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Easy CareEasy-care basics : Plants resist pests

Few true pests | Predators

The necessity for pest control can be largely avoided. We carry the erroneous impression that plants need to be constantly treated with sprays of all kinds against pests and diseases. This is not the case. Although plants appear fairly helpless in defending themselves, they have a formidable range of defences against both pests and diseases. Healthy plants can ward off pest attack; it is nearly always plants growing in unsuitable conditions that succumb.

The most obvious defence against pests are the various kinds of thorns, spines, stinging and irritant hairs. These prevent animals from grazing the stems and leaves of plants. Few gardens have problems with grazing animals, though some have deer and rabbits.

Plants resist pests

Plants resist pests

Many plants have hairy or sticky stems and leaves. These aim to slow down the movement of greenflies and other small pests. Another very effective trick of plants against sap-sucking insects is the natural pressure of the plant sap.

Sucking insects like to use the pressure of the sap to feed themselves but when the plant is growing actively, the pressure can be too great for comfort. When the plant is short of moisture, the sap will be thick with sugars and released at a slower rate after the cells are punctured by the insect’s feeding tube.

Some plants have distasteful substances, even poisons, in the sap. Foxglove contains poisonous digitoxin; rhubarb has poisonous oxalic acid in the leaves. Yew trees contain poisons called taxines. The seeds of spurges contain powerful laxatives.

Oak leaves, indeed many plants, contain tannins that are very bitter and dissuade many animals that would like to eat them. Even so, oak is an important food source for hundreds of animal species that have adapted to the bitter taste.

Few true pests

Despite the effectiveness of physical and chemical defences against the majority of potential pests, most plants are attacked successfully by a variety of animals, especially insects. However, the pests of any one plant species are usually relatively few in number.

Some plants have no pests at all, some are attacked occasionally, a few are prone to more frequent attack. It is important to realise that while an insect, or other pest, might cause light damage to plants on occasion, it cannot be considered a significant pest.

In fact, the number of really significant pests of ornamental plants – that is, those which cause severe damage to plants – is very few. They include slugs and snails, and greenflies, occasionally rabbits are a serious problem in country gardens. Vegetables and fruit are prey to a few others like caterpillars and root flies that frequently cause problems, and can be considered serious pests.

Plants have another solution to the problem of pest attack; they simply outgrow it. Strong-growing healthy plants quickly outgrow the damage caused by pests. A few holes in leaves, even though the plant can look bad, is not significant to a healthy plant. It is constantly producing new leaves that replace any losses.

Predators

Remember, too, that plants are not on their own in this struggle because the pests themselves are prone to the attacks of parasites and predators. There is a long list of these: ladybirds, lacewings, hoverflies, shield bugs(as shown), groundbeetles, wasps, chalcid wasps, capsids, anthocorids, spiders, ichneumon wasps, frogs, hedgehogs and birds.

Plants resist pests

Plants resist pests

In fact, every insect or other animal seen in the garden that is not identifiably a pest is a beneficial predator or parasite. If we favour the beneficial insects and animals, the task of keeping plants pest-free is relatively easy. Finally, we can simply not grow plants that are persistently attacked by pests.

Like to learn more? Go to Garden.ie's Know-How section >

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