Estimates of the number of insects present in a particular environment are not easily made, but for pastures the figure is probably something like 25 million per hectare, in the top 10 - 15 centimetres of soil, or at the surface. At the same time there are probably around 25,000 insects in flight over the same hectare, at any time of the day. In forests or in crops the figures could be larger. For instance I found one estimate of 222 million bean aphids per hectare of sugarbeet in North America.
In the face of these numbers, how are we to survive insect pests?
If we chose to spray them with toxic chemicals, we have to saturate the entire ecosystem of the garden, orchard, crop or pasture, in the full knowledge that only around one percent of the active ingredient will actually reach its target and kill the desired pest. Some of the remaining 99% will kill ‘off target’ organisms, pollute streams, accumulate in wildlife and kill birds or mammals. It is sometimes said that pesticides are applied to ecosystems, not to pests. There are very few ways of deliberately targeting pesticides to reach the pest only. Pheromone traps and other techniques do provide this possibility but they are available for some pest species only.
Organic growers do use toxic chemicals, and should use the same care that any chemical applicators are required to use when applying them. Pyrethrum, derris and other ‘natural’ chemicals can cause poisoning as surely as the synthetic pesticides. ‘Natural’ does not mean safe, or good. Spina bifida may be natural, but it is not good.
The difference between the chemicals organic growers use and the synthetic ones is generally their short life in the environment and the fact that they are so readily broken down that they can not ‘bioaccumulate’ - that is they cannot become concentrated in the food chain, eventually accumulating in higher carnivores such as owls, hawks, fish and hunting animals.
Organic growers should also be using these chemicals as the treatment of last resort, only after a whole range of other protective measures have failed to control the pest to within the damage threshold.
The concept of a ‘damage threshold’ is already different to the familiar idea of a ‘control threshold’. Tolerance of a certain level of pests may be an important aspect of organic pest management. A population of pests may be required to maintain an effective level of predators. Alternatively the disruptive effects of the treatment interventions may outweigh the benefits. Control should be based on a thorough assessment of the consequences of action and inaction including the total economic and ecological ramifications. The consequences of intervention may be remote from the cause, and include encouragement of ‘secondary pests’, which arise when competition from the first pest is removed or when predator populations are killed by chemicals (or other actions).
Some of the common causes of pest problems are:
• Growing monocultures of highly susceptible crop plants - the pest can spend less time searching for an appropriate food source and more time feeding, therefore reaching maturity earlier and going through more generations in one season. Also most insect pests can fulfil all their life-cycle requirements within the crop, whereas the beneficial insects often need a variety of habitat types. Use of tissue culture and reduced genetic variability of crop plants will reduce the possibility for natural resistance.
• Use of pesticides which may kill natural enemies, reduce competing species or stimulate insect reproduction.
• Growing unhealthy plants with poor or unbalanced mineral nutrition, which are unable to resist attack or outgrow damage. Careful attention to complete trace element nutrition, cation balance and soil pH will be greatly rewarded with increased resistance to disease. High additions of soluble nitrogen often expose crops to disease and insect attack. For example higher levels of nitrogen application in apple trees stimulates woolly aphids and other sucking bugs. Low boron and zinc may increase susceptibility to certain bacterial and viral diseases.
• Accidental introduction of pests, by poor quarantine or cleanliness. Pests are often then free of natural enemies which keep them in control in their native habitat.
• Use of ‘cultural practices’ which encourage the development of high pest populations. For example use of fine-tilth seed beds, devoid of plant growth, which become snail highways.
• Removal of natural plant communities, including bushland and weeds or volunteer plants, which may harbour beneficial insects. Beneficials often require different habitats for different life stages (eg dragonflies require water for larval stage, adult predatory wasps need a nectar or pollen source).
• Particularly appropriate weather conditions which favour the rapid growth of pest populations or are unfavourable to natural enemies.
Note that all the above causes are within human control, except for weather, which can only be influenced slightly by crop selection, provision of shelter or irrigation management etc.
Poor mineral nutrition is definitely established as a major contributor to lack of plant resistance or ability to regrow from attack. Plants with a good mineral nutrition and adequate water supply are able to produce thicker cell walls and epidermis layers, and are turgid with strongly flowing phloem. and are able to resist piercing mouthparts. Plants with poor nutrition may also give off chemical signals which indicate their susceptibility to attack.
Environmental enhancement is important to increase the possibility of significant populations of beneficial insects, birds or animals. To quote the agro-ecologist, Miquel Altieri:
“Predators of insect pests suffer more from restricted habitat than the pests. Make sure refuge areas are available to nurture the predators, to enhance natural biological control.”
Beneficial insects such as dragonflies, robberflies, assassin bugs, other predatory bugs and lacewings will be much greater in number where there are healthy natural ecosystems to bring them in. The same is true for other desirable wildlife such as insect eating birds and lizards. Simple methods of increasing biodiversity include planting three or five row wind breaks which include flowering shrubs and ground covers, leaving some fallen wood for habitat, and providing water levels of varying depth in the farm dam.
Selection of strong seed or planting material and appropriate varieties is an excellent aid to problem prevention. When I have started a new garden, I try different varieties of tomatoes, beans and other vegetables for the first few years, to find the ones that perform best. Thereafter I try a few new varieties of tomatoes each year, but concentrate on the proven few. Using correct sowing times, density and sowing depth helps young plants to get away early and establish a good root system. Good water management is also important for a healthy plant, usually that means attention to the wetting and drying cycles of your soil to keep them within the preferred soil moisture range and knowledge of any special requirements of the crop.
Use of cultural techniques such as traps, barriers and repellents may also be important. On a farm that could mean a baited blowfly trap, or a pheromone trap for moth in a fruit orchard, a sticky trap for whitefly in the glasshouse, or snail traps and barriers in the vegetable garden.
Greenhouse release of biological cotrol
Organic sprays include the following:
• Mineral and vegetable oils and soaps. These work by smothering, or forming a film on the plant surface on which fungal spores cannot establish. Oils and potassium soaps may also break down the insect cuticle (outer layer).
• Plant based sprays can be made from many products. Garlic, onion, chilli and rhubarb, tomato-leaf and sabadilla are frequently cited examples. They have a repellent effect but may also work directly on soft bodied insects.
• Plant extracts are more precise in their action and generally more effective. They include azadirachtin (from neem), pyrethrum, quassia, ryania and rotenone (or derris). They are often used with natural carriers and synergists such as sesame oil or codacide oil. Beware of these sprays as they may also be toxic to humans and other animals (rotenone is very toxic to fish). Azadirachtin is unique amongst plant based sprays in that it may have some systemic action.
• Diatomaceous earth is an abrasive dust made from the fossilised skeletons of microscopic organisms, It is used as a dry powder or a slurry (for painting on tree bark or the walls of storage buildings). It works by a physical, abrasive action to kill a wide variety of crawling insects. The dry powder is used as a grain protectant and is safe to humans when eaten, but avoid getting in your eyes or nose. Do not use ‘activated’ diatomaceous earth, which is sold for use in swimming pool filters.
• Essential oils are a special class of plant extracts. Citrus and pine oils are readily available in commercial formulations and peppermint oil, tea tree oil and other products have significant potential for development. Combinations of these oils can be used inside kitchen cupboards to deter or kill cockroaches and other pests. Citrus oils and other potent essential oils can burn skin and eyes - use with care.
• Copper, sulphur, alum (aluminium sulphate), iron sulphate, boric acid and other mineral based sprays are also very effective fungicides, molluscicides (snail and slug killers). They are available from any large garden centre or chemical retailer.
• A variety of other products including rubbing alcohol (70% isopropyl alcohol) or ammonia for thrips, scale, aphids, whitefly and other small bodied pests. Baking soda (sodium bicarbonate) is a fungicide. Be careful with these products as they may damage leaves on sensitive plants.