Aphids: Everyone is familiar with these wide-spread pests that can affect almost any kind of plant. The typical symptoms are deformed, puckered, curled up leaves and stunted shoots until the plant begins to wither in parts or completely.
Aphids usually congregate on the underside of the leaves, preferably young, tender ones, as well as on young green shoots. Because of the secretion of honeydew, an invasion of aphids is frequently followed by sooty mould. In addition, aphids often bring with them dangerous viral diseases. This can be prevented by a well-balanced fertiliser without too nitrogen. If you are dealing with a mild attack, you can just remove the affected parts or dip them in a soapy water. A cold water nettle decoction, sprayed undiluted on the plants, is also very effective. In cases of more severe attacks, a pesticide that will not damage useful insects such as green lacewings and ladybirds should be used.
Tip: Tender houseplants and container-grown plants should not over-winter in an excessively warm room, since that would enable aphids to multiply.
Scale insects: These tend to attack conifers and deciduous shrubs, fruit trees and fruit bushes. They often attack container-grown plants over-wintering indoors, especially citrus plants. The flat or rounded, yellow to brownish insects sit motionless on the branches and leaves. Their secretion of honey-dew often leads sooty mould. In mild cases, you can remove the scale insects by hand or with a toothbrush. In more severe cases, you can spray the plant with a product containing mineral oil and repeat the operation several times. The scale insects will suffocate under the film of oil. However, this should not be done too frequently because the leaves would become too sticky. If the plants are grown in a greenhouse, you buy parasitic wasps in specialist nurseries that will prove very useful in controlling the pest.
Woolly aphids: These aphids covered in tufts of waxy, white, woolly excretions gather on the underside of leaves, leaf stalks, shoots and more especially in the leaf axils. Unlike scale insects they move about but are just as troublesome. The secretion of honeydew is frequently followed by sooty mould. The leaves become sticky and shiny and the needles of conifers turn yellow and drop. They are treated in the same way as scale insects.
Red spider mites: The sucking action of the red spider mites which only measure 0.5 mm (1/50th of an inch) results in a fine mottling of the leaves that discolour and die off. Pelargoniums affected by red spider mites, display a “corklike” mottling of the leaves. Red spider mites sit on the underside of the leaves and will attack a wide range of plants ranging from houseplants to fruit trees. Another indication of their presence is white gossamer-like agglomerations in the leaf axils and on the underside of the leaves.
Red spider mites are more prone to attack in dry, warm weather, which encourages their propagation. That is why excessively dry air should be avoided. One should encourage important rivals in the shape of predatory mites and bugs. Only in severe cases should you a pesticide – of a kind that will not harm useful insects – such as a potash-soap based preparation.
Whiteflies: These winged insects are covered in a white powder and their fixed larvae sit on the undersides of leaves where they lay their numerous eggs. The adult insects will immediately fly away if touched. Their sucking causes yellow spots on the leaves which finally dry up and drop. The secretion of honeydew also leads to fungus. Whiteflies prefer certain plants, which include, verbenas and abutilons.
It is possible to reduce the risk of whiteflies by using a fertiliser that does not contain high levels of nitrogen so that the plant tissues do not become too soft and therefore vulnerable. Whiteflies mainly occur in plants overwintering indoors. This is why it is important to hang up pesticide-impregnated sheets in the room to control possible pests, ventilate the whole area and if necessary introduce parasitic wasps. It the attack is very serious, use a pesticide that does not harm useful insects as soon as you notice the severity of the attack
Thrips: these wingless larvae and adult insects leave fine silvery flecks on the leaves and petals that become discoloured. The leaves become deformed, dry up and die. In the end the whole plant becomes stunted. The insects often suck the leaves and petals from the underside and occur more especially in damp weather and high air humidity. Predatory mites and bugs and green lacewings are natural enemies of thrips.
Check regularly to assess the seriousness of the attack. Mild attacks can be treated by the introduction of parasitic wasps, which can be bought in specialised garden centres. Pesticides should only be used in a serious attack and only of a kind that will not harm useful insects. In a very serious attacks, it is best to get rid of the whole plant. Ideally the affected parts should be burnt. They must not be thrown on the compost because thrips can over-winter in theas well in the discarded affected parts.
Weevils: Weevils have a flat body with a triangular shield between the forewings. The forewings lie flat on the body and unlike beetles, their horny front overlap. They are green to brownish and have very quick with their long legs. Their sucking causes yellow flecks on the leaves that later turn brown and die. The leaves are full of holes. Weevils also attack rosebuds and young shoots which become deformed. If they become troublesome, you can use a potash soap solution that will not harm useful insects. This best done in the morning when the insects when they are still unable to fly because of the low temperatures. You should also introduce natural enemies such predatory insects and birds.
Leaf miners: these pests are widespread. The females lay their eggs directly into the leaf where the larvae then develop undisturbed. Many species pupate there. The larvae eat the leaves, leaving behind clearly visible white blotches caused by their tunnelling through them and leaving excrement behind. If the attack is quite mild, just remove the affected leaves. If the attack is more serious use an ordinary pesticide that will not harm useful insects.
Fickle midge larvae: The larvae live in the soil and damage the roots and base of plants. If you touch the plant you will see small black midges – measuring 2 to 3 mm (about 1/1 Oth of an inch) long – suddenly fly away. The plants should be kept as dry as possible. A layer of sand on the surface of the pots will prevent the laying of eggs and is fatal to the larvae. Pesticide-impregnated sheets will help to prevent attacks while the introduction of parasitic wasps will be beneficial in the case of a mild attack.
Vegetable flies: The group of vegetable flies is very large but the damage they do is always the same. Whether carrot flies, cabbage flies, bean flies – the female maggots always tunnel through the fruit, leaves and often the roots, which causes the leaves to turn yellow and wither. Sometimes the entire plant is affected and dies. Carrots taste bitter when affected. To lay their eggs in late April or early May, flies prefer warm places sheltered from the wind. You can trick them and get round this dangerous period by sowing early or late varieties, or grow the seedlings in the house for the first few weeks. Insect nets are an effective physical barrier. But in the event of an attack it is important to replace the compost completely because carrot flies can over-winter in the soil and move onto other plants such as parsley.
Beetles: Beetles eat the leaves and buds of plants, leaving just the veins behind. The small white larvae eat the roots that leads to the withering and weakening of the plant. You can catch the wingless, nocturnal beetles only in the evening because they remain in the soil during the day. Larvae and beetles also overwinter in the soil. To get rid of larvae, you can introduce parasitic roundworms in spring and autumn. They can be ordered in specialist garden centres and used in roof gardens, containers and window boxes. But the best prevention is clean soil. Frequent victims are rhododendrons and yews, herbaceous perennials and, as well as and houseplants such as cyclamens.
Slugs and snails: Slugs and snails eat fruit and leaves, especially young, tender plant tissue. They can often completely defoliate plants in the course of one night. It is only in rainy weather that they come out during the day.
They leave behind distinctive silvery slimy trails. However, gardeners are as inventive to combat these pests as the slugs are troublesome. Their methods include catching the snails, protecting the plants by surrounding them with a protective ring of sawdust or sand, putting up traps such as bowls of beer and special slug protection barriers. It is helpful to make sure there are no hiding-places for the slugs or snails, to avoid excessive dampness, and to create spaces for natural enemies such as birds, hedgehogs, frogs and toads. If this does not help, you can sprinkle slug pellets around the plants, but you remember that these pellets are poisonous to pets.
Caterpillars: The green caterpillars of the great and small winter moths cause a lot of damage to leaves and young shoots of deciduous plants and trees in spring. Young leaves are spun together in the process.
Butterfly caterpillars are easily identified by their five pairs of legs and typical shape of the back while the great winter moth caterpillar has striking white side stripes. Make use of natural enemies such as parasitic wasps because a couple of titmice can consume up to 30 kg (66 lb) of caterpillars to feed their young. Sticky bands placed around the trunk in autumn will catch the flightless females on their way to laying their eggs at the top of the tree. Hornbeams, oaks and fruit trees are particularly vulnerable.
Stem and bulb eelworms: Typical indications of the presence of eelworms are stunted growth and a swollen leaves and base. In phloxes the roundworms eat the leaves so that only the central vein of the leaf remains. Affected parts of the plant should be removed. An open position will help prevent infestation by eelworms.
Mixed cultivation and a pause in cultivation will also help. Weeds are often host-plants to eelworms and should therefore be removed to prevent new infestation. Besides herbaceous perennials, eelworms also attack bulbs (tulips, narcissi, crocuses) and summer flowering plants (pinks and carnations).
Leafhoppers: Like larvae, fully grown insects cause white blotches on the top side of the leaf, very similar to those caused by the red spider mites. In fact, leafhoppers and red spider mites are often both present on the same plant but they are easy to distinguish from each other. The yellow-green leafhopper jumps away when you touch the part of the plant on which it is sitting after shedding its skin; indeed it leaves behind its very striking white skin. A striking feature of the leafhopper is the slimy foam on the shoots in which the larvae can feed undisturbed. They propagate particularly strongly during warm dry weather.
A well-ventilated position and the application of a fertiliser low in nitrogen can help prevent leafhopper attacks. Spraying with a cold decoction of nettles can be helpful in mild cases. In more severe cases you should repeatedly spray the undersides of the leaves with a standard pesticide against sucking insects, one that will not harm useful insects.