Final report for GS23-274
Project Information
Diamondback moth (DBM), Plutella xylostella (L.), is a ubiquitous pest of Brassicaceae crops, causing a global economic burden of $4-5 billion annually. Although natural enemies can suppress DBM population through high parasitism rates (>80%), growers' reliance on insecticides reduces beneficial insects and drives DBM insecticide resistance. Therefore, a sustainable DBM management approach is needed. This project investigates the impact of sweet alyssum flowers (which attract and enhance the efficiency of natural enemies) in suppressing DBM populations. We conducted trials on small and commercial brassica farms in the southern United States to provide equal benefits to both small- and large-acreage growers. Our results suggested that alyssum could be beneficial in improving biological control in small plots, but selective insecticides (such as Bt) could be necessary if a secondary pest (such as CSCW) is present in the agricultural surroundings. Alyssum could enhance DBM biological control in commercial farms, consequently increasing marketable brassica production. A high parasitism rate (64-78%) results highlighted the crucial role of parasitoid wasps in suppressing DBM in the southern US (South Carolina). To maximize the potential and survival of natural enemies, it is vital to conserve predators and parasitoid wasps during high parasitism periods by minimizing insecticide application and providing them resources (food, shelter) such as refuge or insectary plants (such as alyssum). 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.
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. This sub-objective tested the hypothesis that integrating interspersed sweet alyssum flowers will enhance DBM parasitism rates, decrease DBM densities, and increase marketable yields. We also expected repeated broad-spectrum insecticide applications would eliminate natural enemies and increase DBM pest pressure (for small-acreage growers).
1b. This sub-objective tested 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 (for commercial brassica growers).
2. This objective will test the hypothesis that the parasitism rate would be higher (>80%) during late-spring and late-fall, with Diadegma insulare being the most abundant species, and that parasitism will be directly proportional to DBM density.
Research
Objective 1a: This sub-objective tested the hypothesis that integrating interspersed sweet alyssum flowers will enhance DBM parasitism rates, decrease DBM densities, and increase marketable yields. We also expected repeated broad-spectrum insecticide applications would eliminate natural enemies and increase DBM pest pressure.
Small plot experiments were conducted over a two year period at Clemson’s Coastal Research and Education Center (CREC). Collards were planted on two raised beds (each 20 ft long). All plots had 40 collard plants. The four treatment groups with four replicates arranged in a randomized complete block design (RCBD) were: 1) Untreated control; collards with no flowers or insecticides applied, 2) Alyssum flowers only; collards with interspersed alyssum flowers and no insecticides, 3) Alyssum+Bt; collards with interspersed alyssum and foliar sprays of selective insecticide Bacillus thuringiensis (Bt) subsp. aizawai (Xentari®, Valent, Libertyville, Illinois, 4) Bifenthrin (Sniper®, Loveland Products, Loveland, Colorado); collards with no alyssum flowers and treated with repeated broad-spectrum pyrethroid insecticide sprays. Alyssum treatment plots included 20 interspersed flowering plants (Figure 1). Treatments three and four were sprayed with Bacillus thuringiensis (Bt) and pyrethroid, respectively, biweekly and weekly. A weekly collection of DBM larvae (3rd-4th instar), DBM pupae, and DBM parasitoids was done 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 and natural enemies. 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. In year 2, there was a notable increase in abundance and foliar feeding caused by cross-striped cabbageworm (Lepidoptera; Crambidae: Evergestis rimosalis, “CSCW”), another specialist herbivore of Brassica crops. To account for the effect of CSCW larvae on marketable yield and possible treatment effects on their abundance, we sampled for CSCW larvae on 10 whole plants (similar to the DBM larval sampling protocol). A damage assessment for CSCW and DBM feeding was conducted twice. The damage assessment was done based on counting the number of plants exhibiting more than 70% foliar feeding due to combined CSCW and DBM larval feeding. Plants were considered more than 70% damaged if they had multiple holes and rarely had more than three intact harvestable leaves.
Statistical analysis: DBM pressure and CSCW pressure were analyzed using one-way analysis of variance (ANOVA-PROC MIXED). DBM parasitism rates were compared among all treatments separately each year using generalized linear mixed models with a binomial distribution (PROC GLIMMIX). The marketable yield based on DBM and CSCW larval feeding damage was analyzed separately each year using one-way analysis of variance (PROC MIXED). The number of damaged plants due to combined DBM and severe CSCW larval feeding was analyzed using a one-way analysis of variance (PROC MIXED) to evaluate differences in DBM and CSCW feeding damage among treatments. In year 2, differences in the abundance of Araneae, Carabidae and Dermaptera collected in pitfall traps were analyzed using repeated measures analysis of variance (PROC MIXED). Post-hoc mean separations were performed using Tukey's Honestly Significant Difference (HSD) test when significant differences were detected at a significance level of α < 0.05 (Tukey 1953).
Objective 1b: This sub-objective tested 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 were conducted in collaboration with commercial Brassica growers who were planting large strips of sweet alyssum along the margins of 15–20-acre Brassica fields. Field trials were conducted in the Fall and Spring of 2023 and 2024 and Spring of 2025. Each experimental field included 4-5 rows of alyssum flowers ~300m long, bifurcating two large fields of conventionally managed collards. Experiments were replicated across four separate fields. Approximately 25 DBM larvae (2nd instars) were artificially deployed onto the caged and uncaged sentinel plants (Figure 2). The predator exclusion cages were designed to exclude predator entry into the cages while allowing the entry of parasitoid wasps, so we can use recovered insects from caged plants for parasitism evaluation. These collard plants with sentinel larvae were deployed in fields at 5, 50, 150, and 400 feet away from alyssum (treatment field) or from the edge of the field (control field) to evaluate DBM parasitism and predation rates at different distances. After one week, sentinel plants were collected in labeled plastic buckets and returned to the lab. Recovered DBM (larvae+pupae) were counted and reared until emergence of adult insects (parasitoids or adult DBM). Pitfall traps were also deployed in 2024 and 2025 to assess the predator abundance at different distances in treatment and control fields. 300 collard plants (100 plants sampled at 5, 150 and 450 feet distances) per field were sampled and categorized as marketable and unmarketable (based on feeding damage) to see whether alyssum enhances marketable brassica production.
Statistical analysis: PROC regression was used to assess the role of alyssum on DBM parasitism, DBM recovery and predator abundance (insects recovered in pitfall traps) at variable distances in the alyssum and control fields. PROC TTEST was used to compare DBM parasitism between the control and treatment fields. The differences in marketable yield were analyzed through contingency analysis using the chi-square test (SAS 94).
Objective 2: This objective will test the hypothesis that the parasitism rate would be higher (>80%) during late-spring and late-fall, with Diadegma insulare being the most abundant species, and that parasitism will be directly proportional to DBM density.
The eight staggered plantings of collards were maintained throughout the two-year period (March 2023 to April 2025) to continuously assess the 1) seasonal parasitism of DBM and abundance of parasitoid wasp species, 2) evaluate DBM pressure, and 3) understand DBM parasitism and density relation. The CREC fields were utilized to plant around 850 collard plants (‘Champion’) at each planting date (Figure 3). A biweekly collection of DBM larvae (3rd and 4th instar), DBM pupae, and parasitoid pupae was done to assess parasitism rate. The collected DBM were reared in the lab, and parasitism rates were calculated based on the emergence of adult DBM or parasitoids from collected DBM. A biweekly counting of DBM larvae and pupae was done on 50-70 randomly selected plants to see if the DBM parasitism rate is dependent on DBM pressure. A higher parasitism rate supports our project by planting alyssum or untreated refuge flowers in the fields, further enhancing DBM biological control. Lower biological control would inform guidelines to utilize alternate management strategies, as well as working on increasing parasitoid wasp abundance and efficiency by maintaining an untreated refuge.
Statistical analysis: A two-way analysis of variance (PROC GLIMMIX) was used to assess parasitism differences among years and months. PROC regression (PROC Reg) was used to see if the DBM parasitism rate depends on the DBM pressure. All statistical analysis was done in SAS 94.
The results of the diamondback moth (DBM) biocontrol project were measured through various field trials that involved field development, data collection, data processing and statistical analysis. Here are the key achievements from two years of the project:
Objective 1a: A “small-plot trial” was conducted over two years to evaluate the impact of sweet alyssum flowers in small Brassica fields on increasing DBM parasitism and predation rates, consequently decreasing DBM pressure and increasing marketable Brassica yield.
- DBM pressure: In both years, all treatments suppressed DBM except bifenthrin (a pyrethroid). Bifenthrin flared DBM numbers, likely due to pyrethroid resistance development or the elimination of natural enemies.
- Marketable yield: Bt-treated plots provided a higher marketable yield by suppressing both DBM and CSCW, while bifenthrin suppressed only CSCW, resulting in a reduced marketable yield compared to Bt. The alyssum-only treatment resulted in the lowest marketable yield, probably due to possible attraction of CSCW towards alyssum flowers (for nectar or oviposition site).
- Biological control (parasitism rate and predator abundance): Parasitism rates were higher (66-87%) but similar across all treatments likely due to, 1) the free movement of insects between small plots and/or 2) influx of natural enemies from the nearby untreated refuge plants, which might have maintained a similarly high parasitism rate in all treatments (experimental fields were surrounded by untreated refuge plants). On average, the parasitism rates were 66% in 2023 and 87% in 2024. The highest parasitism rate by species was provided by insulare (70% in 2023, 66% in 2024). Bifenthrin did not reduce parasitism in our study, suggesting that untreated refuge could act as a buffer against the harmful impacts of broad-spectrum insecticides and should be conserved.
- Key takeaway: Alyssum could be beneficial in improving biological control, but selective insecticides (such as Bt) could be necessary if a secondary pest (such as CSCW) is present in the agricultural surroundings. This study underlines the importance of conserving natural enemies in the southern US by reducing the use of broad-spectrum insecticides and by maintaining untreated refuge habitats.
Objective 1b: The “commercial farm” experiments were conducted over 2.5 years in 2023, 2024, and 2025. The objective was to evaluate whether incorporating sweet alyssum flowers increases the DBM parasitism rate and predator abundance, and whether parasitism rate and predator abundance decrease at increasing distances from alyssum strips. The final statistical analysis is in process; however, some key results are mentioned below.
- DBM predation: Lower DBM was recovered near alyssum flowers in treatment (alyssum) fields, which increased away from alyssum, as hypothesized. DBM recovery was greater from caged plants as compared to uncaged plants, likely due to predator exclusion from caged plants. Therefore, reduced DBM recovery near alyssum and a higher DBM recovery from caged plants suggest greater predator activity around alyssum, resulting in increased DBM predation.
- Parasitism comparison at distances: Parasitism did not consistently decrease with distance from alyssum, likely because parasitoids such as Diadegma insulare and Conura spp. can disperse long distances. For example, in a 1000-foot-long field, we recovered Diadegma insulare and Conura spp. in the center of fields, 400 feet away from alyssum flowers or the field edge.
- Parasitism comparison among treatments: Due to similar parasitism rates across distances, we compared parasitism rates between treatment (alyssum) and control fields. The parasitism rate in uncaged plants showed no difference, but caged plants had significantly higher parasitism in alyssum fields (78%) compared to controls (50%). DBM recovery was greater in caged plants, which might have impacted the parasitism rate. In another study conducted at the same location in 1998, the maximum parasitism rate reported was 0-6%, which increased to 78% in our study (Khan 1998). This increase in parasitoid activity may be due to a combination of changes in insecticide use (more selective and effective insecticides became available) and concomitant changes in practices by the grower. Since 1998, growers have adopted a scouting program to reduce overall insecticide use, as well as planting sweet alyssum to enhance the population of natural enemies.
- Marketable yield: The alyssum field had no unmarketable plants and a higher number of undamaged plants (without any holes) as compared to the control field. This suggests less feeding damage in alyssum fields.
- Key takeaway: Alyssum could possibly help in enhancing DBM biological control in commercial farms, consequently increasing marketable brassica production. This will reduce the use of broad-spectrum pesticides, thereby reducing the harmful impact of insecticides on natural enemies and increasing farmers’ profitability.
Objective 2: The “Seasonal parasitism” trial was focused on evaluating the seasonal occurrence of parasitoid species and their contribution to DBM parasitism and whether DBM parasitism is dependent on DBM pressure.
- DBM parasitism: DBM pressure was higher during the late spring, early summer and late fall period. The overall parasitism rate was 64-78%. A higher parasitism rate of 87% was recorded during late spring and early summer periods (April, May, June and July). A comparatively lower parasitism of 35% or less was recorded during the early spring and fall period (January, March, September, October and November), likely due to low temperature, reduced DBM pressure or lag in parasitoid population buildup. Therefore, maintaining refuge could help develop and maintain parasitoid populations during periods of low parasitoid activity.
- Parasitoid wasp species abundance: Diadegma insulare was the most abundant parasitoid wasp species, providing 83% DBM parasitism (larvae+pupae). Four larval parasitoid wasp species (Diadegma insulare, Microplitis, Cotesia spp. and Oomyzus spp.) and six pupal parasitoid wasp species (Diadegma insulare, Oomyzus spp., Ceraphron spp., Pteromalus spp., Conura spp. and Microgastrinae-subfamily) were identified.
- DBM parasitism and DBM density relation: DBM parasitism was positively associated with DBM density in our study. This suggests a higher attraction of parasitoid wasps towards fields when host (DBM) density was high, probably due to increased plant volatile compound emission during DBM larval feeding.
- Key takeaway: A high parasitism rate (64-78%) suggests the importance of parasitoid wasps in suppressing DBM in the southern US (South Carolina). To maximize their impact, growers should conserve parasitoid wasps during high parasitism periods by minimizing insecticide application and providing them resources (food, shelter) such as refuge or insectary plants (such as alyssum). These strategies could help increase the DBM parasitism rate, especially during the months when the parasitism rate was low.
Conclusion: The DBM biocontrol project is based on the “total systems approach”, which considers the well-being of the agricultural system, environmental sustainability and the farming community. The conventional management system typically focuses on a broad-spectrum insecticide-based approach, which is unsuitable for sustaining natural enemies and is no longer effective against DBM management due to the continuous development of insecticide resistance. Therefore, habitat management through the use of insectary plants and maintaining an untreated refuge may promote DBM biological control naturally. This would increase farmers’ profitability, crop yields, and land-use efficiency, ultimately enhancing agricultural sustainability throughout the southern US.
Educational & Outreach Activities
Participation summary:
Manuscripts:
- A manuscript submitted (on 8/1/25) to the Journal of Economic Entomology after completing the small plot experiment (objective 1a). “The role of sweet alyssum flowers (Lobularia maritima) and selective insecticide use on biological control of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)”
- The statistical analyses and manuscript preparation for the seasonal parasitism trial (objective 2) are in progress.
Past Presentations:
- Ghani A, Bilbo TR. October 2025. The South Carolina Entomological Society meeting. Clemson, SC. “The role of sweet alyssum flowers (Lobularia maritima) in enhancing the biological control of diamondback moth (Plutella xylostella) in commercial Brassica farms?”
- Ghani A, Bilbo TR. April 2025. South Carolina Entomological Society (SCES) and Georgia Entomological Society joint meeting. Young Harris, Georgia. “Seasonality of diamondback moth parasitoid wasps and their impact on sustainable Brassica production in the Southeastern United States”
- Ghani A, Bilbo TR. March 2025. The Entomological Society of America Southeastern Branch Meeting (ESA-SEB). Baton Rouge, Louisiana. “Do sweet alyssum flowers (Lobularia maritima) enhance the biological control of diamondback moth (Plutella xylostella) in commercial Brassica farms?”
- Ghani A, Bilbo TR. Nov 2024. The Entomological Society of America (ESA) Annual meeting, Phoenix, Arizona. “Optimizing biological control of the diamondback moth (Plutella xylostella): Evaluating the seasonality of its parasitoids and their impact in the Southeastern United States”
- Bilbo TR, A Ghani, T Sydnor, J Walgenbach, T Kuhar. International Congress of Entomology 2024, Kyoto, Japan. Poster. “Contending with a perennial pest: Developing biologically-based strategies for the diamondback moth (Plutella xylostella) in the United States.”
- Ghani A, Bilbo TR. November 2023. ESA annual meeting. National Harbor, Maryland. “Enhancing the biological control of diamondback moth (Plutella xylostella) through habitat management for sustainable Brassica production in the Southeastern United States”
- Ghani A, Bilbo TR. October 2023. SCES annual meeting. Columbia, South Carolina. “Biological control of diamondback moth (Plutella xylostella) through habitat management for sustainable Brassica production”
Upcoming Presentations:
- Ghani A, Bilbo TR. November 2025. The Entomological Society of America Southeastern Branch Meeting (ESA-SEB). Portland, Oregon. “Do sweet alyssum flowers (Lobularia maritima) improve diamondback moth (Plutella xylostella) biological control?”
Field Day Talks:
- Ghani A, Bilbo TR. June 2024. CREC. “The importance of parasitoid wasps in suppressing diamondback population in South Carolina”
- Ghani A, Bilbo TR. May 2024. Coastal Research and Education Center (CREC), Charleston, SC. Association of Retired Clemson Extension Employees (ARCE). “The role of natural enemies in suppressing diamondback moth population”
- Ghani A, Bilbo TR. December 2023. CREC. Brassica field day. “The importance of insectary plants in enhancing natural enemies’ predation and parasitism efficiency”
- Ghani A, Bilbo TR. June 2023. CREC. “The impact of natural enemies and insecticides in suppressing the diamondback moth population in Brassica”
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 from 2023 to 2025.
Information regarding the role of alyssum flowers in promoting DBM parasitism and predation was shared with farmers and extension agents during Brassica field days and at conferences. Planting interspersed Sweet alyssum flowers (objective 1a) and applying Bt insecticide could benefit small brassica growers by decreasing pest pressure due to increased DBM parasitism and predation rate, thus decreasing plant injury and increasing crop yield. The possible attraction of secondary pest (CSCW) towards alyssum flowers was managed using Bt insecticide. Additionally, our study suggested using selective insecticide (Bt) and avoiding broad-spectrum insecticide (bifenthrin) as it flares up DBM numbers, reduces marketable yield and is not suitable for natural enemy survival. Therefore, the combination of untreated refuge, alyssum flowers and Bt spray could contribute towards sustainable Brassica production by reducing the use of broad-spectrum insecticides, supporting natural enemy survival, and increasing farmers’ profitability. The commercial farm experiments (objective 1b) also suggested that alyssum can improve predator abundance, increase parasitism rate and increase the marketable brassica crop production.
Additionally, the seasonal DBM biocontrol potential (objective 2) informs scientists and growers about high parasitism in the southern US and when it is most effective to rely on biological control for DBM management. Conserving parasitoids by planting alyssum or an untreated refuge will improve parasitoid survival and parasitism efficiency, increasing the parasitism rate. The increased parasitism will ultimately reduce the use of broad-spectrum insecticides, thereby supporting environmental and economic sustainability.
This project yields long-term benefits across economic, environmental, and social fronts.
- 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.
In the past two years, 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. 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 Southern 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.