Insecticides - Best Practice to Minimise the Environmental Impact in Arable Crops

Evaluating the risk of attack

Decisions on the risk of insect attack can be helped by assessing three factors.
All three are best understood by the farmer and by local advisers and can be based on past experience and comprehensive field records.


The location of a farm or position of individual fields can be important when assessing susceptibility to pest infestation.

For example, farms situated in the milder west of the country or in coastal regions are more likely to have an early and prolonged autumn attack from virus-carrying aphids. With less severe winters and more rain, pests can also survive better in the west.

Farms in the eastern counties generally have more diverse rotations including crops like sugar beet, carrots and peas, and therefore have a wider range of pests to worry about. Heavier soils tend to harbour higher slug populations.

Previous field history

Previous cropping regimes can help indicate some of the problems to be expected in the current crop. For instance, wheat bulb flies lay their eggs on bare soil in July and August so wheat sown after crops such as sugar beet, potatoes, onions, peas and set-aside, should be inspected in the spring for attack from this pest. Slugs are more likely following a crop of grass or set-aside, or after crops that produce a lot of trash such as oilseed rape. A cereal crop following grass may well suffer from attacks by leatherjackets, wireworm and frit fly.

Avoid revenge treatments. These are over-reactions to a heavy attack in a field the previous year and unnecessary actions to control last years problem. Where such attacks have occurred, start monitoring earlier and do it more intensively.

Monitoring pests and using threshold levels

Carefully monitoring each crop on a farm for the numbers of pests or damage present is one of the fundamental ways of deciding if an application of insecticide is needed or not. Although time consuming, it is good crop husbandry, can save money and will help to protect beneficial non-target species.

Over the years a great deal of information on likely incidence of pests has been built up on many crops in all parts of the country. This information, together with comprehensive monitoring by various agronomy services, is the basis for the pest warning systems now on offer to farmers and their advisers. However these warning systems should only be used as an indication of likely problems and you should monitor individual crops and then work to threshold levels.

Where possible, always work to action threshold levels. Ask your agronomist for the action threshold of the specific pest you are concerned about.

There are a variety of methods for estimating pest numbers:

  • Pre-cropping assessments
    Soil sampling indicates populations of soil-borne pests such as nematodes. Crops such as potatoes should be grown where cyst nematode levels are low; levels of the pest may determine which variety is grown.
  • Post-establishment monitoring
    Sex pheromone traps allow early detection of pests and are a useful tool in helping to calculate pest numbers of certain species, especially to monitor codling and tortrix moths (in orchards) and to monitor pea moth (in peas). These will give an early indication of future problems and whether a spray is required. Click beetle (wireworm) pheromone traps should be available soon which can be up to 10 times more reliable at estimating densities than soil sampling.
  • Incidence counts can establish levels of infestation, for example the presence of aphids in cereals where the percentage of stems infested at key growth stages can be used as a good guide.
  • Actual counts of pests can establish levels of infestation, such as leatherjackets in a metre-run of row in spring cereals. If counting is difficult as with orange blossom midge, for example, yellow sticky traps can be placed at crop height to spot the start of emergence of winged adults.
  • Slug traps placing tiles, mats or bags to provide shelter for slugs in fields can provide an indication of pest activity. Some people prefer traps baited with a few slug pellets whilst others use unbaited ones and then refer to a threshold level of slugs.
  • Assessment of damage symptoms can be a more cost-effective option in some areas than counts of pest  numbers. Cabbage stem flea beetle, for instance, can be better assessed in November or December by the percentage of leaf petioles that are scarred on a plant.

There are a number of ways to get information on both current and predicted population trends of pests. Some examples are:

  • Various organisations release information on a range of pest species and spray threshold levels. This gives a general picture in a locality and should alert you to check your own crops if you are not already doing so;
  • Agricultural journals such as Farmers Weekly; again the information is a general warning which needs to be checked in your own crops;
  • A qualified agronomist;
  • Many good publications on the use of pesticides from organisations such as the British Crop Protection Council see Further Information for details.

The 'do I, Don't I' dilemma

Thresholds are an invaluable starting point in tackling the dilemma of whether to treat a crop or not but so much of the final decision to spray rests with the weather. If forecasts are favourable for the pest, the decision is easier but if the weather is set to change, it can pay to hold off in the hope that crop growth reaches a less vulnerable stage.

  • Identify the pest correctly; mistaking a rose-grain aphid on wheat for grain aphid, for example, means that the wrong threshold level will be used and spraying could be unnecessary. Pest species are well illustrated in commercial farm management software.
  • Remember, just because one field has reached threshold numbers, it does not mean they all have!
  • If a treatment is delayed by a few days, it is worthwhile rechecking crops. Some pests, such as pea aphid, can decline sharply and populations can crash to below threshold levels.
  • Crops can compensate at later growth stages for damage caused by a pest, for instance cereal plants damaged by wheat bulb fly exploit the remaining tillers and go on to yield equally well.
  • The intended end-use for a crop is a key factor, too. Peas for animal consumption do not need treating against pea moth, as this pest does not affect the yield. Only spray for pea moth when the crop is for human consumption.
  • Some quality standards demand zero tolerance. For instance seed crop potatoes must be substantially or completely virus-free so potato aphid attack must be guarded against.
  • Always make absolutely sure that an insecticide spray is really required; research has shown that some beneficial insects can take up to seven years to recover fully to their former population level following a broad-spectrum spray.
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