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​​​Bees, Beetles, and Bacteria: the Cucurbit Bacterial Wilt Dilemma

Erika Saalau Rojas, Laura Jesse, and Mark L. Gleason

Department of Plant Pathology and Microbiology
Iowa State University
mgleason@iastate.edu

Rojas, E. S., Jesse, L., and Gleason, M. 2014. Bees, Beetles, and Bacteria: the Cucurbit Bacterial Wilt Dilemma. The Plant Health Instructor. 10.1094/PHI-T-2014-0312-01

INTRODUCTION

This case study introduces students to a complex disease management situation in which a muskmelon farmer, Henry DeSoto, is facing pressure to change his management practices against cucurbit bacterial wilt. Cucurbit bacterial wilt is transmitted by striped cucumber beetles (Acalymma vittatum), and Mr. DeSoto relies on weekly or biweekly insecticide applications to control beetles. Beekeepers near Mr. DeSoto’s farm are concerned that the large amount of insecticides that Mr. DeSoto is using on his cucurbit crops may be endangering the health of their honey bees. Unless Mr. DeSoto convinces the beekeepers that he has adopted alternative strategies that significantly reduce the use of insecticides in his farm, he may find himself in a lawsuit.

The cucurbit bacterial wilt case study will challenge students to weigh several integrated pest management (IPM)-based strategies and provide potential solutions to Mr. DeSoto’s problem. In this process, students will recognize some of the advantages and limitations of implementing IPM approaches on a commercial vegetable farm.

OBJECTIVES

The goal of this case study is to challenge students to solve a disease management situation in the context of real world decision-making. Students will recognize that IPM is a holistic approach that needs to consider the cropping system, pathogen biology, economic factors, and surrounding landscape. In addition, this case study illustrates the role of an insect vector in transmission and dispersal of a plant pathogen.

After completing this case study, the student will:

  • Recognize cucurbit bacterial wilt signs, symptoms, and vector.
  • Understand the bacterial wilt disease cycle and the role of the insect vector
  • Identify practical limitations of managing an insect-transmitted disease in a bee-pollinated crop.
  • Evaluate advantages and limitations of IPM-based approaches.
  • Increase awareness of Colony Collapse Disorder (CCD) of honey bees.

CAST OF CHARACTERS

Henry DeSoto: Cucurbit farmer who specializes in muskmelon production.
Deborah Kopper: Beekeeper and chairwoman of the local beekeeping association. She warns Henry that beekeepers blame the poor health of their bee hives on his heavy use of insecticides to control cucurbit bacterial wilt.

THE CASE

Part A
Part B

CLASSROOM MANAGEMENT

Case Summary

This case study is designed to challenge college-level undergraduate students to develop a management plan involving an insect-transmitted disease, cucurbit bacterial wilt. In addition to recognizing the vector, signs, and symptoms of this bacterial disease, students will also identify practical limitations involved in managing an insect-transmitted disease in a bee-pollinated crop. When Henry DeSoto, a local muskmelon grower, is accused of excessive insecticide use by beekeepers in the area, he fears that his reputation and economic viability will be damaged by the complaint. A potential solution is to develop an IPM-based approach to reduce insecticide applications in his farm. Before choosing an alternative management strategy, however, students will have to formulate solutions to overcome some of the practical limitations for each strategy and evaluate the feasibility of implementing an IPM approach.

Suggestions On How To Use This Case

This case can be completed in either a 50-minute class period or a laboratory session of at least 100 minutes. In the former, Part A should be read by students before the class period. Background information about cucurbit bacterial wilt and colony collapse disorder can be presented in a previous class or assigned as a reading beforehand. In a longer class session, background information about cucurbit bacterial wilt and colony collapse disorder may be presented in class and followed by reading Part A of the case study. Before proceeding to Part B of the case study, students should discuss the introductory questions and be familiar with the cucurbit bacterial wilt disease cycle and management practices.

During the first part of the lesson, the instructor can lead the discussion while answering the introductory questions. Students may share their opinion on whether or not Henry should fight the complaint made by beekeepers. At this point we suggest explaining what is known about colony collapse disorder, discussing bee protection rules and the impact of agricultural practices on honeybees and pollinators. Students should discuss the remaining introductory questions and the instructor can finish with a review of the cucurbit bacterial wilt disease cycle and management strategies.

After reading and discussing Part A, divide students into groups of 3 to 4 members. Each student should read and discuss Part B of the case study. The instructor should allow at least 30 minutes for groups to answer questions in Part B and write up a potential management plan. Each group should present their plan on the blackboard for discussion and comparison of management plans among groups. The instructor and classmates should ask each group how they will overcome the limitations associated with each of the strategies. Additionally, each group should explain which factors influenced their decision-making process. In the case of row covers, cost of materials, labor, and timing of removal considerations should be discussed. Students presenting delaying harvest as a management strategy should explain how Mr. DeSoto will overcome the economic tradeoff of fruit harvest later in the season, potentially receiving a lower market value. For perimeter trap cropping, factors such as marketing of a new crop, effectiveness of the strategy, and practical limitations of handling two different crops should be discussed.

Possible Adaptations

The case study “Bees, beetles, and bacteria: the cucurbit bacterial wilt dilemma” presents a storyline that can be adapted to fit into the program needs of students interested in plant pathology, horticulture, sustainable agriculture, entomology, integrated pest management, and plant health management. The case focuses on a complex disease management situation in which potential integrated pest management solutions are emphasized; however, other factors such as economic and social considerations should be examined by students before making a decision. A small amount of comparative cost data is presented in order to help students discuss economic tradeoffs. One or several of these aspects can be highlighted to engage students in discussions beyond basic plant pathology and management concepts.

In our experience in trying out the case, we found that many undergraduate students were particularly interested in learning more about pollinators and colony collapse disorder. This topic can be easily emphasized by assigning more readings about colony collapse disorder and the environmental effects of pesticides. Discussion could be enhanced by dividing a class into two groups: one representing Henry DeSoto, the muskmelon grower, and the other representing Deborah Kopper and the beekeepers. The groups could debate about the effects of pesticides on non-target insects, beekeeping management practices, and agricultural practices that could lead to poor bee colony health. A third group in the classroom could act as evaluators and decide which of the other groups made the more convincing argument to “win” the case.

Given the rapid increase of local agriculture throughout the U.S., this case study can also be adapted to discuss the impact of insect and disease management practices in more densely populated areas. Students could discuss some of the challenges associated with being close to urban centers such as high land prices, on-farm labor shortage, and concerns about the use of pesticides. Students could also discuss some of the benefits such as new marketing opportunities, support of local food systems, and promotion of food security and health awareness.

In a similar context, social perception about pesticide use and insect and disease management strategies could be discussed. Students could investigate whether and how crop management practices and produce marketing strategies have changed due to public awareness of pesticide use and chemical residues.

BACKGROUND INFORMATION

Cucurbit bacterial wilt
Colony collapse disorder
Managing cucurbit bacterial wilt

REFERENCES

Blacquière, T., Smagghe, G., van Gestel, C.A.M., and Mommaerts, V. 2012. Neonicotinoids in bees: a review on concentrations, side-effects and risk assessment. Ecotoxicology. 21: 973-992.

Cavanagh, A.F., Adler, L.S., and Hazzard, R.V. 2010. Buttercup squash provides a marketable alternative to blue hubbard as a trap crop for control of striped cucumber beetles (Coleoptera: Chrysomelidae). Environ. Entomol. 39:1953-60.

Dively, G.P., and Kamel, A. 2012. Insecticide residues in pollen and nectar of a cucurbit crop and their potential exposure to pollinators. J. Agric. Food Chem. 60: 4449-4456.

Latin, R.X. 2000. Bacterial wilt. In: APSnet Features: Scary Diseases Haunt Pumpkins and Other Cucurbits. http://www.apsnet.org/publications/apsnetfeatures/Pages/BacterialWilt.aspx

Saalau Rojas, E., Gleason, M. L., Batzer, J. C., and Duffy, M. 2011. Feasibility of delaying removal of row covers to suppress bacterial wilt of muskmelon (Cucumis melo). Plant Dis. 95:729-734.

Stoner, K.A., and Eitzer, B.D. 2012. Movement of soil-applied imidacloprid and thiamethoxam into nectar and pollen of squash (Cucurbita pepo). PLoS ONE 7: e39114. doi:10.1371/journal.pone.0039114.