When to use pesticides, biological control and IPM

Many attractive and productive gardens exist where no chemicals are used. Other methods can help prevent damage from pests and diseases, or weeds.

Effective ways of avoiding garden problems;

1. Good gardening practice is to grow plants in ways that reduce the threat from weeds, pests and diseases.
2. Choose plants well suited to the conditions and where possible have innate resistance to problems.
3. Providing good drainage, crop rotations, mulching, well-planned pruning regimes and allowing sufficient space for plants can avoid problems.
4. Where problems do arise hand picking of weeds, pests and diseases foliage, hoeing, trapping and even washing off pests with the hose pipe can be sufficient to get acceptable control.
5. Good practice in watering and feeding will promote strong disease resistant growth.

Attitudes to pest and disease damage are also important. For home growers a low level of pest or disease damage is usually acceptable and there is therefore no need to use pesticides.

Biological control and Integrated Pest Management (IPM)

Natural predators and parasites make a useful, if largely unseen, contribution to pest control. Biological controls are effective, especially for indoor pests such as whitefly, red spider mite, mealybugs, aphids and vine weevil.  Other biological controls are available for slugs, caterpillars and an increasing range of other pests.

Integrated pest management (IPM) is standard practice for commercial growers. A combination of practices; biological control, rotation, choosing resistant cultivars, promoting natural predators and parasites of pests, and - where essential - using compatible chemicals.

Gardeners can adopt IPM and have the advantage that they can either use non-chemical hand methods, that would be uneconomical for commercial growers, to eliminate any pests or disease such as picking off pests or can use carefully directed sprays, ideally ones of short persistence such as fatty acids (soap) or oils to only wet pests and nothing else.

Garden chemicals and pesticides

Any chemical or other product used to control pests, diseases and weeds is strictly legislated, under EU and, therefore, UK law. Pesticides are unlikely to be any less strictly regulated after the UK leaves the EU. Pesticides are substances designed to kill or control the growth and behaviour of living organisms and therefore their use carries a degree of risk. Withdrawal of the most potentially hazardous pesticides has reduced this risk, but some risk remains and using pesticides is not something gardeners should undertake lightly.

The legislation, Food and Environment Protection Act 1985 and associated regulations (The Plant Protection Products (Sustainable Use) Regulations 2012 for example) extend to wood preservatives, animal repellents and materials that control plant growth such as hormone rooting powders. Fertilisers and garden disinfectants and cleaning materials are not considered pesticides, although they may be covered by legislation relating to ‘biocides’.

All pesticides must be approved. While it is not illegal to use unapproved materials such as washing up liquid, coffee, vinegar or baking powder, it is good practice to only use products that are officially approved for use in the garden. It is illegal to sell unapproved products for use as pesticides.

The legislation covering pesticides provides that users of products approved for professional use must be trained or experienced to established levels and that they ensure use of pesticides is lawful. Codes of practice provide guidance on how to use pesticides without breaking the law. Products sold to gardeners can be used without training or experience, as long as the directions for use on the label are followed.

A range of products available for UK gardeners can be found on our annually updated lists;

• Pesticides for home gardeners
• Fungicides for home gardeners 
• Weedkillers for home gardeners

How are pesticides developed?

Pesticide manufacturers screen thousands of potentially active molecules for pesticidal activity, and occasionally when they find a promising one spend at least 10 years and very large sums of money indeed in bringing a safe, reliable, usable product to market. The testing process is now very rigorous indeed and relatively few new chemical products are likely to be marketed to gardeners in future; older products will either be retested and reapproved at very great cost or withdrawn from the market. This huge outlay is usually only worthwhile for crops grown on millions of acres, and horticultural crops usually only get products that have a wider use in agriculture. Of the products offered to commercial producers very few are suitable for use by people with no training and without the sophisticated application machines and protective garments used by professionals.

Most products are therefore usually produced for agricultural crops; some of these receive more specialised approval for use on horticultural crops; and finally a very few, especially safe products are offered to home gardeners. A vital part of controlling garden pesticide use is the label. The label directions are designed to ensure that any contamination of food or the environment is within approved limits. All aspects of the label are regulated as part of the approval process.

The approval procedure is covered by European Union law, which is put into effect in Britain by the Chemical Regulation Directorate, part of the Health and Safety Executive.

When to use pesticides?

Before using pesticides consider whether treatment is necessary. Often non-chemical remedies are sufficient.

Even with the best cultivation regime and appropriate choice of plants, pests and diseases can spoil, or even kill plants. Often this is due to weather conditions favouring pest and diseases – for example wet weather leads to slug and downy mildew damage for example, and hot weather favours glasshouse red (two spotted) spider mite and aphids. Other causes can be fluctuations in migration of moths and butterflies and in populations of natural predators and diseases; both largely beyond gardeners’ control. Natural enemies of garden pests often breed too slowly to prevent initial damage and while numbers of natural enemies build up additional protection through chemical use may be needed.

However there are some other factors to be considered before resorting to pesticides:

• Flowering plants, including weeds, should never be sprayed
• Garden chemicals are expensive and often laborious to apply
• Insecticides are usually non-specific in what they kill, so that beneficial organisms are killed with pests
• In some cases insecticides can worsen pest problems
• Regular use of pesticides can lead to the development of resistance - there is a growing list of pests, diseases and weeds which are no longer controlled by chemicals that were once effective against them

If after considering these factors pesticides seem to be required factors that might justify the use of pesticides include:

• Insecticides and fungicides can quickly supress pests and diseases that potentially damage or kill plants
• Pesticides can improve the quality and yield of flowers, fruits and vegetables
• Some pests have no effective natural enemies and it can be difficult to maintain certain plants in good condition without resorting to spraying
• Weeds compete with cultivated plants for water, light and nutrients. Hand weeding, mulching and hoeing control many weeds but herbicides have the advantage in terms of speed and convenience for weed control in lawns and uncultivated ground such as paths. Problem perennial weeds in particular are often much more slowly eliminated where weedkillers are not used

Pesticide labels

Using a pesticide in any way that is not specified on the label is unlawful. If the label is adhered to garden pesticides will be used lawfully. Professional users have a code of practice to help them use the more potent and concentrated materials offered to trained or experienced professional users. Home gardeners are strongly advised not to buy, store or use materials marketed for professional use, and most suppliers are required to check that users possess official certificates of competence.

Amateur users of garden pesticides are only required to follow the label. This will protect them from incidents such as damage to beehives or weedkiller damage to neighbour’s gardens. Failure to follow the label can result in prosecution.

Putting pesticides into unlabelled containers or splitting packs between gardeners is also illegal.

How safe are pesticides?

The safety record of garden pesticides is good with very few accidents reported. However, there is evidence that gardening pesticides can cause pollution where label instructions are not followed or improper disposal methods used.

Not all pesticides are approved for use on food plants, but for those that are exposure to pesticides in food is limited by the Acceptable Daily Intake (ADI), and the Maximum Residue Limits (MRL). The approval process sets conditions under which pesticides can be used so that residues will not exceed these limits. These conditions result from extensive testing by manufacturers who are obliged to submit sufficient data to the authorities before approval will be granted.

Safety to the environment is also a very important part of the approval process. Once diluted and applied pesticides are designed to be broken down in the environment. Again data has to be submitted as part of the approval process. Disposal of undiluted pesticide, spillages and empty containers are especially common ways in which pesticides can cause environmental damage. Label directions for disposal of materials and packaging are designed to help gardeners act responsibly. 

Some steps to the effective and safe use of garden chemicals:

1. Identify the cause of the problem. A comprehensive range of web profiles are produced by RHS Gardening Advice to help accurate identification of problems – inaccurate identification could result in the wrong material being used, if say weather damage was mistaken for disease.
2. Apply the chemical at the right time. Pests often have one or more stages in their life cycle when they are more vulnerable to chemical control; spraying at the wrong time will give little or no control. Inspect plants at regular intervals so that problems are tackled before heavy infestations have developed. Some pests and diseases will need a number of treatments at regular intervals to achieve control.
3. Check the product label before buying. Often conditions on the label limit when the product can be used safely and legally – some insecticides can only be used on young plants for example and are not approved for older plants.
4. To avoid drift and potential plant damage do not use pesticides in wet, windy, completely calm or hot sunny weather, or when plants are suffering from drought. Spray in the early morning, late afternoon or evening. Sensitivity to products should always be checked on a small area or number of plants.
5. When spraying an edible plant, check that the pesticide is suitable for that plant and note the instructions for the period of time that must be left between treatment(s) and harvest (harvest interval) and the maximum number of applications permitted per growing season.
6. Understand and adhere to the label, especially the parts related to safety before using any chemical, read the instructions carefully and apply as directed. As an added safety measure you could wear rubber gloves when applying pesticide and whilst handling concentrates.
7. Apply pesticides thoroughly, diluted at directed and at specified intervals. Spray plants thoroughly, including stems, buds and the underside of leaves but without excessive run-off or drift.
8. Avoid contact with exposed parts of the body, particularly the eyes and mouth. Wash off any splashes immediately. Avoid breathing in dusts or sprays by standing up-wind while treating. Do not smoke, eat or drink while applying pesticides. Wash after using pesticides.
9. Make sure no children or pets are nearby when spraying and keep them away until the foliage is dry.
10. Do not spray open blooms (including flowering weeds) because of the danger to bees, butterflies and other flower visitors (pollinators).
11. Contamination of water must be avoided. Fish and other wildlife in ponds, ditches, streams etc, are very susceptible to pesticides. High risk activities include flushing surplus pesticide down toilets or drains, applying weedkillers to hard surfaces where run off might enter drains and emptying pesticides on to the soil to dispose of them.
12. Do not mix different chemicals together for simultaneous spraying unless the manufacturer’s instructions state that this is permissible. This includes foliar feeds or liquid fertilisers.
13. Use separate equipment for applying herbicides and insecticides/fungicides.
14. Store chemicals safely in a cool, dark, frost-proof place, preferably in a locked container where children and pets cannot gain access. The chemicals must be kept in their original containers with the tops firmly closed. Do not purchase large amounts of pesticide that will take many years to use up. If only a few plants require treatment, a ready-to-use formulation is likely to be the most appropriate.
15. Clean equipment thoroughly after use.

In case of accidents or other aspects of using pesticides, manufacturers provide for contact details of their helpdesk where questions can be answered by specialists trained in all aspects of their products.

RHS policy statements

The RHS believes that avoiding pests and diseases by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. However, garden chemicals can sometimes be appropriate if used correctly and in a discriminating way when other methods fail.

The RHS strongly advises that when a pesticide is required, gardeners should:

1. Identify the problem correctly and choose an appropriate pesticide for the task
2. Read the label before purchase, and follow all the manufacturer’s instructions
3. Only buy the minimum quantity required and store pesticides under secure and safe conditions
4. Dispose of unwanted or out-dated pesticides by consulting the local authority’s waste disposal department

RHS position on weedkillers including glyphosate

The RHS does not encourage the use of weedkillers, including glyphosate, and recommends that wherever possible alternative control methods are used in the first instance. Weed prevention, monitoring and cultivation practices, collectively called IPM (Integrated Pest Management), minimise problems. However sometimes targeted use of herbicides may be required. Our advice reflects this by showing organic and cultural options first, but then provides information on the products which can be legally bought and used by home/amateur gardeners in the UK as a last resort. The RHS works closely with Defra, the Chemicals Regulation Directorate and other organisations to develop best practice based on evidence and research.

RHS use of glyphosate in our gardens is minimal and we have been trialling flame weeders and hot foam weed control methods with a view to eliminating use completely. The RHS has also been active in developing alternative control strategies such as mulching in orchards and other areas to control weeds and seeks to ‘build weeds out’, for example resurfacing gravel and stone paths with asphalt and resin bonded gravel so that weeds cannot grow.