Entomopathogenic Bombs – Sweet Potato Weevils Be Gone

Progress report for SW22-936

SW22-936 (project overview)
Project Type: Research and Education
Funds awarded in 2022: $336,848.00
Projected End Date: 05/31/2025
Host Institution Award ID: G118-23-W9209
Grant Recipient: University of Hawaii
Region: Western
State: Hawaii
Principal Investigator:
Dr. Brent Sipes
Email
University of Hawaii
Co-Investigators:
Roshan Manandhar
Email
University of Hawai'i
Dr. Koon-Hui Wang
Email
University of Hawaii
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Project Information

Summary:

Sweet potato is economically and culturally important in Hawaii. Sweet potato was the highest revenue-generating vegetable crop in Hawaii until challenged by multiple pests, in particular the sweet potato weevil (Cylas formicarius elegantulus). While weevil feeding may cause vine damage, the more serious problem is to the tubers. Larvae tunnel resulting in spongy tuber that is dark in color. Additionally, larval feeding causes tubers to develop a bitter taste and a terpene odor. Losses to the weevil range from 30 to 97% across farms. Producers in Hawaii have turned to pesticide-intensive management tactics spanning the time from planting to harvest. Growers sometimes rely upon weekly insecticide applications to ensure a marketable crop. Non-chemical based integrated pest management strategies are needed and entomopathogenic nematodes (EPN) are a promising candidate. The weevils are susceptible to Heterorhabditis sp. and Steinernema sp. and have met with some success in other sweet potato growing areas. We propose to use EPN living bombs to control the sweet potato weevil. Living bombs are the application of EPN infected insect cadavers rather than direct aqueous application of the EPN. One objective will be to demonstrate the efficacy of H. indica and S. feltiae in reducing weevil damage in sweet potato in Hawaii. Traditional EPN application methods utilize inundative release of billions of EPN/ha in an aqueous solution. Delivery of the EPN in cadavers will allow for lower numbers and a consequently less expensive tool for growers. Another objective will be to compare the efficacy of different delivery methods of H. indica and S. feltiae. All EPN applications (sprays or bombs) will be compared to the standard practices of cooperating sweet potato producers. Since the focus is to transfer pipeline technologies to growers, our final objective is to convey the information to growers and other practitioners. We will develop videos, extension publications, newsletter articles, and scientific publications to share information. Initially, we will serve as a source for the EPN via a fee-for-service while the nascent demand for EPN builds and before commercial operations enter the Hawaii market. The potential impact of the project is the adoption of environmentally safe and sound biological control for sweet potato weevil by the key sweet potato producers that will then model and influence adoption by fellow sweet potato growers. The co-PIs have statewide connections to share project outcomes through social media, e-mail lists, and newsletters. The project team will work closely with leading growers and extension agents to foster adoption. Our longterm goal is to promote sustainable sweet potato cultivation through economic and environmentally sound pest management.

Project Objectives:

We propose to use entomopathogenic nematodes (EPN) living bombs to control the sweet potato weevil. We have two research objectives and three education objectives. Our research objective 1 will be to demonstrate the efficacy of Heterodera indica and Stienernema feltiae, different species of EPN, in reducing weevil damage in sweet potato. Research objective 2 will be to compare the efficacy of spray applications to living bomb delivery of H. indica and S. feltiae documenting that fewer EPN are needed for effective weevil management when delivered as living bombs. Our educational objectives are (1) to impart knowledge about EPN for IPM control of sweet potato weevil, (2) to illustrate the application of EPN living bombs, and (3) to demonstrate the utility of EPN living bombs as an additional IPM tool for control of sweet potato weevils.  

Timeline:

 

Year1

Year 2

Year 3

 

Q1

Q2

Q3

Q4

Q1

Q2

Q3

Q4

Q1

Q2

Q3

Q4

Research Objectives

 

EPN culture

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X

X

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X

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X

 

 

1. Efficacy

 

 

 

 

 

 

 

 

 

Field demo 1

 

X

X

 

 

 

 

 

 

 

 

 

Field demo 2

 

 

 

 

X

X

 

 

 

 

 

 

Analysis

 

 

 

X

X

X

X

X

 

 

 

 

2. Application  Comparison

 

 

 

 

 

 

 

 

 

 

Field demo 1

 

 

 

X

X

 

 

 

 

 

 

 

Field demo 2

 

 

 

 

 

 

 

X

X

 

 

 

Field demo 3

 

 

 

X

X

 

 

 

 

 

 

 

Field demo 4

 

 

 

 

 

 

 

 

X

X

 

 

Analysis

 

 

 

 

 

X

X

X

X

X

X

X

Educational Objectives

 

1. Impart knowledge field days

 

 

 

X

 

 

X

 

 

X

 

 

2. Illustrate living bombs workshops

 

 

 

X

 

 

X

 

 

X

 

 

3. Assess utility

 

 

 

 

 

 

 

 

 

 

Videos

 

 

X

X

X

X

X

X

X

X

 

 

Newsletters

 

X

 

X

 

X

 

X

 

X

 

X

Publications

 

 

 

 

 

 

 

 

 

X

X

X

Reporting

 

Drafting and Writing

 

 

 

X

 

 

 

X

 

 

 

X

The boxes show the initiation and the completion of the different activities.  Field days and workshops boxes represent the quarter in which the one-day events will be held. For newsletters, the boxes represent publication quarter.

Cooperators

Click linked name(s) to expand/collapse or show everyone's info
  • Derek Agader - Producer
  • Tom Huynh - Producer
  • Wally Johnson - Producer

Research

Materials and methods:

Locally isolated EPN H. indica and S. feltiae will be reared on waxworms (Galleria mellonella) or mealworms (Tenebrio molitor) in the laboratory. Mealworm larvae will be exposed to EPN in a filter paper-lined petri dish and cadavers transferred to White traps to collect emerging infective juveniles (IJ). IJ will be held at 15℃ until application. Infected larvae will be desiccated in a saturated KCl solution and the desiccated cadavers applied as living EPN bombs as a treatment to compare against standard water-based application of IJs..

Sweet potato is cultivated continuously in Hawaii, thus allowing demonstrations to be established throughout the year. Growers generally produce their own planting material from cuttings. Fields are usually planted in several rows at a time to allow weekly harvests. Test will be conducted on cooperator farms utilizing the standard practices of that farm.

Demonstration of the efficacy of H. indica and S. feltiae (Objective 1): Demonstrations will be established in each producers’ field. The demonstrations will consist of a no-weevil treatment, standard producer practice (this varies among the 4 producer cooperators), a postplant application of H. indica, and a postplant application of S. feltiae. Plots size and layout will vary and be tailored to each cooperators operation. EPN will be applied 2 weeks after planting at 4 billion IJ/ha as a soil spray. The IJ will be irrigated into the soil. Pit-fall traps will be placed in each plot and monitored weekly to record the weevil population. At harvest, sweet potato yield will be recorded. Weevil damage to the rhizomes will be evaluated and a random 1 kg tuber sample will be assayed for weevil population density and life stages. As  general target, plots will be 2 beds wide by 5 m long. Treatments will be randomized with 4 replications on each farm. Data will be analyzed for variance and differences among treatments. Video will be taken to contrast sweet potato yield and weevil damaged among the treatments. The demonstration will be repeated in a second growing season with each cooperator. A field day will be held to highlight difference and educate other growers of the potential for EPN as a management tool.

Comparison of application methods (Objective 2): Similar to Objective 1, demonstrations will be established in each producers’ field consisting of a no weevil treatment, the standard producer practice, a postplant spray application of H. indica, a living bomb application of H. indica, a postplant application of S. feltiae, and a living bomb application of S. feltiae. EPN will be applied 2 weeks after planting at 5 billion IJ/ha as a soil spray. One-week-old mealworm cadaver bombs will be distributed at 3 bombs/m at the same time as the spray application. The IJ will be irrigated into the soil. At harvest, sweet potato yield will be recorded. Weevil damage to the rhizomes will be evaluated and a random 1 kg tuber sample will be assayed for weevil population density and life stages. The demonstration plots will be 2 beds wide by 5 m long. The design will be a split-plot with EPN species as the main plot and delivery method (spray or living bomb) as the subplot. Main plots will be randomized with 4 replications on each farm. Data will be analyzed for variance and differences among treatments. As a second aspect of this objective, demonstrations will be established in each producers’ field consisting of a no weevil treatment, the standard producer practice, surface living bomb applications of H. indica and S. feltiae individually, and buried living bomb application of each EPN.  S. feltiae, a buried living bomd EPN living bombs will be applied at the time of planting. The surface application will be made immediately after planting at 3 bombs/m for both methods. At harvest, sweet potato yield will be recorded. Weevil damage to the rhizomes will be evaluated and a random 1 kg tuber sample will be assayed for weevil population density and life stages. The demonstration plots will be 2 beds wide by 5 m long. Treatments will be randomized with 4 replications on each farm. Data will be analyzed for variance and differences among treatments.

Outreach: Video will be taken of the spray application and bomb application to educate growers in the appropriate techniques to apply EPN for sweet potato management. The demonstration will be repeated in a second growing season. A field day will be scheduled to highlight efficacy and differences among treatments and to educate other growers of EPN as a management tool. 

Research results and discussion:

The field tests have been slow to start as cooperating growers delayed some plantings for various reasons. One field site was planted in January 2023 with a tentative harvest date in early summer 2023. EPN application in the January tests began in May. Another test was planted in late April 2023 with treatment scheduled for July 2023.

While the field tests were in preparation and plants growing, a series of laboratory experiments were undertaken. EPNs are usually applied as foliar sprays or via irrigation leaving the EPNs exposed to UV radiation and desiccation and thus reduced efficacy. Living bombs protects the EPNs from UV radiation and desiccation but need gentle handling to avoid breakage. Formulating an EPN carrier into a dry cadaver would be beneficial. Galleria mellonella cadavers infected by Heterorhabditis indica were desiccated by incubation in chambers filled with a saturated solution of KCL, providing 85% relative humidity, for 21 days. Cadavers averaged over a 40% loss in weight after desiccation. Desiccated cadavers were leather-like and easier to handle without fear of rupture as compared to the non-desiccated cadavers. The desiccated cadavers produced IJs equal in number to that of non-desiccated cadavers with both averaging over 200,000 IJs/cadaver. The IJ emerging from desiccated and non-desiccated cadavers infected and killed 100% of larvae of G. mellonella that were exposed. EPN infectivity and effectiveness were not impacted by desiccating the host cadavers. Desiccated cadavers will be incorporated as an EPN treatment in future field demonstrations.

Participation Summary
2 Producers participating in research

Research Outcomes

No research outcomes

Education and Outreach

3 Consultations
2 On-farm demonstrations
1 Tours

Participation Summary:

15 Farmers participated
5 Ag professionals participated
Education and outreach methods and analyses:

An early field day was held in November 2022 in conjunction with an organic sweet potato field day. Attendees were introduced to the concept of EPN as a management tool for sweet potato weevils. We are planning summer field days where interested growers can visit and view the treated plots. At the conclusion of the presentation we informally surveyed attendees for their interest and possible use of EPN for insect management.

Education and outreach results:

Our informal survey of the November field day attendees highlighted an interest in the use of EPN for insect management. Several attendees expressed an interest in collaborating in demonstrations. 

7 Farmers intend/plan to change their practice(s)
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Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.