Unit 2 Section 2 Background: Chemical Pest Control
IPM Tactic: Chemical Control
Dedicated
to Reducing
Pesticides
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 participants might investigate the
following questions when considering a pest management scenario
involving the need for a pesticide.
Toxicity Questions
INTEGRATED PEST
MANAGEMENT
1. 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.
Unit 2 Section 2 Background: Chemical Pest Control
Toxicity Questions
(continued)
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.
2. How is toxicity to mammals (including humans) measured?
a) 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 LD50. 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!
LD50 values are reflected on the pesticide labels by the signal word s
which represent increasing levels of toxicity:
Caution - Warning - Danger
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Unit 2 Section 2 Background: Chemical Pest Control
Unit 2 Section 2 Background: Chemical Pest Control
Toxicity Questions
(continued)
b) 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
determine whether the chemical is mutagenic (causes mutations).You
can find information on these factors but it is not clear how to interpret
these results with regard to pesticide use and human exposure.
3. 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
4. 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?
Unit 2 Section 2 Background: Chemical Pest Control
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Unit 2 Section 2 Background: Chemical Pest Control
Toxicity Questions
(continued)
5. 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 formulations 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 s o a k 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!
6. 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 b reathing 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.
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Unit 2 Section 2 Background: Chemical Pest Control
Unit 2 Section 2 Background: Chemical Pest Control
Other Considerations for Pesticide Use
Potential Environmental Effects
The U.S. 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 .
Pest Resistance
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
1.
2.
3.
4.
5.
6.
Quick-acting
Easy to use
Cheap (relatively)
Readily available
Can save lives (vector-borne diseases)
Others?
Unit 2 Section 2 Background: Chemical Pest Control
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Unit 2 Section 2 Background: Chemical Pest Control
Other Considerations for Pesticide Use
(continued)
Disadvantages of Chemical Pesticides
1.
2.
3.
4.
5.
6.
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Potentially toxic to humans
Impact on non-target organisms
Potential negative environmental effects
Unknown cumulative effects
Pest resistance
Others?
Unit 2 Section 2 Background: Chemical Pest Control
Unit 2 Section 2 Background: Chemical Pest Control
Notes
Unit 2 Section 2 Background: Chemical Pest Control
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Unit 2 Section 2 Background: Chemical Pest Control
Notes
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Unit 2 Section 2 Background: Chemical Pest Control