Insecticides, as the name implies, are substances that are meant to kill six legged creatures called insects. Some of them will also kill those with eight legs such as spider mites (technically, miticides do that job better). And, if not used properly, insecticides can also kill four and two legged, take care!


Insecticide come in a wide variety of chemical formulations but the most commonly used in the home landscape are liquids, dusts and granules.

Of course, liquids are most commonly applied with a sprayer or, sometimes, out of a pale as a soil drench. They may also be injected directly into trees with special equipment.

Dusts are best applied with a spreader made for that purpose. Since these are such light particles, it is easy for the wind to blow them around and they could blow back onto you. Duster equipment will help to minimize this problem.

Granules are similar to what you see in a bag of fertilizer. Small, roundish pellets that are easily spread with drop or broadcast spreaders.

Categories of Insecticides

There are many ways to describe the substances we use as insecticides. Some of them have to do with the way they do the job of killing while others describe their chemical classification. Here are some of the more common ones:

  • Spectrum - More traditional insecticides fell mostly in the Broad Spectrum type which would kill anything and everything that it either touched or was consumed by. In the early days of chemical insecticides, this was thought to be a great idea. Imagine spraying once and killing all your insect enemies at once. Of course, this proved to be too good to be true. Eventually, we figured out that we were also killing the beneficial insects and that only made the problem worse. Also, many of these types of chemicals stayed in the environment for years and affect a lot of so called "non-target" organisms too.

    As we enter the 21st century, more and more insecticides are touted as narrow spectrum products. They are meant to kill a very limited group of insect pests and have very short active lives. This is much better for the environment but they require the user to have a lot more knowledge. They need to be able to properly identify the specific pest and the best time to attack it. No more "spray and pray" philosophy.

  • Inorganic Compounds - Until the early to mid 1900s, there were only a handful of substances available to farmers, orchardists, greenhouse growers, and others for the control of insect pests. Such nasty chemicals a lead arsenate, nicotine sulfate, hydrocyanic acid gas and kerosene emulsions were used in the fight against insects.
  • Chlorinated Hydrocarbons - Around the 1930's, the chemists got busy and created a lot of new types of chemicals that seemed to do a good job of killing insects. Perhaps the most famous of these was a little product called DDT which worked wonders against insects and even promised to eradicate mosquitoes that carried malaria in the tropics. Chlordane and benzene hexachloride were valued for their long term effectiveness. Chlordane could be measured in the soil ten years after a single application.

    Of course, once again, unforeseen consequences reared their ugly heads. All of a sudden eagles and other birds started to decline. For a long time, nobody knew why until someone discovered high levels of DDT in the birds and other animals. Then, they found out that the DDT caused the birds to lay eggs with very thin shells that collapsed under little pressure. Chlorinated hydrocarbons were also found to accumulate in animals at the top of the "food chain" in a process called bio-accumulation.

    The Environmental Protection Agency (EPA) finally took DDT off the market. In a few decades, low and behold, the eagles and other birds began to make a comeback. That was great but there is some ongoing debate about the impact of DDT on the mosquitoes and the malaria deaths in third world countries.
  • Organophosphates - These are another group of compounds that make up the active ingredient for many of the common insecticides on the market today. Generally, they are chemicals that act on the nervous system of animals including insects and, if misapplied, humans. They can kill either through absorption by the skin and mucus membranes or by being ingested i.e. stomach poisons.

    Unlike in the past, these compounds must be studied thoroughly for both effectiveness and environmental impacts before they are approved by the EPA. In the end, however, they are still compounds intended to kill forms of animal life. So, safety precautions and proper application techniques are listed on the label of every product. Still, with misapplications, serious problems can still arise.

  • Microbial Insecticides - We have learned how to turn naturally occurring insect diseases against the critters that are bothering our plants. The most common microbes used in the battle with insect pests are bacteria which are available in dusts, wettable powders and liquid forms. These would include Bacillus thuringiensis or Bt which is used against caterpillars and Bacillus popillae sold as milky spore disease for control of Japanese beetles.

  • Botanical Insecticides - As the name implies, these are insecticides that come from the world of botany i.e. plants. As part of their natural defenses against insects, certain plants create chemicals that are poisonous. We have either extracted these chemicals from the plants themselves or learned how to create them in the laboratory and use them in insecticides. Common botanical insecticides include rotenone, pyrethrum, nicotine, ryania and sabadilla.
  • Systemic Insecticides - Systemic insecticides are a relatively recent addition to the arsenal of plant managers. The idea is to get the chemical into the vascular system of the plant so that, when the insect feeds on it, the insect will die. Animals have a nice circulatory system that spreads blood throughout the body. Inject a chemical into a vein and it can quickly move to all parts. Unfortunately, plants do not have exactly the same system. However, they do have pipes which can be used in a vaguely similar manner and that is where systemic insecticides come in.

    Since the plant's vascular system is not the same as an animals, to get good coverage of the entire plant, the systemic product must be introduced through multiple sites. Sometimes, the product is mixed with large amounts of water which are poured over the root system. It is then taken up by many different roots and spread to the various branches and leaves. Another approach is to drill several holes around the base of the tree trunk and, under pressure, deliver the insecticide into each one of the holes.

  • Oil Based Insecticides - Highly refined petroleum oil products are used as insecticides. Unlike other products, they do not poison the insect but, rather, they smother it. The idea behind horticultural oils is that they are sprayed onto the insect and clog up their breathing pores.

    Oil based insecticides fall into two broad categories. One is called dormant oil. These are oils with a thicker viscosity which are applied during the time when plants are without their leaves, thus dormant. They are meant to cover the likes of scale insects or the eggs of others that overwinter on the trunks of trees. These products are sprayed during a warm spell in the winter so that they will spread properly. They must not be used while the plant has leaves or they will cause the death of the leaves.

    The other type is called a summer oil. These are products that are "thinner" or more dilute that may be used while the plants have leaves. Again, they spread over the insects and smother them.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

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