& Lesson Sequence
on Standards & Assessment
Needed, Preparation & Planning, Management Strategies
Why Study Predator - Prey interactions?
of a predator-prey interaction
makes a predator successful?
Designing a predator - prey experiment
Control vs. Pesticides
Rubric for Group Presentation
in the Environment Data Sheet
lacewing Larval Mouthparts
Insect Populations on Single Plants
Insects are small and often numerous. They can hide under
leaves, at the base of stems and other places that make them hard to see
and count. Estimating their numbers on a small plant or in a large
field can be quite a challenge. Researchers solve this problem by
counting the individuals in a "sample" part of the plant or
habitat and then use this sample to estimate the total population. Scientists
use many different sampling methods and no one sampling method is perfect.
Consider the behavior of the insect being counted before choosing a sampling
method. For example, pea aphids tend to drop off the plant when
disturbed, so careful manipulation of the plant while counting is extremely
important. Note the overall distribution of insects to be counted
(base of plant, under leaves, at new growth, etc...). In addition,
the time of day and/or the season can affect results. For example,
tobacco hornworms tend to feed on the plant at night when predators are
less active; attempting to count caterpillars during the day would produce
few individuals and give an inaccurate estimate of the population.
Whichever method is chosen, be sure to state and define your sampling
unit, such as transect length, quadrat size, or leaf position and determine
how many samples will be taken. A hand held counting device (which
can be obtained from a local office supply store) is helpful. You
may use the following suggested methods or develop your own.
randomly selected leaves
For evenly distributed species, count the insects on three randomly
selected leaves on each plant. The "randomly selected leaf"
will be your sampling unit. Average the results and multiply by
the number of leaves on the plant to get an estimate of the total population.
a unit area or quadrat
When individuals are clustered on
one part of the plant, sampling a unit area within that part of the plant
may be necessary. For example, cabbage aphids form tight clusters
on the underside of large cabbage leaves. By counting the aphids
within a square centimeter placed within the cluster of aphids and then
measuring the diameter of the cluster, you can estimate the number of
aphids within a cluster of that size. Perform a similar calculation
for the other clusters on the plant, add them all together in order to
obtain an estimate of the total population. In this case the unit
area is the sampling unit. In another example, milkweed aphids tend
to prefer the new growth on the plant, so the sampling unit might be the
terminal 3 cm of each plant.
a leaf position
Another method for sampling insects that
tend to favor a particular part of the plant, is to use a leaf position
as the sampling unit. To select a leaf position, note where each leaf
leaves the plant stem starting where the stem leaves the soil and moving
up the plant (see figure). Select a leaf position that appears to
have the highest density of insects on most of the plants in your study.
Whatever leaf position you choose, be consistent and count the same leaf
position on each plant. If you must manipulate the leaf in order
to count the insects, do so carefully. As mentioned above,
pea aphids will drop off the plant when disturbed. To avoid this
problem, place a piece of paper or a Petri dish under the leaf while counting.
Include any insects that drop off the leaf in your census. Note
that this method does not give you an estimate of the total population.
It is best used to determine the relative level of insect population growth
on a plant. Data such as this can be compared to similar data collected
from other infested plants or used to monitor the growth of a pest population
of insects within an agricultural field where counting insects on every
plant is impossible.