Experimental design with bean beetles

Overview

Understanding the natural world requires not only observation, but also well-designed experiments in order to draw accurate conclusions. In this lab, you’ll practice designing experiments to understand the effects of resource quantity and quality on bean beetle reproduction.

Background

Bean beetle basics

Objectives

Students should be able to

• Describe the key elements of a well-designed experiment

• Identify independent, dependent, and controlled variables

• Develop a testable hypothesis about effect of resource quantity and quality on beetle oviposition

• Apply experimental design principles to a real organism

• Collect, organize, and visualize data using a spreadsheet

• Interpret results and propose future research

Introduction

Ecology involves the study of the interactions among organisms and their environment. One fundamental ecological interaction is the relationship between consumers and their resources. The availability of resources is an important consideration when studying the behaviors of individual organisms and the dynamics of populations. When environmental conditions result in a change in the quantity or quality of resources, juvenile growth and offspring survival may be affected, for example due 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. For example, 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 resource quantity and quality on egg production in the bean beetle, Callosobruchus maculatus. Bean beetles (Callosobruchus maculatus) are small beetles (adults are ~ 2 mm in length) that lay eggs on dry bean species. Eggs are deposited (“oviposition”) one at a time, and several days after oviposition, a beetle larva (maggot) burrows into the bean. At 30°C, pupation and emergence of an adult beetle occurs 25-30 days after an egg was deposited. Adults are mature 24 - 36 hours after emergence and they do not need to feed. Adults may live for 7-10 days during which time mating and oviposition occurs (Mitchell 1975). This life history strategy has led to the beetles becoming a major agricultural pest in some areas of the world. 

Since larvae cannot move from the bean on which an egg was deposited, the oviposition choice of a female determines the future food resources available to their offspring (Brown and Downhower 1988). As a result, it is the most critical choice a female makes for her offspring, because it will influence their growth, survival, and future reproduction (Mitchell, 1975; Wasserman and Futuyma, 1981). Although females can be induced to oviposit on a wide range of bean species, very few bean species result in normal development and the successful emergence of adults. Some bean species are very clearly toxic to Callosobruchus maculatus larvae (Janzen 1977).

You will design an experiment to test the effect of either 1) resource quantity (i.e. number of beans) or 2) resource quality (i.e. type of bean) on beetle oviposition behavior. You will then create live cultures, following your experimental design, and compare the oviposition rate of beetles in your control and experimental groups.

Methods

You'll now apply these ideas about designing experiments and consumer-resource interactions to test the effects of resource quantity and quality on bean beetles. The materials available to you are listed below:

• Live bean beetle cultures (males and females)

• Plastic petri dishes

• Microscopes

• Timers

• Dried mung beans (Vigna radiata)

• Dried adzuki beans (Vigna angularis)

• Dried cowpeas (Vigna unguiculata)

• Dried black beans (Phaseolus vulgaris)

• Dried wheat kernels (Triticum aestivum)

• Forceps and/or paint brushes (for moving beetles)

Part 1: Experimental design

1. At your table, develop a research question you want to investigate about the egg-laying behavior of female bean beetles.

2. Is your question about resource quantity or quality?

3. Propose an experiment to test your research question.

   • Describe the experimental setup.

   • What is your dependent/response variable (variable that you will measure)?

   • What is your independent/predictor variable (variable that you will manipulate to differ between control and treatment)?

   • What are your controlled variables (variables that will be held constant)?

   • What will be your treatment levels (different instances of the independent variable)?

   • How many replicates will you use (how many trials will you do)?

   • Make a prediction about the outcome of your experiment (what do you expect the results to be, and why?)

4. Set up your experimental cultures, following your chosen design as you described above. Take a photograph or draw a diagram of your experiment.

Part 2: Data collection and analysis

5. Create a data sheet in Google Sheets where you will enter your data for visualization and analysis. You can use the spreadsheets from the climate data summaries or butterfly phenology labs as examples of how to format your data sheet. Provide a screenshot or a link to the data sheet here (make sure that the sharing settings are set to “Anyone with the link can view”).

6. Once the experiment is complete, record your data in the spreadsheet. Create a graph in Google Sheets to visualize your findings, and upload a copy here. Be sure that your graph includes all necessary components (descriptive title, axis labels, legend, etc.)

7. Based on your graph, how do your actual results compare to your expectations? If they match, what does this tell you about bean beetle behavior and resource use? If they do not match, what might be some possible explanations for the discrepancy?

8. What additional research question(s) are suggested by these results? Briefly describe the design a follow-up experiment that could be conducted to address this questions.

References

Beck, C.W., & Blumer, L.S. (2014). A handbook on bean beetles, Callosobruchus maculatus. National Science Foundation.

Beck, C.W. & Blumer, L.S. "Natal Bean Discrimination by Bean Beetles" Retrieved 8/15/2025 from https://www.beanbeetle.org/protocols/natal-preference/ 

Boggs, C. L., & Freeman, K. D. (2005). Larval food limitation in butterflies: effects on adult resource allocation and fitness. Oecologia, 144(3), 353-361. https://doi.org/10.1007/s00442-005-0076-6

Brown, L., & Downhower, J. F. (1987). Analyses in behavioral ecology: a manual for lab and field.

Byrne & Heiman, "Effects of Resource Limitation on Bean Beetle Reproductive Strategies." Retrieved 8/15/2025 from https://www.beanbeetle.org/protocols/resource-limitation/

Clifford, L. D., & Anderson, D. J. (2001). Food limitation explains most clutch size variation in the Nazca booby. Journal of Animal Ecology, 539-545. https://doi.org/10.1046/j.1365-2656.2001.00521.x 

D'Costa, A. & Schlueter, M. "Using bean beetles to teach experimental design and experimental variables." Retrieved 8/15/2025 from https://www.beanbeetle.org/protocols/experimental-design/

Janzen, D. H. (1977). How southern cowpea weevil larvae (Bruchidae: Callosobruchus maculatus) die on nonhost seeds. Ecology, 58(4), 921-927. https://doi.org/10.2307/1936229

Mitchell, R. (1975). The evolution of oviposition tactics in the bean weevil, Callosobruchus maculatus (F.). Ecology, 56(3), 696-702. https://doi.org/10.2307/1935504

Smith, C. C., & Fretwell, S. D. (1974). The optimal balance between size and number of offspring. The American Naturalist, 108(962), 499-506. https://doi.org/10.1086/282929

Wasserman, S. S., & Futuyma, D. J. (1981). Evolution of host plant utilization in laboratory populations of the southern cowpea weevil, Callosobruchus maculatus Fabricius (Coleoptera: Bruchidae). Evolution, 605-617. https://doi.org/10.1111/j.1558-5646.1981.tb04923.x