Chemical tactics to manage pests can include many types of compounds. Some merely repel or confuse pests. Some interfere with weed photosynthesis, insect molting processes or development in some way. Others, including some botanical and most conventional insecticides, are broadly toxic to living systems. The term pesticide literally means "pest killer". The specific type of organism killed is revealed in the name; i.e. insecticides kill insects, herbicides kill plants, fungicides kill fungi, rodenticides kill rodents and so on.
Various characteristics of pesticides
Generally speaking, when using pesticides, we are trying to garner the advantages of using pesticides while minimizing the risk to humans and other non-target organisms. Risk to humans of pesticide use is often described as:
Risk = Toxicity X Exposure
The simplest way to safely use pesticides is read the label, be sure you understand the label and follow ALL instructions. Exposure can be minimized by simply following the directions about precautions. Both toxicity and exposure are eliminated if you choose not to use a pesticide as one of your IPM tactics.
A number of characteristics of pesticides should be understood in order to consider whether or not to use a pesticide, which one to use and how it might be safely used. You and your students might investigate the following questions when considering a pest management scenario involving the need for a pesticide.
How widely toxic is the pesticide?
If a chemical is "broad spectrum", that means it kills lots of different organisms within its category. In contrast, "narrow spectrum" chemicals affect a more narrow range of organisms. So, for example, if your IPM approach was going to include limited use of an insecticide to kill caterpillars but you wanted to leave their predators and parasites unharmed, you would choose a narrow spectrum product targeted at caterpillars only. Other "non-target", or unintentional, victims of an insecticide's use might include bees, fish, birds, pigs, or aquatic organisms. These risks are usually listed on the pesticide label. There are also lists that show relative toxicity of specific compounds toward these organisms (see resource materials in this section).
How is toxicity to mammals (including humans) measured?
Acute Toxicity: Toxicity that causes harm or death from a single exposure.
Each pesticide contains a small percentage of the chemical component responsible for the killing activity. This component is called the "active ingredient" or a.i. for short. Before release for sale, pesticides are required to undergo testing to determine how much of the active ingredient will kill a mammal. Laboratory rats of known weights are fed specific quantities of the pesticide active ingredient, measured in milligrams of a.i. per kilogram of body weight (mg/kg). The number of milligrams/kilogram required to kill half of the experimental population is referred to as the "Lethal Dose at which 50% of the animals die" abbreviated as LD 50. The assumption is that as mammals, both humans and rats share common physiological characteristics. Thus, LD50 data are used to estimate toxicity of pesticides to humans. Also, the pesticide can be administered by mouth (oral LD50), on the skin (dermal LD50) and by breathing (inhalation LD50). So for example, some active ingredients are more dangerous when breathed rather than when ingested. High LD50 values are better than low ones from an acute toxicity point of view, it means it would take more of it to kill you!
LD 50 values are reflected on the pesticide labels by the "signal words" which represent increasing levels of toxicity:
Caution --> Warning --> Danger
(See "Lessons from Labels" for how to use this information in a classroom activity.)
Chronic Toxicity: harmful effects of exposure over long periods of time.
Long-term effects of pesticide exposure, or of exposure to multiple pesticides are poorly understood. Laboratory tests with rats measure various factors over time such as whether the chemical is carcinogenic (causes cancer) or is teratogenic (causes birth defects). Other tests test whether the chemical is mutagenic (causes mutations). You can find information on these factors (see websites below) but it is not clear how to interpret these results with regard to pesticide use and human exposure.
Which types of pesticides are most and least toxic to humans?
Generally speaking, from least to most toxic common pesticides are:
fungicides --> herbicides --> rodenticides --> insecticides
Within the insecticides, which chemical categories are most toxic?
(You could ask this question within each type of pesticide. Here we are using insecticides as an example since they are the most dangerous.)
Generally speaking, from least to most toxic common insecticides are:
soaps -->microbial products -->insect growth regulators -->botanicals pyrethrins -->chlorinated hydrocarbons -->carbamates -->organophosphates
These toxicities are generally related to the "mode of action" of the insecticide; how does it kill? For example, all nervous systems use the same chemicals to transmit (and stop) nerve impulses. If the pesticide "mode of action" is to interfere with nerve transmissions, it is potentially more toxic to all organisms than a pesticide that is more specific to just insects. Insecticidal soaps' mode of action, for example, is to interfere with the function of insects' waxy outer covering, the cuticle, having little or no effect on mammals. So would you choose a soap or an organophosphate to control aphids on your houseplants? Lists of common household chemicals and their chemical categories are included in background materials.
What is the formulation of the insecticide?
Insecticides and other pesticides come in various kinds of mixes. These can affect both the toxicity per se and how likely you are to be exposed. Generally least to most toxic formuations of insecticides:
enclosed bait -->granular -->dust -->water solution -->water emulsion
oil solution -->aerosol -->emulsifiable concentrate -->liquid concentrate
Toxicity of a formulation is related to how easily a dangerous amount might be able to get into your system. Liquid concentrates, if swallowed or contacting skin, are more toxic than a dilute or premixed version of the same chemical. Oils "soak" into the skin more. Fine particles of aerosol formulations can pass through the lungs into the bloodstream. Dusts can be breathed if you are not careful but they usually carry relatively little active ingredient. There are, of course, exceptions!
How does the insecticide get into the insect?
Insecticides work on insects in specific ways. Once you know the pest and have identified your management goals, this knowledge is important in your choice of insecticides. There are 4 basic "routes of entry" of the poison into the insect:
- Stomach poison (insect must eat it)
- Contact poison (just has to hit them)
- Systemic poison (absorbed by the plant and then eaten by insect)
- Fumigant (kills by "breathing" or vapors absorbed across membranes)
Each mechanism has relative advantages and disadvantages, and different efficacy on particular groups or stages of insects. For example, a stomach poison will not work on sucking insects. It works on caterpillars but not their adult moths. A contact poison will kill individual worker ants in your kitchen but they will keep coming back. Why? Systemics generally should be avoided on food plants in your garden. Why? Fumigants can also be quite toxic when breathed by mammals (us!) and should be used with extreme caution.
Other Considerations for Pesticide Use
Potential Environmental Effects
The US uses over a billion pounds of pesticide active ingredient each year. Homeowners apply at least 76 million pounds of these pesticides. Where do all these chemicals end up? Some chemicals persist in the environment for relatively long time periods, to unknown effects. Others "biomagnify", that is, chemicals get passed along the food chain so animals high on the food chain (predators) can end up with toxic levels that can kill them or interfere with reproduction. Other chemicals are highly water-soluble and may leach into the ground water or run-off the land when it rains and end up in surface waters (streams, lakes, rivers). Generally speaking, we do not measure these potential effects when we use a pesticide. We expect that somehow, the chemicals just "go away".
Some chemicals are relied on so heavily that pest populations have been exposed long enough to develop resistance. In this situation, it takes higher and higher concentrations of the chemical to kill the pest until the pesticide becomes useless. This is another reason it is helpful to use multiple tactics in an IPM approach. When pesticides are used, different chemical categories should be applied over time rather than always using the same one over and over. The harsher the pesticide, the more quickly resistance develops in the pest population.
Advantages and Disadvantages of Pesticide Use
As we saw with biological controls, there are distinct advantages and disadvantages to pesticide use as well. Some of these are listed below.
Advantages of Chemical Pesticides
- Easy to use
- Cheap (relatively)
- Readily available
- Can save lives (vector-borne diseases)
- Potentially toxic to humans
- Impact on non-targets
- Potential negative environmental effects
- Unknown cumulative effects
- Pest resistance
Your course materials provide an abundance of information about pesticides in the pages that follow. Pick and choose what you might need to develop lessons for this unit.