Biological pest control

Suitable for...

• Biological controls are an alternative to the use of pesticides for certain specific pests
• This is a safe method of pest control for both edible and ornamental plants that can give excellent results under the right conditions
• Biological controls are not available for all pests, although most greenhouse pests can be tackled in this way. The biological controls available, and the types of pest they deal with, are listed below in the section "Which biological control do I choose?"

When to use biological pest control

The pest must be present, otherwise there is nothing for the predators to feed on or the parasites and nematodes to develop in. However, predators and parasites cannot give an instant reduction in pest numbers; they need several weeks to get established and build up sufficient numbers to reduce the pest population. Predators and parasites should therefore be introduced into a greenhouse before a heavy and damaging infestation has developed. Pathogenic nematodes infect  slugs and certain insect larvae with fatal bacterial diseases, which can give a rapid reduction in pest numbers. Nematode biological controls used against insect larvae are best applied while the target pest grubs are still small and before they are causing severe damage.

Predatory mites and insects and parasitic wasps generally need a good light intensity and daytime conditions of 21°C (70°F) or more if they are to breed faster than the pests and so achieve control. For this reason, their use in greenhouses and on house plants is restricted to April to September. Some pathogenic nematodes, such those used against slugs and vine weevil grubs, are active in the soil at temperatures above 5°C (41°F) and have a longer season of use. Other nematodes, such as those used against leatherjackets, chafer grubs, ants, fungus gnat larvae, carrot fly, onion fly, cabbage root fly and caterpillars, need warmer soil temperatures of 12°C (54°F) or more.

Using biological control

Biological controls can sometimes be bought or ordered at garden centres (including the Wisley Plant Centre), 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.

• The controls should be released or applied as soon as possible after they have been received
• Predatory insects and mites or parasitic wasps  are vulnerable to most insecticides, especially synthetic compounds
• Biological controls are an alternative to pesticides, rather than an additional treatment
• If it is necessary to reduce an infestation of whitefly, aphids, mealybugs or red spider mites before a biological control is released, use a short persistence natural pesticide, such as a spray containing plant oils or fatty acids

Which biological control do I choose?

There are many biological controls available, and the types of pest they deal with, are listed below. The letters (a to h) refer to the companies that supply the controls – see the corresponding suppliers in the list below.

1. Aphid midge larvae predator: b,c,g,h
2. Aphid parasite*:  a,b,c,d,e,f,g,h
3. Caterpillar bacterium: no longer available in retail packs for garden use
4. Fungus gnat (sciarid flies)predatory mite: b,c,e,f,g
5. Lacewing larvae (mainly aphid predator): a,f,g
6. Mealybug ladybird (Crptolaemus): a,b,c,d,e,f,g,h
7. Red spider mite predator (Phytoseiulus):  a,b,c,d,e,f,g,h
8. Soft scale parasite*:  b,c,g, h
9. Slug nematode: a,b,c,d,e,f,g,h
10. Thrips predatory mite:  a,b,c,d,e,g
11. Vine weevil nematode: (mostly Steinernema kraussei; Agralan supplies Heterorhabditis megidis, which requires warmer soil temperatures of 12°C (54°F) or more) a,b,c,d,e,f,g,h
12. Whitefly ladybird: Not available
13. Whitefly parasite (Encarsia):  a,b,c,d,e,f,g,h
14. Aphid ladybird larvae:  a,d,e,f,g
15. Glasshouse leafhopper parasite*: b,h
16. Chrysanthemum leafminer parasite*: b
17. Mealybug parasite*:  b,c, d
18  Chafer grub nematode: a,b,d,e,f,g,h
19. Leatherjacket nematode:  a,b,d,e,f,g,h
20. Fruit tree red spider mite predator:  g
21. Fungus gnat nematode: b,d,g
22. Hard scale beetle predator:  g, h
23. Fungus gnat (sciarid flies) rove beetle predator: g
24. Thrips predatory bug (Orius): b, g
25. Mixed nematodes for carrot fly, onion fly, cabbage root fly, cutworms, swift moth larvae:, leatherjackets, fungus gnats (sciarid flies), caterpillars, gooseberry sawfly larvae, codling moth, ants a, b,d,e,f
26. Caterpillar/codling moth nematode: b,c,d,e, f
27. Ant nest nematode: a,b,d,e,f, g

* When applying for these parasites it is advisable to send to the firm samples of the pest(s) on their host plants in a dry polythene bag in a crush-proof container to ensure that they are species that can be dealt with by the parasites.

List of suppliers

a) Agralan, The Old Brickyard, Ashton Keynes, Swindon, Wilts SN6 6QR. tel: 01285 860015 Agralan products are supplied by Pest Free Gardening.
tel 07540 622216
[5, 6, 7, 9, 11 (Heterorhabditis megidis), 13, 14, 18, 19]

b) Biowise, Hoyle Depot, Graffham, Petworth, West Sussex GU28 0LR.
tel: 01798 867574
[1, 2, 4, 6, 7, 8, 9, 10, 11 (Steinernema kraussei), 13, 15, 16, 17, 18, 19, 21,24,25, 26]

c) Defenders, Coldharbour Farm, Amage Road, Wye, Ashford, Kent TN25 5DB.
tel: 01233 813121
[1, 2, 4, 6, 7, 8, 9, 10, 11 (Steinernema kraussei),13, 17]

d) Green Gardener, Chandlers End, Mill Road, Stokesby, Great Yarmouth, NR29 3EY.
tel: 01493 750061
[1, 2, 5, 6, 7, 9, 10, 11 (Steinernema kraussei), 13, 14, 18, 19, 21, 25, 26]

e) Organic Gardening Catalogue, Riverdene Business Park, Molesey Road, Hersham, Surrey KT12 4RG.
tel: 01932 253666
[2, 4, 6, 7, 9, 11 (Steinernema kraussei), 13, 14, 18, 19, 25, 27]

f) Ladybird Plantcare, The Glasshouses, Fletching Common, Newick, Lewes, East Sussex BN8 4JJ.
tel: 0845 094 5499
[2, 4, 5, 6, 7, 8, 9, 10, 11 (Steinernema kraussei), 13, 14, 18, 19, 25, 26, 27]

g) Just Green, Unit 14, Springfield Road, Springfield Business Park, Burnham on Crouch, Essex CM0 8UA.
tel: 01621 785088
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 (Steinernema kraussei), 13, 14, 16, 18, 19, 20, 21, 22, 23, 24, 26, 27]

h) The Natural Gardener, The Steppes, Hope under Dinmore, nr Leominster, Herefordshire HR6 0PP.
tel: 01568 611729
[1, 2, 6, 7, 8, 9, 11 (Steinernema kraussei), 13, 15, 17, 18, 19, 22]