Biological control can give effective management for some plant damaging garden invertebrates. The biological controls available to home gardeners are either predators or pathogenic nematodes.
Biological controls can have advantages over

Pesticides are substances designed to kill or control the growth and behaviour of living organisms. They are also known as plant protection products and include insecticides, fungicides, herbicides and molluscicides. The RHS doesn’t support the use of pesticides and recommends that gardeners use non-chemical control options whenever needed.

• Biological controls cause no damage to plants and do not leave residues
• Once established natural enemies can breed and increase in numbers until the damaging invertebrates has been reduced to an acceptable level 
• The parasitoids are usually specific to certain prey species and will not become a nuisance or cause harm to non-target creatures
• Some of the predatory mites, bugs and beetles are generalist predators and so should be used with care
• Nematode biological controls are usually either insect or mollusc specific. They therefore have the potential to infect non-target animals and so should be used in a targeted manner
• Biological controls can be used where pesticides cannot be used, for example on plants that may be scorched by sprays and there are no synthetic pesticides available for some food plants
• The presence of biological controls should not interfere with normal gardening activities such as ventilating glasshouses and watering, although it is necessary to avoid the use of pesticides
• The target animals do not develop resistance

To get the best results it is important to know how biological controls behave. Many of the predators used for biological control are more accurately termed parasitoids as they kill the host whereas a true parasite does not; these animals lay eggs in a host, the grub grows and feeds inside its prey, only killing the host when it completes its development.

Using them correctly
Predatory and parasitoid biological controls rarely give instant reductions in prey populations; they need time to multiply. It is therefore necessary to introduce them before large populations of the target invertebrates have developed. Conversely, there is no point in introducing the biological control before the prey becomes active, since they can only breed when their prey is present.
The effective season for biological control with predators and parasitoids in glasshouses is late March/April to September. Glasshouse predators and parasites, just like the animals they control, require warm conditions, if they are to thrive and breed rapidly. They generally require daytime temperatures of at least 21ºC (70ºF) and high light intensity to stimulate breeding, although they can usually survive at temperatures of 13ºC (55ºF). Glasshouse biological controls will often die out during the winter and reintroduction may be necessary each year.

Biological controls and pesticides
Predators and parasites are very susceptible to most pesticides and these should be avoided. Pesticides will kill biological controls and can persist for at least six weeks, for example ten weeks for deltamethrin based products; but natural organic pyrethrum can be used up to seven days before predator release. Pesticides based on fatty acids and plant oils/extracts, can be used with care up to the day before biological controls are introduced. Fungicide use should also be kept to a minimum.

Using biological controls: nematodes
Nematodes (phylum Nematoda) are a very diverse group of animals. There are more than 25,000 described species, and they are found in almost every habitat. Most are microscopic and they are important components of soil and marine ecosystems. Many nematodes are free living, but more than half of described species are parasitic on plants or animals. A few species such as the potato cyst nematodes (Globodera species), leaf and bud nematodes (Aphelenchoides species) and stem and bulb nematodes (Ditylenchus dispasci) can feed on garden plants.

Invertebrate pathogenic nematodes
Some of the nematode species that infect insects and molluscs have been developed as biological control. These species pose no risk to plants or vertebrates. They work by entering the invertebrate’s body and releasing bacteria. This results in an infection causing the death of the invertebrate, the nematodes then feed and multiply on the decomposing body.

Using nematodes correctly
Biological control nematodes come in packs that are mixed with water and watered onto affected plants and soil. They are available via mail order and some are formulated with a longer shelf life and supplied via garden centres. There are conditions which must be understood if they are to work well. Being living organisms they should be used as soon as possible after they are purchased or received and all manufacturers’ instructions followed. In general these nematodes require moist conditions and so are best applied in cool and damp conditions. Temperature is also important with different species requiring temperatures above 5ºC (41ºF) or 12ºC (54 ºF). Whilst the nematode biological controls are usually either insect or mollusc specific, they  therefore have the potential to infect non-target animals within those groups. They should therefore be used with care and only when there is a specific problem to treat. 

Biological controls can sometimes be bought or ordered at garden centres but are usually supplied by mail order. Being living animals they cannot be stocked on shop shelves. The exceptions are some of the nematodes used against ants, chafer grubs leatherjackets, slugs and vine weevil larvae, formulations of these nematodes are, stocked by some retail outlets. For success, the supplier’s instructions regarding release and subsequent care of the predators or parasites must be followed carefully.

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Biological controls and their suppliers

Below is a list of some garden problems and the biological controls available for them. More details are available on the relevant (linked) webpage.

Ants in lawns
Control: nematode Steinernema feltiae, labelled for ant control. Water in during the summer months. Ants in the garden border should be tolerated.

Aphids  (See also Aphid predators)
Controls: There are a range of biological controls available for aphids (greenfly and blackfly), which can be introduced indoors. Many are native species that commonly occur in gardens these often naturally reduce aphid problems by mid-summer in the garden.
Predatory midge, Aphidoletes aphidimyza. The larvae prey on aphids and can give effective control in glasshouses from April to September. These tiny flies (< 3 mm) lay red eggs on foliage, these are laid singly or in clusters of up to 40. Each female lays about 100 eggs over two weeks. Eggs hatch after three days and the small orange-white larvae feed for up to two weeks before pupating in silk cocoons in the soil. Adults emerge about three weeks later. Pot plants should be stood on trays of damp sand, to provide pupation sites. The larvae feed by attaching themselves to an aphid's leg joint and suck out the body contents, each larvae consuming up to 80 aphids. Supplied as larvae, which should be gently transferred to aphid-infested plants with the aid of a soft paint brush, or as pupae which should be put in a cool damp place at the base of plants.
Green lacewing larva, Chrysoperla carnea. The adult has a slender pale green body, about 10mm long, with long thread-like antennae and transparent wings with many veins. The adults feed on pollen, nectar and honeydew, the larvae are voracious predators of aphids, other small insects and mites. The females lay white eggs that are raised off the leaf surface on 5-7 mm long silk threads. This lacewing overwinters as adults in sheltered places.
Two-spot ladybird, Adalia bipunctata. Larvae and adults can be purchased; they should be placed on aphid affected plants. Adult ladybirds tend to disperse but the larvae stay where they have been put if aphids are present. Each ladybird will consume many hundreds of aphids during its lifetime.
Parasitoid wasps, Aphidius, Praon or Aphelinus species. These lay eggs inside aphids, the larvae then eat the internal contents of the aphid, leaving a hollow swollen straw-brown or black skin known as an aphid mummy. Aphidius and Aphelnius species pupate within the hollowed out mummy, the adult wasp emerging from a circular hole in the dorsal surface. The larvae of Praon species emerge from under the aphid spinning a silken cocoon that fastens the mummy to the leaf. Supplied as adult wasps or infected aphids, these should be released or placed on plants in the evening.
Rove beetle Atheta coriaria. This 3-4 mm long dark brown beetle is predatory as an adult and larvae, it feeds on a wide range of invertebrates including aphids, fungus gnats, thrips and red spider mite.
Larvae (maggots) of a hoverfly Sphaerophoria ruppelli. About half of the hoverfly species found in Britain are predatory on aphids as larvae. The predatory green maggots of S. ruppelli are sometimes sold comercially and can be released on aphid affected plants.

Caterpillars including Box tree caterpillar, Cabbage caterpillars, Codling moth, Cutworms and Plum moth
Control: nematodes Various products containing Steinernema feltiae and S. carpocapsae, including Nemasys Fruit and Veg protection and Box tree caterpillar killer.

Carrot fly 
Control: nematodes Steinernema feltiae (spring application) and S. carpocapsae (summer application). 

Chafer grubs in lawns 
Control: nematode Heterorhabditis bacteriophora best applied in late summer early autumn.

Chrysanthemum leaf miner 
Control: nematode Steinernema feltiae, labelled for leaf miner control.

Diaspid scale insects (indoors) 
Control: Black ladybird, Chilocorus nigritus. This 2 mm long ladybird is sometimes available for use in glasshouses and can help to control these scales during the growing season. Another ladybird Rhyzobius lophanthae is also sometimes available, a scale insect eating 2 mm long ladybird with black wing cases and a brown head.

Fuchsia gall mite
Control: predatory mite, Amblyseius andersoni. This mite will also feed on pollen and thrips larvae allowing the population to survive mite populations are low.

Glasshouse mealybug (Pseudococcus and Planococcus species)
Controls: 
Mealybug ladybird, Cryptolaemus montrouzieri. A native to Australia, adults and larvae feed on mealybugs and their eggs. Adults are 3.5 mm long with grey-black wing cases and a reddish-brown head and thorax. The larvae resemble mealybugs, but when fully grown are more than twice as large as mealybugs. The ladybird can lay up to 500 eggs. Cryptolaemus does best when daytime temperatures are around 27ºC (80ºF). It should be released in the evening to reduce the risk of adult ladybirds flying up and escaping through vents.
Parasitoid wasps, Leptomastix species, Leptomastidea species and Anagrus species. These are often available in combinations and with Cryptolaemus for mealybug control. These parasiotoids are good at searching for mealybugs and can find them when populations are small.
Lacewing larvae, Chrysoperla carnea. Larvae of this native insect are sometimes sold for mealybug control (more information under Aphid biological controls).

Glasshouse red (two-spotted) spider mite (Tetranychus urticae)
Controls: 
predatory mite, Phytoseiulus persimilis. Originally from Chile, this mite feeds on all life stages of glasshouse red spider mite. Dispatched as nymphs and adults, they should be released in sheltered positions on infested plants. Phytoseiulus is about the same size as glasshouse red spider mite (0.5 mm), it has an orange-red pear-shaped body and is more active than glasshouse red spider mite. Glasshouse red spider mites are rectangular and are usually yellowish-green with two dark patches.
Predatory mite, Amblyseius species. These pear shaped pale yellow mites predate on a range of glasshouse invertebrates. These can be used early in the season when temperatures are as low as 10ºC.
Predatory gall midge, Feltiella acarisuga. The 2 mm long pale yellow maggots of this midge can find small hidden populations of red spider mite and when available can be used early and late in the season.
Rove beetle Atheta coriaria. A 3-4 mm long dark brown beetle. Both adults and larvae will feed on a wide range of invertebrates in addition to mites.

Glasshouse whitefly (Trialeurodes vaporariorum)
Controls:
Chalcid parasitoid wasp, Encarsia formosa. Native to South America and discovered in Britain in 1926. Adults are 0.6 mm long; females have a dark coloured head and thorax with a yellow abdomen; males are entirely dark. Females lay 50-100 eggs singly in whitefly scales. Parasitised scales turn black, healthy scales are whitish-green. Adults emerge through circular holes cut in the upper surface of the dead scales. Adults feed on the honeydew. Encarsia is supplied as infected black whitefly scales on pieces of leaf or card. These should be fixed securely in a shaded position on infested plants. Adult parasitoids emerge within a few days and the first blackened scales should appear after three weeks. This parasitoid is not effective against other whitefly species.
Predatory mite Amblyseius andersoni is sometimes sold as a whitefly egg predator.

Leatherjackets in lawns
Control: nematodes Steinernema feltiae and S. carpocapsae, labelled for leatherjacket control.

Sciarid fly/fungus gnat larvae
Controls: The predators fungus gnat mite (Hypoaspis miles), mighty mite (Macrocheles robustulus) and rove beetle (Atheta coriaria) can be used in greenhouses. A pathogenic nematode (Steinernema feltiae) is also available for watering into composts.

Soft scale (Coccus hesperidum) and hemispherical scale (Saisettia coffeae) in glasshouses
Controls:
nematode Steinernema feltiae. This nematode is sometimes supplied for control of these scale insects.
Parasitoid wasps Metaphycus helvolus, Encyrtus species and Encarsia citrina are sometimes available. These parasitoids lay eggs in scale nymphs, which become darkened and are killed. At 27ºC (80ºF) the life cycle from egg to adult wasp takes about 12 days but is slower at lower temperatures. These parasitoids should be introduced in May to July for the best results.

Slugs
Control: nematode, Phasmarhabditis hermaphrodita. This can be watered into the soil anytime when the soil is moist and above 5ºC (41ºF). Most effective if applied on several occasions between spring and autumn. Light sandy soils allow nematodes to move more freely through the soil; poor results may be achieved on heavy, poorly drained soils. Snails can be affected by the nematode but are less likely to come into contact with it as snails spend most of their lives on or above the soil surface.

Thrips
Controls: predatory mites (Hypoaspis and Ambyseius species) and mighty mite (Macrocheles robustulus) can be used. The predatory bug Orius laevigatus is also sometimes available. The nematode biological control (containing Steinernema  feltiae and S. carpocapsae) sold as Fruit and Vegetable Protection can be used indoors and outside.

Tarsonemid mite and citrus red spider mite
Control: predatory mites (Amblyseius species)

Various fruit and vegetable problems including cabbage root fly, onion fly, cutworms, leatherjackets, caterpillars, gooseberry sawfly, thrips and codling moth.
Control: A mixture of nematode species, sold as Fruit and Vegetable Protection, is available, may give some control of other insects.

Vine weevil (Otiorhynchus sulcatus)
Controls: 
Larvae: Vine weevil grubs can be controlled by watering into compost or soil a suspension of pathogenic nematodes. For best results nematodes need to be used in a well-drained potting compost or light soil, which must be moist. Treatment in August to early September should control vine weevil grubs before they are large enough to cause serious damage to plant roots.
Several species and combinations of species can be available including, Steinernema kraussei,  S. feltiae, Heterorhabditis megidis and H. bacteriophora. Steinernema species can remain active at soil temperatures down to 5ºC (41ºF) whilst Heterorhabditis require temperatures above 12ºC (54ºF).
Adults: A trap containing nematodes (S. carpocapsae), is available for adult vine weevil. The traps should be placed on the ground below plants damaged by the weevils during the summer. The adults enter the trap during the day and are infected by the nematodes.

Various beetle larvae including VIburnum beetle and Asparagus beetle
Control: nematode – S. carpocapsae

Wireworm
Control: nematode Heterorhabditis bacteriophora, labelled for wireworm control.