UNIT
3: LESSON 2
"WEATHER" OR NOT TO ACT
SUBJECT: Science, Math
OBJECTIVE: Students will learn how to count the number
of black cutworms in a pheromone trap. They will also learn how to use
daily temperatures to calculate the Growing Degree Days in order to
predict when the worms will cause problems in corn fields.
MEASUREMENT: Students will understand the process of insect
monitoring.
BACKGROUND FOR TEACHERS:
Black cutworm moths do not overwinter in the Corn Belt but are brought
north on winds from Mexico and the southern United States. At dusk,
emigrating moths fly upward, are caught by surface winds and rising
air in advance of thunderstorms into the lower-level jet stream. The
can fly from Texas to Minnesota in as little as two days. Moths are
carried along until they decide to "drop out," encounter cold air or
thunderstorms.
After she finishes her migration, the female moth releases a sex attractant
to draw in males. That same sexual pheromone is used as a lure in a
sticky trap (see picture). The moths caught in that trap are then used
to determine the date of arrival and the number of moths in each flight.
Traps are checked daily and the numbers recorded.
Once the arrival date is known, it becomes possible to predict what
day the eggs will hatch, and what day the worms will start eating, and
how fast the worms will grow. This predictive ability is an important
tool! It indicates when to start looking for problems, so no time is
wasted, and no problems are missed. It helps prevent economic loss and
unnecessary use of pesticides.
This prediction is possible because of two things-time and temperature.
The importance of time is obvious. Animals obviously do not change
from infant to adult overnight; neither do insects. It takes time.
Temperature is also important to growth of organisms. However, in humans
it is not very apparent. That is because humans and other mammals maintain
a constant body temperature. That's not true of plants and insects.
Their temperatures vary with their environment, so, for predicting growth
of plants and insects, it becomes important to find a way to combine
both time and temperature.
The method used to measure their development is called "physiological
time." Growing degree days, or heat units as they're sometimes called,
measure both time and temperature. They represent the number of degrees
above some minimum temperature necessary for growth multiplied by time
in days.
For example, 10 degrees above the minimum for 5 days represents 50
degree days (10 x 5) just as does 2 degrees above the minimum for 25
days (25 x 2). Both represent the same amount of physiological time-an
insect would have grown the same amount under either condition!
STUDENT ACTIVITIES:
1. Ask students to read Elizabeth Sees a Bad Side to Her Favorite
Creatures, paying close attention to the discussion of the sticky
trap (that looks like a flying sandwich and catches worms), and the
discussion of Growing Degree Days. Discuss how this knowledge helps
farmers protect corn plants from bad insects. (Example 1)
2. Then, younger students can complete the worksheets in which they
count and record the number of moths flying into a region over a week-long
period. (Unit 3, Lesson 3 has more information
on the life cycle of the moths and the appearance of the larvae.) (Worksheet
1)
3. Older students can complete the Growing Degree Day calculations
that will predict when the eggs will hatch and the worms begin feeding.
(Worksheet 2)
4. The Growing Degree Day concept could be demonstrated to younger
children using a thermometer to read the high temperature of the day
and the low temperature, explaining that a formula uses those two values
to determine how much an insect grows on that day. Remind them that
an insect (and a plant) can only grow at certain temperatures. They
grow slowly on cold days and quickly on hot days. ("You can hear the
corn grow on warm summer days" has a lot of truth to it!) But if it's
too hot, or too cold, no growth occurs at all.
5. It would be good to include a discussion of predictability of
growth.
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Can we predict how fast we will grow? How do we make those predictions
about ourselves? (Do we use time and temperature?) How are those
predictions useful?
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This story illustrates the benefits of predicting the growth of
insect larvae. (It tells farmers when they need to look for problems.
It prevents problems from happening without their knowledge.) We
can also use this same method to predict how fast the corn will
grow. How would that be beneficial?
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What else would the students like to be able to predict? Why?
