The ‘cultural’ measures gave control inferior to that given by modern insecticides; they were also labour-intensive and therefore became highly expensive as labour costs rose. Moreover, many were obstacles to farmers’ aspirations to intensify and further mechanise their holdings. As a consequence, the availability of cheap and efficient synthetic insecticides caused farmers largely to abandon cultural controls, particularly in temperate countries where labour was becoming so expensive. However, there are now attempts to raise some cultural controls as part of pest management programmes. In the tropics, where peasant farmers often cannot afford insecticides and labour is still cheap, cultural control is still a major pest control weapon.
Cultural control, though providing control inferior to that of pesticides, is a valuable restraint on the average pest density, and therefore is valuable in reducing the challenge that insecticides may be called upon to meet in the future.
Cultural control aimed against one pest may well improve conditions for another. The potential impact of cultural control is sometimes best seen where management practices change for agronomic reasons, and new pest problems are created as a result.
Soil Cultivation
Many insects live or hibernate in suitable temperature and humidity conditions relatively near the soil surface. These conditions can be disturbed by ploughing, which creates temporary drought conditions in the upper soil layers and may even expose larvae and pupae to the full radiation of the sun.
Compacting the soil with a roller is a cultural measure for limiting the between plant movement of some larger soil insects such as beetle larvae. Another problem with minimum tillage has been that of the tem-boring fit fly. This insect can develop large populations in grasses and migrate into winter wheat seedlings when wheat is drilled into a herbicide-treated sward in the common rotation of cereals after grass. This problem was never serious when the old sward was ploughed in late summer and left fallow until the sowing of spring wheat the following year.
Sanitation
Farm hygiene often has a pest control purpose. The destruction of crop residues removes residual pest populations (e.g. stalk-boring grubs in maize) and eliminates plant debris on the soil surface in which many pests find shelter for hibernation (e.g. flea beetles and white flies of brassicas).
Another aspect of clean cultivation is ‘roguing’- the removal and destruction of infected growing plant material where there is danger of spread to other parts of the crop. Before the advent of adequate plant resistance to the pest, the control of reversion virus spread by the blackcurrant gall mite was largely dependent on the removal and burning of infested bushes; roguing plants attacked by the sisal weevil are still a component of control of this pest in the tropics.
Manuring
The belief the vigorous plants are less attacked by pests is one of the foundation stones of so-called ‘organic’ farming, and it is far from being an erroneous concept. Rapid, healthy plant growth can reduce pest damage in four ways:
a) Rapid growth shortens any susceptible stage. It therefore induces resistance against pests such as stem borers, to which seedlings have a relatively short window of susceptibility before the tissues harden;
b) It may well lead to the maximum expression of some chemical resistance factors;
c) It will allow maximum compensation for damage by the plant. For example, good root system would clearly withstand root grazing by pests where weak root systems would not. Another example concerns the shot hole borer (Xyleborus fornicatus) on tea Sri Lanka, where damage was successfully reduced by fertilizing the bushes with nitrogen. The stimulation in growth enabled the bush to form new tissue as a support bracket over the beetle gallery so that breaking of the branches at the gallery as tea pluckers passed through the plantation no longer occurred
d) It can promote uniformity and density of crop stand. This can discourage pests such as the chinch bug (Blissus leucopterus), which is most abundant where the crop stand is somewhat thin. Aphids occur in smaller numbers where the crop is more dense; this is because fewer winged immigrants land where less bare ground is exposed.
Water and Humidity Management
Irrigation is a common practice in many crops, and it can be manipulated for pest control purpose. Small pests such as aphids are easily washed off plants by overhead irrigation, and soil insects may be killed by the pressure of swelling soil particles in saturated soils. Additionally, ample water availability causes physiological changes in plants; some sucking insects such as aphids and thrips tend to do badly on well-irrigated plants and benefit from periodic wilting of the plants.
Another approach to retaining moisture in the soil is to cover it with a mulch, often composed of plant debris. In coffee, thrips are rarely a problem in the more humid conditions of mulched plantations; just one season without mulch may elevate this insect to pest status. Damp conditions created by mulching may also be favourable to insect parasitoids’ thus mulching in coffee increases the biological control of the Antestia bug. Still with coffee, pruning management is a further weapon against Antestia. This bug does less well where humidity in the canopy is reduced by pruning. Lower humidity, unfortunately, also makes the environment less suitable for parasites of the pest, but this can be compensated for by leaving the prunings on the ground as a mulch, and the reduction of biological control is minimised by careful timing of the pruning operation.
Strip Farming and Intercropping
Before intensive agriculture, farmers tended to grow several crops on one unit of land. Such multiple cropping is still common in peasant agriculture in the tropics. Either the area is divided into relatively narrow strips of different crops, or low crops are grown either under or in between the rows of taller crops. In intercropping, the low crop reduces weed competition by covering the ground rapidly, and prevents soil erosion and water loss.
Both strip farming and intercropping reduce pest attack. In strip farming and intercropping often reduce pest attack. In strip farming, the intervening strips of a non-suitable food may prevent movement of pests from one strip of a crop to another or from one suitable crop to a different one. Moreover, where two crops harbour unspecialised natural enemies, these can move over on to a neighbouring strip if pests build up there.
Intercropping may reduce pest damage by attracting the pests to a less valuable crop, or one
The host-plant-finding behaviour of insects may be disrupted by the close comparison of two plant species. Several crop pests, such as cabbage white butterflies and cabbage aphids are very much influenced by the crop background in their colonisation behaviour, and intercropping removes the contrast between seedlings and bare soil in the same way as dense planting does. Weeds, of course, have the same effect as a low intercrop; it has been shown that very few immigrant aphids were trapped over weedy plots of Brussels sprouts. Additionally, the mixture of odours from an intercrop, particularly any strong smell from a non-host plant for the pet masking the odour of the host plant, can disrupt the host-finding behaviour of pets. This has been shown, for example, by work at Cambridge in relation to carrot fly on carrots interplanted with onions. There are many other, mainly anecdotal, records of aromatic plants repelling insect pests, particularly those of vegetable crops, and this is another area, which is very amenable to a little experimentation.
Intercropping may also increase the impact of natural enemies. This may be because one of the intercropped plants provides a honey or nectar source, which attracts natural enemies for adult feeding, or because the shelter and humid conditions near the ground provided by the intercrop encourage ground-living predators. Work at Rothamsted Experimental Station has shown that many predatory beetles are more abundant in weedy rather than clean plots of winter wheat. There is evidence that some ladybirds prefer ground cover to row of plant sin bare ground. Some hover fly adults, whose larvae are important predators of aphids, also lay more eggs where the ground is covered than where it is bare; unfortunately other hover flies have the reverse behaviour. Another experiment at Rothamsted Experimental Station had endeavoured to establish aphid parasitoid populations by under sowing cereal crops with rye grass and liberating an aphid, which lives on the grass but does not attack wheat, together with its parasite. The parasite, however, also attacks grain aphids, and there is some evidence that this procedure improved biological control of the aphid. Work on cabbage root fly in plots of cabbages either grown traditionally or undersown with clover has shown that the clover undersowing greatly promotes the number of ground beetle predators of cabbage root fly eggs.
where the pest is less serious for some reason. One example is the mixing of maize and cotton to achieve control of the shared lepidopteran pest Heliothis. Heliothis is attracted to the maize tassels, but is a much less serious pest on maize than cotton, because attack on the cobs is reduced by cannibalism when larvae meet within the tight husks. Intercropping cowpea with sorghum can attract polyphagous pests onto the sorghum, which is less valuable crop.
Crop Rotation and Isolation
Attempting to separate the pest from its host plant in time or space is one of the oldest and most widespread farm practices often directly motivated by pest control, and it is still one of the most effective controls of some eelworm problems. Crop rotation normally reduces and delays attack rather than giving complete control because, although control may be significant within a given field, it is a less effective restraint over an area as a whole. Most pests have strong migratory powers or, if not, can frequently survive rotation on wild host plants. Moreover, crop rotation usually means that a particular crop is nevertheless grown somewhere close-by in the area. Thus the common rotation for a field of grasses or cereals, followed by legumes and then root crops does not result in the absence of any of these crops on the farm as a whole. Yet the rotation is effective in reducing the many soil pests (e.g. wireworms, chafers, and shoot-boring flies) which multiply most successfully under grass. The various crop midges (e.g. pea midge and bladder pod midge) are weak fliers and also are affected by crop rotation. However, just to emphasise the point that cultural controls can often be a two-edged sword, it is worth giving the example of the wheat bulb fly (Leptohyemyia coarctata), which strangely does not lay eggs in wheat crops, but in any fallow ground. The pest is therefore not a problem when cereals follow cereals, but only when cereals follow, through a rotation, bare fallow or a crop such as a root crop which leaves a fair amount of the surface exposed in late summer.
Attempts to avoid pests by isolating crops from regularly infested sites are frequently designed to prevent diseases from reaching the isolated crop. Because wild plant form reservoirs of both the insect vectors and the diseases they carry, the method has rarely proved successful on a regional scale.
Trap crops
If insect pests can be concentrated in particular small areas of a field, they can then be destroyed with locally applied pesticide or some other technique to which insects are unlikely to develop resistance, such as ploughing in, feeding the vegetation to livestock, or the use of a flame gun. Such concentrations of pests may be induced by position; by exploiting the crop zone in which most insects are deposited behind windbreaks by planting taller plants at the edge of the crop to filer out flying insects; by earlier sowing. A particularly ingenious example of a trap crop is the use in Canada some 30 years ago of a non-crop trap plant (brome grass) planted in a 15-to 20-m strip around wheat fields to control the stem-boring sawfly Cephus cinctus. The adults did not penetrate into the wheat crop but laid eggs on the brome grass in which many larvae developed per stem. It was not necessary or even advisable to destroy the grass, for the grubs cannibalised one another, and even most of the eventual survivors failed to survive to maturity in the grass, although their parasites were able to emerge. Thus the brone grass filtered out the sawflies and effectively converted them into biological control agents for the crop.
Trap crops are therefore either non-crop plants or crop plants intended to be especially heavily damaged by pests and often destroyed well before harvest. The land occupied, which in some instances may need to be in the order of 5 to 10% of the whole crop, inevitably involves some reduction in crop yield.
Sowing and harvesting practices
Variation of sowing date can control pets, most of which show some seasonal predictability, either by avoiding the egg-laying period of the pest or by allowing the plants to reach an age where they are resistant by the time the pest appears. For example, the hessian fly (Mayetiola destructor), has a predictable flight peak of limited duration; thus a few days delay in sowing wheat can make all the difference between a good and a bad crop. Changes in the sowing date for agronomic reasons have sometimes caused new pest problems. The increased growing of winter wheat, sown even earlier to exploit non-tillage systems, has created a whole range of new problems related to the changed timing of the crop. A stem-boring fly (Opomyza florum) now finds crereals at the right stage of growth when it lays its eggs in late summer. Also the winter cereals are infested by the aphid Rhopalosiphum padi in autumn and can be infected with barley yellow dwarf virus by this aphid. The virus then multiplies in the plants for the rest of the crop season and can cause severe symptoms the following year. Spring wheat avoids this autumn aphid infestation and, although the virus may be brought in by other aphids much closer to harvest, the virus is then not much of a problem.Early harvesting may remove pests (especially cereal pests in the straw and grains) from the field before they can emerge and perpetuate the population in the area. Damage to wheat caused by the wheat stem sawfly can be minimised by harvesting earl, before the weakened stems lodge in wind and rain.
Ag.
Technologies
(Pest Mgmt.)
