Enhancing the Biological Control of the Diamondback Moth (Plutella xylostella) Through Habitat Management for Sustainable Brassica Production

Progress report for GS23-274

Project Type: Graduate Student
Funds awarded in 2023: $12,341.00
Projected End Date: 08/31/2025
Grant Recipient: Clemson university
Region: Southern
State: South Carolina
Graduate Student:
Major Professor:
Dr. Tom Bilbo
Clemson University
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Project Information


Diamondback moth (DBM), Plutella xylostella, is a ubiquitous pest of Brassicaceae crops that leads to a global economic burden of $ 4-5 billion annually from yield losses and control costs. Over the past few decades, it has gained importance as a key pest of brassica crops due to resistance development against over 98 active insecticide ingredients, including novel diamide insecticides. Several natural enemies of DBM attack its larval and pupal stages and alone can suppress DBM populations through high parasitism rates (>80%). However, grower reliance on insecticides eliminates beneficial natural enemies and drives the development of insecticide resistance. Sustainable biocontrol tactics, such as enhancing conservation biological control (through habitat management), could bring promising results if strategically introduced. This project will investigate the impact of sweet alyssum flowers (which attract and enhance the efficiency of natural enemies) in suppressing DBM populations. We will conduct trials on commercial and small brassica farms in the Southern United States to provide equal benefits to small and large-acreage growers. Preliminary results revealed increased DBM parasitism nearer to alyssum flowers; however, more trials are required to determine how to most effectively incorporate alyssum flowers and determine its effects across spatial scales. Project findings will be shared with stakeholders through broad Extension programs. This project addresses environmental, economic, and health sustainability by developing a total-system approach that relies on naturally occurring resources (natural enemies) that will reduce chemical use, thus increasing profitability, yield, and land use efficiency, ultimately improving the quality of life for farms and communities.

Project Objectives:

The objectives of this project are:

1. Develop a habitat management strategy that uses sweet alyssum flowers to improve biological control of DBM in the Brassica fields of the southern United States.

1a. Conduct small plot experiments to determine how incorporating alyssum flowers influences predation and parasitism of DBM (to benefit small growers).

1b. Conduct large on-farm trials to assess how alyssum flowers influence DBM parasitism and predation at different spatial scales realistic to intensive brassica production systems in order to optimally distribute insectary plants (to benefit large growers)

2. Evaluate seasonal parasitoid species and parasitism rates of DBM, which would serve as a valuable tool for understanding biological control potential and developing DBM management strategies based on the occurrence of DBM parasitoids during different brassica growing seasons throughout the year.


Materials and methods:
Small-plot trail
One of the treatments of the small-plot experiment is attached.

Objective 1a: This sub-objective will test the hypothesis that incorporating interspersed Sweet alyssum flowers will increase the rate of DBM parasitism and predation and that applications of broad-spectrum insecticides will suppress parasitoid wasps and lower the DBM parasitism rate.  

In years 1 and 2 at Clemson’s Coastal Research Station (CREC), small experimental plots of collards will be established on two raised soil beds (each 20 ft long). All plots will include 40 collard plants. In year 1 of experiment, a small plot experiment was conducted with four treatment groups with four replicates arranged in a randomized complete block design (RCBD): 1) Collards with interspersed alyssum, 2) collards with interspersed alyssum and foliar sprays of Bacillus thuringiensis (Bt), 3) Collards treated with broad-spectrum pyrethroid sprays, 4) untreated control group having only collards. Alyssum treatment plots included 20 interspersed flowering plants. Treatments two and three were sprayed with Bacillus thuringiensis (Bt) and pyrethroid biweekly and weekly. A weekly collection of DBM larvae (3rd- 4th instar), DBM pupae, and DBM parasitoids was carried out to evaluate parasitism rates. The number of DBM larvae and pupae was monitored by weekly sampling to assess the influence of each treatment on DBM populations, natural enemies, and marketable yields. Two pitfall traps per plot were deployed to collect and monitor ground predators. Three harvests were conducted to evaluate which treatment provided the maximum marketable yield. The collected data is being cleaned and analyzed. The DBM abundance will be analyzed using ANOVA, and the parasitism rate will be evaluated using logistic regression (SAS 94).

Objective 1b: This sub-objective will test the hypothesis that incorporating Sweet alyssum flowers will increase the rates of DBM parasitism and predation on large commercial farms but that parasitism and predation will decrease at increasing distances from alyssum strips. Large on-farm trials will be conducted in collaboration with a commercial Brassica grower who currently plants large strips of Sweet alyssum along the margins of 15–20-acre Brassica fields.  

Field trials will be conducted in the Fall of year one and in the Spring and summer of year 2. Each experimental field includes 4-5 rows of alyssum flowers ~300m long, bifurcating two large fields of conventionally managed collards or kale. In year 1, experiments were replicated across four separate fields, each 15-20 acres. Approximately 25 DBM larvae (2nd instars) were artificially deployed onto the sentinel plants of collards that were deployed in fields at varying distances to evaluate DBM parasitism and predation rate at different distances from alyssum flowers. Distances include 5 feet, 50 feet, 150 feet, and 400 feet away from alyssum strips. After one week, sentinel plants were collected in labeled plastic buckets and returned to the lab. For the control group, the above-explained procedure was repeated for at least three non-alyssum fields. Additionally, an exclusion cage study (4 replicates) was carried out to evaluate the role of predators in DBM recovery from sentinel plants using plants enclosed in a cage with a coarse mesh screen that excludes only larger predators (e.g., hoverflies, beetles, spiders) and not small parasitoid wasps. Four caged-sentinel plants and four uncaged sentinel plants were used to perform this study. The total number of insects recovered (larvae, pupae, and predators) were counted and maintained on collard leaves in separate petri dishes until adult insects (parasitoids or adult DBM) emerge. The parasitoids were identified based on their specific characteristics. The experimental design is in the process of refining, and data is being cleaned and organized for further analysis. The final abundance of DBM will be analyzed using ANOVA and parasitism rate will be evaluated using logistic regression (SAS 94).

Preliminary data suggested that the alyssum flowers promote the DBM parasitism rate near the alyssum flowers, with the most abundant parasitoid species recovered being Diadegma spp. However, more trials need to be conducted at greater distances (>150 ft) and during different seasons (fall and spring) along with a control experimental unit (non-alyssum fields) and caged plants to understand predation better. This will allow a valuable evaluation of alyssum flowers in enhancing DBM parasitism at different distances to better understand the parasitoid foraging range.

Objective 2:  This objective will test the hypothesis that the key parasitoid species Diadegma insulare, Microplitis plutellae, Cotesia spp., and Oomyzus sokolowskii will vary in their seasonal occurrence, that parasitism will increase later in the season, and that D. insulare will be the most common and important species.

In years 1 and 2, a field will be planted with three to four staggered plantings of collards from February through October to assess the seasonal parasitism of DBM, providing information regarding the occurrence of DBM parasitoid species and their abundance during different seasons. In year 1, the CREC fields were utilized to transplant around 1000 collard plants (‘Champion’) at each planting date to evaluate DBM parasitism. A biweekly sampling of DBM larvae (3rd and fourth instar), DBM pupae, and parasitoid pupae was carried out. Insects were transferred to the lab for DBM adult or parasitoid emergence, providing information about the percent parasitism rate during different seasons. The PROC MIXED model will analyze the final data (SAS 94). A higher biological control would support our project by planting alyssum flowers in the fields, further enhancing DBM parasitism. On the contrary, lower biological control would step up a guideline to work on other sustainable strategies to control DBM abundance instead of relying on alyssum only. Currently, the trial is in process, with additional modifications in the experimental design that include counting DBM (larvae and pupae), which will help to understand the relation between parasitism rate and DBM pressure.

Expected Outcomes:  

Objective 1a and 1b: Strategically incorporating Sweet alyssum flowers in small and commercial Brassica farms will benefit growers by increasing the DBM parasitism and predation rate, thus decreasing plant injury and increasing crop yield. This will reduce the use of broad-spectrum pesticides, thereby reducing the mortality rate of natural enemies and increasing farmers’ profitability.

Objective 2: Seasonal DBM biocontrol potential will provide information regarding the parasitism rate, the abundance of parasitoid species, and their relative contribution to parasitism throughout the year. This information would provide baseline data to scientists and growers about when biological control has the maximum potential to be enhanced by conservation efforts (by planting alyssum) to achieve maximum profitability.

Research results and discussion:

The initial results of the diamondback moth (DBM) biocontrol project were measured through various field trials that involved field samplings, data cleaning and organizing. Here are the preliminary key achievements and measurements from year-one of the project:

A small-plot trial (Obj. 1A) was conducted at coastal REC to evaluate the impact of Sweet alyssum flowers in Brassica fields on increasing DBM parasitism and predation rates, consequently decreasing DBM larval feeding damage and increasing Brassica marketable yield. The abundance of DBM larvae, pupae, and parasitoid pupae was monitored weekly in different treatments to evaluate how each treatment affected the DBM parasitism and predation rate and yield. Initial results suggest that the abundance of DBM was similar across all treatments except the pyrethroid treatment, which indicates a disruption of biological control in that treatment. DBM counts were low in all other treatments, indicating a high baseline level of biological control. Yield and parasitism data is in the process of being analyzed.

The commercial farm trial (Obj. 1B) was conducted at commercial farms of Brassica. Initial data suggests that the alyssum flowers enhance the DBM parasitism rate in their vicinity, with the predominant parasitoid species recovered being Diadegma spp. Currently, the experimental design is being refined to execute additional trials in replicated Brassica farms (with and without alyssum flowers). 

The “Seasonal parasitism” trial (Obj. 2) was focused on evaluating the seasonal occurrence of parasitoid species and their relative contribution to DBM parasitism. So far, three staggered collard plantings have been planted for the biweekly collection of DBM larvae and pupae to evaluate the DBM parasitism rate. Parasitism rates were calculated based on the emergence of adult DBM or parasitoids from collected samples. The initial results suggested that the Diadegma insulare is the most abundant parasitoid wasp species and provides more than 90% DBM parasitism during certain times of the year. So far, approximately seven parasitoid wasp species have been reared from collected samples. Currently, the DBM larvae and pupae are being collected and counted (to evaluate pest population trends and to correlate pest and parasitoid wasps’ occurrence in the agricultural system) from the “fourth planting” of the Collards.

The DBM biocontrol project is based on the “total systems approach” that considers the well-being of the agricultural system, environmental sustainability and the farming community. However, the conventional management system typically focuses on a broad-spectrum insecticide-based approach that is not suitable for natural enemies' sustainability and is no longer effective against DBM management due to continuous insecticide resistance development. Therefore, habitat management through insectary plants would possibly promote DBM parasitism and predation process naturally. This would increase farmers’ profitability, crop yield, and land use efficiency, ultimately increasing agricultural sustainability throughout the southern US.

Participation Summary
1 Farmers participating in research

Educational & Outreach Activities

1 Tours
2 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

1 Farmers participated
Education/outreach description:

Past Presentations:

  • Presented initial results at the Entomological Society of America (ESA) Annual Meeting in National Harbor, Maryland in November 2023.
  • Delivered oral presentation at the South Carolina Entomological Society (SCES) in Columbia, SC in October 2023.

Upcoming Presentations:

  • Will present initial results at the Entomological Society of America (ESA) Annual Meeting in Phoenix, AZ 2024.
  • Planning to share results from objective two at the South Carolina Entomology Society (SCES) in 2024.

Past Field Day Talks:

  • Shared initial results on the biological control of the Diamondback moth at the Brassica field day held at the Coastal Research and Education Center (CREC), Charleston, in December 2023.

Upcoming Field Day Talks:

  • Results from the seasonal parasitism trial will be presented at the Clemson University CREC field day in 2024.

Project Outcomes

Project outcomes:

A graduate student and a part-time summer hire focused on developing DBM biological control strategies (by planting Sweet alyssum flowers) for the small Brassica fields and large commercial farms during the fall of 2023 and spring of 2024.

In Fall 2023, the small-plot (objective 1a) experiment was conducted. The initial results regarding the importance of alyssum flowers in promoting DBM parasitism and predation were shared with farmers and extension agents during Brassica field day. Initial results suggested that planting interspersed Sweet alyssum flowers (in combination with weekly Bt spray) in small Brassica fields would benefit growers by decreasing pest pressure due to increased DBM parasitism and predation rate, thus decreasing plant injury and increasing crop yield. Additionally, the increased DBM abundance was observed in the treatment sprayed with broad-spectrum pesticides (due to the harmful impact of pesticides on natural enemies). Therefore, utilizing alyssum could contribute towards sustainable Brassica production that will reduce the use of broad-spectrum pesticides, thereby reducing the mortality rate of natural enemies and increasing farmers’ profitability. Currently, the collected data from the small plot is being refined and analyzed. The experimental trials for commercial Brassica farms (objective 1b) are in the execution process.

Additionally, seasonal DBM biocontrol potential (objective 2) will provide information regarding the parasitism rate, the abundance of parasitoid species, and their relative contribution to parasitism throughout the year. This information would provide baseline data to scientists and growers about when biological control has the maximum potential to be enhanced by conservation efforts (by planting alyssum) to achieve maximum profitability. Currently, the project is in the execution phase (picture attached). DBM parasitism data has been collected from the past three staggered plantings of collards and is in the process of refining.

This project yields long-term benefits across economic, environmental, and social fronts. 1) Economically, it helps farmers save on pest management costs by reducing reliance on broad-spectrum insecticides. 2) Sustainable pest management practices enhance crop yields, boosting farmers’ profitability. Environmentally, 1) it promotes biological control methods, reducing synthetic pesticide use and chemical residues while improving soil and water quality. 2) Introducing insectary plants conserves beneficial insects and native pollinators, enhancing ecosystem health. 3) Diversifying pest management strategies helps prevent pesticide resistance in target pests. Socially, 1) reduced pesticide exposure improves farmers' and workers' health and safety. 2) The project fosters knowledge exchange, empowering stakeholders to adopt sustainable practices for profitable Brassica crop production.

Knowledge Gained:

The previous year, a graduate student (and Ph.D. supervisor) gained insights into integrating biological control methods into farming practices. It helped them understand optimal ways of incorporating insectary plants in agricultural systems. Methodology was developed and refined in studying DBM parasitism in commercial fields, and this approach can be utilized by other researchers throughout the country. The practical implementation of the project enhanced their appreciation of the complexity of ecological interactions and the importance of biological control agents (specifically parasitoid wasps) in pest suppression. Furthermore, collaborating with farmers and extension agents facilitated the exchange of knowledge and perspectives, enriching their awareness of real-world challenges and solutions to achieve sustainable agriculture. Overall, this project fostered a more profound commitment to promoting environmentally friendly and economically viable agricultural practices, reflecting a positive evolution in their attitudes toward sustainable agriculture.


For future studies, it would be beneficial to conduct long-term monitoring to assess the sustainability and effectiveness of implementing the DBM biocontrol strategy. Collaborative research involving different regions of the Southeastern U.S. would also be beneficial, as this would provide information regarding DBM biocontrol potential across various regions simultaneously, following similar research strategies. This will help scientists and researchers better understand how and when to optimally utilize insectary plants in their regions to benefit their farming communities.

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.