphids, potato aphids, potato leafhopper nymphs and European corn borers. Minute pirate bugs can even deliver harmless but temporarily irritating bites to humans.
Because they depend on pollen and plant juices to tide them over when their preferred prey are scarce, minute pirate bugs are most prevalent near spring- and summer-flowering shrubs and weeds.
Big-eyed bugs (Geocoris spp.). Named for their characteristically large, bulging eyes, big-eyed bugs are key and frequent predators in cotton and many other U.S. crops, including warm-season vegetables. Geocoris punc- tipes and G. pallens are the most common of the roughly 19 Geocoris species found in North America.
Adult big-eyed bugs — normally yellow or brown but sometimes black — are oval and small (0.12 to 0.16 inch, or 3–4 mm, long). Their un- usually broad heads are equipped with piercing, sucking mouthparts. The similarly armed nymphs look like smaller, grayer versions of the adults.
Big-eyed bugs are omnivorous. Their diet includes plants but they pre- fer to prey on smaller insect and mite pests. They have been observed charging their intended victims, stabbing them quickly with their extend- ed beaks and sometimes lifting them off the ground in the process.
Big-eyed bugs attack the eggs and small larvae of bollworm, pink boll- worm and tobacco budworm and most other lepidopteran pests. They also target all life stages of whiteflies, mites and aphids and the eggs and nymphs of plant bugs. Laboratory studies indicate that a ravenous, growing nymph can exterminate 1,600 spider mites or about 250 soybean looper eggs before reaching maturity; adults have bolted down 80 spider mites or four lygus bug eggs a day.
Syrphid flies. Also known as hover flies because they hover and dart in flight, these brightly colored bee and wasp mimics are unusually vora- cious predators, as larvae, of aphids and other slow-moving, soft-bodied Insects.
Depending on the species, many syrphid flies over-winter, giving rise to adults in spring. Adult syrphid flies feed on pollen, nectar and aphid honeydew. Each female lays hundreds of white, football-shaped eggs, about 0.04 inch (1 mm) long, amidst aphid colonies. The narrow, tapered slug-like larvae that hatch from these eggs can pierce and drain up to 400 aphids apiece during the two to three weeks it takes them to complete development. Unable to perceive their prey except through direct contact, syrphid fly larvae find their dinners by flinging their forward ends from side to side.
Parasitoids
Most parasitoids — parasitic insects that kill their hosts — live freely and independently as adults; they are lethal and dependent only in their im- mature stages. Parasitoids can be specialists, targeting either a single host species or several related species, or they can be generalists, attacking many types of hosts. Typically, they attack hosts larger than themselves, eating most or all of their hosts’ bodies before pupating inside or outside them.
When the parasitoid emerges from its pupa as an adult, it usually nour- ishes itself on honeydew, nectar or pollen — although some adults make meals of their host’s body fluids and others require additional water. Adult female parasitoids quickly seek out more victims in which to lay their host- killing eggs. With their uncanny ability to locate even sparsely populated hosts using chemical cues, parasitoid adults are much more efficient than predators at ferreting out their quarry.
Different parasitoids can victimize different life stages of the same host, although specific parasitoids usually limit themselves to one stage. Thus, parasitoids are classified as egg parasitoids, larval (nymphal) parasitoids or adult parasitoids. Some parasitoids lay their eggs in one life stage of a victim but emerge at a later life stage. Parasitoids are also classified as either ectoparasites or endoparasites depending, respectively, on whether they feed externally on their hosts or develop inside them. Their life cycle is commonly short, ranging from 10 days to four weeks.
Principal Parasitoids
Dipteran flies. Entomologists have described more than 18,000 species of dipteran, or fly, parasites, which have diversified over an expansive range of hosts (Table 5). Unlike parasitic wasps, most species of parasitic flies lack a hardened structure with which to deposit eggs inside their hosts. Instead, they lay their eggs or larvae on plants, where the parasitoid larvae can easily penetrate the host but also where their target victims can eat them.
Individual species of parasitic flies are extraordinarily capable of sur- viving on many kinds of foods. The tachinid Compsilura concinnata, for example, has been successfully reared from more than 100 host species and three different host orders. Members of other Diptera families — such as big-headed flies in the Pipunculidae family, which are endoparasites of leafhoppers and planthoppers, and the small-headed Acroceridae, which only target spiders — are generally more specialized. However, some at- tack hosts from several families or subfamilies.
Chalcid wasps. For both natural and applied biological control, the chal- cid wasps of the superfamily Chalcidoidea are among North America’s most important insect groups. About 20 families and 2,000-plus species have been found on the continent — among the smallest of insects.
Because they are so diminutive, chalcid wasps are often underestimated in their numbers and effectiveness. They can be seen tapping leaf surfaces with their antennae in search of their host’s “scent,” but their presence is most commonly revealed by the sickly or dead hosts they leave in their wake. They parasitize a great number of pests, and different species attack different stages of the same host.
The following six families have proven especially useful in managing pests.
Fairyflies (Mymaridae). At between 0.008 and 0.04 inches (0.2–1 mm) long, these smallest of the world’s insects can fly through the eye of a needle. Viewed under the microscope, the back wings of fairyflies contain distinctive long hairs.
Fairyflies parasitize the eggs of other insects — commonly flies, beetles, booklice and leafhoppers. Many fairyfly species, especially those belonging to the genus Anaphes, play crucial roles in biological control. The intro- duced egg parasite A. flavipes, for example, is one of two parasites that have been established for cereal leaf beetle management in small grains. In pes- ticide-free California vineyards with ground vegetation, the tiny Anagrus epos wasp can make a big dent in grape leafhopper densities.
Trichogramma wasps (Trichogrammatidae). Trichogramma wasps are the most widely released natural enemies. The tiny female wasp — gener- ally less than 0.04 inch (1 mm) long — lays an egg inside a recently laid host egg, which blackens as the larva develops.
The host range of many Trichogramma wasps spans numerous species and families of insects. Moths, butterflies, beetles, flies, wasps and true bugs are all frequent victims. Some Trichogramma wasps even use their wings in a rowing motion to reach aquatic hosts.
Among commercially available species in the U.S. are T. minutum, T. platneri and T. pretiosum, which are released into fields on cards loaded with the parasitized eggs of non-pest hosts. Some foreign species — including T. ostriniae, T. nubilale and T. brassicae — also are being evaluated for augmentation biocontrol against European corn borers.
Eulophid wasps (Eulophidae). Eulophid wasps number more than 600 in North America, making theirs one of the largest chalcid families. About 0.04 to 0.12 inches (1–3 mm) long, they are often brilliant metallic blue or green.
Some species of eulophids are mite predators while others attack spider egg cases, scale insects and thrips. Most eulophids, however, parasitize flies, other wasps or the larvae or pupae of beetles or moths. Leaf-mining and wood-boring insects are frequent hosts.
Eulophids destroy many major crop pests. In the Midwest alone, Sympi- esis marylandensis is an important parasite of spotted tentiform leafminer in apples. Diglyphus isaea — available commercially — is a primary parasite of agromyzid leafminers in greenhouses. Edovum puttleri attacks the eggs of Colorado potato beetles. Finally, Pediobius foveolatus — introduced from India and also available commercially — parasitizes Mexican bean beetle larvae.
Pteromalid wasps (Pteromalidae). This large family of wasps assaults many types of insects, including the larvae of moths, flies, beetles and wasps. Several pteromalids target scale insects and mealybugs and some even act as “hyperparasitoids” — parasitizing other parasites within their hosts.
In the upper Midwest, Pteromalus puparum is a key enemy of imported cabbageworm pupae, each of which can involuntarily host more than 200 Pteromalus offspring. Anisopteromalus calandrae, which attacks the larvae of beetles that infest stored grain, impressed scientists several decades ago with its ability to suppress 96 percent of rice weevils in wheat spillage in small rooms. A. calandrae can now be purchased for release in grain storage and handling facilities.
Encyrtid wasps (Encyrtidae). Responsible for much of the classical biological control of scale insects and mealybugs in fruit trees, this impor- tant family of natural enemies encompasses about 400 species in the U.S. and Canada. Its extensive host range includes soft scales, armored scales, mealybugs and the eggs or larvae of insects in about 15 families of beetles, 10 families of flies and 20 families of moths and butterflies.
Several commercially available encyrtids now help manage scale and mealybugs in greenhouses: Leptomastix dactylopii, for example, parasitizes citrus mealybug, while Metaphycus helvolus attacks black, hemispherical, nigra, citricola, brown soft and other soft scales.
Other noteworthy encyrtids include Ooencyrtus kuwanae, an introduced parasite of gypsy moth eggs, and Copidosoma floridanum, a native parasite of cabbage looper larvae.
Aphelinid wasps (Aphelinidae). The effectiveness of aphelinids in managing scale insects has earned them one of the best reputations in bio- logical control. They also destroy mealybugs, whiteflies, aphids and other families of Homoptera.
Aphelinus varipes parasitizes greenbugs, A. mali targets the woolly apple aphid, and members of the genus Eretmocerus attack silverleaf whitefly. Encarsia formosa, in commercial use since the 1920s, is now released into greenhouses worldwide ; it kills almost 100 greenhouse whitefly nymphs during its 12-day life span.
Principal Insect Pathogens
Just like humans and other vertebrates, insects are susceptible to many disease-causing organisms known as pathogens. Thousands of species of bacteria, fungi, viruses, protozoa and nematodes can sicken or kill insects. Even if the insects survive, the pathogens’ “sub-lethal” effects can keep their victims from feeding or reproducing.
Bacteria. Most bacteria infect specific insect orders. Some naturally oc- curring insect-pathogenic bacteria have been isolated and mass-produced for commercial use. One of these, Bacillus thuringiensis or Bt, is the world’s most widely applied biological control agent. It exerts its toxicity only after plant-eating insects actually consume it. A highly dense protein crystal, the Bt toxin kills victims by first paralyzing their mid-gut, then their entire bodies. Like most other bacterial pathogens, Bt is specific to certain insect orders. Its short residual period also makes it an ideal candidate for pest management in fruits and vegetables.
Fungi. Although an estimated 700-plus species of fungi can infect in- sects, fewer than 20 have been developed for insect management. Most in- sect-pathogenic fungi need cool, moist environments to germinate. Com- pared to most other insect pathogens, they have an extensive host range. Beauveria bassiana, for example, can help manage beetles, ants, termites, true bugs, grasshoppers, mosquitoes and mites as well as other arthropod pests. It unleashes a toxin that weakens its host’s immune system, then over- whelms its dead host’s intestinal bacteria with an antibiotic. The tell-tale sign of B. bassiana’s carnage is its victim’s “white bloom” of fungal spores.
Fungi can invade their insect host through natural openings in its cu- ticle. Thus, hosts need not consume pathogens but only come into direct contact with them. Although some fungi can take up to several weeks to kill their hosts, most infected insects die within three to seven days.
Viruses. Most viruses that attack insects belong to a group called nucle- ar polyhedrosis viruses or NPVs. Their victims are usually young larvae of butterflies and moths, which become infected by eating NPV particles and typically die within several weeks. Some infected larvae hang limply from the tops of crop canopies, prompting the common name “caterpillar wilt” or “tree top” disease.
Prevailing environmental factors heavily influence the insect-killing ef- ficiencies of viruses. For example, they are adversely affected by sunlight, while the relatively slow speed at which they kill has also hindered their widespread acceptance for biocontrol.
Nematodes. Nearly 40 known families of nematodes parasitize and consume insects and other arthropods. Some are hunter-cruisers while others are ambushers. The most beneficial of these “entomopathogenic” nematodes belong to the Heterorhabditidae and Steinernematidae families. Both families are “obligate” parasites: their survival depends on their hosts and on the symbiotic relationships the nematodes have evolved with dis- ease-causing Xenorhabdus and Photorhabdus bacteria.
Parasitic nematodes transport bacteria inside their host, penetrating the host via the mouth, anus, spiracles or cuticle. Once inside, the nematodes release the bacteria, which quickly multiply and kill the host. In turn, the nematode uses the bacteria and insect cadaver for food and shelter, matur- ing, mating and reproducing inside it. Infective-stage juvenile nematodes eventually emerge from the cadaver and seek out another host.
Because they are highly mobile and can locate and destroy new victims in just a few days, entomopathogenic nematodes make outstanding can- didates for all kinds of biological control. Some are applied to soils to suc- cessfully manage the underground life stages of insect pests.