Biological control is the safest protection for plants. Kathryn Bradley-Hole
reports on pest predators
I WONDER how many viewers of David Attenborough's recent television series, The Private
Life of Plants, are still reeling from the extraordinary revelations of the plant world's hidden
agenda. For me, one of the most memorable (although not very pleasant) sequences explained
the way that a particular tribe of ants protects its host acacia tree from potential sap-suckers and grazers. As the programme showed again and again, the symbiotic relationship between plants and creatures is staggeringly finely tuned.
Now "biological control", the bug -eats-bug syndrome, can be safely applied in the greenhouse
and garden to deal with a broad range of common pests. This year gardeners will notice greater than ever availability of pest predators on sale in garden centres and by mail.
But why choose a biological solution rather than zap the bugs with a range of garden chemicals?
One very good reason is that some pests, particularly whitefly, can rapidly evolve "super" strains, immune to approved pesticides. A more general concern for the environment is the other reason most people give for using pest predators: broad-ranging chemical insecticides are indiscriminate, killing beneficial insects as well as the baddies.
Fortunately, common pests such as slugs, aphids, vine weevils, red spider mites, whitefly,
caterpillars and mealy bugs can be controlled by their natural predators. This may conjure
up visions of having to look at even more creepy-crawlies going about their business on
one's treasured plants; but many predators are so small they are invisible to the naked eye
and others are no less acceptable than a ladybird.
Greenhouses and conservatories provide ideal breeding grounds for several major
pests, one of the most common being the whitefly, the adults of which erupt in clouds
of fluttering white "ash" when disturbed. Encarsia formosa is its exotically titled predator,
a tiny black and yellow wasp, just one millimetre long. Ideally, these wasps should be
introduced as soon as whitefly are discovered. They are usually supplied at the pupal
stage on a card, which is placed among the infestation.
At Hatfield House, in Hertfordshire, Encarsia has been successfully used for several
years by head gardener David Beaumont. He checks over plants to see which are most
badly infested, for these are the ones that will provide the predators with their essential
early food supply while they are breeding.
Three applications, each two weeks apart in early spring, are recommended to gain
good control over the sap-sucking juveniles. Encarsia is not effective against adult
whitefly, however, so yellow sticky card traps are also suspended just above the
affected plants. They catch whitefly on the wing every time the plants are disturbed,
as well as other flies and aphids. Blue sticky cards, often used in commercial nurseries,
are effective in trapping thrips.
Glasshouse red spider mite is another sap-sucker, commonly attacking house and
greenhouse plants, causing mottling of the leaves and eventual death of the host plants.
A tiny, predatory mite, Phytoseiulus persimilis, gives control when the air temperature
is about 16-18OC so it is effective from spring to autumn. A humid atmosphere benefits
the predator, while providing unfavourable conditions for the spider mite.
Phytoseiulus, which is often supplied on a withered-looking leaf, is also used in the
Hatfield greenhouse. "You just hang the leaf on a plant that is infested with the mite
and leave them to it," says Beaumont.
Mealy bugs are particularly difficult to control by ordinary methods; they are coated in a
white, waxy material that somewhat resembles small pieces of cotton wool. They multiply
quickly and large colonies weaken plants as well as looking unsightly.
There are several effective predators, including Cryptolaemus montrouzieri, an
Australian ladybird which needs temperatures above 20OC and good sunlight to be effective.
Both indoors and outdoors, the vine weevil has become a major pest in recent years.
Soil-less composts have provided ideal breeding grounds for the small, dark beetles
which have pale mottling on their backs. They lay eggs on the soil surface around
plants and the white larvae grubs rapidly kill a huge range of plants by eating the
roots. Nematodes can be watered into the soil to attack the grubs and are most
effective in spring and autumn, provided the soil is moist and 12OC or more. They
do not attack adult weevils, however, which need to be crushed whenever they are found.
Slug-killing nematodes were introduced to the garden scene last year, marketed as
Nemaslug. They come in sachets and are mixed with water to make a soil drench.
Effective within a temperature range of 5-20OC, they are said to attack all slug species
found in the UK, although they are not effective against snails. The suppliers are careful
to point out they are harmless to humans, pets, birds, hedgehogs and frogs.
With spring now under way, pest populations will be increasing rapidly, but may I make
one request? I do hope that readers of this newspaper will not be inspired to send me
unidentified beetles, worms and other creepy-crawlies through the post. There is nothing
more horrible than receiving suspicious packets of this nature with one's breakfast mail.
