Effects of Resource Limitation on Bean Beetle Reproductive Strategies
Objectives

Design and perform a set of experiments to observe the impact of limited
resources on female bean beetle (Callosobruchus maculatus) egg laying behavior.
Introduction
The availability of resources is an important consideration when studying the
reproductive behaviors of animals. When environmental conditions result in limited
resources, juvenile growth and offspring survival may be poor due, at least in part, to
maternal and/or offspring competition for those resources. One aspect of reproduction
that can be altered when the availability of resources changes is the amount of parental
resources (metabolic energy and/or time) invested in each offspring. This investment can
be crucial for the reproductive success of the animal by enhancing or reducing the
survival of the offspring. Many studies in seabirds have shown that secondary offspring
are only produced and nurtured to maturity when food resources are abundant. In food
limited situations, seabirds, such as Nazca booby, only produce one fledgling per
breeding season (Clifford and Anderson 2001).
Numerous studies on insects have examined how resource allocation and
reproductive output differ under optimal and food limited scenarios. For example, Boggs
and Freeman (2005) have shown that female Speyeria mormonia butterflies have smaller
body size, forewing length, and decreased survival if they are food-limited as larvae.
Similarly, early work by Smith and Fretwell (1974) predicted that where juvenile growth
and/or survival are poor, larger propagules (e.g. eggs or seeds) would be produced.
In this laboratory exercise, you will design and implement an experiment
examining the effects of limited resources on egg production in the bean beetle,
Callosobruchus maculatus.
Materials
In class, you will be provided with live cultures of bean beetles containing adults
that have been raised on mung beans (Vigna radiata). Supplies of organic mung beans
and 60mm petri dishes will be available. Dissecting scopes can be used for identifying
the sex of the beetles. You will also be provided with a Motic camera and laptop
computer to photograph and measure the size of eggs laid. Your laboratory instructor will
demonstrate the use of the Motic camera and program to you.
Experimental Design
Since larval bean beetles have no means of moving out of the bean they are living
in, choices made and resources provided by their mothers greatly influence their survival
and future success. We know that resource availability potentially impacts the amount
and size of eggs laid by female bean beetles. Given this, female bean beetles may be very
sensitive to the species and condition of the beans on which they are depositing eggs,
including whether those beans already have an egg on them. As a class, we will discuss
possible experimental approaches to examine whether female bean beetles change the
number, size, or some other aspect of how they lay eggs under situations where resources
are abundant or limited. We will design an experiment as a class so that enough data can
be generated for statistical analysis.
In groups of 4 discuss the following:
 What resources could influence female egg laying behavior if limited?
 Describe at least TWO experimental designs for evaluating how female bean
beetles' egg laying behavior differs under scenarios with abundant and scare
resources.
 Predict the outcomes for each experiment.
 Identify and list the variables you would manipulate in each experiment.
 Identify and list the variables you would keep constant in each experiment.
 List the data you would collect to determine if your predictions were true.
Be prepared to discuss your group's ideas with the class.
Set up the experiment designed by the class. Once your cultures of beans and beetles are
ready, incubate the cultures at 30°C. During next week's lab we will collect and analyze
the data.
Data Collection and Analysis
Observe your bean beetle cultures and record all data in your lab manual. We will pool
the class data for class discussion and statistical analysis.
From each of your cultures:
1. Observe any patterns of oviposition. How many eggs are in the cultures? Are
there more than one egg per bean? How do the different cultures compare?
2. Randomly select 5 eggs from each culture and measure the length and width
of each egg using the Motic camera system.
Laboratory Discussion
Using the class data write a brief discussion of the data including the following:
1. How did female bean beetles respond to your experiment?
2. Conduct a t-test on the class's results. Do you see differences between your
class's experimental and control groups? Are these differences statistically
significant? Discuss the meaning of your statistical findings.
3. Discuss three ways to refine your experiment to get at more specific questions
of bean beetle responses to population size and/or resource limited situations.
Literature Cited
Boggs, C.L. and Freeman, K.D. 2005. Larval food limitation in butterflies: effects on
adult resource allocation and fitness. Oecologia 144:353-361.
Clifford, L.D. and Anderson D.J. 2001. Food limitation explains most clutch size
variation in the Nazca booby. Journal of Animal Ecology 70: 539-545.
Smith, C.C. and Fretwell, S.D. 1974. The optimal balance between size and number of
offspring. American Naturalist 108:499-506.
This study was written by M. Myrne and K. Heiman, 2010 (www.beanbeetles.org).