Pesticides

Karen Delahaut, Outreach Specialist, UW-Madison

If you choose to use chemical pesticides to control diseases and insect pests in the greenhouse, there are some things you must know in order to use them safely and effectively.

There are several types of pesticides available and they can be categorized by several methods. The simplest method classifies all products as either organic – those products that contain carbon; and inorganic – those that don’t contain carbon. Most of the inorganic products are naturally-occurring and include such things as copper hydroxide while most of the synthetic pesticides are organic but there are some naturally-occurring organic pesticides such as rotenone, pyrethrum, and neem.

Recently, a new term has emerged for the group of pesticides that are less toxic than some of the older products. These are the biorational pesticides. Many are derived from natural sources but some are now synthesized in the lab more cost effectively than they can be obtained from nature. Within the group of biorational pesticides are the botanicals, microbials, soaps, oils, antibiotics, and insect growth regulators. Examples of botanical insecticides include pyrethrum, neem, sabadilla, rotenone, and azadirachtin. Microbial insecticides include Bacillus thuringiensis, Beauvaria bassiani, and spinosad. The antibiotic insecticide abamectin is isolated from the natural fermentation product of Streptomyces.

Before you buy any pesticide, you must first ask yourself the following questions. If you are unable to answer “yes” to all 6 questions, you should reconsider pesticide use.

1. Is the site listed on the label?
2. Is the target pest listed on the label?
3. Can the pesticide be used when you need it?
4. Do you have the necessary equipment to use the product?
5. Is the pesticide practical for the particular situation?
6. Did you read the entire label.

You’ve answered yes to all of the above questions and now have several products that are registered for your site and pest problem. How do you select which product is the best for your situation? The first consideration is which product is less toxic to people. There are two types of toxicity: acute and chronic. Acute is the immediate effect while chronic toxicity happens over a long period of time and includes things such as cancer, birth defects, and sterility. The potential for a pesticide to make you acutely ill can be measured through the use of what’s called an LD₅₀. This is the lethal dose that will kill 50% of the test animals. It’s expressed in milligrams of pesticide per kilogram of body weight. The lower the LD₅₀ the greater the toxicity. To better understand this concept, manufacturers put a signal word on the product label. Those products with the signal word “Danger” are highly toxic and have an LD₅₀ between 0-50 mg/kg and only a few drops to 1 teaspoon will cause the death of a 150 lb person. Pesticides that carry the signal word “Warning” are considered moderately toxic and 1 teaspoon to 1 ounce will cause death. LD₅₀ for products labeled with “Warning” are between 51-500. Slightly toxic, and relatively non-toxic, pesticides are labeled with the signal word “caution”. You would have to ingest 1 ounce to 1 pint of these products to cause death while non-toxic products can be ingested in amounts of 1 pint to 1 pound.

The other consideration to be made when choosing between several pesticides is economics. Let’s look at a couple of examples here.

Example 1

Product A costs $400/lb while Product B costs $200/lb.

At first glance it looks like Product B is the better buy but after reading the directions for use you learn that you need to apply ¼ pound in 100 gallons of water for Product A but you need 1 pound in 100 gallons of water for Product B. By doing some quick math, you realize that although Product B is cheaper to buy, you will actually spend twice as much per application.

Example 2

Product A costs $35/100 gallons of water

Product B costs $25/100 gallons of water

Here again it looks like B  is the better buy until you find out that Product B doesn’t have much residual activity and has to be reapplied every week while Product A is applied every 2 weeks.  Again, Product A is the best buy in the long range.

Other considerations when choosing a pesticide are whether the product is effective for the pest type and life stage. Is the pesticide preventative as are most fungicides or does it provide curative properties which are most insecticides but few fungicides. Also, does the product kill on contact such as oils and synthetic pyrethroids or is it systemic in the plant.

Remember that chemical pesticides aren’t without their problems. Controlling a pest problem may allow a previously, non-economically important pest to become a problem once the competition is removed. Pesticide use can also result in an increase in the target pest population once the pesticide has worn off. And most seriously, resistant pest populations can develop. Because pest populations are so large and have a relatively short generation time, it is likely that there are some individuals within a population that are resistant to a particular class of pesticide. It is these individuals that survive the pesticide application and reproduce thus increasing the percentage of resistant individuals. As time goes on, if you don’t rotate between pesticide classes, you will end up with a pest population for which a particular class of pesticide is relatively useless. So, if you are regularly applying pesticides, make sure you switch classes every once in a while to break up the resistance cycle.

With a little knowledge, you can use pesticides safely and effectively as part of your integrated pest management program. Remember, however that pesticides should not be your first line of defense against pests but your last.