It has been difficult to utilize the full potential of biological control in tree fruit and other crops that receive periodic sprays of broad-spectrum pesticides and/or have high quality standards.The best pest targets for biological control in tree fruits are generally the secondary foliage-feeding pests that do not cause direct fruit injury (e.g., mites, aphids, and leafminers). Populations of pests that feed directly on the fruit (e.g., codling moth, Oriental fruit moth, and plum curculio) generally cannot be tolerated at levels high enough for special biological control agents to reproduce.
Natural enemies and environmental factors limit populations of insect and mite pests in natural ecosystems. When natural enemies are killed by human's actions in any habitat or when pests are introduced to new habitats without their natural enemies, natural control often fails and results in pest outbreaks.
While biological control is often thought of as a biopesticide where a single species of beneficial arthropod is released or conserved, the best results are most often achieved where a complex of many species of natural enemies, including predators and parastioids, each contribute to reducing pest populations at different times of the season and on different developmental stages. While the development of pesticide resistance (mainly to organophosphates) has occurred in Stethorus punctum, the black ladybeetle predator, and several species of predatory mites, such resistance is generally much slower to develop in beneficial arthropods. The biological control potential of the vast majority beneficial arthropods is not realized unless they develop resistance to pesticides, no pesticides are used, or only pesticides that are selective and nontoxic to these arthropods are used.
Complex of natural enemies for woolly apple aphid.
Types of Biological Control Agents
- Consume many prey during development.
- Generally larger than prey
- All stages may be predators.
- Are often generalists rather than specialists on any one prey type and eat both adults and immatures.
- Immatures feed only on a single host and almost always kill it.
- Are smaller than the host.
- Are often specialized in their choice of host species and life stages thereof.
- Only the female attacks the host and lays eggs or larvae on or in the host.
- Immatures remain on or in the host, adults are free living and mobile and may be predaceous, feed on nectar, or not feed at all.
- Smaller than host and don't generally kill it (e.g., mites)
- Diseases caused by fungi, bacteria, and viruses that kill the host.
- Some are naturally occurring and some have been commercially developed.
- Bacillus thuriengensis (Bt) toxins and spores--Dipel, etc.
- Fermentation products from fungi are precursors to making abamenctin and spinosad.
- Codling moth polyhedrosis virus available commercially for control.
- Naturally occurring Beauvaria and Hirsutella fungal pathogens.
Biological Control of Mites
The most successful biological control programs in eastern tree fruits have centered on the conservation of native species of mite predators to control the European red mite and twospotted spider mite. After 40 years of use, some of these predators have developed resistance to organophosphate insecticides (e.g., Stethorus), but are suppressed or eliminated when broad-spectrum carbamate and pyrethroid insecticides are used. The use of pheromone mating disruption, horticultural oils, and some of the more selective reduced-risk insecticides and miticides will allow a natural increase of predators capable of regulating pest mite populations to tolerable levels without the use of miticides. Mite control through biological control has the additional advantage of stopping the development of miticide resistance and, once established, is sustainable long-term if the use of certain harmful pesticides is avoided. The routine use of carbamates and pyrethroids in stone fruits, pears, grapes, and small fruits currently prevents reliable biological mite control agents even though many of the same predators found in apples can be present.
Listed below are descriptions of the main biological mite predators found in Pennsylvania apple orchards:
Typhlodromus pyri (Phytoseiidae)
Discovered in Pennsylvania for the first time in 2003, this predatory mite is currently the most reliable and effective mite predator in eastern U.S. apple orchards. Pear shaped and slightly larger than a European red mite adult, they are white/translucent until they feed. When feeding on adult red mites or apple rust mites, its abdomen may appear reddish. T. pyri is very active and moves rapidly to consume up to 350 mite prey in a lifespan of about 75 days. Females may lay up to 70 eggs each and have several generations per season. Populations, therefore, can build rapidly in response to pest mite populations. Most effective in the cooler weather of the spring and fall, T. pyri is somewhat less effective in the summer months. It overwinters on the apple tree under the bark where it is less susceptible to dormant oil applications and is very tolerant of Pennsylvania's winters.
T. pyri for biological control of European red mite.
Neoseiulus fallacis (Phytoseiidae)
Almost indistinguishable from T. pyri except under a microscope, this predator is currently more widespread in distribution in Pennsylvania apple orchards than T. pyri, due to a higher tolerance for some pesticides and the use of alternative plant hosts. Like T. pyri, N. fallacis is also very active, but is able to build populations three times faster during the hotter summer months. This predator lives only about 20 days with each female laying 40 to 60 eggs and may have 6 to 7 generations/year. Like T. pyri, N. fallacis is resistant to organophosphate insecticides, but it is very susceptible to pyrethroids and carbamates. This predator is not as tolerant of cool weather in the spring and fall and is susceptible to winter kill in Pennsylvania. Purely a predator, N. fallacis is not able to coexist on apple trees without pest spider mite populations to consume and will often leave the tree to feed on mites in the orchard ground cover.
Zetzellia mali (Stigmaeidae)
An omnivore like T. pyri that is able to exist on pollen, fungi, and rust mites when spider mite populations are absent, Z. mali is very slow moving and feeds only on the eggs of pest and predatory mites. Its diamond shape and bright yellow coloration (turning more reddish after feeding) make it easy to distinguish this predator from other predatory mites. It is smaller in size than the European red mite. Because it is less active, it is able to exist on pest mite populations even lower than T. pyri. Like T. pyri, it is also more active in the cooler spring and fall months. However, with only a couple of generations each season and a consumption rate of only two to three eggs per day, it cannot usually be relied on to control mite pests alone. It is a valuable supplement to control by other mite predators and is much more tolerant of most pesticides, including carbamates and pyrethroids. Generally, populations of more than one per leaf are necessary to exert significant control of spider mite populations.
Stethorus punctum (Coccinellidae)
Once the cornerstone of biological mite control in Pennsylvania apple orchards, this small, black ladybeetle predator has greatly declined in importance over the last five years. Although one of the smallest of all ladybird beetles, S. punctum was the most important beneficial insect in Pennsylvania apple orchards starting in the mid-1970s and conservation of this predator reduced miticide use by 50 percent for over 30 years. While tolerant of many organophosphate insecticides, the decline of this predator was mainly due to the greater use of pyrethroids and the introduction of several new neonicotinoid and IGR insecticides that are toxic to various life stages of this predator. Reproducing only when populations of pest mites exceed eight to ten mites per leaf, relying on S. punctum alone requires grower tolerance of some foliar mite injury. With the registration of newer, more effective miticides in recent years, most growers are not willing to tolerate this injury, despite the high cost of miticides. S. punctum is now much less common in orchards and generally in small localized "hot spots" of mites. The main advantage of this predator is its ability to fly and quickly colonize areas of high mite populations.
Biological Control of Aphids
Aphid Midge--Aphidoletes aphidimza (Cecidomyiidea)
The aphid midge, Aphidoletes aphidimyza, often contributes to biological control of spirea and green aphids in pome fruits. They feed on many species of aphids on many type of crops, but are not generally found in stone fruits because of their susceptibility to pyrethroids. Generally tolerant to organophosphate insecticides as immatures and slightly less so as adults, all stages are susceptible to carbamates, pyrethroids, neonicotinoid, and certain miticides. This species can be reared and is sold from biological control companies for mass releases in many crops, but especially for aphid control in greenhouses.
Ladybird Beetles (Coccinellidae)
Adults from these easily recognized beetles are oval, often brightly colored and spotted, and vary in size from 1.5 mm to 6 mm. Approximately a dozen of the 450 species found in North America are found in fruit with most feeding primarily on aphids, but some like Stethorus specialize on mites while others specialize on scales and mealybugs. A number of species require pollen as adults to reproduce and some can be important predators of moth eggs.
Multicolored Asian Ladybird Beetles-Harmonia axyridis
The multicolored Asian ladybird beetle has recently become the most common and most effective aphid predator in Pennsylvania orchards, replacing Coccinella septumpunctata and several native species. H. axyridis is native to Asia, but was released in Pennsylvania in 1978 and 1981. However, overwintering individuals were not recorded until 1993, and the populations that have become established may have resulted from an accidental introduction by an Asian freighter in New Orleans.
Multi-colored Asian Ladybird beetle.
Green and Brown Lacewings (Chrysopidae and Haemorobiidae)
Green lacewing adults are 6/10 to 9/10 inch in length, green with transparent wings with an interconnecting network of fine veins. The many different species are difficult to distinguish, but the adult of the most common green lacewing species has golden eyes. The adults feed on nectar, honeydew, and pollen with females producing 400 to 500 eggs each over a relatively long life of up to 3 months. Green lacewing eggs are laid on the tips of long, white, hair-like stalks to prevent cannibalism. The larvae (called aphid lions) are generalist predators of mites, thrips, soft scales, and almost any other soft-bodied prey. They are voracious aphid predators, eating 100 to 600 aphids during a 1 to 2 week development period and can be important predators of moth eggs and larvae as well. Prey are seized in hollow, sickle-like jaws protruding from the head and sucked dry. The larvae make a small, round, and white pupal case, often on the stem or calyx end of the fruit where they overwinter or, in the case of one species, overwinter as adults in bark crevices and other protected places. Brown lacewings are smaller (1/5 to 6/10 inch long) and are predatory, both as adults and larvae. They are much more tolerant of colder weather than the green lacewings and are more useful predators early in the season. Females lay 100 to 460 eggs, but not on stalks like the green lacewings. Larvae may consume more than 20 aphids per day or 30 to 40 mites per day. Developmental times are slower with most species only having two generations per season. Both types of lacewings have some tolerance to organophosphate insecticides, but should be conserved by selective pesticide use.
Minute Pirate Bug-Orious insidiosus
Generalist predators of aphids and mites, these are very small 1/10 inch, black, somewhat oval-shaped bugs that look like miniature, dark, tarnished plant bugs. They are most easily recognized by white, shiny wing patches on the adults. Able to feed on a wide variety of small prey, including thrips, leafhoppers, moth eggs, and young larvae, they are able to subsist on pollen or plant juices when prey are not available. This habit of feeding on plant juices may make them more susceptible to plant systemic products like some neonicotinoid insecticides. They are efficient at searching out high-prey densities and will aggregate where there is an abundance of prey. When handled, Pirate Bugs are capable of causing a mild sting with their beak. Orius has several generations/year and take about 20 days to develop from egg to adult. The adults live about 35 days with each female inserting about 130 eggs into plant tissues. Immature stages and adults can eat about 30 mites/aphids per day. Adults appear in late April, continue to feed all season until early fall, and then overwinter in the leaf litter both inside and outside orchards. They have some tolerance to organophosphate insecticides, but should be conserved by selective pesticide use.
Several species of syrphid flies are among the most voracious of aphid predators in Pennsylvania orchards. Adults are known as hover flies and resemble bees except that they have only one pair of wings. They are generally brown to black with yellowish areas. Their food source is pollen, nectar, and aphid honeydew, which is necessary for proper development of the eggs. Eggs are white, elliptical, and less than 4/100 inch long. The larvae, or maggots, are elongate, tapering gradually toward the head end and may be cream, yellow, gray, or a combination of these colors. Adults lay eggs in the midst of aphid colonies. Larvae cast their head side to side to locate aphids, which they pierce and consume. A single larva may destroy hundreds of aphids as it completes its three development stages in about 3 weeks. There may be five to seven generations per year with most species overwintering as adults or last instar larvae. Check for the presence of eggs and larvae in aphid colonies. Control of green aphids may result if 20 percent of the aphid colonies have syrphid larvae present.
Several species of these wasps are specialist predators of stink bugs and actively queue in on their smell to find their host. Bycertes quadrifasciata is a species that looks like a smaller version of the cicada killer wasp whose nests in the sand have been found to have prey consisting almost entirely of brown marmorated stink bug nymphs from which the wasps develop quite normally. Each female is capable of collecting up to 50 nymphs, which they paralyze and place in several burrows 6-8 inches into sandy ground. Another species, Astata unicolor, has also been seen to collect nymphs, but nests have not yet been found. This species is commonly found around our orchards as it prefers to nest in heavier ground. Like most predatory and parasitic wasps, it needs to visit flowers as adults to obtain nectar for food to mature its eggs.
Lepidopteran Predators-Ground Beetles
(Carabidae) and Rove Beetles (Staphylinidae)
These are two of the largest families of beetles with 1,500 ground beetle and 3,000 rove beetle species in North America. Many are generalist predators that are effective in controlling pests that pupate or overwinter in the ground cover or on the trunks (e.g., codling moth, Oriental fruit moth, apple maggot, plum curculio, European apple sawfly, leafrollers). Many live in the ground cover away from pesticide applications made to tree foliage, but some may climb trunks. All are very pesticide susceptible and are often used as indicators of environmental quality.
Woolly Apple Aphid Parasitoid-Aphelinus Mali
These adult wasps are very small and they insert their eggs singly into the body of aphids, where they will develop internally to kill the host. There are six to seven generations each year with each generation taking about 20 to 25 days to develop. Larvae or pupae overwinter within the mummified body of the aphid. A. mali are most effective in reducing small woolly apple aphid colonies in the spring when colonies are small. If biological control is disrupted with toxic pesticides, A. mali are less effective in controlling larger colonies later in the season. These very small wasps are able to attack only the aphids on the periphery of the colony and cannot successfully penetrate the wax and mass of aphid bodies to attack the center of the aphid colony, thus the percentage of parasitism actually decreases as the aphid colonies get larger in size. From midsummer to late season woolly apple aphid colonies are usually brought under control by a complex of syrphid fly species and generalist predators, such as brown and green lacewings. Ladybug larvae and adults are occasional predators of woolly apple aphids but do not appear able to deal with the waxy covering and give little control.
Lepidopteran and Stink Bug Parasitoids
Tachinid Flies--important parasitoids of leafrollers in the spring. One species, Actia interrupta, is currently the most important parasitoid of the obliquebanded leafroller. Eggs are laid on the skin of larvae to hatch and develop externally on the larvae to eventually leave just an empty husk of skin. Pupae are generally found near the host remains and resemble a grain of wheat in size and shape. Trichopoda pennipes, an important parsitoid of the squash bug, is also now undergoing a host shift to attack the brown marmorated stink bug with eggs being present at levels of up to 20 percent in some locations, but successful development on this host remains low so far. All species appear to be very susceptible to pesticides and are important only in pheromone disruption or orchards with minimal pesticide sprays.
Scelionid Wasps--several species of Trissolcus and Telenomus wasps are egg parasitoids of our native stink bug species, are the primary regulatory agents of these pests outside our orchards, and reduce their numbers so they are of minor importance when they move into the orchards from other hosts. Several species are in the process of undergoing host shifts to the brown marmorated stink bug, but current rates of parasitism are under 5 percent. A specialist Trissolcus parasitoid of BMSB in Asia, where it parasitizes up to 80 percent of eggs, is being evaluated by USDA-ARS for possible mass release sometime in the future.
Egg parasitoids of stink bug species.
Braconid and Ichneumon Wasps--with approximately 120,000 known species and many as yet undescribed, this is a virtually untapped source of biological control in modern agriculture. With various complex life histories, often alternating between several hosts and attacking specific life stages, these wasps have not been important sources of biological control in tree fruit since the introduction of disruptive broad-spectrum insecticides. Previous to this, however, they provided almost complete control of many of the leafroller species. Currently, there are more than 40 different wasp parasitoids capable of attacking tufted apple bud moth in Pennsylvania apple orchards. All species appear to be very susceptible to pesticides and are important only in pheromone disruption or orchards with minimal pesticide sprays. Braconid species appear to be most important late in the growing season.
Trichogramma Egg Parasitoid--most commonly employed as a biopesticides obtained from biological supply houses for mass releases into many crops. These tiny wasps complete their development inside a single egg of their moth or butterfly host. Native populations of mostly T. minutum attack many different orchard pests in Pennsylvania (most important are the several species of leafrollers, codling moth, and oriental fruit moth). The life of the adults and the number of eggs laid are greatly increased with the provision of nectar sources and females may then live up to 2 weeks and lay over 80 eggs. Although present during most of the growing season, populations generally do not build to be significantly important in controlling these orchard pests until late summer. Trichogramma is very susceptible to pesticides and is important only in pheromone disruption or orchards with minimal pesticide sprays.
More detailed information on biological control is in the Penn State Tree Fruit Production Guide.