No matter what views one may hold about pests and diseases or poison sprays, the golden rule under glass is to start clean. With the new greenhouse there is no problem, but in the year-round warmth of heated greenhouses there is bound to be a gradual build up of unwanted guests unless preventive steps are taken. Some may be engaged in eating others and there is much talk of preserving beneficial predators and parasites.
We want to prevent pests or diseases from consuming or disfiguring our plants and to do so without endangering ourselves or our children. This is not as simple as it sounds, because some of the most damaging insects gradually become resistant to remedies that are regularly applied. Varying the chemicals used prevents a build-up of pest resistance.
Our first thought must always be to avoid introducing pests or diseases into the greenhouse. It is worth being quite ruthless about this. Every plant needs the most careful examination before being added to a collection. The soft growing points and the undersides of the leaves are the places to look. This may sound fussy, but two weeks isolation followed by another close examination is even more effective. If in doubt spray with pesticide.
Some pests will always be with us, unless we are willing to have a continuous discharge of some chemical or a frequent and regular spraying or fumigation in the greenhouse. I would rather keep a sharp look out and spray the enemies when seen,
There are four extremely persistent greenhouse pests that one needs to keep under control, and recognition of these is essential. These are mealybugs, red spider mites, whitefly and aphids. Among the lesser evils to lookout for are ants, earwigs, scales and thrips.
Mealybugs. These creatures which look like tiny white woodlice are clearly visible to those with normal sight, and their presence is usually first noticed as a small patch of white wool,which means they are breeding. Fortunately they do not spread rapidly in the early stages, but they are hard to kill. Picking off each insect with a pin or painting them and their eggs with methylated spirits are two effective methods for small infestation. This pest is the only major pest of cacti and succulents and can spread to woody plants if allowed to do so.
In heavier infestations, nicotine, malathion or one of the newer systemic insecticides, such as dimethoate, may be used either as sprays or, in the case of the systemics, as soil drenches.
Red spider mites. These are serious pests, particularly as the early stages are hard to spot, since the individual mites can barely be seen. Affected leaves become pallid due to the thousands of bleached spots where the mites are feeding on the under surface. The mites are straw-coloured, turning in autumn to minute red spider-like creatures that can clearly be seen with a hand lens, weaving webs over the lower surfaces of the leaves. This pest revels in a hoi dry atmosphere and spraying the under surfaces of the leaves of woody plants with water in warm weather helps to prevent trouble. The plants most likely to be attacked are shrubs and carnations, but almost ail greenhouse plants are susceptible.
Red spider mites quickly become resistant to any chemical that is used regularly but some reduction of infestations may be obtained by spraying thoroughly with derris, pirimiphos-methyl, malathion or dimethoate.
Alternatively, biological control may be attempted during the summer by introducing the red spider mite predator, Phytoseiuius persimilis (for suppliers see opposite).
Glasshouse whiie/Iy. These are tiny snow-white insects that rise into the air when the plants are disturbed. Unfortunately, their larvae are sucking insects with a complicated life cycle and all remedies have to be applied several times to eradicate this persistent pest. Many plants are affected including regal pelargoniums, fuchsias, cucumber, tomato and poinsettia.
Whitefly can be controlled by spraying or fumigating the plants with permethrin. This is a persistent but safe chemical that can be used on a wide range of plants, including tomatoes and cucumbers. Less persistent alternatives are bioresmethrin, pyrethrum, HCH, malathion and pirimiphos-methyl. In some areas there are strains of whitefly that have gained resistance to some of these insecticides.
Biological control may be attempted using a parasite, Encarsia /ormoso, but it only operates effectively above 21 °C (70°F).
HCH dusts or proprietary ant baits may be used in areas where they are troublesome, and these control measures should be applied in and around the nesting sites rather than on plants which ants are visiting.
Earwigs. It is in hot dry summers that earwigs often cause mysterious punctured holes to appear in the leaves of succulents and they also damage chrysanthemum and other flowers.
They can be trapped under old sacking and seed-boxes or in flower pots stuffed with straw or similar materials. HCH dust applied to their hiding places also reduces numbers.
Scales. The protective coverings of scale insects are sometimes found on the backs of the leaves of oleanders and other evergreens. From this safe hiding place they suck the sap of shrubby plants.
Thorough spraying of the undersides of leaves with malathion or nicotine in spring and summer will check infestations, and small pot plants can be hand cleaned using soft rags dipped in soapy water.
Tfirips. Thrips are minute blackish grey, winged insects that are hard to find, but the damage they cause is rather similar to that of red spider. They will not become established unless the atmosphere is too dry and spraying with clear water discourages them. If an insecticide is necessary, they can be controlled by spraying with malathion, derris or HCH. Suppliers of biological predators:
Bunting and Sons, Biological Control, The Nurseries, Great Horkesley, Colchester, Essex
English Woodlands Ltd., Graffham, Petworth, Sussex
H R Thrussell, Pike Plant Products. Antwerp Cottage, Mankinoles, Todmorden, Lanes., OL14 6HR
Natural Pest Control (Amateur), Watermead, Yapton Road, Barnham, Bognor Regis, West Sussex, P022 OBQ
Noianvale Ltd., 31 Springfield Road, Bury St. Edmunds, Suffolk
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