Advanced Gardening

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The predatory mite Phytoseiulus persimilis can eat up to five adult red spider mites per day

Red spider mites can cause untold damage to crops, particularly tomatoes. The tell-tale webbing usually means there is a large infestation

Biological control for indoor plants

Steve and Val Bradley explain the biological methods for controlling red spider mite and whitefly that involve the use of predatory mites and parasitic wasps

IOLOGICAL pest control is a ‘bug eats bug’ approach that involves the use of natural

Benemies to help control pests on your plants. There are predators for almost all situations now, not just outside in the garden but also in enclosed places such as indoor rooms, conservatories, greenhouses and polythene-clad structures where edible plants may grow alongside ornamental and tender ones.

Selective organic control This method of pest control is suitable for organic gardeners and anyone who is reluctant to use pesticides, especially indoors or on plants being grown for food. The predators or parasites are bred to be host-specific, so they will only prey on the insect pest you wish to control and are harmless to other wildlife and pets. whole pest population with one or two applications of a chemical. Biological control is actually about pest management, rather than outright control, so the aim is to keep the pest population at a manageable level, rather than eliminating it altogether. If the pest population disappears completely, the control agent will die out, too, due either to a lack of food or a disrupted lifecycle. So, the aim is to have a balance at all times, with a

Management not elimination Dealing with pests biologically is quite different from the traditional practice of aiming to wipe out the low population of pests supporting a low population of control.

The two ways biological controls work 1. The pest is controlled by introducing insects that are predators and feed on the targeted pest, often eating adults, young and even eggs. This is the case with red spider mite control. Some predators can feed on other things, such as pollen or dust on the leaves of plants, so they can survive for a time even if the pest is only present in low numbers. 2. The pest is controlled by introducing a parasite, where the control will lay eggs in the pest at some stage of its lifecycle, usually the eggs or young. The predator’s young develop inside the pest, killing it. This is the case with whitefly control.

n Other pests that can be controlled biologically Citrus red spider mite, fuchsia gall mite, fungus gnats, mealybugs, scale insects, thrips

Whitefly control

THE most common insect used to deal with glasshouse whitefly is Encarsia formosa, a small wasp. The females lay 50-100 eggs singly by injecting them into whitefly scales. The scales turn black as the wasp egg hatches and the young feed on them before emerging as adults by making a hole in the dead whitefly scales. Adult wasps feed on the honeydew produced by whiteflies. The wasps are supplied as parasitised whitefly scales on a cardboard square.

Use a yellow

1sticky trap to get some idea of the level of whitefly population before ordering the control. Hang the card

2containing the juvenile wasps on the plants where the whiteflies are feeding. The parasitised scales show as lots of little back dots in the centre of the card.

Whitefly can

3be a problem on tomatoes and fuchsias. Growing marigolds in front of tomatoes or between fuchsias helps, as whiteflies feed on them, but they will eventually migrate to the crop. Having the control ready can save a lot of damage. With all these

4pests and controls it is worth having a 10x hand lens to see what is happening, so you can check on a regular basis.

Red spider mite control

THE most commonly available control used to deal with red spider mites is another mite, called Phytoseiulus persimilis, which feeds on the red spider mites.

They can eat around five adults and 20 young per day, and breed much more quickly than the red spider mite, so they can potentially double their numbers every two days and reduce the red spider mite population very quickly if the conditions are good.

Red spider mites usually feed at

1the top of a plant on the soft, young growth, so spray first to moisten the leaves. This stops the predators rolling off the leaves as they are distributed. Shake the container before

2scattering the predators over the leaves, because the insects often settle into the base of the container during transit. Alternatively, place them in

3distribution boxes, which act as breeding sites for them to start to hunt for prey. Place these among high populations of spider mites.