Performance of Two Solid Fumigants 

in Covered Bridges

Matthew J. Konkler

Mark A. Newbill

Stan T. Lebow

Jeffrey J. Morrell

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Note

FPL–RN–0346

February

2017
Abstract

The potential for two solid wood fumigants, dazomet and 

methylisothiocyanate (MITC), to move through wood at 

levels sufficient to arrest decay in covered bridges was 

assessed. Dazomet alone failed to decompose to MITC and 

move into the wood at effective levels, probably because too 

little moisture was available for breakdown. Adding a 

copper accelerant improved dazomet decomposition to the 

point at which it produced an effective level of chemical up 

to 600 mm from the point of application. The MITC levels 

in bridges receiving the active chemical in an aluminum 

tube were 15 to 20 times the protective levels within 2 years

of treatment. Although both chemicals eventually provided 

protective levels, MITC in aluminum tubes produced the 

most consistent chemical distribution in the shortest time.

Keywords: covered bridge, wood decay, fumigant, dazomet, 

methyisothiocyanate

February 2017

Konkler, Matthew J.; Newbill, Mark A.; Lebow, Stan T.; Morrell, Jeffrey J. 

2017. Performance of two solid fumigants in covered bridges. Research 

Note FPL–RN–0346. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 5 p.

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Introduction

Covered bridges have a long history of successful use in 

North America as a means of protecting the bridge deck 

from wetting and decay (Pierce and others 2005). The roof

decreases but does not completely prevent wetting, which 

can still occur particularly near the bridge approaches and 

through any windows or other penetrations in the roof or 

siding. Water ingress can also occur through leaks or from 

underneath during flooding. The resulting moisture can create conditions conducive to decay development, which

shortens bridge service life.

Establishing a routine bridge inspection program can help 

detect decay before it decreases bridge capacity, but it is 

equally important to arrest and remediate any decay as it is 

discovered (Morrell and Lebow 2005). For many years, the 

options for arresting decay in service were limited. Surface 

treatments lacked the ability to penetrate deeply into the 

wood of most species to the point where fungi were actively 

growing. In addition, the complex designs of many covered 

bridges creates obstacles that making it difficult to effectively treat some members. During the past 20 years, a number of diffusible treatments have emerged that can address 

some of these challenges. Water-diffusible boron compounds can be applied as either solid rods to holes drilled in 

timbers or as liquids. Boron diffuses with moisture and is 

effective against fungi and insects. Boron also has exceptionally low toxicity to birds and mammals. One drawback 

of boron in covered bridge applications is that it needs water 

to diffuse and wood in a properly maintained covered bridge 

should not be wet enough for diffusion to occur. As a result, 

boron is not useful in covered bridge applications except as 

a preventative treatment in locations where the wood is destined to become wet. Fumigants, which are solid or liquid 

compounds with high vapor pressures, are another common 

type of diffusible preservative system. These systems volatilize and can move through wood as gases to control fungi as 

far as 900 to 1,200 mm away from the point of application, 

depending on the dosage and wood species. More importantly, these treatments interact and remain in the wood 

for several years after treatment, providing long-term protection against renewed fungal attack.

Originally, fumigants used for bridge applications were liquids that were either caustic or had strong odors that made 

them difficult to work with, especially over water (Morrell 

and Corden 1986, Morrell and others 2013). More recently, 

two solid fumigants have emerged as safer alternatives. Methylisothiocyanate (MITC) is a solid at room temperature 

but sublimes directly to a gas. It is applied in aluminum

tubes and volatilizes through the tube top into the wood.

This system has provided 7 to 12 years of protection against 

fungal attack in utility poles (Morrell and others 1992, 1998; 

Ziobro and others 2004). The other fumigant is dazomet

(tetrahydro-3,5-dimethyl-2-H-1,3,5, thiodiazine-6-thione), 

which is a solid crystalline material at room temperature. 

Unlike MITC, dazomet is not volatile or highly fungitoxic. 

Instead, it decomposes to produce MITC which then diffuses through the wood (Forsyth and Morrell 1993, 1995). 

Dazomet decomposition is highly dependent on the presence 

of free water, although it can be accelerated by the co-application of copper compounds such as copper naphthenate

(Love and others 2010). The fact that dazomet and MITC 

are easy to handle makes them appear to be ideal treatments

for bridges, but little information exists on the ability of 

these compounds to perform in these applications. In this report, we examine the ability of dazomet and MITC to diffuse through covered bridges in various parts of the United 

States.

Materials and Methods

Bridge Selection

Six bridges located in five states were selected for treatment. The bridges contained four different wood species and

were made using four different construction types (Table 1).

Performance of Two Solid Fumigants

in Covered Bridges

Matthew J. Konkler, Faculty Research Assistant

Oregon State University, Corvallis, OR

Mark A. Newbill, Former Field Manager

National Wood Treating, Corvallis, OR

Stan T. Lebow, Research Forest Products Technologist

USDA Forest Service, Forest Products Laboratory, Madison, WI

Jeffrey J. Morrell, Professor

Oregon State University, Corvallis, OR
Research Note FPL–RN–0346

2

Treatment

Chemicals were applied by drilling 19-mm-diameter by 

200-mm-long holes at various angles (depending on the 

bridge configuration) into the timbers or chords at 1.2-m

intervals. Dazomet is a powder and was poured to within

50 mm of the hole top. The top of the MITC tube (MITCFUME, Osmose Utilities Inc., Peach Tree City, GA) was removed and the tube was placed, open end downward, into 

the treatment hole. Approximately 30 g of dazomet or a single vial of MITC-FUME (~32 g of MITC) was applied to 

each treatment hole. The holes were plugged with tight-fitting preservative-treated wooden dowels that were slightly 

larger in diameter than the treatment hole to retard chemical 

loss. Four (Honey Run and Oregon Creek bridges) or five 

(Cedarburg, Cooley, and Red bridges) members were 

treated with each chemical system (Lebow and others 2012),

except for the Pengra Bridge for which all members were 

treated with dazomet and then floor beams (bottom chords) 

were later randomly sampled across the bridge.

Wood species and moisture content at the time of treatment 

were determined by collecting drill shavings from the treatment hole and sealing them in plastic bags for all bridges 

except the Pengra Bridge. The shavings were weighed, ovendried, and reweighed. Mass differences between pre- and 

post-ovendried weight were used to calculate wood moisture 

content. This method may have slightly underestimated 

moisture content because of moisture loss from the shavings 

during drilling. Wood species was determined by microscopic analysis of the shavings.

Fumigant Analysis

The presence of MITC in the wood was assessed 1 to 

2 years after treatment by removing 150-mm-long increment 

cores at selected distances from the treatment site. The 

MITC levels in the Pengra Bridge were assessed 300 and 

600 mm away from the point of application 18 months after 

treatment, whereas MITC levels in the remaining bridges 

were assessed 300, 600, and 900 mm away from the treatment site 12 and 24 months after treatment.

The outer and inner 25 mm were cut from the increment 

cores removed from the Pengra Bridge. These 25-mm-long 

segments were placed into glass vials that were tightly 

sealed with Teflon-lined screw caps (Teflon, Chemours 

Company, Wilmington, DE). The entire core was analyzed 

without segmentation for cores removed from the other four 

bridges. Five milliliters of ethyl acetate were added to each 

vial, and the vial was stored at room temperature for 48 h. 

The resulting extract was decanted, and a subsample of that 

extract was analyzed for MITC by gas chromatography 

(GC). Briefly, 2 μL of extract were injected into a Shimadzu 

GC equipped with a flame photometric detector with filters 

specific for sulfur (Shimadzu Corp., Kyoto, Japan). A glass 

column (2.6-mm inner diameter by 2.1 m long) packed with 

10% carbowax on 80/100 Supelcoport was used to separate 

materials, and nitrogen was used as the carrier gas at a flow 

rate of 60 mL/min. Injector, oven, and detector temperatures 

were 100, 150 and 200 °C, respectively. The MITC content 

was determined by comparison with prepared standards.

The increment core segments were ovendried and weighed 

Table 1. Characteristics of covered bridges used to assess dazomet and MITC movement

Bridge Year built Type Wood species

MC

(%) Treatment(s) Treatment pattern

Honey Run Bridge, Butte 

County, CA

1896 Pratt and kingpost trussDouglas-fir 11.3 Dazomet, MITC Downward holes at 1.2-m

intervals in 288- by

375-mm- or 300- by

450-mm-deep beams

Oregon Creek Covered 

Bridge, Yuba County, CA

1860–

1862

Howe pony truss Douglas-fir 13.0 Dazomet, MITC Sloping holes in 238-mm 

square or 238- by

300-mm columns

Cooley Covered Bridge, 

Pittsford, VT

1849 Town lattice 

truss

Eastern spruce 12.9 Dazomet, MITC Holes drilled downwards at 

1.2-m intervals into narrow face of 75- by

288-mm-wide chords

Pengra Covered Bridge, 

Fall Creek, OR

1938 Howe truss Douglas-fir — Dazomet Downward holes at 1.2-m 

intervals on timbers

Red Covered Bridge, 

Princeton, IL

1863 Howe truss Red pine 12.6 Dazomet, MITC Sloping holes in 163- by 

225-mm diagonal supports

Cedarburg Covered 

Bridge, Cedarburg, WI

1876 Town lattice 

truss

Eastern white 

pine

13.7 Dazomet, MITC Holes drilled downward at 

1.2-m intervals in narrow 

face of 75- by 275-mm 

bottom chords near ends 

of the bridge
Performance of Two Solid Fumigants in Covered Bridges

3

to allow MITC content to be expressed on a microgram of 

MITC per ovendried gram of wood weight basis.

Results and Discussion

Dazomet Treatment

No MITC was detected in wood removed from the Red, Cedarburg, Cooley, Oregon Creek, or Honey Run covered 

bridges in zones that had received dazomet alone. The absence of MITC in wood removed from these locations was 

consistent with the moisture contents measured. Moisture 

levels in these bridges at the time of treatment ranged from 

11% to almost 14%. Dazomet generally requires liquid or 

free water to decompose to produce MITC, and this does not 

occur in most wood species until moisture content exceeds 

28% to 30% (or the fiber saturation point) (FPL 2010). 

From a protective perspective, this is not necessarily bad,

because dry wood is not likely to experience fungal attack, 

which also requires that free water be present. The dazomet 

could provide insurance should a bridge element become 

wet. However, the absence of substantial decomposition to 

MITC could be an issue if dazomet were placed near a decaying area but in a drier zone. In that case, little or no decomposition to MITC would occur and the decay would 

continue.

The addition of a copper-based accelerant is typically recommended when dazomet is used to protect utility poles in 

drier climates. The copper accelerates decomposition and 

provides a boost in MITC release into the surrounding wood 

that can result in protective levels for 2 years after treatment. Treatment with dazomet and 1% copper naphthenate 

in the Pengra Bridge resulted in detectable MITC at most locations 300 and 600 mm from the treatment point, although 

the levels were much lower than those found in the MITCFUME-treated bridges (Tables 2 and 3). The MITC threshold for protection against fungal attack is 20 μg/g of ovendried wood, based on an analysis of numerous previous field 

tests of Douglas-fir utility poles (Morrell and others 2012). 

Average MITC levels in the inner 25 mm of the increment 

cores removed from the timbers were at or above that level 

300 and 600 mm from the treatment point, although there 

was a considerable amount of variation in individual values.

The MITC levels in the outer zones of the wood were much 

lower and were mostly below the protective threshold. The 

differences between inner and outer chemical levels reflect, 

in part, the tendency of the treatment holes to direct chemical toward the center of the timber. Volatile chemicals such 

as MITC are also more likely to be lost near the wood surface. The presence of higher MITC levels in the interior regions of fumigant-treated wood is consistent with previous 

studies in poles and illustrates the fact that these treatments 

are best suited for controlling internal not surface decay.

MITC-FUME Treatment

The MITC levels in the covered bridges treated with MITCFUME were almost all well above the protective threshold

1 and 2 years after treatment (Table 3). The exception was 

the Oregon Creek Bridge, where levels were only above 

threshold 300 mm from the treatment site 2 years after treatment. Unfortunately, inner and outer analyses of increment 

cores were not possible because of handling logistics, making it impossible to determine if MITC levels in the outer 

zone were above the threshold. However, this treatment is 

not intended to protect surfaces, and it would be impractical 

to depend on the presence of low levels of MITC near the 

surface of these timbers to provide consistent protection 

against fungal attack.

The MITC levels 300 mm from the treatment holes in the 

Cedarburg, Cooley, Honey Run, and Red bridges were 14 to 

23 times the threshold level at 2 years. Levels were also 

above threshold 600 mm from the treatment zone 2 years after application but were below threshold at most locations 

900 mm from the treatment site. The elevated levels of 

MITC are consistent with previous studies of this system, 

which tends to release very high quantities of MITC that remain in the wood at protective levels for 5 to 7 years. Fungi 

then very slowly recolonize the wood, resulting in a total 

Table 2. Methylisothiocyanate (MITC) content in Douglas-fir chords in the Pengra Bridge 18 months after application of dazomet with 1% copper naphthenate as a decomposition accelerant

Location

MITC content

(μg/g of wood)a

300 mm from treatment site 600 mm from treatment site

Inner 25 mm Outer 25 mm Inner 25 mm Outer 25 mm

Mean Range Mean Range Mean Range Mean Range

Floor 19.2 (9.9) 0–48.3 1.0 (1.0) 0–4.4 20.4 (10.8) 0–43.9 1.5 (2.3) 0–10.6

Diagonal 77.6 (57.8) 17.2–203.4 14.5 (22.7) 0–59.7 80.6 (30.1) 25.4–130.6 10.0 (7.7) 0–21.2

a

Values represent means of 5 to 11 analyses depending on location. Figures in parentheses represent one standard deviation. Values in bold are above the 

threshold for protection against fungal attack.
Research Note FPL–RN–0346

4

protective period that is typically between 10 and 12 years, 

depending on the environmental conditions at the site (Morrell and Corden 1986). The MITC levels in these bridges 

were consistent with this behavior and suggest that this system should perform at least as well in current timber bridges 

constructed with pressure-treated wood. An outer layer of 

creosote or oil-based preservative treatment may slow volatilization of MITC from the interior of timbers and extend 

the period of protection (Morrell and others 2013).

Conclusions

Dazomet performed poorly in covered bridges when applied 

without a copper-based accelerant, probably because too little moisture was available to hasten decomposition. The addition of copper naphthenate markedly enhanced decomposition to MITC in one bridge evaluated. Application of 

MITC-FUME resulted in exceptionally high MITC levels 

for 2 years after treatment and was unaffected by the low 

wood moisture content. The results suggest that MITCFUME produced more consistent MITC levels in dry applications such as untreated covered bridges, whereas the application of dazomet required the use of a copper-based accelerant to achieve protective levels.

Literature Cited

Forsyth, P.G.; Morrell, J.J. 1993. Preliminary field trials using the solid fumigant Basamid amended with selected additives. Forest Products Journal. 43(2): 41-44.

Forsyth, P.G.; Morrell, J.J. 1995. Decomposition of 

Basamid in Douglas fir heartwood: Laboratory studies of 

potential wood fumigant. Wood and Fiber Science. 27: 183-

197.

FPL. 2010. Wood handbook: wood as an engineering material. Gen. Tech. Rep. FPL-GTR-190. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Lebow, S.; Kirker, G.; White, R.; Amburgey, T.; Barnes, H. 

M.; Sanders, M.; Morrell, J. 2012. Guide for in-place treatment of wood in historic covered and modern bridges. Gen.

Tech. Rep. FPL-GTR-205. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory.43 p.

Love, C.S.; Freitag, C.; Morrell, J.J. 2010. Effect of copper 

on long-term performance of dazomet as an internal remedial treatment in Douglas-fir poles. Forest Products Journal. 

60(2): 194-199.

Morrell, J.J.; Corden, M.E. 1986. Controlling wood deterioration with fumigants: a review. Forest Products Journal.

36(10): 26-34.

Morrell, J.J.; Freitag, C.M.; Newbill, M.A.; Connelly, A.;

Chen, H. 1998. Seven-year performance of glass encapsulated methylisothiocyanate. Forest Products Journal. 48(1):

65-71.

Morrell, J.J.; Lebow, S.T. 2005. Initial preservative treatment of wood in covered bridges. In: Pierce, P.C.; Brungraber, R.L.; Lichtenstein, A.; Sabol, S., eds. Covered bridge 

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Table 3. Methylisothiocyanate (MITC) levels in covered bridges 12 or 24 months

after application of MITC-FUME

Bridge

Time after 

application

(months)

MITC levels

(μg/g of wood)a

300 mm from treatment site600 mm from 

treatment site

900 mm from 

treatment site

Mean Range Mean Range Mean Range

Cedarburg 12 242 (136) 57–448 37 (44) 0–119 6 (17) 0–44

24 469 (266) 163–891 90 (99) 0–256 19 (45) 0–111

Cooley 12 216 (195) 0–594 197 (123) 41–338 18 (28) 0–66

24 469 (242) 96–946 82 (53) 8–131 5 (8) 0–19

Honey Run 12 327 (468) 0–1373 29 (64) 0–184 13 (20) 0–43

24 358 (345) 65–1098 65 (116) 4–339 3 (13) 0–13

Oregon Creek 12 12 (27) 0–78 0 0 0 0

24 51 (68) 0–165 3 (4) 0–10 0 0

Red 12 280 (361) 60–1265 13 (10) 0–31 2 (5) 0–12

24 452 (309) 40–913 86 (75) 2–234 42 (99) 0–99

a

Values represent means of 6 to 9 analyses, and figures in parentheses represent one standard deviation. Values 

in bold are above the threshold for protection against fungal attack.
Performance of Two Solid Fumigants in Covered Bridges

5

Morrell, J.J.; Love, C.; Freitag, C.; Schindler, J.; Leavengood, S. 2012. Utility Pole Research Cooperative, 32nd annual report. Corvallis, OR: Department of Wood Science 

and Engineering, Oregon State University. 124 p.

Morrell, J.J.; Newbill, M.A.; Sexton, C.M. 1992. Remedial 

treatment of Douglas-fir and southern pine poles with methylisothiocyanate. Forest Products Journal. 42(10): 47-54.

Pierce, P.C.; Brungraber, R.L.; Lichtenstein, A.; Sabol, S.,

eds. 2005. Covered bridge manual. U.S. Federal Highway 

Administration Report FHWA-HRT-04-098. McLean, VA: 

U.S. Federal Highway Administration.

Ziobro, R.J.; Fomenko, J.; Herdman, D.J.; Guzzetta, J.;

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