Predators y
lacewings are available from biocontrol dealers. Predators are mainly free-living species that directly consume a large number of
prey during their whole lifetime. Given that many major crop pests are insects, many of the predators used in biological control are insectivorous species.
Lady beetles, and in particular their larvae which are active between May and July in the northern hemisphere, are voracious predators of
aphids, and also consume
mites,
scale insects and small
caterpillars. The spotted lady beetle (
Coleomegilla maculata) is also able to feed on the eggs and larvae of the
Colorado potato beetle (
Leptinotarsa decemlineata). The larvae of many
hoverfly species principally feed upon
aphids, one larva devouring up to 400 in its lifetime. Their effectiveness in commercial crops has not been studied. The running crab spider
Philodromus cespitum also prey heavily on aphids, and act as a biological control agent in European fruit orchards. y
Polistes wasp searching for bollworms or other
caterpillars on a cotton plant Several species of
entomopathogenic nematode are important predators of insect and other invertebrate pests. Entomopathogenic nematodes form a stress–resistant stage known as the infective juvenile. These spread in the soil and infect suitable insect hosts. Upon entering the insect they move to the
hemolymph where they recover from their stagnated state of development and release their
bacterial symbionts. The bacterial symbionts reproduce and release toxins, which then kill the host insect.
Phasmarhabditis hermaphrodita is a microscopic
nematode that kills slugs. Its complex life cycle includes a free-living, infective stage in the soil where it becomes associated with a pathogenic bacteria such as
Moraxella osloensis. The nematode enters the slug through the posterior mantle region, thereafter feeding and reproducing inside, but it is the bacteria that kill the slug. The nematode is available commercially in Europe and is applied by watering onto moist soil. Entomopathogenic nematodes have a limited
shelf life because of their limited resistance to high temperature and dry conditions.
Neoseilus californicus, and a ladybird
Stethorus punctillum. Predators including
Cactoblastis cactorum (mentioned above) can also be used to destroy invasive plant species. As another example, the
poison hemlock moth (
Agonopterix alstroemeriana) can be used to control
poison hemlock (
Conium maculatum). During its larval stage, the moth strictly consumes its host plant, poison hemlock, and can exist at hundreds of larvae per individual host plant, destroying large swathes of the hemlock.
Aleiodes indiscretus parasitizing a
spongy moth caterpillar, a serious pest of forestry For
rodent pests,
cats are effective biological control when used in conjunction with reduction of
"harborage"/hiding locations. While cats are effective at preventing rodent
"population explosions", they are not effective for eliminating pre-existing severe infestations. Although there are no quantitative studies of the effectiveness of barn owls for this purpose, they are known rodent predators that can be used in addition to or instead of cats; they can be encouraged into an area with nest boxes. In Honduras, where the mosquito
Aedes aegypti was transmitting
dengue fever and other infectious diseases, biological control was attempted by a community action plan;
copepods, baby
turtles, and juvenile
tilapia were added to the wells and tanks where the mosquito breeds and the mosquito larvae were eliminated. Even amongst arthropods usually thought of as obligate
predators of animals (especially other arthropods),
floral food sources (
nectar and to a lesser degree
pollen) are often useful adjunct sources. Parasitoids are most effective at reducing pest populations when their host organisms have limited
refuges to hide from them. '', widely used in
greenhouse horticulture, was one of the first biological control agents developed. '' Parasitoids are among the most widely used biological control agents. Commercially, there are two types of rearing systems: short-term daily output with high production of parasitoids per day, and long-term, low daily output systems. In most instances, production will need to be matched with the appropriate release dates when susceptible host species at a suitable phase of development will be available. Larger production facilities produce on a yearlong basis, whereas some facilities produce only seasonally. Rearing facilities are usually a significant distance from where the agents are to be used in the field, and transporting the parasitoids from the point of production to the point of use can pose problems. Shipping conditions can be too hot, and even vibrations from planes or trucks can adversely affect parasitoids.
Gonatocerus ashmeadi (
Hymenoptera:
Mymaridae) has been introduced to control the
glassy-winged sharpshooter Homalodisca vitripennis (Hemiptera:
Cicadellidae) in
French Polynesia and has successfully controlled ~95% of the pest density. The
eastern spruce budworm is an example of a destructive insect in
fir and
spruce forests. Birds are a natural form of biological control, but the
Trichogramma minutum, a species of parasitic wasp, has been investigated as an alternative to more controversial chemical controls. There are a number of recent studies pursuing sustainable methods for controlling urban cockroaches using parasitic wasps. Since most cockroaches remain in the sewer system and sheltered areas which are inaccessible to insecticides, employing active-hunter wasps is a strategy to try and reduce their populations.
Pathogens Pathogenic micro-organisms include
bacteria,
fungi, and
viruses. They kill or debilitate their host and are relatively host-specific. Various
microbial insect diseases occur naturally, but may also be used as
biological pesticides. When naturally occurring, these outbreaks are density-dependent in that they generally only occur as insect populations become denser. The use of pathogens against
aquatic weeds was unknown until a groundbreaking 1972 proposal by Zettler and Freeman. Up to that point biocontrol of any kind had not been used against any water weeds. In their review of the possibilities, they noted the lack of interest and information thus far, and listed what was known of pests-of-pests – whether pathogens or not. They proposed that this should be relatively straightforward to apply in the same way as other biocontrols. And indeed in the decades since, the same biocontrol methods that are routine on land have become common in the water.
Bacteria Bacteria used for biological control infect insects via their digestive tracts, so they offer only limited options for controlling insects with sucking mouth parts such as aphids and scale insects.
Bacillus thuringiensis, a soil-dwelling bacterium, is the most widely applied species of bacteria used for biological control, with at least four sub-species used against
Lepidopteran (
moth,
butterfly),
Coleopteran (beetle) and
Dipteran (true fly) insect pests. The bacterium is available to organic farmers in sachets of dried spores which are mixed with water and sprayed onto vulnerable plants such as
brassicas and
fruit trees.
Genes from
B. thuringiensis have also been incorporated into
transgenic crops, making the plants express some of the bacterium's toxins, which are
proteins. These confer resistance to insect pests and thus reduce the necessity for pesticide use. If pests develop resistance to the toxins in these crops,
B. thuringiensis will become useless in organic farming also.
Bacillus spp.,
fluorescent Pseudomonads, The largest-ever deployment of
Wolbachia-infected
A. aegypti mosquitoes reduced dengue incidence by 94–97% in the Colombian cities of
Bello,
Medellín, and
Itagüí. The project was executed by non-profit World Mosquito Program (WMP). Wolbachia prevents mosquitos from transmitting viruses such as dengue and
zika. The insects pass the bacteria on to their offspring. The project covered a combined area of , home to 3.3 million people. Most of the project area reached the target of infecting 60% of local mosquitoes. The technique is not endorsed by WHO.
Fungi , a pest in its own right and a vector of plant viruses, killed by the fungus
Pandora neoaphidis (
Zygomycota:
Entomophthorales) Scale bar = 0.3 mm.
Entomopathogenic fungi, which cause disease in insects, include at least 14 species that attack
aphids.
Beauveria bassiana is mass-produced and used to manage a wide variety of insect pests including
whiteflies,
thrips, aphids and
weevils.
Lecanicillium spp. are deployed against white flies, thrips and aphids.
Metarhizium spp. are used against pests including beetles,
locusts and other grasshoppers,
Hemiptera, and
spider mites.
Isaria fumosorosea (formerly known as
Paecilomyces fumosoroseus) is effective against white flies, thrips and aphids;
Purpureocillium lilacinum (formerly known as
Paecilomyces lilacinus) is used against
root-knot nematodes, and 89
Trichoderma species against certain plant pathogens.
Trichoderma viride has been used against
Dutch elm disease, and has shown some effect in suppressing
silver leaf, a disease of stone fruits caused by the pathogenic fungus
Chondrostereum purpureum. Pathogenic fungi may be controlled by other fungi, or bacteria or yeasts, such as:
Gliocladium spp.,
mycoparasitic Pythium spp.,
binucleate types of
Rhizoctonia spp., and
Laetisaria spp. The fungi
Cordyceps and
Metacordyceps are deployed against a wide spectrum of arthropods.
Entomophaga is effective against pests such as the
green peach aphid. Several members of
Chytridiomycota and
Blastocladiomycota have been explored as agents of biological control. From Chytridiomycota,
Synchytrium solstitiale is being considered as a control agent of the
yellow star thistle (
Centaurea solstitialis) in the United States.
Viruses Baculoviruses are specific to individual insect host species and have been shown to be useful in
viral biological pest control. For example, the
Lymantria dispar multicapsid nuclear polyhedrosis virus has been used to spray large areas of forest in North America where larvae of the
spongy moth are causing serious defoliation. The moth larvae are killed by the virus they have eaten and die, the disintegrating cadavers leaving virus particles on the foliage to infect other larvae. A mammalian virus, the
rabbit haemorrhagic disease virus was introduced to Australia to attempt to control the
European rabbit populations there. It escaped from quarantine and spread across the country, killing large numbers of rabbits. Very young animals survived, passing immunity to their offspring in due course and eventually producing a virus-resistant population. Introduction into New Zealand in the 1990s was similarly successful at first, but a decade later, immunity had developed and populations had returned to pre-RHD levels. RNA
mycoviruses are controls of various fungal pathogens.
Competitors The
legume vine
Mucuna pruriens is used in the countries of
Benin and
Vietnam as a biological control for problematic
Imperata cylindrica grass: the vine is extremely vigorous and suppresses neighbouring plants by
out-competing them for space and light.
Mucuna pruriens is said not to be invasive outside its cultivated area.
Desmodium uncinatum can be used in
push-pull farming to stop the
parasitic plant, witchweed (
Striga). The Australian bush fly,
Musca vetustissima, is a major nuisance pest in Australia, but native decomposers found in Australia are not adapted to feeding on cow dung, which is where bush flies breed. Therefore, the
Australian Dung Beetle Project (1965–1985), led by
George Bornemissza of the
Commonwealth Scientific and Industrial Research Organisation, released forty-nine species of
dung beetle, to reduce the amount of dung and therefore also the potential breeding sites of the fly.
Combined use of parasitoids and pathogens In cases of massive and severe infection of invasive pests, techniques of pest control are often used in combination. An example is the
emerald ash borer,
Agrilus planipennis, an invasive
beetle from
China, which has destroyed tens of millions of
ash trees in its introduced range in
North America. As part of the campaign against it, from 2003 American scientists and the Chinese Academy of Forestry searched for its natural enemies in the wild, leading to the discovery of several parasitoid wasps, namely
Tetrastichus planipennisi, a gregarious larval endoparasitoid,
Oobius agrili, a solitary, parthenogenic egg parasitoid, and
Spathius agrili, a gregarious larval ectoparasitoid. These have been introduced and released into the
United States of America as a possible biological control of the emerald ash borer. Initial results for
Tetrastichus planipennisi have shown promise, and it is now being released along with
Beauveria bassiana, a fungal
pathogen with known insecticidal properties.
Secondary plants In addition, biological pest control sometimes makes use of plant defenses to reduce crop damage by herbivores. Techniques include
polyculture, the planting together of two or more species such as a primary crop and a secondary plant, which may also be a crop. This can allow the secondary plant's defensive chemicals to protect the crop planted with it. ==Target pests==