Biological pest control

Biological control is the use of natural enemies to control pests. In the home garden this can be by introducing predators or pathogenic nematodes. This technique is mainly used in greenhouses, but some biological controls, can be used out of doors.

Mealybug ladybird (Cryptolaemus montrouzieri) on the right with its prey, a mealybug, on the left.

Quick facts

Suitable for: Biological control can be an effective alternative to pesticides
Timing: Mainly April to September

Suitable for...

Biological control can be an effective alternative to pesticides for some problems.

  • They can reduce or eliminate the need for pesticides
  • Biological controls cause no damage to plants and do not leave residues
  • Pests do not develop resistance
  • Once established natural enemies can breed and increase in numbers until the pest has been reduced to an acceptable level 
  • The predators and parasites are usually specific to certain pests and will not become a nuisance or cause harm to non-target animals
  • Biological controls can be used in situations 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 restrict the use of pesticides and, to a lesser extent, fungicides.

Using biological controls: Parasites and predators

To get the best results it is important to know how biological controls behave. Many of the parasites 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 developes inside only killing the host as it completes its development.

Using them correctly

Predatory and parasitic biological controls rarely give instant reductions in pest populations; they need time to multiply. It is therefore necessary to introduce them before plants become heavily infested. Conversely, there is no point in introducing the biological control before the pest becomes active, since they can only breed when their prey or host is present.

The effective season for biological control in glasshouses is late March/April to September. Careful use of pesticides may be required at other times of year. 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 for at least six weeks before biological controls are introduced, (e.g. ten weeks for deltamethrin based products; but natural organic pyrethrum can be used up to seven days before release). The safest pesticides are those based on fatty acids and plant oils/extracts, which can be used up to the day before biocontrols are  introduced. Fungicides are generally safe but use should be kept to a minimum.

Using biological controls: Nematodes

The Nematoda, known as nematodes are a very diverse phylum of animal. There are more than 25,000 described species, and they are found in almost every habitat. Most are microscopic and many are important components of soil and marine ecosystems. More than half of the described species are parasitic on plants or animals and some species such as the potato cyst nematodes (Globodera species), leaf and bud nematodes (Aphelenchoides species) and stem and bulb nematodes (Ditylenchus dispasci) cause problems in plants.

Beneficial nematodes

Some of the nematode species that infect insects and molluscs have been developed for pest 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

These nematodes come in packs that are mixed with water and watered onto affected plants and soil. There are limitations 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.

The nematodes require moist conditions and so are best applied in cool and damp conditions. There are also temperature restrictions with different species requiring temperatures above 5ºC (41ºF) or 12ºC (54 ºF).

Biological control for major horticultural pests

Some of the most damaging pests can be controlled using biological control. 

1. Glasshouse whitefly (Trialeurodes vaporariorum):

CONTROL: A small parasitoid chalcid wasp (Encarsia formosa), thought to have originated from South America, but discovered in the UK in 1926. The adult parasitoids are 0.6mm long; the female has a dark coloured head and thorax with a pale yellow abdomen; the males are entirely dark. Females lay 50-100 eggs singly in whitefly scales. Parasitised scales turn black distinguishing them from whitish-green healthy scales. Adults emerge through circular holes cut in the dorsal surface of the dead whitefly scales. Adults feed on the sugary excretion (honeydew) produced by whiteflies.

Encarsia is supplied in the form of black scales on pieces of leaf or on card. These cards or leaf portions 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 on the plants after about three weeks. This parasite is not effective against cabbage whitefly or other whitefly species.

2. Glasshouse red (two-spotted) spider mite (Tetranychus urticae)
The mite is a common problem that breeds rapidly and is difficult to control effectively with pesticides.

CONTROL: There are several biological controls available for this mite.

Predatory mite (Phytoseiulus persimilis), originally from Chile, is now widely used in preference to pesticides.

  • The predatory mites feed on all life stages of glasshouse red spider mite
  • The predatory mites are dispatched as nymphs and adults which should be released in sheltered positions on infested plants
  • The predatory mite is about the same size as glasshouse red spider mite (0.5 mm), but they can be distinguished if examined with a hand lens. The predatory mite has orange-red pear-shaped bodies and are more active than glasshouse red spider mite. Glasshouse red spider mites are rectangular inn shape and despite its common name, they are usually yellowish-green with two dark patches. They may be entirely dark, or in the autumn they may become reddish-orange
  • The predatory mite does not control fruit tree red spider mite on apple and plum, but can be successful against glasshouse red spider mite on outdoor plants in the summer.

Other mites, Amblyseius spp., are  sold for control of red spider mite and some other glasshouse problems. These can be used earlier in the season when temperatures are as low as 10ºC.

A midge, Feliella acarisuga, is also available this can be used early and late in the season and can find small hidden populations of red spider mite.

A 3-4mm long dark brown rove beetle Atheta coriaria is a predatory insect as an adult and larvae it will feed on a wide range of invertebrate problems including fungus gnats, thrips and is sometimes sold for control of red spider mite

3. Glasshouse mealybugs (Pseudococcus and Planococcus spp.)
The waxy fibres that mealybugs secrete over themselves and their habit of infesting relatively inaccessible places on plants, reduces the effectiveness of insecticides.

CONTROL: A ladybird predator (Cryptolaemus montrouzieri) native to Australia. The adults and larvae feed on mealybugs and their eggs. The adults are 3.5mm long and are of the typical ladybird shape with grey-black wing cases and a reddish-brown head and thorax. The ladybirds’ larvae resemble mealybugs, but when fully grown, the ladybird larvae are more than twice as large as most mealybugs and have a more extensive wax coating.

This ladybird can lay up to 500 eggs. The mealybug ladybird needs warm conditions and does best when daytime temperatures are around 27ºC (80ºF). The predator should be released onto infested plants in the evening to reduce the risk of adult ladybirds flying up and escaping through vents. The beetles will generally stay on the plants once they have settled.

Parasitoid wasps, Leptomastix and Leptomastidea sp. and Anagrus sp. are also available often in combination with Cryptolaemus for mealybug control.  These can help reduce mealybug populations when the problem is at low levels. 

4. Scale insects

CONTROL: Some glasshouse species, such as soft scale (Coccus hesperidum) and hemispherical scale (Saisettia coffeae), can be kept in check by releasing parasitic wasps Metaphycus helvolus, Encyrtus spp. and Encarsia citrina. Thes controls are sometimes available. These 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.

The nematode Steinernema feltiae is also sometimes recommended for control of these scale insects.

A small black ladybird, Chilocorus nigritus is sometimes available for Diaspid scales in glasshouses and can give good control during the growing season.

5. Aphids

CONTROL: There are a wide range of biological controls available for aphids (greenfly and blackfly). Many are native species that commonly occur in gardens. These parasitoids and predators often naturally reduce aphid problems by mid-summer in the garden but may need introducing into the glasshouse.

A small midge, Aphidoletes aphidimyza, whose larvae prey on aphids can give effective control in glasshouses from April to September. These tiny flies (< 3 mm) lay red eggs at night on foliage, these may be laid singly or in clusters of up to 40. The females lay a total of about 100 eggs over two weeks. The eggs hatch after three days and the small orange-white larvae take seven to fourteen days to complete their feeding before pupating in silk cocoons in the soil. Adults emerge about three weeks later. Pot plants should be stood on trays of damp sand, otherwise fully fed Aphidoletes larvae will drop down and fail to find anywhere to pupate.

The larvae feed by attaching themselves to an aphid's leg joint and sucking out the body contents. The larvae often feed underneath the aphids' bodies and can be difficult to see in dense aphid colonies. Sixty to eighty aphids can be killed by a single larva. Control may be less satisfactory on plants with hairy leaves. Aphidoletes are 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.

A species of 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 but 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 silk threads 5-7mm long. This species of lacewing overwinters in sheltered places as adult insects.

Larvae and adults of the two-spot ladybird, Adalia bipunctata, can be purchased and should be placed on aphid infested plants from spring onwards. 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.

Parasitic wasps, Aphidius, Praon or Aphelinus species are also available. 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 fully grown larvae of Praon species emerge from under the aphid spinning a silken cocoon that fastens the aphid mummy to the leaf. The parasites are supplied as adult wasps or parasitised aphids. These should be released or placed on plants in the evening.

A 3-4mm long dark brown rove beetle Atheta coriaria is a predatory insect as an adult and larvae it will feed on a wide range of invertebrates including aphids, fungus gnats, thrips and red spider mite.

The predatory green larvae (maggots) of a hoverfly Sphaerophoria ruppelli are also available for control of aphids

Aphid predators, such as the larvae of ladybirds, hoverflies and lacewings, can also be collected from garden plants and released into a glasshouse.

6. Vine weevil (Otiorhynchus sulcatus)
The larvae of vine weevil feed on the roots of many plants, especially those being grown in pots or other containers.

CONTROL: Vine weevil grubs can be controlled by watering into the potting compost a suspension of the pathogenic nematode, Steinernema kraussei. 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.

Steinernema kraussei will remain active at soil temperatures down to 5ºC (41ºF), so control can also be achieved later in the autumn to spring period, although late treatment may fail to prevent some damage occurring. A different species of nematode, Heterorhabditis megidis, is available from some suppliers, this requires soil temperatures of 12-20ºC (54-68ºF).  The nematode Heterohabditis bacteriophora is also available for vine weevil grub control.

A trap containing nematodes (Steinernema carpocapsae), is available for controlling 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.

7. Slugs
Several species of slugs cause damage in gardens, especially to seedlings, herbaceous plants and potato tubers.

CONTROL: They can be controlled by a pathogenic nematode, Phasmarhabditis hermaphrodita. It can be used anytime when the soil is moist and above 5ºC (41ºF), but is best 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.

8. Other biological controls – predators and parasites

Other controls are sometimes available for use in gardens and/or glasshouses:

  • Sciarid fly/fungus gnat larvae: Fungus gnat mite (Hypoaspis miles), Mighty mite (Macrocheles robustulus), rove beetle (Atheta coriaria), predatory mite Stratiolaelaps scimitus
  • ThripsA predatory bug (Orius laevigatus), Predatory mite (Hypoaspis species), Mighty mite (Macrocheles robustulus)
  • Thrips, including western flower thrips, glasshouse red spider mite, fruit tree red spider mite, tarsonemid mite and citrus red spider mite: predetory mites (Amblyseius and Hypoaspis species)
  • Caterpillars: Trichogramma egg parasite
  • Cabbage white butterfly eggs: Parasite Trichogramma brassicae

9. Other biological controls – nematodes

Other controls are available for use in gardens and/or glasshouses:

Suppliers of biological controls

Biological controls can sometimes be bought or ordered at garden centres but are usually supplied by mail order. Being living animals with limited lives they cannot be stocked on shop shelves. The exceptions are some of the nematodes used against slugs, vine weevil larvae, chafer grubs and leatherjackets, 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.


Biological controls and their suppliers

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