Insecticides

Insect pests can damage garden plants by feeding on them, burrowing into them, or by carrying diseases. ​

FLIT manual spray pump for insecticides from 1928 (Bullenwächter) CC BY-SA 3.0

Ants, aphids, scale insects, mites, thrips, psyllids… the list never seems to end! Practically any time of year, one sort of insect or another is trying to take a bite out of your garden plants. If that weren’t bad enough, many of these pests spread diseases as they feed and travel around. There isn’t a gardener alive who hasn’t wished for an easy solution to the constant onslaught. But easy solutions often backfire and insecticides are a perfect example.

How insecticides work

Insecticides are formulated to repel, kill, or otherwise harm insects. These agents are classified as either systemic or contact insecticides. Systemic insecticides are absorbed by the plant, making it toxic to anything that eats it. Systemic insecticides have residual, long term actions, while contact insecticides have no residual actions. Contact insecticides simply have to come into contact with an insect to be toxic.

The mode of action by which an insecticide works is important as it determines which other living things may be affected. Some insecticides work by damaging an insect’s nervous system, interrupting feeding and reproductive behaviors, while other insecticides attack the exoskeleton. A damaged exoskeleton allows insects to dry out, causing death by desiccation. Growth regulators (e.g., pyriproxyfen, methoprene) stop insects from molting or laying eggs. Ovicides kills eggs. Larvicides kill larvae.

Insecticides also come in several forms: sprays, dusts, baits, and gels. Depending on which form you use and how you use it, the poisons intended for pests can harm pets, people, and the environment.

Pesticides to be sprayed on food crops (USDA, Public Domain)

Insecticides can be repellent or non-repellent. Repellents discourage insects from bothering a plant in the first place. Non-repellents are especially effective against social insects, such as ants. Being a non-repellent, the insecticide is not offensive to the pest, so they walk through it and end up carrying it back to their nest, ultimately killing the entire colony.

Types of insecticides

There are three basic types of insecticide: natural, inorganic, and organic. Natural insecticides are enzymes and other protective substances made by plants as part of their arsenal against insect pests. These natural insecticides include nicotine, neem, and pyrethrum. Pyrethrum is made from the dried flower heads of two chrysanthemum species: Chrysanthemum cinerariifolium and Chrysanthemum coccineum. Other natural insecticides include the chemical that gives horseradish its fiery bite, rosin, and wintergreen.

Inorganic insecticides are made from metals. Organic insecticides are organic chemical compounds that are generally work by making contact with an insect.

The problem with insecticides 

Being poisons, insecticides can affect our health, as well as kill insects. Insecticides can also remain in the food supply, increasing in concentration as they move up the food chain. This is important because we are at the top of our food chain. [It is a lot like mercury in fish.] Many of these problems can be reduced or eliminated by understanding the different types of insecticides and using them responsibly.

Broad-spectrum insecticides

Like weedkiller sprays, broad-spectrum insecticides are very appealing. A problem appears. You spray it. The problem is gone. Fantastic. Except that the problem is not gone. In fact, the problem just got worse. By spraying broad-spectrum insecticides, all insects are affected. Beneficial predatory insects, pollinators, burrowing soil arthropods, and our beloved honey bees are all subject to the same poisoning.

There are several type of broad-spectrum insecticides, in order of toxicity, all of which interfere with nerve cell transmissions:

• organochlorides (e.g., DDT)
• most organophosphate (e.g., malathion, dimethoate)
• carbamate (e.g., carbaryl–Sevin, methomyl–Lannate)
• pyrethroids (e.g., beta-cyfluthrin–Baythroid, fenpropathrin–Danitol) 
• neonicotinoids (imidacloprid–Admire, acetamiprid–Assail)
• ryanoids (e.g., Rynaxypyr)

Educate yourself about ingredients

Before applying insecticides, you can protect yourself and the environment by learning more about the ingredients. For example, some beneficial insects, such as lacewings, are tolerant of pyrethroids, while beetles, parasitic wasps, and predatory mites are very sensitive to the same chemicals.

You should also ask yourself how long the ingredients of any particular insecticide will remain in the environment. Insecticides are commonly grouped by persistence: short (days), intermediate (up to 6 weeks), or long (months). Finally, are insects developing a resistance to an insecticide? If so, it should be avoided and another product used.

Reducing risks associated with insecticides

Before resorting to the use of insecticides, be sure you have done the following:

1. Properly identify the pest in question.
2. Decide on a tolerance level.
3. Practice Integrated Pest Management.
4. Consider the area - who and what will be exposed if insecticides are used?
5. Select a product specific to your pest.
6. Educate yourself about the ingredients before applying the product.
7. Read the label completely and follow the directions exactly, including safety equipment.
8. Buy only what you will use during the current season.
9. Always keep insecticides away from pets and children.

In many cases, it is against federal law to use insecticides improperly, and for good reason.

As quickly as insects can reproduce, it often feels like a losing battle. This is what makes the use of modern insecticides so appealing. Insecticides are an easy way to kill insects. But not all insecticides are safe to use on edible plants. And many insecticides interfere with the delicate balance that exists in a healthy environment. Knowing more about the different ways insecticides work, and how and when to apply them properly, can prevent longterm problems while still reducing the damage done to your plants by insect pests.