Biological Control of Garden Pests

 

Biological control involves keeping down pests by the introduction of natural enemies: predators, parasites and diseases. This technique may be essential in the greenhouse, where pests may have developed immunity to chemicals.

Control in the Greenhouse

The specificity of biological control is very attractive to the gardener, and it is becoming more and more popular: there are now several different pest predators available to kill whiteflies, red spider mites, mealybugs, leaf miners, scale insects, thrips and vine weevils. Biological controls are particularly useful in the greenhouse; its controlled, warm environment and excellent light allows predators and parasites to breed faster than pests. However, it is important to introduce the control before plants are heavily infested, as it may take a number of weeks before it becomes effective.

The following table lists just a few of the biological controls available by mail order from specialist firms:

Pest Control
Aphids Aphidoletes aphidimyza
(Fly larva predators)
Caterpillars Bacillus thuringiensis
(Bacterial disease)
Mealybugs Cryptolaemus montrouzieri
(Mealybug ladybird)
Red spider mites Phytoseiulus persimilis
(Predatory mites)
Soft scale insects Metaphycus helvolus
(Parasitic wasp)
Thrips Amblyseius spp.
(Predatory mites)
Vine weevil larvae Heterorhabditis spp.
(Nematode predators)
Whiteflies Encarsia formosa
(Parasitic wasp)

The majority of parasites and predators need temperatures of at least 21°C (70°F) for at least part of the day, as well as good levels of sunlight. They should be released onto the affected plants on the day of receipt, preferably in the cooler part of the day. Some predators, such as Cryptolaemus are strong fliers, so open doors and windows may need to be netted to prevent them from escaping.

Biological controls are living creatures and can be affected by chemical usage. As a general rule, you should stop using derris 1-2 weeks before, pyrethrum 4 days before and soft soap insecticides 1 day before releasing biological controls.

Control in the Garden

Although many of the biological controls are only suitable for use in the greenhouse or conservatory, some, such as the nematodes used to kill vine weevil grubs, may be released in the open ground. The nematodes live in moisture surrounding soil particles and enter the grubs, releasing bacteria that kills them. The nematodes then reproduce in the dead grub and re-enter the soil to infect more vine weevil grubs. If there are no grubs present in the soil, the nematodes die out after a few weeks.

There are a number of other biological controls that may be used outdoors; these are natural predators such as birds, hedgehogs and frogs that should be encouraged to visit the garden. Harmful insect populations may be reduced by spiders, frogs, ants, wasps and birds, whilst hedgehogs, shrews, frogs and toads feed on many ground-dwelling plant pests. The centipede is also a useful ally, feeding on soil-dwelling pests. Unfortunately, it is often mistaken for the harmful millipede, so always check carefully before removing. One way to tell them apart is to look at their legs; millipedes have two pairs of legs per body segment, whilst centipedes have just the one.