Overview & Lesson Sequence

Focus on Standards & Assessment

Materials Needed, Preparation & Planning, Management Strategies

Background Info

Intro Activity
Why Study Predator - Prey interactions?

Activity 1
Observation of a predator-prey interaction

Activity 2
What makes a predator successful?

Research Project
Designing a predator - prey experiment

Optional Activity
Biological Control vs. Pesticides

Case Study
Rabbits in Australia




Resource Sheets
-Plant Propogation
-Rearing Aphids
-Eyelash Brush
-Petri Dish Habitat
-Sampling Methods
-Observation Check Sheet
-Sample Rubric for Group Presentation
-One Gallon Cage
-Predators in the Environment Data Sheet
-Predation Inquiry
-Green lacewing Larval Mouthparts
-Chewing Mouthparts

-Hemipteran Mouthparts

-Green lacewing
-Ladybird Beetle
-Big eyed Bug
-Praying Mantid
-Fruit Fly

-Green lacewing
-Ladybird Beetle
-Praying Mantid
-Big eyed Bug

Enforcers Home



Sampling Methods

Sampling 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.

Using 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.

Using 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.

Using 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.


    Center for Insect Science Education Outreach
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