University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article


Dr. Leonard Perry, Extension Professor
University of Vermont

If you garden you invariably will have pests, insects you don’t want and that may damage your indoor plants, ornamentals outside, or crops.  Knowing some basic facts about the many types of insecticides now available, even organic ones, should help you garden safer and with better pest control.  An extension leaflet on “How Insecticides Work” from the University of New Hampshire provides a summary of details on the most common types of products.

It is useful to know the various types or “classes” of insecticides, and which ones your pesticides are in.  Insects often build up resistance to repeated use of the same insecticide, or ones from the same class, so one of the principles of pest management is to rotate products from a different class every third or fourth time they are used.
The organophosphates, such as the common malathion, act by interfering with the transmission of nerve impulses.  Basically it causes the nerves to keep acting continuously, rather than stopping as normally, resulting in symptoms such as tremors, convulsions, or even paralysis. These can be some of the most toxic pesticides.
The carbamates, such as carbaryl, act similarly with similar symptoms to the organophosphates.  However, unlike the former, action of this class can be reversed.

Similar action is from the former chlorinated hydrocarbons, now called organochlorines.  These include such as DDT and chlordane, and because of their toxicity to humans or the environment most are no longer found in the U.S.
Pyrethroids, such as permethrin, were first made based on chemicals naturally occurring in the seeds of certain chrysanthemums.  These act similarly to their natural relatives, disrupting the transmission of nerve impulses, but last longer.
Insect growth regulators include many products that only affect immature insects, not the adults.  Insects go through stages in growth and development, often molting or shedding one skin and growing a new one.  These products imitate the hormones that control this process in insects, causing changes when they aren’t ready.  This usually results in their death.  Since humans don’t have these insect hormones, these products are relatively safe for people.
Microbial insecticides actually are made from microorganisms that attack insects.  Since they are so specialized, such as attacking the cuticle (covering) of insects, they pose relatively little risk to humans.  These include viruses and bacteria that invade insects and multiply, or fungi that attack from the outside.  The various versions of Bt (Bacillus thuringiensis) are in this class.
Then there are various other classes such as the horticultural oils, which smother insect eggs, or the common insecticidal soaps.  The latter are chemicals, although often not thought of as such, usually being potassium salts of fatty acids.  They destroy the outer covering of insects, causing body fluids to leak out, and the insects die from dehydration.
Keep in mind several points relating to your safety and that of the environment.  Many insecticides are chemicals, even so-called “organic” ones, and can affect humans much as they affect pests.  Read and follow all label precautions when using any insecticide product, organic or otherwise.  Even if relatively safe when mixed, handling the concentrated products can be much more risky.

When reading the label, also check for the active ingredients.  Look for the official common chemical name, not just the trade name which products often are known by.  For example, on the product Sevin, the common name you should see is carbaryl.  There are at least 300 products with this active ingredient.

You can search all the details about a specific product or active ingredient online at one of several sites, good starting points being the National Pesticide Information Center ( or a site from Cornell University (  There you can learn, among other details, that this product is a carbamate and that it is highly toxic to bees.  As a general rule of safe use for the environment, don’t apply any pesticides where, and when, they may interfere or kill beneficial wildlife as well as your target pests.

Do some research on pesticides for specific plants and pests, assemble two or three from different classes, rotate their use, and use exactly according to label directions.  The result should be good pest control with least toxic impacts on the environment.  Don’t forget to explore cultural and other non-pesticide controls which may reduce your need even to use pesticides.

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