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The fight against flatheaded borers

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Researchers study nursery and orchard pest with an eye toward better management methods

Fig. 1 Pacific flatheaded borer larvae (Chrysobothris mali) as seen on a branch. The life cycle of this pest still not well understood. Researchers hope to get answers that will help them better manage the pest. Photo by Chris Hedstrom.

Oregon State University researchers from the Nackley (nursery) and Wiman (orchards) programs have teamed up with scientists from across the country to study the biology and management of flatheaded borers.

Flatheaded borers are beetles, from the order Coleoptera and the Buprestidae family, that can be devastating pests of ornamental trees grown in nurseries, transplanted landscape trees, trees grown for wood or other products, and fruit and nut crops.  

This four-year project is part of a $6 million federal project titled Flatheaded Borer Management in Specialty Tree Crops (SCRI # 2020-51181-32199) led by Karla Addesso, Department of Agricultural and Environmental Sciences at Tennessee State University and 25 other researchers from the University of California, Clemson University, University of Florida, North Carolina State University, University of Georgia, University of Tennessee, Rutgers University, Texas A&M, and USDA-ARS.

Flatheaded borers can be serious pests of nursery trees and tree nut and fruit crops in the Pacific Northwest. 

Fig. 2 Pacific flatheaded borer adult  is a nursery pest only found west of the Rocky Mountains. Two other C. species are also found in the region. Photo by Chris Hedstrom.

In particular, two flathead borers in the genus Chrysobothris — C. femorata (flatheaded apple tree borer) and C. mali (Pacific flatheaded borer) — damage many different deciduous trees and shrubs grown commercially, weakening or even killing trees by girdling the trunk or branches. A third species, C. nixa, attacks cedar and juniper nursery trees. 

The flatheaded apple tree borer is a common pest in the Eastern and Central United States, while the Pacific flatheaded borer and flatheaded cedar borer are only found west of the Rocky Mountains. The Pacific Northwest region is home to all three species. 

The project focuses on six strategies:

1. Identification. Currently there is great difficulty in identifying Chrysobothris to species, particularly those in the C. femorata complex, whose economically important members remain unresolved taxonomically. From a management perspective, it is imperative to know what species we are targeting, as closely related species may have different host preferences, seasonal phenologies, and thus management requirements. In addition, eastern flatheaded borer species could threaten commercial nursery production and landscapes in the western U.S., and vice versa, if introduced and established. Consequently, accurate diagnostic identification of adult and larval life stages for Chrysobothris is critical to regulatory efforts at safeguarding interstate commerce of nursery and orchard stock. Our project is developing new molecular tools for species identification.

2. Life-cycle biology. Currently there is limited understanding of the basic ecology and life history of key Chrysobothris pests. The oviposition timing of flatheaded borer females is still poorly understood, hindering our ability to accurately time pesticide sprays. Likewise, the relationship between female host selection behavior and larval success is unknown. We do not know if females are targeting specific stressed trees displaying cues more suitable for larval survival or if they are randomly ovipositing in crops and larval infection success is an independent process. The answers to these questions will direct future management strategies. Our project is developing improved knowledge of the phenology, life history and spatial distribution of flatheaded borer species. 

Fig. 3. Researchers have noted that damage from flatheaded borers occurs near bud unions and other wounds on branches or trees. Photo by Chris Hedstrom.

3. Monitoring. Currently it is unknown how flatheaded borers target trees. We know that Chrysobothris color preferences in the red and violet range can be exploited in trapping strategies. In addition, Chrysobothris are regularly captured on freshly cut stumps, suggesting that volatiles may play some role in the ecology of host location. But to date, no lure tested has performed consistently better than a blank purple panel or prism trap. This project is developing new lurers and trapping methods.

4. Improved cultural practices. Understanding what makes a tree susceptible or resistant to flatheaded borers can lead to production practices that minimize the likelihood of a crop being attacked by flatheaded borers. For example, it has been consistently observed that damage often begins near a bud union or other wound. It also has been observed that attacks are concentrated on the sunny (south) side of trees. By orienting the bud unions of test plants to the north, LeBude and Adkins reduced attacks compared to trees with bud unions facing south, southeast, or southwest. Fast-growing cultivars of red maple may suffer less damage than slower-growing varieties and stress from drought and defoliation appear to contribute to attacks. Therefore, improving plant vigor may reduce susceptibility to
flatheaded borers.

Fig. 4 Swelling associated with borer larvae in red maple. Photo by Lloyd Nackley.

5. Identify new conventional, organic and biological management options. Insecticide efficacy trials have been conducted in nursery trees, but not in orchard systems. Over-reliance on an insecticide-only management strategy rather than an integrated approach increases the likelihood of poor control and insecticide resistance. Because imidacloprid has been the only consistent, long-term flatheaded borer treatment option, amounts applied to specialty crop acreage may have already exceeded the label per acre limits. This project is investigating the efficacy of new sprays and improved timing of spray applications

Fig. 5 Researchers are looking at how to best trap flatheaded borers and have experimented with different color traps to see what works best.  Photo by Erica Rudolph.

6. Consumer preferences and economic impacts. The costs and benefits of any particular management practice depend on the crop in question, the age of the trees, the size of the operation and other market factors. Cost-benefit budgets associated with each management method will allow growers to optimize management practices based on their particular needs. Consumer responses to changes in management practices, particularly with the use of neonicotinoid replacements, will also need to be addressed. This project also is evaluating consumer and grower preferences to optimize impact of research products.

If your nursery is impacted by borers and you would like to participate in this research, please contact us at Lloyd.Nackley@OregonState.edu. We are scouting on farms all summer and would like to work with you too. 

This work is supported by Specialty Crop Research Initiative (grant no. 2020-51181-32199) from the USDA National Institute of Food and Agriculture. 

Co-authors on this article include Melissa Scherr, Erica Rudolph and Heather Andrews. Dr. Lloyd Nackley is an assistant professor and researcher at Oregon State University. Melissa Scherr is a research associate in entomology at OSU and can be reached at melissa.scherr@oregonstate.edu. Erica Rudolph is a graduate research assistant at OSU and can be reached at rudolphe@oregonstate.edu. Heather Andrews is an orchard crops faculty research assistant at OSU and can be reached at heather.andrews@oregonstate.edu. Nik Wiman is an associate professor at OSU and can be reached at nik.wiman@oregonstate.edu.