UNIT
3: LESSON 3
PROLEGS, HEAD CAPSULES, AND WARTS
SUBJECT:
Science
OBJECTIVE:
Students will learn some identifying features of the black cutworm
larvae and understand why such small details are so important.
MEASUREMENT:
Students know that a worm is not just a worm. If they look closer, it
might be a larva with many specific, identifying factors!
BACKGROUND FOR
TEACHERS:
The scientific name
for Black Cutworm is Agrotis ipsilon. This insect is in the Order Lepidoptera
which includes butterflies and moths, and the Family Noctuidae which
is the largest family in the order, with over 20,000 species, over 2700
in North America. Many are common moths, and although they vary in size
and color, most have a wingspread of 20-40 mm and are dark-colored.
Noctuid larvae are
smooth and dull-colored and most have five pairs of prolegs; a few,
called loopers, have only three pairs and move like inchworms. Larvae
of some species (cutworm) feed on roots and shoots of plants, and often
cut off the stem just above the ground. Larvae of other species (corn
earworm) feed on growing ears of corn, tomatoes, and cotton bolls.
In terms of economic
damage to crops, the Noctuidae family is easily the most significant.
Noctuids are nocturnal
and the moths constitute a major proportion of insects attracted to
lights. The larvae are also nocturnal, emerging at night to chew through
stems or other food source.
The life cycle is
one of complete metamorphosis, meaning it progresses from egg to larvae
to pupa to adult. The larvae are the destructive stage for corn producers
and the only stage diagramed in this lesson. (See Unit
3, Lesson 2 for information on the time required to move from
one growth stage to the next.)
The size of the
head capsule is an indicator of the size of the larva, and how much
damage it can do before it quits eating corn, and pupates.
Proper identification
of insects is essential for proper management. In cornfields, black
cutworms are often confused with larvae of the crane fly, which have
no legs as compared to the prolegs of the black cutworm. They're also
confused with dingy cutworms, which are less aggressive feeders and
so do not require the same degree of control. The paired tubercles,
or "warts" on dingy cutworms are both the same size, as compared to
black cutworms, in which one is much larger than the other.
STUDENT ACTIVITIES:
1. Ask students
to read the story, Elizabeth Sees a Bad Side
to Her Favorite Creatures, paying close attention to the descriptions
of the black cutworm moths and worms (larvae).
2. Show students
the picture of the moth and explain that the moth lays the eggs that
hatch into the larva (worm) you're going to study. Explain that after
the larva grows to its full size it pupates into another moth. The cycle
of complete metamorphosis is therefore: egg to larvae to pupa to adult.(Example
1) Again, the size of the head capsule is a way of measuring a larva's
size, and knowing how soon it will pupate. In the case of black cutworm,
the moth does not overwinter in the north, but relies on strong southern
winds to bring it back in the spring.
3. Use the example
of student's fingerprints to emphasize that small details can be very
important for proper identification. Explain that there can be more
kinds of insects in one acre of land than species of birds in the entire
United States, as many as several million, and it is very important
for farmers to know exactly what insect they need to manage. There are
different management systems or pesticide treatments for different insects.
4. Mistaken identities
can cause lots of problems. Students can discuss some of the possible
consequences. They can also understand why farmers hire crop consultants
or other advisors to help identify insects, weeds, and diseases in their
cornfields.
