In an initial 3-hour lab, students first design experiments to examine intraspecific and interspecific competition using two species of parasitoid wasps. Second, students are guided to a consensus experiment that examines the effect of both types of competition on reproductive output in the parasitoids. Third, the students conduct the consensus experiment in which one or two females are placed on a single host, alone, with conspecific competitors, or with interspecific competitors. Working individually, students set up replicate cultures of the experimental treatments. In subsequent labs, students check cultures for emergence of new adults and record date at first emergence. Cultures are frozen after full emergence, approximately 21 days for Nasonia cultures and 40 days for Melittobia and mixed species cultures. In a final, 3-hour lab, six weeks later, students gather data on the number of offspring produced by females under each condition. The resulting data are used to estimate the parameters of the Lotka-Volterra competition model. The predictions of the model are then compared to the outcome of interspecific competition treatments.
At the conclusion of this lab, students will be able to...
Equipment/ Logistics Required:
Materials for a class of 24 students (working in pairs):
Order the living wasp cultures and fly pupae to arrive at most one week before class. Wasps are shipped as late pupal stages and should be beginning to emerge upon arrival. If emergence appears complete upon arrival (i.e., numerous adult wasps crawling in culture container), cultures can be maintained fresh for short periods of time by storing them in refrigerator dairy compartment until day of class.
Note: if you need a large number of parasites, you may wish to rear your own. See “Maintaining parasitoid wasp cultures,” below, for details.
The Neobellierria (=Sarcophaga) pupae must be placed in the refrigerator immediately upon arrival and kept there until just before class use. Otherwise, they will begin to develop into flies and if this happens they are unsuitable as hosts for the wasps.
The day before class, you (or the lab prep person) need to sort through the Nasonia culture removing all males, so that the wasps provided to the students are entirely female. This is necessary because the sexes are similar in appearance, and if the students are asked to distinguish between the sexes they are not always reliable. However, with a little practice males can be readily distinguished. Because the Melittobia digitata culture is always about 95% female and the tendency of males to remain inside the host pupal skin, there is no need to remove the males. There is little chance that a male would end up in an experimental vial. Male Melittobia are also extremely different from females, so in the unlikely event that one is found and chosen by a student it would be readily apparent.
See Notes to Faculty: Preparing Vials with Wasps of Each Sex to help students differentiate the sexes.
See Notes to Faculty: Instructions on Maintaining Parasitoid Wasp Cultures.
Summary of What is Due:
During the first lab period, students will produce an experimental design to examine the effects of intraspecific and interspecific competition on offspring production in two species of parasitoids. After collecting and analyzing the data, the students will write scientific papers based on their results.