How Pesticides Work

By Toni Salter

We often start a vegetable garden because we have decided to eat organic produce and reduce our chemical exposure.

Inevitably, However, we are faced with pest and disease problems and with the dilemma of controlling them without poisoning ourselves in the process.

Sometimes our best effort at organic solutions simply doesn't cut it and we might be tempted to turn to pesticides.

Pesticide is a generic term for any substance that interferes with the physical, chemical or biological mechanisms of the invading insects, weeds or fungus affecting the garden.





The term "pesticide" covers a number of poisons specific to each organism. An "herbicide" is designed to control weeds, a "fungicide" is designed to control fungus or disease and an "insecticide" is designed for insects or arachnids.

Most pesticides are lethal when applied at the rate specified on the label. This includes those that affect the nervous system of an insect. The mode of action of a pesticide is either contact or systemic.

If contact is the mode, it is only effective in controlling the pest if direct contact is made with the chemical. Systemic pesticides, in contrast can be sprayed on any part of the plant and are translocated through the vascular system of the plant and become effective on all parts of the plant.

In other words, a systemic chemical can be sprayed on just the lower leaves but will become effective in the roots, leaves, trunk, branches, fruit and even seeds. If you think that peeling your fruit or vegetables will reduce your exposure, think again when it comes to systemic pesticides.





Not all pesticides are designed to kill, however. Some will affect the reproductive cycle, making the pest sterile so that no further attack can occur.

Others are defoliants, which cause the leaves to drop without killing the plant. Some foster the action of another pesticide without being particularly toxic themselves (such as piperonyl butoxide which is present in many pyrethrum based insecticides).

It is also important to remember that even "organic" pesticides can be lethal. Chilies, kerosene, methylated spirits and salt are often used in homemade pesticides and will burn, harm or kill pests.

Mineral oil and vegetable oils, will suffocate soft-bodied pests. Sometimes, the indiscriminate nature of pesticides (organic or otherwise) may affect other species.




This is where nature's guardians are adversely affected. Bees, for example, are often casualties in the pesticide war. To limit this fallout on beneficial insects, choose to spray only at dusk, when bees have gone back to their hive.

Minimize crop loss with these natural techniques instead

  • Repel - garlic or other herbs are strong smelling and will often confuse pests or repel them away from your crop.

  • Attract - flowers bring in natural predators who will help do the pest cleanup naturally. Look for wasps, lady birds/beetles, dragonflies and hover flies in your garden.

  • Feed - Biological farmers maintain that pest and disease is merely a sign of a sick plant and focus on good nutrition instead. High nutrient value is achieved through natural fertilizers like compost, rock dust and minerals from the sea.

  • Problem Solve - Treat the problems that bring on pest and disease attack. Poor drainage can lead to wet areas that attract fungus, so build raised beds. Poor plant selection can attract unwanted pests, so make sure to grow according to your soil type and the season and consider planting endemic species.

Toni Salter, 'The Veggie Lady', is an Australian registered horticulturist living in Sydney who runs therapeutic gardening programs. Follow Toni on Twitter and 'Like' her on Facebook

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