Wednesday, March 20

Understanding how sprays and other chemical treatments work


CHEMICALS for use in the garden are a plenty and, as gardeners, we need to know exactly how each chemical treatment works as well as their side effects. It is true that modern urban living means having to deal with several pests or diseases that were not serious in the past.

It has become so difficult to find useful insects in the garden today. As such it is hard to rely just on biological controls. Take for example ladybird beetles – you simply do not find them living on vegetables and other greenery in backyard gardens.

In many cases, it is not just not enough to rely on non-chemical means to get rid of pests. We can pick off caterpillars that devour the leaves by hand if there are just a few. However, when the problem gets severe, applying chemicals seems to be the only option.

However, the confusing game starts right there – which chemical to use or buy? There is a whole range of sprays, granules, bait, and even hormonal treatments on offer. The terminology can be rather confusing. Take the words like systemic, broad spectrum, and so on. What about organic remedies? This week I’ll try to decode some of the chemicals available.

What is a pesticide?

It is a chemical formulation either in powder or liquid form. Common insecticides control insect pests, fungicides control fungal diseases, while miticides control mites. There are also molluscides, bactericides, and herbicides, etc.

Labels tell us what the product and dosage recommendations are for target pests. They also detail the active ingredients contained in the product. Labels should also inform us about the safety precautions and recommended rate or mixture percentage necessary for control.

How do pesticides work?

Pesticides act on different problems at different stages of development and in different ways. Most are broad spectrum (killing a range of pests or diseases). Selective chemicals target a single pest or disease, leaving the rest unaffected. Systemic pesticides move about within the plants and only remain on the surface of the plants.

Most pesticides kill by disrupting the nervous system or by suffocation (such as oil sprays including white oil). Contact pesticides kill through bodily contact or are ingested through treated materials and act through the digestive system. Systemic pesticides kill hidden pests inside plants or those covered by protective scales.


Combing two treatments, such as insecticide and fungicide to treat aphids and black spots on roses, can save time and effort. Remember only some compatible ones are suitable for cocktails.


Spreaders, stickers, and wetting agents are added to pesticides to be more efficient as the leaves can absorb the active ingredients of the chemical or just keep it on the surface to kill the pests.

Organic options

Many gardeners prefer to use natural products made from plants or naturally-occurring materials. The term organic depends on how we define it, and some could accept treatments such as mineral oils, pyrethrum sprays, copper oxychloride, and sulphur.


It is a natural reaction to develop an immunity or tolerance towards a chemical so that the same treatment becomes less effective over time. To reduce this problem, alternative treatments or non-chemical treatments are used intermittently to avoid resistance from developing.


We have a whole set of rules for manufacturers, users, and sellers in Malaysia. Before a pesticide can be sold in Sarawak, it must comply with the Pesticides Act 1974 and be approved by the Pesticides Board. Manufacturers must provide evidence of toxicity, residue, effect on the environment, and efficacy. New chemicals must also be registered with the Pesticides Board.

There are five categories of toxicity reflected by the colour of the labelling, such as acute level of LD 50 per cent, which means the killing intensity is 50 per cent during tests. The lowest toxicity is given the white level meaning safe to any stem, followed by yellow, blue, red, and black.

Any chemical can be hazardous if not properly used. For home gardens, only the white and yellow labels are less toxic, and the rest should be avoided. Poisons can be used safely if we follow the regulations and the right precautions. Always use safety gear and never misuse chemicals for other non-intended uses.

Formulations for control:

Root level – Uses soil

drenches, where chemicals go through the soil to treat underground problems such as root rot. Uses granules for soil problems like grass grubs and borers.

Ground level – Uses bait

to control snails and slugs.

Leaf level – Uses contact insecticides (such as spraying oils) to kill pests and pathogens through contact or consumption. Uses fungicides to control diseases like powdery mildew and protect the foliage from further attacks.

Uses dust to control caterpillars. This can be applied during damp leaf conditions.

Do note that plants can suffer damage such as burning from sprays if applied incorrectly.

Send me an email for questions, comments, or suggestions. Happy gardening.