Abstract

The United States is the largest industrial timber producer in the world, with almost 20 to 25 percent of the world's production prior to the 2009 recession and 15.9 billion ft³ harvested in 2017 (Howard and Liang 2019). Overall, forest products comprise about 1.5 percent of the total US economy and contribute about 5 percent of the total manufacturing output in the country. Furthermore, the timber industry is one of the top three contributors to most southern state economies (Alvarez 2018). In 2017, 80 percent of US lumber production was from softwoods, which are primarily grown in the southern United States. After harvest, timber is often transformed into lumber, wood pulp, or wood chips. Approximately 87.5 percent of softwood timber is processed prior to export (Export Market Analysis for Selected Texas Commodities 2020).

Due to the minimally processed nature of forest products, the international trade of this commodity has great potential to lead to the introduction of nonnative invasive pest species (Westphal et al. 2008; Work et al. 2005). This risk has great potential to lead to serious negative impacts on the local biodiversity and economy, with economic costs estimated at hundreds of billions of dollars to economies globally (Lopian and Stephen 2013). Once introduced, invasive pest species often have few, if any, competitors and predators among highly susceptible hosts, resulting in unchecked reproduction and spread.

The introduction of invasive pests through international trade of wood-based products has risen as forest insect pests continue to spread (Brockerhoff et al. 2006). Among the major pests of concern is the pinewood nematode (PWN) (Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle 1970), the causal agent of pine wilt disease. Pine wilt disease has severely damaged exotic pine (Pinus spp.) trees in Europe and Asia; however, North American pine trees are naturally resistant (Dwinell and Nickle 1989, Bonifácio et al. 2013). Damage caused by PWN in Japan, China, and Korea has been extensive, with more than 2 million hectares affected by this disease (Yun et al. 2012).

To mitigate the risk of invasion by PWN and other invasive pests, many countries have implemented strict phytosanitary measures for wood chips exported from North America. Treatment prior to export must be carried out using heat treatment or fumigation with methyl bromide (MB). The use of heat treatment is typically uneconomical for commodities such as wood chips; therefore, fumigation with MB is the primary treatment option. Due to its ozone-depleting properties, the use of MB is being phased out globally under the Montreal Protocol and is restricted in the United States under the Clean Air Act (UNEP 2006, Hagstrum et al. 2012). Currently, only quarantine and preshipment (QPS) uses of MB are authorized in the United States; however, to reduce global MB usage, the European Union does not accept product treated with MB. In addition, the phaseout of MB has led to an increase in input costs (Goodhue et al. 2005).

Few fumigant alternatives are currently available for the treatment of wood chips. Sulfuryl fluoride (SF) has been used as a fumigant for more than 50 years but is generally only effective at temperatures above typical fumigation conditions (>20°C) (Buckley 2010, Ren et al. 2011). When applied to blocks and wood chips, SF was ineffective against PWN at all dose rates during 24-hour treatment times but was effective when treated for 48 hours with high dose rates (Seabright et al. 2020). In addition, SF is a potent greenhouse gas with an estimated lifetime of 36 ± 11 years in the atmosphere (Mühle et al. 2009). The global warming potential of SF is reported to be similar to that of CFC-11 (trichlorofluoromethane), which is already banned under the Montreal Protocol (Papadimitriou et al. 2008).

Another alternative control measure, phosphine (PH₃), has also been used as an alternative fumigant. While PH₃ is generally an effective alternative, its use requires an extremely lengthy treatment period (>5 days) to control some insect egg and pupal life stages (Ren 2013). In addition, some insects have been found to be resistant to PH₃ (Opit et al. 2012)_. The volatile nature of gasses often requires lengthy treatment with PH₃ and additional fumigant to be applied to the commodity to maintain the appropriate concentrations necessary to effectively control pests. Furthermore, treatment of PWN-infested wood chips with PH₃ over 25 days was not 100 percent effective, which is required by the importing country to accept the commodity (Leesch et al. 1989).

Ethanedinitrile (EDN) is a broad-spectrum fumigant with efficacy against a wide range of insect pests, including PWN (Douda et al. 2015, Seabright et al. 2020, Uzunovic et al. 2022). Due to its highly volatile nature, EDN can penetrate both across and with the grain of timber quicker and further than MB (Ren et al. 2011, Park et al. 2014). EDN is also highly sorptive, with less than 1 percent of the applied concentration remaining following 10-, 16-, 20-, and 24-hour treatments (Hall et al. 2017, Najar-Rodriguez et al. 2020, Park et al. 2021, Uzunovic et al. 2022). These data indicate that EDN can be safely released into the atmosphere without the need for recapture technology, as it is not an ozone-depleting substance nor a greenhouse gas. EDN was found to be an economically viable fumigant with minimal impact to the environment by the Solar Impulse Foundation (Anonymous 2020).

While laboratory-scale studies have confirmed the efficacy of EDN against PWN in wood chips, its use in a commercial setting has not been reported. The objective of this study was to examine the efficacy of EDN in wood chips against PWN on a large, commercial scale with artificially and naturally infested wood chip samples.

Materials and Methods

Sample preparation and placement

Wood chips sourced from the southern yellow pine species, loblolly pine (Pinus taeda), were sized 0.45 cm by 4.5 cm, 2 percent maximum bark, and had a density of 170 kg/m³. The moisture content of wood chips for these experiments was determined by weighing 1000 g of wood chips, drying in an oven for 16 hours, reweighing wood chips to calculate moisture loss, which was 51 percent on an ovendry basis. Prior to each experiment, samples were sent to the lab for confirmation of the presence of live PWN, which revealed an average of 1 PWN per gram of wood chip. In addition to wood chip samples, three water samples were included with different types of nematodes in each sample. Water samples included (1) a mixture of live PWN recovered from the naturally infested wood chips 1 day prior to EDN application; (2) a mixture of live soil nematodes including root-knot (Meloidogyne incognita), lesion (Pratylenchus spp.), lance (Hoplolaimus galeatus), spiral (Helicotylenchus spp.), and free-living nematodes (Rhabditis sp., Oscheius sp., Cruznema tripartitum) sourced from field crops; and (3) live lance nematodes sourced from turfgrass. These nematode samples were extracted in the lab prior to the application and placed in 50-mL falcon tubes with water. Immediately prior to application and after wood chip loading, each sample was uncapped and placed in each container near the doors during Experiment 2. These water samples allowed for immediate preliminary results of the trial without further nematode extractions. Nematodes in wood chips were extracted by soaking the wood chip in water for 24 hours, followed by counting live, mobile nematodes using a pie-pan method.

Six mesh bags filled with wood chip samples collected from the same wood chips to be treated (2 kg and 4 kg for Experiments 1 and 2, respectively) were placed in each container. Samples were placed in the center of each container during wood chip loading at 2.01-m and 0.43-m intervals, laterally and vertically, respectively, with the first sample placed on the container floor (Figs. 1a and 1b). This sampling scheme ensured that each sixth of the container was sampled. As the container was filled with wood chips, samples were placed accordingly until the container was deemed full. The filled container was then reweighed. During Experiment 1, the equipment used for wood chip loading broke beyond repair; therefore, only two wood chip samples were placed in the front of that container.

View Figure

• Download as Powerpoint
• Download Figure

Results and Discussion

Preapplication samples for both experiments confirmed the presence of live PWN infesting the wood chips (Tables 1 and 2). During Experiment 1, two of the preapplication samples were free of PWN; however, nonparasitic nematode species were present in both samples. Samples with PWN contained 10 to 12 PWN/g wood. During Experiment 2, preapplication samples averaged 1 PWN/g wood.

Table 1. Incidence of PWN in southern pine chips and water samples before and after exposure to 75 or 120 g/m⁻³ EDN.

View Fullscreen

Table 2. Incidence of PWN in southern pine chips before and after 24 hours of exposure to 75g/m³ EDN.

View Fullscreen

Postapplication wood chip samples in Experiment 1 revealed no live PWN or free-living nematodes at the low and high EDN application rates. In addition, all water samples in EDN-treated containers resulted in 100 percent mortality of PWN, lance nematode, and other soil nematodes. Live nematodes were observed in all samples collected from the container that did not receive treatment. The same trend was revealed during Experiment 2, with the EDN-treated container resulting in complete control of PWN.

These results agree with those of Seabright et al. (2020), who reported 100 percent control of PWN in pine wood chips when EDN was applied at 30 g/m³ for 24 hours. In the same study, EDN was found to effectively penetrate larger blocks of pine wood to control PWN. When applied at 40 g/m³ for 24 hours, EDN provided 100 percent control of PWN in 75 by 75 by 150-mm wood blocks and 1.5-m-long logs by 19.1- to 39.1-cm diameter with the bark intact, while sulfuryl fluoride and phosphine only provided control in the smaller wood chips. Uzunovic et al. (2022) tested the efficacy of 50 and 100 g/m³ EDN against PWN in 17-cm long pine logs and 2.5 by 3.8 by 0.64-cm pine wood blocks at 10°C and 20°C for 1-, 3-, 6-, 12-, 18-, and 24-hour treatment time. Their study found that EDN was 100 percent effective at all temperatures, treatment times, and rates for wood blocks. Both EDN rates provided complete control of PWN at 20°C when applied to pine logs for 12 and 24 hours and at 10°C when applied for 3, 12, 18, and 24 hours. EDN also provided control of PWN in 2-cm-thick log blocks with >13-cm diameter when applied at rates of 97g/m³ and above (Chung et al. 2007). Previous research by Malkova et al. (2016) reported 100 percent control of PWN when treated with 50g/m³ EDN for 6, 12, and 18 hours. Furthermore, EDN applied at 100, 120, and 150 g/m³ at temperatures of 21 to 33, 6 to 12, and −1 to 3°C, respectively, resulted in complete mortality of PWN (Lee et al. 2017).

These findings and those of previous research indicate that EDN is a suitable alternative to MB for the control of PWN in pine wood chips of these specifications. As international timber trade continues, so too will the risk of invasive species both domestically and internationally. EDN provides effective control of invasive pests for QPS purposes.