Plant Pests and Diseases – Biological Control

Plant Pests and Diseases

Taking action with biological methods

An important aspect of organic gardening is the encouragement of natural predators and parasites to keep pests and diseases in check. These naturally occurring pest controllers can be bought as commercially produced “biological control agents”. The use of such agents is known as biological control. The agents are all tiny or microscopic, and are very specific in their action.

plant pests and diseases - biological control - parasitic wasps Biological control agents are available to help control a range of pests and one disease. They include: parasitic wasps that kill their prey by laying their eggs in them; predatory mites that eat other mites; and microscopic nematodes that infect pests with lethal diseases.

The majority of biological control agents are only suitable for use in a conservatory or greenhouse as they require warm conditions to be effective. Some are available for outdoor use. Trichoderma and Bacillus thuringiensis (Bt) are two biological control agents that differ somewhat from the others in their mode of use. Trichoderma – a fungus that will repel other fungi, is used to control silver leaf and also as a wound paint). Bacillus thuringiensis (Bt) is a bacterium used for caterpillar control. It is applied to plants in the same way as more conventional sprays.

Level of control

The aim of biological control is to reduce pest levels and related damage rather than to eliminate them completely. In some cases this can be achieved with one application; sometimes a further batch of biological control agents may have to be introduced.

Sources of supply

Most biological control agents are purchased by mail order as they do not have a shelf life. Shops and garden centres also sell a biological control card which you then send off to arrange delivery of the agent when it is required.

Tips on using biological control

(Note that these guidelines are not applicable to all biological control agents)

Watch out for the first appearance of the pest. Biological control is most effective where the agent is introduced when the pest level is low. There is no advantage in introducing the agent before pests are present. If pest levels are high, try to reduce them using non-chemical means before introducing the agent. If you plan to use biological control, do not use any persistent pesticides. Some can still harm the agents several weeks after spraying.

As soon as the pests are seen, order the relevant biological control agent. Before ordering, check the following:

• That you have not used a pesticide that will harm the agent within the last 6-8 weeks. If necessary ask your supplier for details.

• That you can meet the required conditions of temperature (minimum and maximum), humidity and daylight.

• The delivery date. Often, the package containing the agents must be opened on arrival. Try to use the agents as soon as they are delivered. Read the instructions carefully before opening the container. Some may be stored in a fridge if delay in using them is unavoidable.

If you should need to use a pesticide to control other pests or diseases, use one that will not harm the natural predator or parasite.

Biological Control Agents

Pest: Whitefly

Predator/parasite: Encarsia formose

Type: Parasitic wasp

Optimal temperature: 18°-25°C (64°-77°F)

Pest: Red spider mite

Predator/parasite: *Phytoseiulus persimilis

Type: Predatory mite

Optimal temperature: 18°-25°C (64°-77°F); humidity 60%

Pest: Mealy bug

Predator/parasite: Cryptolaemus montrouzieri

Type: Predatory beetle

Optimal temperature: 20°-25°C (68°-77°F); humidity 70%

Pest: Aphid

Predator/parasite: Aphidiius matricariae/Aphidoletes aphidimyza

Type: Parasitic wasp/predatory midge larva

Optimal day temperature: 21°C (70°F); minimum night temperature: 16°C (61°F)

Pest: Vine Weevil

Predator/parasite: * Heterorhabditis megidis

Type: Parasitic nematode

Optimal soil temperature: 14°-25°C (57°-77°F); minimum: 10°C (50°F)

Pest: Slugs

Predator/parasite: * Phasmarhabditis hermaphrodita

Type: Parasitic nematode

Optimal soil temperature: 15°C (59°F); minimum: 5°C (41°F)

* Suitable for outdoor use


(in pots outdoors)

This predator is a microscopic nematode used on soil or compost in pots where vine weevil action is suspected. Apply from early to late spring and late to mid-autumn provided the soil or compost temperature is over 10°C (50°F). It comes in a sachet containing millions of nematodes and can be stored for up to six weeks in the fridge but is best used fresh. To use, mix the contents of the sachet with water and apply to moist soil or compost.


As soon as you see the first whitefly in the greenhouse order a supply of Encarsia formosa (parasitic wasps). There must be plants in the glasshouse and make sure you have not used any chemicals within the last month that could harm the wasps. The night-time temperature inside must be over 15°C (59°F) and the day-time temperature 20-26°C (68-79°F).

Remove any yellow sticky traps before introducing the wasp and only open the package in the greenhouse. If essential, the box can be stored unopened for one day in the fridge and no longer.

The box should contain small cards, each bearing a cluster of tiny black “scales”. These are parasitized young whitefly, containing eggs of the Encarsia wasp. Hang these cards up on infested plants out of the sunlight.

Within a few days the wasps should have hatched out. Hold a card up to the light; if wasps have hatched, you will see a tiny hole in the back of each blackened scale. You are unlikely to notice the wasps themselves. Within a couple of weeks you should be able to find blackened scales on the underside of leaves. This shows that the wasps are doing their work. Do not remove leaves from plants that show blackened scales underneath because you will be removing new supplies of wasps.

30. January 2011 by Dave Pinkney
Categories: Organic Gardening, Pests and Diseases, Plant Care | Tags: , | Comments Off on Plant Pests and Diseases – Biological Control


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