Insects and People

Ent. 109 Insects and People Predation Lab

1. Principles of Predation

Some insects catch and kill other species for food. This sort of association is known as predation. Predation is an extremely common way to make a living among arthropods--it is known to occur in at least 19 orders. Irrespective of the taxonomic affiliations, predators share certain similarities in the way they capture prey. One of these similarities is the functional response curve; as more prey are available, predators consume more, up to the point at which they are either satiated or unable to handle more prey.

In this exercise, you are going to act as a predator and estimate your own functional response curve. Work in groups of two. One person is blindfolded and one person records the data. The person without the blindfold should place four M&Ms on the lab bench. The blindfolded person should take a pencil and tap the table with the pencil for one minute trying to locate the candies. The partner should then record the number of candies located in one minute. Then the partner sets up another trial with twice as many candies. Record and repeat until there are 32 candies on the benchtop. Plot your results in graph form on the back of this page.

Partners should then change places and repeat the experiment, only this time the blindfolded person not only has to find the candies but also crush them with a thumb to crack the thin candy shell (“killing” the prey) before eating them. The blindfolded person should stop looking for candies when he or she no longer wants to eat any more. Plot your data and compare to your earlier graph. How do they differ? How does handling time (the amount of time required to find, capture, and consume prey) affect the functional response? How does satiation affect the functional response?

Now, repeat this entire exercise once again, only this time using Skittles instead of M&Ms. These candies have a much thicker candy shell than M&Ms. How does handling time differ between the two different kinds of candies? What effect does this difference have on the function response? II. Prey defense

Most insects are not content to allow themselves to be eaten by predators; many different defense mechanisms are used by potential prey species to avoid being eaten. One such defense strategy is to produce distasteful substances. The milkweed bug Oncopeltus fasciatus produces a noxious odor which is emitted through glands; in addition, it feeds on the poisonous seeds of milkweed plants and stores in its body the toxins (cardiac glycosides) from its hostplants.

The predator to be used in this experiment is the wolf spider, Lycosa. Work in groups of four. Each of you will receive two containers, each of which houses a single spider. Place a cricket in one of the containers. Time how long it takes the spider to find, capture, and consume the cricket. In the second container, place a milkweed bug nymph and time how long it takes the spider to find, capture, and consume the milkweed bug.

Cricket Milkweed bug

Time to notice Time to catch Time to consume Time to discard remains

What differences in prey handling can you notice?

To what do you think you can attribute these differences?

III. Adaptations for predation With predation a way of life for at least 19 orders of arthropods, it's not surprising that different approaches are taken to prey capture. These different approaches are often reflected in the external morphology of the predator.

Ambush predator 1. To what orders do these insects belong?

2. Are they brightly colored or cryptically colored?

3. Do you notice any unusual features of their morphology that may be related to prey capture? Pay particular attention to legs.

Visually orienting active predator 1. To what orders do these insects belong?

2. Do you notice any unusual features of their morphology that might be related to prey capture? Pay attention to legs and head.

Predator on sedentary prey 1. To what orders do these insects belong? 2. In what ways do these insects resemble other predators? In what ways are they different?

Wolf spider Based on your observations of pinned specimens, what kind of predator would you expect wolf spiders to be? Watch wolf spiders in pursuit of prey (crickets, milkweed bugs); how would you describe their mode of predation?