Pesticides and Insecticides By.......Mick Allsop
|The subject of pesticides is a complex one and can baffle even the keenest gardener. Gone are the days when there was only one chemical, DDT, which could be used for everything. A highly sophisticated armoury is now available to tackle all the different possible fungal infections and other garden pests. |
The term 'pesticide' in fact covers a wide range of chemicals that have a number of different uses. Pesticides include the following:
Insecticides - These include many different chemicals and are applied to the plant or compost to control pests such as aphids (greenfly/blackfly), whitefly, caterpillars and other insects.
Acaricides - These are specialised chemicals used for controlling members of the spider family which are dangerous to plants, particularly under glass, and can be difficult to eliminate. Examples are red spider mite and tarsonemid mite. Specific acaricides tend to be restricted to commercial use, but those that also have wider insecticidal properties include pirimiphos-methyl, malathion and dimethoate.
Nematicides - These chemicals are even more specialised and have been specifically designed to control nematodes (worms) such as eelworms. Eelworms are microscopic worms which live inside the affected plant tissue and, in order to deal with these pests, nematicides are highly toxic. They are not generally available to the public and their use is usually restricted to professional growers. Some nematodes are regarded as 'friendly', such as the nematodes which are used to destroy vine weevil, slugs and snails.
Aphicides - These are targeted at aphids such as greenfly and blackfly. Some are formulated to kill only aphids; these are particularly useful for plants in flower that are likely to be visited by other harmless insects such as butterflies, bees and predatory friends like ladybirds and lacewings (or their larvae). One of the best specific aphicides is pirimicarb, which acts selectively against only aphids. There are many other aphicides that also act as general insecticides and will control many pests. These include malathion, dimethoate, permethrin and pyrethrum.
Molluscicides - These are used to control pests such as slugs and snails. Molluscicides, either in the form of poisonous bait or in liquid form which is coated onto plant tissue, control these pests quite effectively. Chemicals such as metaldehyde and methiocarb are very efficient.
Fungicides - These are a very useful group of chemicals that prevent and, in some cases, control and kill fungal disorders. The wide range of fungal diseases which attack many plants include leaf spot, stem and root rot, rust and mildew. Not all fungi actually kill plants - indeed some will only survive on living tissue - but they can seriously reduce the vigour of the affected plant and also deform growth. Fungicides are generally best applied as a preventative measure.
Although the effect of any particular pesticide or insecticide depends on its chemical activity, it can also be influenced by the method of its application. The most common way of applying these are:
Sprays, drenches, dust, aerosols, misters, chemical baits, granules, powder or 'smoke bombs'.
HOW THEY WORK
Contact - These are applied directly onto the pest which means that good coverage of the infected plant is essential. The insect may then be poisoned either by the direct effect of the chemical on it, or by eating or walking over plant tissue that has been poisoned.
Systemic - These include both insecticides and fungicides and are extremely effective. Systemic chemicals may be sprayed or even watered onto the plants and rapidly enter the plant tissue where they are able to move freely to all parts of the plant. Pests attempting to suck the sap are then killed. Fungal diseases may also suffer the same fate when it attempts to draw nutrition from the plant.
Preventative - The term 'preventative' is applied to those fungicides which help to control fungal diseases such as mildew, rust, grey mould, damping off and leaf spot. Preventative fungicides, which are normally sulphur or copper based, are used to inhibit the germination of fungal spores, thus helping to reduce the spread of the disease. Preventative treatment should therefore be taken before any sign of disease, or at the first sign, before the disease reaches an advanced stage whereby the plant may be severely damaged.
REMEMBER - PLEASE TAKE NOTE OF THE DOSAGE AND SAFETY PRECAUTIONS RECOMMENDED BY THE MANUFACTURER AS REGARDS THE MIXING, USE AND STORAGE OF ALL INSECTICIDES/PESTICIDES. MOST PESTS SOON BUILD UP RESISTANCE TO CHEMICALS AND THESE SHOULD BE ALTERNATED TO HELP PREVENT THIS OCCURRING.