Te Kura Horticulture
Home
Horticulture Home
Course Information
Course Information
Horticulture 1000
HT1103 - Plant Management 3 - Chemical Control
Cultural control Integrated pest management (IPM)
6

Chemical Control

Pesticides

Pesticides are used to control pests. These chemicals are also called insecticides.

Deciding which pesticide to use is determined by:

There are three main types of pesticides: contact, stomach and systemic.

Contact poisons

These are sprayed onto the pests and are absorbed all over the body surface. Pyrethrum is a contact poison used against caterpillars. Pyrethrum is a naturally occurring chemical extracted from some plants in the daisy family. It is also available in a synthetic form.

Stomach poisons

These are usually laid as bait. Slug and snail pellets are forms of stomach poison. These are eaten (ingested) by animals and kill them.

Systemic poisons

These are sprayed onto the plant surface and absorbed in the plant tissues. Sucking pests absorb the pesticide when they feed on the plant. They suck the pesticide up with the sap.

Death by suffocation

Oils can be used to suffocate pests because they coat the pest with an airtight film of oil that blocks their breathing holes. Sucking insects like scale can be controlled this way. Oils are useful to get rid of pests overwintering on plants. Overwintering pests don't move much. Oils sprayed on dormant plants with no leaves like apples reach and cover the target pests easily.

How pesticides act on pests

How pesticides act on insect pests

Fungicides and bactericides

Protective fungicides and bactericides are chemicals that prevent fungal and bacterial spores from germinating. They form a protective layer over the plant so that spores landing on the surface of the plant cannot grow.

Fungicides and bactericides are best applied before the fungi and bacteria grow. Compounds of metals such as copper and zinc are commonly used.

Some oils are used to protect plants from fungal and bacterial disease and act in a similar way as protective fungicides and bactericides.

Controlling large pests

Large chewing pests like rabbits and possums can be controlled with poison baits or traps.

Calendar spraying

Calendar spraying is where the grower sprays at regular intervals and control of pests and diseases is very effective. This method of control has become less popular because it is costly, wasteful and can have harmful effects on the environment.

Summary of advantages and disadvantages of using chemicals

AdvantagesDisadvantages
  • appearance of fruit maintained
  • usually cost-effective
  • quick-acting
  • increased yields – important for economic reasons
  • required for access to some overseas markets
  • can leave toxic residues – chemicals damage the environment or remain in food, which can cause consumer resistance
  • can be expensive if calendar spraying is used
  • useful insects may be killed, for example pollinators such as bees often suffer during spraying
  • pests can develop resistance with repeated use
  • can harm the person applying them

To stop residue problems the chemical should be applied at the correct rate and any withholding periods should be observed. The withholding time or period is found on the label. This is the minimum time needed between applying the chemical to a crop and the time it is safe to harvest the crop and eat it. The withholding period ensures there is minimal or no poison or residue left in the vegetable or fruit.

Compatibility

If two or more chemicals can safely be mixed together, they are compatible. Some chemicals can be mixed and applied together. Chemicals that are not compatible will react in ways we don’t want, or do not mix readily and therefore cannot be applied together.

Rules for storing and using chemicals

Always read the label and do what it says Keep sprays in a locked cupboard out of reach of children and pets Don’t put sprays in a different container Always read the label and do what it says. Keep sprays in a locked cupboard out of reach of children and pets. Don’t put sprays in a different container from their usual one. Never put sprays in a drink bottle!
 
Do not smoke, eat or chew while spraying Always wash and change into clean clothes after spraying Do not smoke, eat or chew while spraying Always wash and change into clean clothes after spraying
 
Correct protective gear for sprayingCorrect protective gear for spraying.

The person in the photo is correctly dressed for spraying. He is wearing protective clothing, gloves, boots and mask.

 

Soil sterilisation

Soil sterilisation is used to control many soil pests and diseases including nematodes, insect pests and fungal diseases such as phytophthora.

Steam treatment

Sheet steaming of soil
Sheet steaming of soil.

High-pressure steam can be used to steam the inside of greenhouses. It penetrates the soil that is covered by heat-resistant plastic. This is called sheet steaming.

Sheet steaming is effective to a depth of 15 cm. Its effects are reduced when soil crumbs are large and hard to penetrate, and it is less effective in damp soils.

Chemical treatment

This involves using chemicals that are toxic to most living organisms. A period where no crops are grown in the soil is required, as residues can remain and contaminate the crop.

Methyl bromide has been used on soils prior to planting strawberries. It has become less popular as it is an ozone-depleting toxic gas. Chloropicrin is an alternative chemical to use.

Green houses are sterilised with formaldehyde, formic acid or burning sulphur. Pests such as red spider mite, whitefly and grey mould can be greatly reduced by these methods of control.

Activity 6A Read this chemical label for the spray STRIKE and answer the quiz.

Directions for use

Crop Pests Rate of application Time of application
Citrus Leafroller, mealy bug 100 ml per 100 litres of water Apply pre-blossom and then at 4–6 weekly intervals to within 14 days of harvest.

STRIKE may cause slight injury to the new soft growth of sensitive roses and ornamental plants.

Withholding period: Citrus – 14 days

Method of application:

Strike may be applied using ground and aerial spray equipment.

Mixing and compatibility:

Strike mixes with water and is compatible with commonly used fungicides except Bordeaux mixture and lime sulphur.


Complete Activity 6B in your workbook

Key points   Key points

  • Chemical control is using pesticides, fungicides and bactericides to control pests and diseases.
  • Pesticides may be contact, stomach or systemic poisons.
  • Problems with chemical control include residues, crop damage, killing of beneficial insects and poisoning of humans and their animals.
  • You must use and store chemicals safely.
 

What's next?

Go to: 7 Integrated pest management (IPM).

Cultural control Integrated pest management (IPM)