A Recent History of the Verbenone Suppression Tactic:

Advances and Developments of an Inhibitor-Based Integrated Pest Management Strategy for the Southern Pine Beetle

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Contents

Introduction

The southern pine beetle (SPB), Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), is the most destructive forest insect pest in the southern United States (Payne 1980). Control methods for suppressing infestations have evolved over the past century. Early techniques involved salvage of infested trees, burning infested material, injection of chemicals into the sap of standing trees, and exposure of felled trees to solar radiation (St. George and Beal 1929, Craighead and St. George 1938). In the 1940s these tactics gave way to pesticide applications on felled trees, and the objective of control tactics was elimination of epidemics (Billings 1980). By 1970 it had become clear that pesticides were eliminating natural enemies of SPB (Williamson and Vité 1971), and epidemics continued to occur. As a result, the emphasis shifted to integrated control. Forest managers utilized silvicultural practices to reduce SPB hazard, and implemented direct control measures to suppress infestations and reduce losses. Four suppression methods were authorized by the Final Environmental Impact Statement for the Suppression of the Southern Pine Beetle (USDA 1987): 1. cut-and-remove (fell and remove attacked trees plus a buffer strip of unattacked trees); 2. cut-and-leave (fell infested trees plus a buffer strip of uninfested trees and leave them at the site); 3. cut-and-hand spray infested trees with insecticide; and 4. cut, pile, and burn infested trees.

Verbenone Technology Development Project

While the approved direct control methods are generally efficacious for suppressing individual SPB infestations (Redmond and Nettleton 1990, Billings 1995), they all involve tree-felling and therefore cannot be implemented in certain areas or at all times. Also, cut-and-remove and cut-and-leave include felling a buffer of uninfested trees. This led researchers to search for methods that would not require tree felling or sacrificing uninfested timber. Vité (1971) suggested the use of semiochemicals for the management of SPB, and techniques involving attractants and inhibitors have since been tested. The most promising tactic tested thus far has been the deployment of the inhibitor pheromone, verbenone, around the active front of an infestation to prevent its continued expansion.

Verbenone is a pheromone produced primarily by male SPB. At high concentrations it inhibits aggregation of both male and female SPB and prevents overcolonization of individual hosts (Rudinsky 1973). During the summer, SPB are generally aggregated within infestations. Emerging and reemerging beetles cue into aggregation pheromones emanating out of trees currently under attack, whereupon landing on the tree, males release verbenone and other inhibitory pheromones. As concentrations of these inhibitory pheromones increase, beetles are deterred from these trees and fly to nearby uninfested pines and begin a new mass attack. By placing synthetic verbenone on fresh attacked and uninfested trees at the front of a SPB infestation, it was theorized that spot expansion would be disrupted. Emerging and reemerging beetles would disperse, resulting in the infestation becoming inactive. After continued field testing and refinement of application methods (Payne and Billings 1989, Salom et al. 1992, Payne et al. 1992, Billings et al. 1995), standardized and effective operational techniques have been devised which will suppress SPB infestations within specified size categories (Salom et al. 1996a).

With the development of operational techniques for suppressing SPB infestations using verbenone, the following steps are necessary to put this tactic into practice: 1. further refinement of methods to increase ease of application and reduce costs; 2. obtain final registration of verbenone from the U.S. Environmental Protection Agency (EPA); and 3. technology transfer of this tactic to potential users.

Common Name of Biopesticide: Verbenone dispensed from slow-release polyethylene pouches attached to trees located around the active front of southern pine beetle infestations.

Chemical name: 4,6,6-trimethylbicyclo [3.1.1] hept-3-en-2-one

Commodities and Sites Protected: Southern pine stands (loblolly pine, Pinus taeda L.; shortleaf pine, P. echinata Mill.; pitch pine, P. rigida Mill., Virginia pine, P. virginiana Mill.; pond pine, P. serotina Michx.; and longleaf pine, P. palustris Mill.).
The general range of susceptible trees in the U. S. is from Maryland south to Florida and west to Texas.

Protocol for Verbenone Suppression Tactics

Generalized procedures used in applying verbenone and evaluating success of two verbenone suppression tactics follow:

1. Identify active SPB infestations with < 120 currently infested trees.
2. Count the number of currently infested trees and verify the presence of fresh attacks.
3. Assess whether the infestation will continue to grow. Continued growth can be determined based in part on the number of fresh-attacked trees (trees where eggs have not yet been laid) and continued availability of susceptible trees in front of the active head (zone of most recent SPB attacks) (see Billings and Pase 1979).
4. Make decision whether or not to treat.
5. If the decision is to treat, apply one of the following available options:
• Verbenone-only: treat fresh-attacked trees and a buffer strip of unattacked trees with verbenone.
• Verbenone-plus-felling all actively infested trees: Fell all actively infested trees and treat buffer strip of unattacked trees with verbenone.
• Verbenone-plus-felling fresh-attacked trees: fell all fresh-attacked trees and treat buffer of unattacked trees with verbenone
6. Monitor infestation for 6 weeks to determine treatment effectiveness. Categories include:
• Total suppression: no new trees attacked beyond treated buffer strip during the 6-week post-treatment period.
• Partial suppression: new trees attacked beyond treated buffer strip, yet infestation growth decreased by > 50%.
• No suppression: new trees attacked beyond treated buffer strip and infestation growth decreased by < 50%.
  

Verbenone Elution Device. Verbenone with the enantiomeric ratio of 34%(+): 66%(-) was used for all treatments (Salom et al. 1992). 5 ml of verbenone was place in white 1.2 mil polyethylene pouches (8 x 13 cm) provided by Phero Tech, Inc., Delta B.C. The bag provided an elution rate of ca. 9.2 mg/h for 40 to 50 days during summer months (C.W. Berisford et al., unpublished data).

The verbenone suppression tactic has proven effective for a number of years (Payne et al. 1992, Billings et al. 1995). But since the application procedures continue to evolve, we present only the latest procedures and data set on efficacy (1994 - 1996).

Treatment Options Using Verbenone

Verbenone Only. All fresh-attacked trees plus a buffer strip (3 trees deep and 10 m surrounding the active head) were treated with verbenone. Pouches were attached to trees at a height of 3 m . The number of pouches attached to trees were based on tree diameter at breast height (d.b.h.) (Table 1).

Verbenone-Plus-Felling All Actively Infested Trees. All actively infested trees were felled toward the older part of the infestation. A buffer strip of unattacked green trees, similar in size as used in the treatment above, was treated with verbenone. With the exception of the buffer, this treatment mimics a conventional cut-and-leave treatment that serves to open up the stand. The drastically different environmental conditions that result from opening the stand is believed to promote dispersal rather than aggregation. Brood in felled timber may be susceptible to increased radiant heat near the opened forest floor, resulting in a higher level of mortality. In addition, felling fresh-attacked trees reduces the production of secondary attractants.

Verbenone-Plus-Felling Fresh-Attacked Trees. In this option only freshly-attacked trees were felled. The same sized buffer strip of unattacked trees at the active head of the infestation was treated with verbenone. This treatment is aimed primarily at eliminating sustained production of beetle- and host-produced attractants within the treated infestation. The felling of freshly-attacked trees open up the stand, but to a lesser degree than when all infested trees are felled.

Check. SPB infestations that met treatment criteria but could not be treated due to various constraints were left untreated and monitored for 6 weeks.

Table 1. Recommended number of verbenone pouches placed on trees in southern pine beetle infestations during 1994 field season.

Comparisons Among Verbenone Treatment Options in 1994

The collaborators in this project were the Texas Forest Service (TFS), University of Georgia (UGA), and Virginia Tech (VT). The TFS evaluated sites in Texas, UGA in Alabama and Georgia, and VT in Virginia, North Carolina, and South Carolina. Three verbenone treatment options plus a check were evaluated as follows: Verbenone-Only (5 sites), Verbenone-Plus-Felling All Active Trees (9 sites), Verbenone-Plus-Felling All Fresh Attacked Trees (8 sites), and no treatment checks (13 sites). The number of pouches attached to trees for each treatment and each site were based on tree diameter at breast height (d.b.h.) (Table 1).

Results

Verbenone-Only. Infestation growth was reduced by an average of 62% in 5 infestations (Table 2, Figure 1). Four infestations were completely suppressed while one was unaffected by the treatment.

Verbenone-Plus-Felling All Actively Infested Trees. Infestation growth over all 9 infestations was reduced 70% (Table 2, Figure 1). Although the percent reduction in spot growth was less than in previous years (Billings et al. 1995), the tactic was completely effective in 8 of the 9 infestations by the end of the post-treatment monitoring period.

Verbenone-Plus-Felling Fresh-Attacked Trees. This tactic was completely effective in 4 of 5 infestations in Texas and partially effective in the other (Table 2, Figure 1). In Alabama, the treatment was effective in one of two infestations.

Check. Thirteen infestations were used as controls to ensure that most untreated infestations would continue to expand. Nine infestations increased in size by > 1 tree/day, and 10 added more than 10 trees during the monitoring period (Table 2).

Comparisons Among Verbenone Treatment Options in 1995

Treatments evaluated in 1995 were Verbenone-Only (20 sites), Verbenone-Plus-Felling All Active Trees (19 sites) , and 8 check sites. Application rates of verbenone and buffer sizes were increased for both Verbenone-Only and Verbenone-Plus-Felling treatments for the 1995 season (see below).

Standardization of Verbenone Application Rates. Application rates of verbenone were increased for both Verbenone-Only and Verbenone-Plus-Felling treatments for the 1995 season. The rates were modified from previous procedures (Billings et al. 1995) and based partially on data from Figure 2. Verbenone application rates were established to deliver 40 ml/ft² of basal area of actively infested trees (number of pouches = (mean d.b.h/2)² * no. of active trees * 0.1745) for verbenone-only and 25 ml/ft² (number of pouches = (mean d.b.h/2)² * no. of active trees * 0.1091) for Verbenone-Plus-Felling All Active Trees (Table 3). Table 3 was designed to provide potential users with the number of pouches required to treat infestations based on number of trees and mean tree diameter. Minimum buffer strip sizes of 12.2 m and three trees deep were used for Verbenone-Only and 7.6m and two trees deep for Verbenone-Plus-Felling All Active Trees. If the application rates listed above failed to meet the minimum buffer requirements, additional pouches were added to treat all trees within the buffer. The number of pouches attached to each tree ranged from 1 - 6, based on tree diameter size (Table 1).

Results

Verbenone-Only. Infestation size averaged 42 trees, ranging from 15 - 88 trees at the time of treatment. Overall, infestation growth was reduced by 77% (Table 4, Figure 3). Of the 20 infestations treated at all locations, 16 (80%) were completely suppressed. In Texas this tactic completely suppressed 10 of 12 infestations. The application rate requirement of 40 ml/ft² was met in 14 of the infestations.

Verbenone-Plus-Felling All Actively Infested Trees. Infestation growth rate was reduced 89% (Table 4, Figure 3). This was not significantly different from the Verbenone-Only treatment (t _{0.05 (2), 37} = 1.99; p = 0.054). Out of 19 infestations, 16 (84%) were completely suppressed. Standardized application rates appeared to improve results for this treatment as with the previous one. Despite similarity in treatment effectiveness between Verbenone-Only and Verbenone-Plus-Felling, it should be noted that the average infestation size for the latter treatment (77 trees) was almost twice as large as for the former one (42 trees).

Check. Of the eight infestations used as controls, six expanded by > 1 tree/day (Table 4). Check infestations grew an average of 3.1 trees killed/day, versus 0.3 and 0.1 trees killed/day for Verbenone-Only and Verbenone-Plus-Felling active trees treatments, respectively.

Comparisons Among Verbenone Treatment Options in 1996

Southern pine beetle activity was down throughout the South. Thirteen infestations were evaluated in North Carolina and Texas. In North Carolina, all infestations were in pond pine, P. serotina, stands. The verbenone treatment had not previously been evaluated for this species. All procedures and application rates developed for the 1995 season were used in 1996.

Results

Verbenone-Only. Eight of nine infestations were completely suppressed (Table 5, Figure 4). Infestation growth was reduced by 76%, very similar to results obtained in 1995.

Verbenone-Plus-Felling All Actively Infested Trees. Both infestations treated in North Carolina were completely suppressed (Table 5, Figure 4). Infestation growth rate was reduced by an average of 66%, a bit lower from 1995.

Check. The one untreated check site in North Carolina grew an average of 1.9 trees/day (Table 5). This compares to post-treatment growth rates of 0.2 and 0.5 for the Verbenone-Only and Verbenone-Plus-Felling treatments, respectively.

The verbenone tactics have proven to be very effective and results have been quite consistent, especially after the application rates were standardized in 1995. We have also shown that this tactic can be effective in pine infestations that occur in host species other than loblolly and shortleaf pine.

References Cited

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