>Weed Topics

Using Pesticides

Sooner or later most of us find we have an intractable weed, insect, or plant disease problem that doesn't respond to non-chemical methods of control. This leaves us with two options; 1) live with the pest, or 2) use pesticides. Pesticides are expensive and can be inconvenient to use, so to get the best results for your effort and money, use techniques that ensure they will be the most effective. They will be most effective if the right product is selected, if it is applied at the right time, and if enough (but not too much) is used. When all label instructions are followed carefully, the product will provide better control of the pest and effects on people, animals, and the environment are minimal.

The term "pesticides" is an umbrella term that includes any product used to kill pest organisms, including herbicides (for weeds), fungicides (for plant diseases), insecticides (for insects), rodenticides (for rodents), etc. In order to select the right product, first have the pest positively identified by a qualified person, such as your Extension agent or a knowledgeable nursery or garden center employee. Remember, just because a sales person sells pesticides does not mean that person is knowledgeable about either pest identification or pesticides. What works for one pest, won't necessarily work for another, so correct identification is important. For example, quackgrass is often mistaken for crabgrass, which is rare in Montana. But herbicides that work for crabgrass won't have any effect on quackgrass.

Once you have the pest correctly identified and know the kind of plant or site on which it is causing the problem, you have enough information to select the correct pesticide yourself. The next step is easy- read product labels to find one that lists your pest on it and says it can be used where the pest is found. For example, for aphids on tomato plants, select an insecticide with a label that specifies it will control aphids, and that it can be used on vegetable crops including tomatoes. If aphids are not included in the list of insects the insecticide will control AND tomatoes are not included in the list of plants on which it can be used, select another insecticide. Always select the least-toxic pesticide possible that will control the pest. Labels that have the word "Caution" on them are the least toxic. Labels with the word "Warning" are more toxic, and labels that say "Danger" or "Poison" are the most toxic. In many cases, there are non-toxic or least-toxic alternatives to the pesticides.

For pesticides to work best, they should be used when the pest is most susceptible to the product. The label should specify the best time for application. For example, some weeds are controlled better by fall applications of herbicides, but others may be more susceptible in spring, or just before flowering. Insects are often vulnerable at one stage in their life cycle and relatively unaffected in another. Plant diseases are usually not affected by fungicides after they have established; for fungicides to work they usually must be applied before the disease develops. This doesn't mean you should spray everything with fungicide "just in case"; it simply means if a disease was present last year, or if you know it is present in your immediate neighborhood, then preventative applications should be made. When using herbicides, make sure the weeds are not drouth-stressed because the physiological processes of plants slow down when deprived of water, which in turn inhibits the effectiveness of the herbicide. You may even want to water your weeds before you spray them! So read the label to determine the best time to apply the pesticide.

The amount of pesticide used can also determine how effective it will be. Use specified on the label. Twice as much pesticide won't do twice as much good! Too much pesticide can damage the plant and leave toxic residues with which children, pets, and wildlife can come in contact. Excess pesticides can also leach through soil into groundwater or be carried in surface runoff into streams or ponds. Pesticides work best when used in amounts specified on the label and stay in the sites where they are needed.

When you have selected the right pesticide, read the label again, then gather together everything you will need. When mixing and applying the product you should wear the proper clothing and equipment to keep the pesticide from coming in contact with your body, including your eyes and lungs, which can transport the product directly into your bloodstream. The minimum clothing to be worn during pesticide use includes long pants, a long-sleeved shirt, unlined rubber boots and rubber gloves. A wide brimmed, washable hat should also be worn to keep the pesticide off of your head and neck. If the label says to avoid eye contact, wear goggles with side shields. If it says to avoid inhalation of the pesticide, wear a face mask. If it says to avoid inhalation of the vapor, wear an approved respirator. The label will tell you what the risks are, then it is up to you to protect yourself from them. When they are applied properly, your body does not come in contact with pesticides.

For assistance with pest identification, non-toxic pest control methods, and pesticide selection and use, contact your county Extension agent, listed in the phone book under county government and ask to borrow video No. 021, "9 Steps to Safe Pesticide Use: A Guide for Homeowners". A free companion bulletin of the same title is also available. These will show you how to select, mix, and apply pesticides. Names and addresses of additional sources of assistance and materials are also included.


Bohmont, B. 1990. The Standard Pesticide User's Guide. Prentice-Hall, Inc., Englewood Cliffs, NJ 07632. 498 pp.
Lajeunesse, S., G. Johnson, J. Jacobsen, and K. Johnson. 1993. Video. Nine Steps to Safe Pesticide use: A Guide for Homeowners. Montana State University Extension Video 021. MSU, Bozeman, MT 59717. 14 min.
Marer, P. 1988. The Safe and Effective Use of Pesticides. Univ. Of California. Statewide
IPM Project. Publication 3324. 387 pp.
Olkowski, W., S. Daar, and H. Olkowski. 1991. Common-Sense Pest Control; Least-toxic Solutions for Your Home, Garden, Pets and Community. Taunton Press, Newtown, Connecticut. 715 pp.

Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997