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

Planning to prevent pest problems

Crop rotation

Rotation can disrupt the natural life-cycles of pests and can play a major part in helping to reduce the impact of pest outbreaks.

For soil inhabiting pests, an obvious example of rotational control would be leaving a sufficiently long interval between root crops to break the life-cycle of pest nematodes so that populations fall to an acceptable level, ie one year in five for potatoes.

Where pests are more mobile, rotational breaks in a field are equally essential but the effects will be further enhanced when one-years cropping is out of flying range of the same crop the next year. Carrot fly, although a weak flier, can cross boundaries into neighbouring fields but would struggle to get much further, for example.

General farm hygiene

Potato and sugar-beet clamps or spoil-heaps left after handling can harbour aphids which can then spread to neighbouring crops. To prevent carry-over, these sources should be cleared thoroughly before the new crops emerge.

Varietal choice

Pest-resistant or pest-tolerant varieties have characteristics which help to minimise pest problems, for example:

Non-waxy (non-glaucous) wheat varieties are thought to be less susceptible to aphid attack than glaucous (waxy) varieties;

Bearded wheats, such as Soissons, are less likely to suffer from aphids on the ears;

The wheat varieties Apollo, Beaver or Riband, are better suited to dealing with wheat bulb fly attack;

Hunter wheat has some resistance to orange blossom midge;

Some varieties of wheat are more palatable to slugs and show much higher levels of hollowing, for instance Riband;

Optic spring barley is resistant to BYDV;

Oats are immune to wheat bulb fly attack and the variety Gerald is resistant to stem eelworm;

The potato variety Cara is tolerant to potato cyst nematode attack, whereas Pentland Dell and Record are non-tolerant.

Check the NIAB varietal ratings for crop susceptibilities and tolerances to pest attack.


The key action is to create the optimum seedbed; one of the advantages of giving a crop a flying start is that it is better able to resist pest attacks.

Some insect attack can be reduced by the right method of cultivation at the right time but some can be increased. Knowing from field records which pest is likely to be a threat in a particular field helps the selection of the most appropriate cultivation method.

Minimal cultivation can increase numbers of pest invertebrates which can remain active in mild autumns. Good, well-consolidated seedbeds help to control the movement of frit fly larvae.

The best defence for slugs is good seed-to-soil contact. Drill winter wheat at 4cm depth and roll behind the drill. Increase seed rates if losses are anticipated. When emergence is achieved, subsequent grazing is unlikely to cause economic loss except in poor growing conditions, where further treatment may be required.

Remember poor, cloddy seedbeds, full of trash, attract slugs, so time spent on seedbed preparations can be a useful tool in controlling slug damage.

Surface trash can confuse winged aphids seeking cereal plant hosts emerging in the autumn. Hence in min-till, direct-drilled seedbeds or sprayed-off stale seedbeds, there may be much less need for autumn insecticide treatments.

Where wheat bulb fly problems are expected, try to keep green cover for as long as possible without inducing pest carry-over. Wherever practical, do not cultivate in July or August, as freshly cultivated soil attracts egg-laying flies where the pest is endemic.

Where a cereal crop follows a grass ley, destroying the grass six weeks before drilling will help to alleviate frit fly problems.

Drilling dates

Sowing dates are dependent on soil type and weather and can influence which pests are likely to become a problem.

Early-sown wheat crops, for instance, avoid damage from wheat bulb fly, whereas they are more vulnerable to BYDV. If this is likely to occur, consider using an increased sowing rate on the fields which are prone to bulb fly attack. If soil type and weather conditions allow, consider delaying the planting of other wheat crops until after the middle of October when the risk of aphids diminishes.

Late-sown crops are, however, most susceptible to slug attack. Drill slightly deeper if slug attack is expected.

General habitat management:
Beetle banks

Research has shown that many of our beneficial insects leave their over-wintering quarters in the grassy margins of fields around March to spend the summer months among the crops.

However the number of insects found in the middle of some of our larger fields is much reduced; it appears they are not prepared to travel much more than 200 metres or so, out into the field. Therefore the middle of fields over about 16 hectares (40 acres) may not benefit from these aphid-eating predators, and some birds that frequent open areas, such as lapwing, skylark and stone curlew, may struggle to find enough food during the

nesting period.

Beetle banks are designed to provide an over-wintering habitat for insects across the middle of these larger fields by planting tussocky grasses on a bank. Usually, within three years, the winter populations of some insect species in the beetle bank match those existing in the headlands, leading to useful numbers of these beneficial insects in larger areas of the field.

Many of the above conservation techniques can now be funded under Government schemes such as the Countryside Stewardship Scheme.

Seed treatment

If a pest outbreak is predicted in certain crop situations, treating the seed offers more precise targeting than an overall spray. Treated seed must be incorporated carefully so that it is well covered with soil and wildlife cannot reach it. If left on the surface, it can be harmful to wildlife so any spills of treated seed must be immediately cleared up.

Good candidates for seed dressing are early sown cereals where BYDV protection is needed, or late sown cereals after an open-ground crop, which is vulnerable to wheat bulb fly. Some wheat seed treatments can also deter slugs. Oilseed rape seed can be treated to combat flea beetle attack.

Providing these application guidelines are followed, there has been no evidence in recent years that autumn seed treatments have any effect on birds. It is better to treat seeds in autumn if wildlife-damaging sprays are likely to be applied in spring or summer.

General habitat management:

When spraying with a summer insecticide, try to leave cropped headlands unsprayed as a reservoir for beneficial insects. Wherever possible a whole sprayer width should be left. However this may not be possible when spraying some seed crops or crops grown for human consumption such as peas.

Many insecticides have restrictions for use on headlands during the summer, so always read the label before applying a pesticide.

Conservation headlands (selectively sprayed cereal headlands) benefit from allowing a scattering of broad-leaved weeds to remain, so they can act as host plants to a wide range of insects. Butterflies seek nectar on these headlands and many small mammals and birds use them to feed in. They also give a lifeline to some of our rare arable flowers.

Non-cropped field margins can also play an important role in helping to control pests and

provide valuable feeding grounds for many declining farmland bird species.

Tussocky grass margins, including such grasses as cocksfoot and Yorkshire Fog, are used as overwintering sites for beneficial insects like carabid beetles.

Flower-rich areas next to the crop can provide a nectar source attractive to species such as hoverflies. They lay eggs in aphid colonies which the larvae then attack, reducing their numbers. Hoverflies especially like to seek nectar on umbellifers (plants such as wild carrot and hedge parsley, for example) which are commonly found in field margins. So take care to prevent herbicide and fertiliser drifting into these areas.

Consider planting strips of native annual flowers next to crops to attract insect species. These flower strips could be placed on set-aside, but advice should be sought first.

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