Activity
Engage
Yesterday’s homework asked students to assess
the state of pest control in their lives by opening up
the yellow pages to ‘Pest Control’, looking
in their cupboards for pesticides and asking their families
for their input.
1. Today ask students what they found out about pest control in their homes.
Are pesticides everywhere?
2. On the board or overhead share with students the following statement: “The
US uses over a billion pounds of pesticide-active ingredients each year. Homeowners
apply at least 76 million pounds of these insecticides.” Ask students
how many pounds of pesticides is that per person? What happens to all these
chemicals? Throughout the class they should consider that question, “Where
do all the pesticides go?”
3. Pass out copies of different insecticide labels to each group. Ask them
what they can tell you about this insecticide based on the label. Students
should take notes as they identify qualities of the insecticide including definitions.
4.
Start by asking how the insecticide gets into the insect? From the labels
students should be able to identify that there are
4 basic “routes of
entry” of the poison:
- 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
5.
Does this insecticide kill just one specific insect? What
other organisms does it affect? Is it
a broad or narrow spectrum
insecticide? Students should
note that a narrow spectrum insecticide might just attack
one type of insect; a broad spectrum may have affect lots
of different
animals.
6. Have students find the active ingredient (a.i.) in
the insecticide. Explain that before they can be sold,
specific
quantities of
the active ingredient
is fed to mammals, specifically rats, to see how much
will kill of 50% of the experimental population. This
amount
of the active ingredient
is known
as the
Lethal Dose at which 50% of animals die, abbreviated
to LD50. Ask students, ‘Why
do they test the active ingredient on rats and not on insects?” Rats
are used instead of insects as a way to estimate the impact on humans. This
assumes a rat’s physiology is similar to a human’s. Let students
know that while the LD50 might not be written on the label they are reflected
by the use of “signal words” that show increasing
levels of toxicity:
Caution <Warning< Danger
7.
While the LD50 represents what happens if the rats eat
the active ingredient at one time, what would happen
if they kept ingesting it at lower levels for longer
periods?
8. Students should identify what is the formulation of the
insecticide. Explain that insecticides come in different
forms and those different forms might have different toxicities
dependent on the toxicity of the chemical and how likely
you are to be exposed. Ask students what form their pesticide
is in. After listing the different forms have students reason
which is most toxic of these.
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!
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
9. Students share within their group information about their
pesticide. What is it used for, how strong is it, what might
it best be used for.
Students are presented with this scenario:
”
Your dad is insisting on buying a pesticide at the store
to get rid of the caterpillars that are infesting his citrus
plant. You’re not particularly happy about this, since
the tree has yet to produce oranges that can be eaten, you’re
concerned. What type of pesticide would you use? This can
be an imaginary pesticide or an actual pesticide that you
have a label for. Justify your reasons.
Let students discuss as a group which they would choose and
present to the class. Do they have other suggestions other
than use of a pesticide? The following questions can help
students get started:
- Will
these products actually control the pest I have?
(Am I sure I know what the pest is??! Do I know its
life
cycle?)
- Efficacy;
short or long term control?
- What
are the active ingredients?
- What
are the relative acute toxicities to humans by signal
word? By actual LD50s?
- What
are the risks of exposure due to formulation?
- What
precautions do the labels tell you? How, exactly, would
you apply
the products safely?
Now
that the students know a little more about pesticides
have them begin to
answer the initial question, “Where
do these chemicals go after they are sprayed, ingested or
poured into the environment to contact insects?” Students
pick one target insect and a pesticide and draw out at least
one possible route for the chemicals to travel into the environment.
Encourage diverse thinking by questioning students as they
draw different routes. Did they consider decomposition of
the insect? Where does the chemical go then? Is it still
active?
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Embedded
Assessment
The
justifications provided during the scenario provide insight into
students’ ability to apply their understanding of the different
pesticides.
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