Integrated Crop Management (CPA Book)

Crop Protection


Protection Against What?

Cultivated land left unattended will quickly grow a variety of plant life. Although seemingly haphazard, this invasion by plants is strictly self-regulated. The plants that appear will not be all of the same type, nor will they grow in straight lines. Some individuals will survive at the expense of others. Prostrate species become intermingled with more erect plants so that not only does the soil surface become fully utilised,  but so too is the space for several centimetres above it.

In this way nature exploits all the available living space, or niches. The species that grow first are those best adapted to occupy these niches and best able to compete for the available resources of nutrient, light and water. Collectively they are called weeds - plants growing where they are not wanted. The density and composition of the weed population will be governed by a whole range of factors such as previous cropping, nutrient availability and soil acidity.

Pests and Crops

  • Most weeds grow very quickly and are prolific seed producers.
  • Weeds compete vigorously for nutrients, light and water;

  • Crop yields and quality suffer when invaded by weeds therefore control is essential;
  • Other organisms, such as eelworms, insects, fungi, rodents and birds invade crops and damage them;
  • All organisms that hamper man's efforts to grow crops are termed pests;
  • Pest invasion of crops is inevitable because monoculture leaves biological gaps, or niches, available for them to occupy.
 

A cultivated field sown with a crop, all of the same type, all germinating at the same time and all growing to the same height, is very different to this natural state. Monoculture is a man-made system sometimes likened to a housing estate with only half the properties inhabited. It is obvious that the available niches are not fully occupied and nature's squatters quickly start filling the gaps. Weed species arrive, and, because they are vigorous competitors, they will grow at the expense of the sown crop. The result is that, at best, the yield and quality of the crop suffers or, at worst, it is completely wiped out, while at the same time the weeds flourish and deposit huge reserves of seed into soil ready to compete with the next crop sown.

Weed invasion into cultivated crops is therefore inevitable and control is usually essential.

Other organisms also invade crops. These may range from bacteria, or even viruses, to soil-living eelworms, flying or crawling insects, fungal diseases and higher animals such as rodents and birds. The presence of at least some of these in a monoculture is as inevitable as the presence of weeds. If they live on the crop and use it as a food source, they will cause damage and, along with, weeds, they need to be controlled.

For convenience, organisms, whether weeds, insects, animals or diseases, that damage crops are collectively described  as pests. The chemicals that are used to control them are therefore called pesticides, whether they are weedkillers, or products to control insects or diseases. Crop protection is the science of controlling pests safely, effectively and economically, not just by using chemical control measures, but by considering all the available options. Indeed, the best exponents of ICM are those who optimise the use of non-chemical methods but integrate the judicious use of pesticides where necessary.

Crop Protection in ICM

It is not sufficient merely to accept the inevitable presence of pests that must be dealt with as they appear. Economically this would be unwise in any system, but ICM demands that farmers and growers adopt a structured and long-term approach to pest control. Prevention is always better than cure, but if this is not possible, the cure must take account of the overall aim of ICM to balance economic production with environmental sensitivity.

Crop Protection and ICM

  • Prevention is better than cure;
  • Prevention is sometimes possible and cure is not always necessary;

  • Cure must balance economic production with environmental sensitivity;
  • Pest control strategy should keep pests below economically damaging levels and use the best combination of available control methods;
  • Crop protection in an ICM system must be structured, long-term and involve the whole farm.
 

Best prevention is sometimes possible, while cure is not always necessary. Those who practice ICM should aim to devise a management strategy to keep pest levels below economically damaging thresholds by using the most appropriate combinations of biological, cultural, mechanical and chemical methods. The process involves:

  • Planning;
  • Evaluation of non-chemical pest reduction strategies;
  • Pest prediction and identification;
  • Use of rotations;
  • Choice of most appropriate pesticide and its application;
  • Keeping records.

Planning

Those who start considering pest control only when they see the pest (or, more likely, the damage it has caused) will never succeed. Like all living organisms, pests thrive in some circumstances and struggle to survive in others. Just as one swallow does not make a summer, so one aphid does not make an infestation that requires treatment. Some pests are a greater threat to one crop than to another or they may be suited to particular soils or climates. Presence of a pest (especially a weed) in one year is a good (but not totally reliable) indication of invasion in the following year, or, perhaps, when the same crop is next grown in the field.

Planning starts with understanding the farm, its history and its crops. Awareness of the pests likely to appear and their economic threshold levels is an essential first step. Rotations and choice of pest and disease resistant varieties can do much to reduce the chances of pest damage long before the crop is ever sown. Keeping up-to-date with technological progress in pest control is a continuous process. Those practising ICM keep a special look-out for developments, such as satellite mapping or genetically engineered pest-resistant crops, which may reduce the use of chemical pesticides,

Finally, the planning stage includes maintenance of the equipment that will be used to apply pesticides, together with training of the staff who operate it. Methods of storage and disposal of chemical pesticides, including the emergency procedures for dealing with accidents, should be reviewed and updated.

Planning

  • Don't wait for pests;
  • Create a long-term strategy taking account of past history and present cropping;

  • Be aware of what pests may appear and their economic thresholds;
  • Adjust rotations and variety choice if necessary;
  • Keep up-to-date with developments;
  • Maintain equipment and train staff;
  • Review storage, disposal and accident procedures.
 

Evaluation of non-chemical pest reduction strategies

In any farming system, total reliance on chemical pest control is unwise, and unlikely to succeed. By definition, pests are adaptable organisms. Over the 50 years since chemical pesticides began to be used on any scale, there have been numerous instances of resistance, sometimes leading to wide-scale control failures and changes in control practice. Examples of this are the resistance of the peach-potato aphid (Myzus pericae) to organophosphorus insecticides, and the rapid spread of resistance to mbc-fungicides in cereal eyespot (Pseudocercosporella herpotrichoides).

Pests have natural enemies and, as indicated above, have climatic and host preferences. By exploiting these features much can be done to reduce the incidence spread and impact of pest species long before chemical control needs  to be contemplated. Indeed, careful management of the non-chemical options can sometimes eliminate the need for further control measures, although some degree of chemical pest control is usually necessary.

There are a number of non-chemical options that should be considered in an ICM system.

...crop rotation

Surveys have shown that the composition of weed populations is largely unaffected by annual cropping sequences - in general the same weed species are likely to appear each year regardless of the crop being grown unless a switch is made to a perennial crop like grass. However, the  abundance of some species can be greatly influenced by crop rotation. For example, continuous runs of cereals are likely to result in a build-up of grass weeds, especially blackgrass and wild oats. Some of these are, however, significantly influenced by cultivation practices (see below), so an option exists to reduce them by other means.

Insect and disease pests are different. They get their food from the crop plants, not from the soil, and very often they have specific preferences in this regard. Their incidence is therefore dependant on the presence of a suitable host crop. This is why growing successive crops of winter wheat results in a build-up of eyespot and take-all, and a break from this crop is an effective non-chemical means of reducing these pests. Some pests such as eelworms or Sclerotinia disease (oilseed rape, peas, beans) or white rot (onions, leeks) can survive in soil for several years and attack when the host plant is planted again.

In all these cases awareness of the problems and careful planned rotations incorporating suitably long breaks between host crops can do much to reduce pest incidence.

...attention to farm hygiene

Annual weeds spread by seed. The soil carries a huge 'bank' of weed seeds some of which can survive in a dormant state for several years until favourable conditions for germination and growth occur. Indeed, this is one of the characteristics of a successful weed species. The only way to reduce this seed burden in the field is by a slow war of attrition, but a key to success is to reduce as far as possible any return of additional seed to the bank. This means not only attending to the control of the weeds in the crop, but making sure that weeds elsewhere on the farm, on non-cropped land, on roadways and round the buildings are not allowed to flower and set seed.

While some insect and disease pests live in the soil in the field, many do not. These invade crops from outside. Obviously little can be done about invasion of flying insects or wind-borne spores from neighbouring farms, counties or even continents. However, the risk can be greatly reduced by eliminating sources of infection or infestation on the farm. For example, diseases such as take-all use weed grasses as alternative hosts. Potato camps and dumps are the most usual farm infection focus for potato blight, and good hygiene, including prompt destruction of any spring growth, is an essential component of control.

...use of resistant varieties

Sometimes the host preference of the pest relates to a particular variety or group of varieties. This can occur because of some natural differences, such as hairiness or leaf wax. More usually it is because some varieties have an inbred resistance (tolerance) to the pest, normally fungal diseases. Conventional plant breeding, is however, a slow process, and it takes along time to combine a range of desirable attributes (which must also include yield and quality factors) in one plant. It is rare for a variety to carry effective resistance to more than two or three of the major diseases that may attack it.

In order to make best use of the varieties available, it is therefore vital to be aware of the particular disease risks on the farm before deciding which varieties to grow. It is equally important to keep informed about new varieties.

Genetic engineering may soon change the whole scene. New transgenic crop varieties can be produced much more quickly than by conventional breeding, and, provided the appropriate genes can be found, it may well be possible to combine specific insect or disease resistances with tolerance to a herbicide. These developments are still in their infancy but nevertheless they should be watched.

...cultivations

All pests must survive the winter somewhere. Weeds do so as seeds in the soil. Insects pests generally take shelter on an alternative host but some also move down into the soil. Fungal diseases either produce resting bodies which fall to the soil, or they survive on dead and decaying organic matter, or they live on autumn sown crops - the 'green' bridge.

In all these cases it is clear that management of the soil can play a key part in non-chemical pest control. Nothing survives burial indefinitely. For example, wild oat seeds can survive for many years buried in soil, but they will not germinate unless brought back to the surface layers. Eyespot disease in cereals can survive on the base of the stems in a stubble, but it is eliminated by deep burial. However, the benefits of ploughing must be set against the possible disadvantages. For example, on heavy soils, ploughing can lead to cloddy seedbeds which may not suit the crop, will hinder herbicide performance and may encourage slugs.

Some weeds germinate mainly (but unfortunately never exclusively) at a particular time of year. For example, blackgrass and cleavers mostly appear in the autumn. Where this occurs the 'stale seedbed' technique (cultivating and then leaving time for weeds  to germinate before killing them with a further cultivation before drilling) can achieve major reductions before the crop is even sown. Similarly, research has shown that delayed drilling of autumn sown cereals significantly reduces the likelihood of infection by some fungal diseases.

...biological control

There are few animal organisms that do not have natural enemies. This notion is implicit in the fundamental concept of food chains. Similarly, the vast majority of the world's insects feed on plants. It follows the isolation and exploitation of the natural enemies of pest organisms is a non-chemical control option that must be considered. However, there are practical limitations to any system of biological control.

Firstly, the system is self-regulating. The predator cannot survive without the presence of the victim - the pest organism - which may well have caused damage to the crop before it is itself killed. Secondly, as the pest population decreases so must the number of predators. The populations of each track each other in a cyclical manner in which neither is totally eliminated. The result is that the degree, duration and reliability of control achievable in a biological system is normally inferior to that obtained with chemicals.

Nevertheless, there are opportunities for biological control, particularly in glasshouse crops, where there may not be suitable chemical products. The majority of commericalisations so far have been produced for the control of insects. with the bacterium Bacillus thuringiensis (Bt ) a leading example. Bt controls caterpillars in brassicas, vegetables, soft fruit and ornamentals. Other examples in use in the UK are Encasia formosa, a parasitic wasp for control of whitefly in glasshouses, and the fast-moving predatory mite Phytoseiulus persimilis for red spider mite control in glasshouses. Verticillium lecanii is a fungal parasite used to control aphids and whitefly in protected crops, and Trichoderma viride is an example of the control of a fungal disease (Silver leaf in plums) by a fungal parasite.

Biological control of weeds is not yet a realistic option although considerable work has been done on the use of caterpillars and flea beetles for the control of ragwort.

...encouragement of natural predators

Many pests, but especially aphids, have natural predators such as ladybirds and parasitic wasps, living with then in the crop and the immediate surroundings. These predators themselves cannot adequately reduce a heavy pest invasion, but they make a significant contribution. Measures to encourage them, therefore form an important part of an ICM pest control strategy.

If they are managed correctly , the margins of arable fields provide ample havens for these predators, some of which are known to have host preferences. For example, Phacelia tanacetifolia is favoured by the hoverfly predators of aphids.   There can be problems with very large fields because it can take predators , many of which are flightless, a long time to reach the middle.  In these cases, a simple alternative method of encouraging these predators is to create mid-field refuges, or 'beetle banks', from where they can easily spread across a crop.

An additional benefit from the creation of species-rich field margins is a general increase in the supply of insect food and seeds for birds (see Wildlife and Landscape Management).

...other considerations

For some weed problems, for example roguing light infestations of wild oats manual labour should be considered, but the number of cases where this is both an economic and effective option will be few.

Pest control does not end when the crop is harvested. Indeed for many crops the greatest losses to pests occur in store at a time when there are far fewer chemical options and when chemical treatments are least desirable. In these circumstances, careful attention to store hygiene and environmental control is essential.

Evaluation of other options:

  • Never rely entirely on chemical pest control;
  • Crop rotations are an essential means of reducing the impact of many insect and disease pests, and some weeds;

  • Farm hygiene helps prevent replenishment of the soil 'seed bank' and delays appearance of some insects and diseases;
  • Resistant varieties are useful if they also meet the commercial requirements of yield and quality;
  • Cultural methods such as burial by ploughing, delayed drilling or use of stale seedbeds can help reduce some pests;
  • Biological control opportunities are generally limited to glasshouse crops;
  • Consider using manual labour.
 

Pest prediction

Forewarned is forearmed. Of all pests, the incidence of weeds is less effected by outside influence. Previous experience and existing records will be a reliable guide to the species that will normally appear and plans should be made accordingly. Predictions should always be confirmed by regular field inspections and maintenance of weed maps will aid decisions in future seasons.

Pest predictions:

  • Forewarned is forearmed;
  • Experience and records, including maps, are especially useful in forecasting weed infestations;

  • Long- and short-term weather forecasts will give a good prediction of invasion by some pests;
  • Take note of official warnings (e.g. potato blight, apple mildew, sugar beet aphids).
 

By contrast, insect pests and diseases are much more influenced by weather conditions. For example, cutworms and aphid pests like greenfly and blackfly generally flourish in hot, dry summers; yellow rust disease of wheat spreads in cool humid conditions; snow rot disease of barley favours, as its name implies, cold moist weather following a covering of snow.

Awareness of these conditions clearly assists in predicting pest attack and this should be backed up by taking note of long-range weather forecasts. In addition, official warnings are issued when weather conditions have favoured the spread of problems such as potato blight, apple mildew or sugar beet aphids.

Computerised models can be used to predict some pest problems and this area of technology is soon likely to produce some sophisticated decision support systems.

Identification and evaluation

Having predicted what pests may appear, correct identification is the next step. Even more important is an evaluation of whether the infestation merits treatment. Total control is rarely needed and treatment thresholds must always by considered.

Weeds are static and relatively easy to identify using various books and guides that are available, or by a competent BASIS registered advisor. Nevertheless it is important to do this as soon as the weeds are seen. Weeds get more difficult to control as they grow larger which may mean using a higher dose than would have been the case with prompt action. Furthermore, as long as they remain untreated they are competing with the crop for light, water and nutrients.

Insect pests present a bigger challenge because of their mobility and ability to build up extremely rapidly. Trapping is the best method of quantifying an infestation. There are numerous types, for example sticky traps (for whitefly in glasshouses), suction traps (aphids), bait traps (slugs), and pheromone traps (pea moth, cabbage seed weevil and flour beetles in stored grain).

Disease identification presents a particular challenge. Visual diagnosis, even by experts of, for example stem base disease in cereals, can be unreliable and yet correct choice of control measures depend on correct identification.

Identification and evaluation

  • Identify weeds when small so as to minimise herbicide dose;
  • Use traps to identify and quantify insect pests;

  • Diagnostic kits help the identification of some diseases but mere presence is not always a signal to spray;
  • Soil analysis gives important guidance on soil-living pests;
  • Walk fields regularly and be aware of pest thresholds.
 

A range of diagnostic kits is now available to assist in this area. However, accuracy and reliability remains a problem, and they are all qualitative rather than quantitative assessment tools. In other words, diagnosis of the presence of a disease is no indication that it will become a problem.

Other methods of identification and evaluation include soil analysis for soil living pests such as wheat bulb fly and eelworm.

Efficient pest control management cannot be carried out entirely in the laboratory or in front of the computer screen. Regular field inspections, use of competent professional advice if in doubt, and use of pest thresholds (where they are known) are all essential in the process of deciding what a pest is, and whether it needs treatment.

Pesticide choice

In many cases use of a chemical pesticide will not be necessary, but the challenge for the ICM practitioner is in making he most appropriate choice. The key aspects revolve around a product's selectivity for the target pest or weed, its persistence in activity and its impact on non-target organisms.

A pesticide should always be regarded as an aid to solve a problem and its use should be integrated with other measures to minimise the problem both at the time and for the future.  Most pest problems now have several chemical options available. For most people professional advice from a BASIS registered advisor is essential at this point. However, there are some basic rules to follow.

For example, where a choice of product does exist the decision will be governed by factors such as the proximity of wildlife habitats and water courses, the time of application and the nature of neighbouring crops. The chosen material must be approved for the job and, as far as possible, specific for the problem and not harmful to natural predators.

Pesticide choice

  • Integrate chemical pesticides with other pest control methods;
  • Do not regard chemical pesticides as a universal panacea for all problems;

  • Use BASIS registered advisors;
  • Always consider location, neighbouring crops, wildlife habitats and watercourses before making the choice;
  • Use computerised systems as decision aids but never without physically inspecting the problem.
 

Computerised decision support systems will undoubtedly make the choice of product easier in future, but there is always the danger that a wealth of additional information may serve to cloud, rather than clarify, the process. n any event, pesticide choice should never be made without physically inspecting the field.

Pesticide application

Most of the difficulties that arise with the use of pesticides are normally concerned with the way they are applied.

Crop sprayers of any age are precision machines that require skill to maintain and operate efficiently. This includes regular maintenance, calibration and cleaning. Operators should be trained and most need to hold NPTC certificates of competence. Pesticide applications should aim to contain a problem below a damaging threshold, rather than eliminate it. This can be achieved by following the maxim to use as much as necessary but as little as possible . However, it is important to remember that it is as much of a sin to underdose (thereby leaving the problem uncontrolled) as it is to overdose. In both cases chemical is introduced into the environment with no benefit.

Pesticide application

  • Machinery is sophisticated: skilled and regular maintenance is essential;
  • Operators must be trained with NPTC certificates if necessary;

  • Aim to contain a problem rather than eliminate it;
  • Use as much as necessary but as little as possible;
  • Carry out COSHH assessments beforehand: prevent operator and bystander exposure;
  • Observe Codes of Conduct and be especially vigilant of the weather;
  • Spray accurately: avoid overlapping or missed strips;
  • Be responsible about disposal of surplus, waste and the containers.
 

The fundamental requirements of pesticide application is accurate placement of the correct dose in the target area without escape sidewards or upwards by drift, or downwards by leaching and run-off. Furthermore, it must be done without contaminating users or bystanders. The Control of Substances Hazardous to Health Regulations (COSHH) are designed to control exposure to substances like pesticides by a combination of measures. They require any operator to carry out a COSHH assessment of the hazards before commencing work. Once spraying is started, following the manufacturer's instructions during use, and observing the various Codes of Conduct are all essential components of correct application.

This will include keeping a careful eye on  the weather, especially wind direction and strength, and ensuring accurate matching of spray swaths to avoid overlapping or missed areas. Finally it is essential to adopt responsible practices for the disposal of surplus or waste and used containers.

Keeping records

Record keeping

  • Pesticide applications (date, dose, volume, timing);
  • Weed mapping;

  • Cropping records, including details and dates of pest problems, provide continuous farm summary;
  • Maintain regular field monitoring.
 

It is a legal obligation to keep records of pesticide applications. In addition, however, such records provide a valuable continuous summary of pest problems on the farm. These should be supplemented by weed mapping and the normal farm cropping records. Regular field walking and simple environmental measurements can give an indication of the extent to which the overall aims of ICM are being achieved.

Conclusion

Pest control is vital in any farming system if yield, quality and profit are to be maintained. Reduction of chemical inputs is usually possible by evaluation of all available options and use of appropriate measures. The nature and size of the problem will dictate the solution, but the benefits that accrue include reduced costs, improved margins and a better safeguard for the environment.

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