As blackberry season peaks in Arkansas, research is shedding light on a trellising system that could improve pest management for blackberry growers.

A study on insecticides to control spotted-wing drosophila in blackberries showed that the rotating cross-arm trellis exhibited four times fewer spotted-wing drosophila larvae in blackberry fruit than the more commonly used T-trellis. The data also show that the canopy structure created by the rotating cross-arm trellis, also known as RCA, offers more light penetration and airflow, resulting in more heat and less humidity, effectively dissuading spotted-wing drosophila fruit flies from laying eggs in berries. 

Aaron Cato, a researcher for the Arkansas Agricultural Experiment Station and an extension specialist in integrated pest management in the Horticulture Department, discussed these findings in a recent Food, Farms, and Forests episode.

His study, titled "Impact of Trellising on Spray Coverage in Spotted-Wing Drosophila Infestation: Comparing Rotating Cross-Arm Trellis to the T-trellis," explored the benefits of the rotating cross-arm trellis system compared to the traditional T-trellis. This research focused on controlling the spotted-wing drosophila, an invasive fruit fly species that has become a significant pest in U.S. blackberry and blueberry crops since they arrived in 2012.

Earlier observations had shown fewer spotted wing drosophila eggs on blackberries on the RCA trellis. Cato's research is about determining what aspect of the RCA drives this benefit.

"We don't use the RCA for pest control; we use it for fruit quality, and we use it especially for winter injuries or impacts," Cato said. "So, that's really what we were trying to answer and figure out: what aspects of [the RCA] are helping us manage spotted-wing?"

Improved Spray Coverage

One of the study's most significant findings was that the RCA trellis system improves the spray coverage of insecticide sprays.

One of the reasons for this is the RCA's thinner canopy, Cato said. The T-trellis can provide a canopy of 3 to 4 feet, whereas the RCA is around 1 foot in width, allowing for better reach of spray coverage and more air movement through the canopy.

"Regardless of what spray material you use, there were fewer larvae in the RCA. It was, on average, one in every 40 berries versus one in every 10 berries in the T-trellis."

This increased coverage means growers can potentially use less water and still achieve effective pest control, reducing costs and environmental impact.

"When you put out 60 gallons per acre, we got good coverage in both the RCA and the T-trellis," Cato said. "However, on the rotating cross-arm trellis, our data indicated that we got the same coverage on 40 gallons per acre as 60, while we saw a significant decrease in the T-trellis. This gives us an opportunity to increase efficiencies in the RCA and asks whether growers who currently don't reach 60 gallons of water per acre would get better pest protection if they used the RCA."

Microclimate and Pest Management

Research published in a separate paper by Mataya Duncan, Amanda McWhirt and Cato also revealed that the RCA trellis likely creates a less favorable microclimate for spotted-wing drosophila. The thinner canopy allows more sunlight exposure, which increases ambient temperature and decreases humidity. These conditions are less conducive to the survival and reproduction of spotted-wing drosophila, which Cato calls "SWD" for short, because the bugs prefer high humidity and lower temperatures as an environment shelter during the day before laying eggs at dusk and dawn on berries.

These findings highlight the importance of integrating cultural controls with chemical treatments for blackberry producers.

"There's more than just insecticides to control pests like SWD," Cato said. "I hope that growers see that, especially in specialty crops, you have to rely on your cultural controls for a long-term approach to best management. Because spotted-wing drosophila is becoming resistant to insecticides. So, you can't rely just on those things alone, or you're not going to be able to keep them out of folks' berries in the future."

Looking Ahead

Though slightly more labor-intensive than traditional methods, the RCA trellis system has shown promising results in Arkansas and other states. The benefits of higher fruit quality and yields and reduced pest pressure can outweigh the initial costs and labor requirements.

"It's a lot of hands-on work," Cato said, "and if you ask almost any blackberry grower out there what their major hold-up is in production, it's probably labor."
 
However, Cato says these results do not mean the industry can discount the T-trellis just yet. It remains useful, inexpensive and valuable for growers and scientists. Meanwhile, researchers continue to refine their recommendations for using the RCA trellis.

"The reality is the RCA has saved crops in extreme cold and frost events in Arkansas; if you ask any of these growers, it always ends up paying for itself," Cato said. "In the future we hope to provide precise guidelines on spray volumes and other practices to maximize its value for growers."

Recommendations

Every year, researchers make a few recommendations for growers, including picking more often, getting berries into cold storage right away, opening up the canopy, letting more light and heat in, and pruning out as much plant material as possible in the winter.

As spotted-wing drosophila has already been spotted in blackberries this summer, Cato adds another one.

"I recommend you go out there and spray for spotted-wing and try to make sure you get a good application that covers it because you'll have some little worms in your berries if you don't," he said.

Resources on the RCA trellis are available here. You can also access this Guide on the RCA for Blackberry Production.

To learn more about the Arkansas Agricultural Experiment Station, visit our website. Follow us on X, Facebook and Instagram.

To learn more about the Division of Agriculture, visit uada.edu and follow on X. To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit www.uaex.uada.edu.

About the Division of Agriculture: The University of Arkansas System Division of Agriculture's mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation's historic land grant education system. The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on five system campuses. The University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.