"I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars." - Charles Darwin
In a mature, balanced ecosystem the flora and fauna usually reach an equilibrium - this is natural control where no one species becomes dominant. As intelligent gardeners we design our gardens in such a way that we are continually upsetting this by creating monocultures of plants in close proximity which allow pests and diseases to thrive beyond their natural potential. To counter this we can use chemical controls, but as nature has taken millions of years to evolve, it is risky to accept the evidence of a few years' use to prove that they do not cause damage now or at some time in the future. Take for example DDT, banned from use nearly half a century ago, but still persisting in our environment - a recent study found traces of it in children.
Natural controls can be encouraged by having shelter and plants that provide food or that they may need to complete their reproductive cycle. For example adult Hover Flies feed on nectar while their larvae feed on Aphids and other sap-sucking insects, so growing nectar-rich flowers close to plants which are attacked by aphids will attract the flies and they should lay their eggs on affected plants..
Although hygiene around cultivated plants is usually a good idea, being too tidy can discourage beneficial creatures which feed on pests, eg. Ground and Rove Beetles which feed on Slug and Snail eggs need cover to shelter during the day. Other plants can provide shelter for overwintering or the reproduction of natural controls. Having done as much as possible to encourage natural controls, the pests can still thrive because of the artificial conditions for cultivating plants or because they are not native and lack their own natural enemies.
Biological control is an artificial way to reduce or eliminate an unwanted organism using a natural agent. The introduced organisms usually range from predatory insects to much smaller single cell pathogenic bacteria and viruses. They can be introduced in small numbers which reproduce rapidly to overwhelm the pest - inoculative release. Or where their reproductive cycle is slow, large numbers are released and after reducing the pest population they die out due to lack of food - inundative release. Because they are not in their native environment, the biological controls may need special conditions to be present before they can be used, eg. the nematode Heterorhabditis megidis needs soil temperatures around 12°C to work effectively so can only be used outdoors for a limited period.
Present day biological controls are subjected to rigorous testing for at least three years in quarantine conditions, exposing any species related to, or associated with the target organism, to the potential agent. If carried out successfully the new agents can provide a safe means of controlling pests without the threat of further problems in the future. Recently a Tasmanian gnat which paracitises the New Zealand Flatworm has been identified and may become a control if it can be shown to be benign to our native flora and fauna.
Biological Control Target Pest Modus Operandi Paracitic Wasp Encarsia formosa Greenhouse Whitefly lays eggs in pest larvae which develop inside, killing it Paracitic Mite Phytoseiulus persimilis Red Spider Mite nymphs and adults feed on the mite Ladybirds Aphids, Thrips adults and larave feed on pests Lacewings Chrysopa sp. Aphids, Thrips, whitefly, mealybugs, leafhoppers larvae feed on pests Nematode (eel worm) Heterorhabditis megidis Garden Chafer Grub, Leatherjacket, Vine Weevil grub infects the pest with a bacteria which kills it and the nematodes reproduce using the grub as food Nematode - Steinernema kraussei Vine Weevil grubs infiltrate the pest and reproduce using the grub as food Nematode Phasmarhabditis hermaphrodita Slugs and Snails infects the pest with a bacteria which kills it and the nematodes reproduce using the pest as food Paracitic Wasp Apantales glomeratus Large Cabbage White Caterpiller lays eggs in pest which develop inside, killing it
There is a new control which contains a number of nematode species aimed at a number of pests in their larval stage. It is claimed to treat Ants, Carrot rootfly, Cabbage Root Fly, Caterpillars, Codling Moth, Cutworms, Gooseberry Sawfly, Leatherjackets, Onion Fly, Sciarid and Thrips, but has to be applied throughout the growing season to catch the target species when they are present so works out quite expensive. Perhaps one season's treatment would reduce the pests for a few years and give some respite. Or it would be more efficient to use single treatments if only one pest is causing a problem.
Weeds can also be attacked with a biological control. Many diseases and species of insect attack just one plant species, so unlike with weedkillers, a single weed can be targeted. The control does not necessarily remove the weed completely, but by damaging it, surrounding, more desirable plants can outcompete and overwhelm it. This method is being used to remove plants which have outgrown their welcome in new terratories where they were taken as ornamentals, crops or by accident.
A leafspot fungus and a psyllid or plant louse, which attack Japanese Knotweed in its native environment have been investigated as controls. After careful research the psyllid, Aphalara itadori, was released in a test area in the UK in March, 2010.
It could be said that this phenomenon has been exploited for thousands of years already, the rolling countryside would soon revert to scrubland if it were not grazed by cattle and sheep. Even some so-called wild countryside is kept in order by the periodic introduction of grazing animals to remove developing scrub and allow some more desirable or rare flora and fauna to survive.
This method of control is not a panacea for the problem of garden pests and diseases. After the introduction of alien species, either accidentally or deliberately, it is not always known what will result. In the past controls were introduced with little or no testing and some are now pests themselves.
- Cane Toads (Bufo marinus) - Native to the southern United States and tropical South America, were known to prey on French's Cane Beetle and the Greyback Cane Beetle, so sugar cane growers in Australia introduced the toads to stop the larvae of these beetles damaging the roots of the plants. Out of their normal surroundings their reproductive rate is extremely high and in North Eastern coastal areas of Australia they have reached pest levels themselves. They compete for the food of native species and exude toxins which are poisonous to other animals that might prey on them.
- Harlequin ladybird (Harmonia axyridis) - native to Asia is used to control aphids. It has escaped to the wild in the countries where it was introduced as a biological control and has overrun the native species of ladybirds to such an extent that it is now the dominant species in North America. They are still being sold as pest controllers.
Every living organism is part of a food chain, so upsetting one of the links in this chain can alter the ecosystem of an area. If the caterpillers which are the main food source of a bird species are eradicated, they may die away or take the food important to another animal. The lower down a food chain this occurs the more devastating the results could be. There has been some concern recently about the reduction of some bird species in the British countryside, and this is probably be due to the removal of their food source by modern agricultural techniques.
The use of any artificial pest control method should be used very carefully, if at all. By having a mixture of crops there is a better chance of arriving at a balance. Allowing some losses to pests may not be as bad as it sounds, after a while natural predators take over and the problem is kept at a low level. Removing the pest completely means any natural control is lost as well - to keep the natural control in an area there must also be a supply of the pest. A case of "live and let live" - a practice some of our own species would do well to adopt!
The demand for mass-production of cheap food does not support this natural control method. Single crops are produced year after year in the same fields, so the area must be kept free of all pests. If left to their own devices they would increase in numbers from year to year. This removal affects the higher species in the food chain which have previously relied on the pest species and other prey which have been removed in the broad sweep of modern pest control. In earlier times farms were run on a mixed basis - a number of crops were raised along with livestock. These could be rotated around the farm, so the pests did not build up too badly in particular fields. The same principal is now mainly restricted to domestic vegetable plots and in the Biodynamic and Permaculture movements. The capital outlay for agricultural equipment and labour costs make such culture unsustainable if the price of food is to be kept low.
The artificial conditions of a glasshouse are probably the most suited environment for using a biological control, if it is temperature sensitive, it will not be able to survive outside, so it will be less of a threat to native organisms.
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