Biological pest control

Biological control is the use of natural enemies to control pests. In the home garden this can be done by introducing predators or pathogenic nematodes that infect the pest with a fatal bacterial disease. This technique is mainly used in greenhouses, but some biological controls, including pathogenic nematodes, 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: Most glasshouse plants and house plants, some also for outdoor use
Timing Mainly used April to September

Suitable for...

Biological control is a viable and effective alternative to pesticides for some pests. It can have several advantages over chemical control. Populations of some pests such as glasshouse whitefly and glasshouse red spider mite have developed resistance to some pesticides. While pesticides cannot be used on certain plants because they may be scorched by the sprays and there are no synthetic pesticides available for some food plants.

Biological control avoids these problems by using natural enemies to combat pests. These cause no damage to plants, do not leave residues and 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 or is eliminated. The predators and parasites are usually specific to certain pests and will not become a nuisance or cause harm to non-target animals.

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 insecticides and, to a lesser extent, fungicides.

Using biological controls: Parasites and predators

It is important to know how biological controls work in order to get the best from them. Most of the parasites used for biological control are more accurately termed parasitoids, as they kill the host whereas a true parasite does not.

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.

Glasshouse predators and parasites, like the pests they control, require warm conditions, if they are to thrive and breed more rapidly than the pests. They generally require daytime temperatures of at least 21ºC (70ºF) and high light intensity to stimulate breeding, although they can usually survive at temperatures of13ºC (55ºF). The effective season for biological control in glasshouses is therefore late March/April to September. Outside of this period careful use of pesticides may be required to keep pest populations in check.

Glasshouse biological control will often die out during the winter and reintroduction may be necessary each year.

Biological controls and pesticides

Biological controls  are very susceptible to most pesticides and there use should be avoided for at least six weeks before they are introduced, (e.g. ten weeks for deltamethrin based products; but 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, roundworms or eelworms 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) and leaf and bud eelworms (Aphelenchoides species) are plant pests.

Beneficial nematodes

Some of the microscopic species of nematode that can 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. Like other biological controls 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 temperatures 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 thankfully be controlled using biological control. Here are the major pests and their controls;

1. PEST: Glasshouse whitefly (Trialeurodes vaporariorum):

CONTROL: A small parasitic 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. Adult wasps emerge through circular holes cut in the dorsal surface of the dead scales. Adult parasites feed on the sugary excretion known as honeydew produced by whiteflies.

Encarsia is supplied in the form of black scales on pieces of leaf or pasted onto cards. These cards or leaf portions should be fixed securely in a shaded position on infested plants. Adult parasites 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.

A tiny 1.2 mm long shiny black ladybird beetle, Delphastus catalinae is also available for glasshouse whitefly control. Both adults and larvae feed on all stages of the glasshouse whitefly’s.  This ladybird is recommended for plants where a heavy whitefly infestation has developed.  The adult ladybirds can live for up to 60 days and will lay about three eggs a day

2. PEST: Glasshouse red (two-spotted) spider mite (Tetranychus urticae)

CONTROL: This common pest breeds rapidly and is difficult to control effectively with pesticides, especially if resistant strains of the mite are present. A predatory mite (Phytoseiulus persimilis), originally from Chile, is now widely used in preference to pesticides.

  • The predator 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 predators have orange-red pear-shaped bodies, are more active than the pest, and can often be seen tapping the leaf surface with their front legs in search of prey
  • Glasshouse red spider mites are more lethargic than the predator and their body shape is rectangular. 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 predators feed on all life stages of glasshouse red spider mite.
  • The predators are dispatched as nymphs and adults which should be released in sheltered positions on infested plants.
  • The predator 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 also sold for control of red spider mite and some other glasshouse pests. 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 its prey.

3. PEST: Glasshouse mealybugs (Pseudococcus and Planococcus spp.)

CONTROL: The waxy fibres that mealybugs secrete over themselves and their habit of infesting relatively inaccessible places on plants, reduces the effectiveness of insecticides. A ladybird predator (Cryptolaemus montrouzieri) native to Australia can overcome these problems. Both the adult ladybirds 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 if there is a plentiful supply of food. 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 ventilators. The beetles will generally stay on the plants once they have settled, although they will wander about if there is insufficient food.

Parasitic wasps, Leptomastix and Leptomastidea sp. are available in combination with Cryptolaemus for mealybug control from one supplier.

4. PEST: 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. 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.

5. PEST: 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 parasites 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 a period of 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 larva during its development. 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.

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

6. PEST: Cutworms and Caterpillars

CONTROL: A pathogenic nematode, Steinernema carpocapsae, is available for the control of caterpillar pests, such as those on brassicas and for cutworms and other soil-dwelling caterpillars. When nematodes are watered onto the foliage, they need damp conditions to allow them time to infect the caterpillars with a bacterial disease, so apply during dull, cool weather.

7. PEST: Vine weevil (Otiorhynchus sulcatus)

CONTROL: The larvae of vine weevil feed on the roots of many plants, especially those being grown in pots or other containers. The 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 Agralan Ltd, this requires soil temperatures of 12-20ºC (54-68ºF).

8. PEST: Slugs

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

9. Other biological controls – predators and parasites

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

  • Sciarid fly/fungus gnat larvae: the mite, Hypoaspis miles
  • Thrips and other small insects in glasshouses: A predatory bug, Orius laevigatus
  • Thrips, including western flower thrips, glasshouse red spider mite, fruit tree red spider mite, tarsonemid mite and citrus red spider mite: a predetory mite (Amblyseius and Hypoaspis species)

10. Other biological controls – nematodes

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

    Biological control scattered on greenhouse melon leavesWhitefly biological control for gardenersGlasshouse whitefly nymphs (black ones) killed by biological control Encarsia FormosaBiological control for glasshouse red spider mite Phytoseiulus persimilis (orange-red pear-shaped mites)Glasshouse mealybug ladybird larvae Cryptolaemus montrouzieri (larger white larvae on right)Predatory midge (Aphidoletes aphidomyza) larvae feeding on aphids2-spot ladybird (Adalia bipunctata) biological control for aphid on Broad beanAphid parasitic wasp (Aphelinus sp.) on carnation

    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 the nematodes used against slugs, vine weevil larvae, chafer grubs, leatherjackets and some other pests. These are stocked in refrigerated cabinets in some garden centres.

    For success, the supplier’s instructions regarding release and subsequent care of the predators or parasites must be followed carefully.

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