United States

Department of

Agriculture

Forest Service

Southern 

Research Station

General Technical 

Report SRS–120

Xylem Transport Models Optimize

Effectiveness of Systemic

Insecticide Applications for

Controlling Hemlock Woolly Adelgid

(Adelges tsugae)

Chelcy R. Ford, Barbara C. Reynolds, and James M. Vose
May 2010

Southern Research Station

200 W.T. Weaver Blvd.

Asheville, NC 28804

Authors: 

Chelcy R. Ford, Ecologist, U.S. Forest Service, Southern Research 

Station, Coweeta Hydrologic Lab, Otto, NC 28763; Barbara C. Reynolds, 

Associate Professor, University of North Carolina at Asheville, Department 

of Environmental Studies, Asheville, NC 28804; and James M. Vose, Project 

Leader and Research Ecologist, U.S. Forest Service, Southern Research 

Station, Coweeta Hydrologic Lab, Otto, NC 28763.

Cover photos: Sap flow probes in hemlock trees (left) and tree selected for 

stem injection of imidacloprid (right).

Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not 

contain recommendations for their use, nor does it imply that the 

uses discussed here have been registered. All uses of pesticides 

must be registered by appropriate State and Federal agencies 

before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic 

animals, desirable plants, and fish or other wildlife—if they are 

not handled or applied properly. Use all pesticides selectively 

and carefully. Follow recommended practices for the disposal of 

surplus pesticides and pesticide containers.
Xylem Transport Models

Optimize Effectiveness of

Systemic Insecticide Applications for 

Controlling Hemlock Woolly Adelgid 

(Adelges tsugae)

Chelcy R. Ford, Barbara C. Reynolds, and James M. Vose

Abstract

The hemlock woolly adelgid (HWA, Adelges tsugae Annand) is 

causing widespread decline and mortality of eastern hemlock trees 

(Tsuga canadensis (L.) Carr.). Stem injection of insecticide is widely 

used as a control measure, but its effectiveness depends on individual 

tree hydraulic characteristics. Recent work has shown that eastern 

hemlock daily water use is exponentially related to tree diameter, 

with smaller-diameter trees using significantly less water than largerdiameter trees. In this study we modeled daily water use for 20 eastern 

hemlock trees across a range of diameters. Based on expected daily 

water use and, thus, potential xylem transport of insecticide, we 

applied a dosage estimated to achieve a lethal and uniform xylem 

sap concentration of imidacloprid to half the trees (xylem transport 

treatment), and treated the remaining half based on the manufacturerrecommended dosage (MFR treatment), which is a linear function 

of tree diameter. At 4 and 56 weeks after treatment, we assessed all 

trees for the presence or absence of new shoot growth and live HWA 

population density. We found that both treatment dosages significantly 

reduced live HWA populations; however, 4 weeks after treatment, 

live HWA population density was 32 percent lower (LSMEANS) on 

xylem transport treatment trees compared with MFR treatment trees 

(treatment by time interaction, P = 0.006). Both treatment dosages also 

significantly increased the proportion of new shoot growth; however, 

over time xylem transport treatment trees had a significantly greater 

increase in new shoot production compared with MFR treatment trees. 

We conclude that dosages based on a xylem transport model not only 

significantly improved tree health more than the trees receiving the 

current recommended dosage, but also caused greater reductions in 

the live HWA population. Improving treatment dosages would reduce 

treatment cost, nontargeted effects, and would allow land managers to 

treat a greater number of trees.

Keywords: Eastern hemlock, hemlock woolly adelgid, imidacloprid, 

insecticide efficacy, sap flow, systemic insecticide, transpiration.

Introduction

Eastern hemlock (Tsuga canadensis (L.) Carr.) trees of all 

ages and sizes in the eastern and southernmost portion of 

their range are declining and dying as a result of attack by 

a nonnative invasive insect, the hemlock woolly adelgid 

(HWA, Adelges tsugae Annand) (Orwig and others 2002, 

U.S. Department of Agriculture 2006). Eastern hemlock 

decline and mortality has been rapid especially in the 

Southern Appalachians of Virginia and North Carolina 

(Nuckolls and others 2009, Skinner and others 2003). 

Because eastern hemlock has no known resistance to HWA, 

mortality can occur after 3 or more years of infestation 

(Nuckolls and others 2009, Orwig and others 2002). Eastern 

hemlock trees infested with HWA display progressive 

canopy thinning from decreased needle, bud, and new shoot 

production (Cobb and others 2006, Eschtruth and others 

2006, Jenkins and others 1999, McClure 1991, McClure 

and Cheah 1999, Orwig and Foster 1998, Stadler and 

others 2005). 

Chemical treatments have been effective in controlling HWA 

on individual trees, and application methods include foliar 

sprays and soil and stem application of systemic insecticides 

(e.g., imidacloprid, a chloronicotinyl (1-[(6-chloro-3-

pyridinyl) methyl]-N-nitro-2-imidazolidinimine) insecticide) 

(Cowles 2009, Cowles and others 2006, Fidgen and others 

2002, McAvoy and others 2005, Steward and Horner 1994, 

Webb and others 2003). The effectiveness of soil and stem 

applications, however, has varied based on injection method, 

timing of application, and other tree-specific characteristics 

(Cowles and others 2006, Doccola and others 2003, McAvoy 

and others 2005, Tattar and others 1998). Because uptake 

and transport of systemic insecticides occur via mass flow 

in the transpiration stream (Byrne and Toscano 2006, Castle 

and others 2006, Tattar and others 1998), treatment efficacy 

may be linked to the volume and velocity of xylem water 

movement, and thus the concentration of the insecticide in 

the xylem sap. 

Differences in tree size and climate affect the amount and 

timing of water use among trees. For example, recent work 

(Ford and Vose 2007, Ford and others 2007) has shown 

that eastern hemlock daily water use is exponentially 

related to tree diameter, with smaller-diameter trees using 

significantly less water than larger-diameter trees. In 

addition, models have been developed that predict daily 

tree water use (and thus potential imidacloprid uptake and 

transport) from climatic variables (average daytime air 

temperature and vapor pressure deficit), day of the year, 

and tree diameter at breast height (d.b.h.) (Ford and others 

Xylem Transport Models Optimize Effectiveness of

Systemic Insecticide Applications for Controlling

Hemlock Woolly Adelgid (Adelges tsugae)

Chelcy R. Ford, Barbara C. Reynolds, and James M. Vose
2007). The exponential relationship between mass flow 

of water and tree size in eastern hemlock means that an 

exponential volume or mass of insecticide should also be 

applied to trees per unit diameter to achieve a consistent 

and known concentration in the tree xylem sap across tree 

sizes. Most previous studies of imidacloprid effectiveness 

used a constant dosage per unit diameter, consistent with 

the manufacturer-recommended dosage (MFR) (Doccola 

and others 2003, 2005; Fidgen and others 2002, McAvoy 

and others 2005, Webb and others 2003). Based on our 

earlier research on hemlock water use (Ford and others 

2007), we predict that the dosages used in those studies 

would have resulted in imidacloprid concentrations higher 

than required for a lethal dose for smaller-diameter trees 

and lower than required for lethal concentrations in larger 

trees (Cowles 2009, Cowles and others 2006). For example, 

if the recommended dosage is 0.55 g active ingredient (AI) 

cm-1 d.b.h., an 80-cm tree would receive 44 g of AI, while 

a 20-cm tree would receive 11 g of AI. Under the same 

climatic conditions, estimated daily water use by those trees 

would predict mean imidacloprid concentration of 0.22 g 

l

-1 for the 80-cm tree and 2.2 g l-1 for the 20-cm tree. This 

tenfold difference in imidacloprid concentration could affect 

the effectiveness of HWA treatments and result in variable 

efficacy among trees of different sizes (Byrne and Toscano 

2006, Doccola and others 2003). 

Imidacloprid is a highly effective insecticide (Elbert and 

others 1991, Nauen and Elbert 1994), and several studies 

have shown that HWA populations are significantly reduced 

Table 1—Characteristics of experimental trees

Treatmenta Replicate d.b.h.b

Expected 

daily water 

usec Treatment 

dosage

cm kg mg AI tree-1

1 10.9 2.64 221

2 17.4 3.93 443

3 20.9 4.86 443

4 24.6 6.09 664

5 30.8 8.89 996

6 38.5 14.22 1328

7 41.2 16.77 1661

8 52.2 32.80 3321

9 59.5 51.20 4982

Xylem transport

10 61.4 57.49 5646

1 11.6 2.76 664

2 13.3 3.06 886

3 20.9 4.86 1328

4 26.2 6.72 1661

5 31.0 9.00 1993

6 35.5 11.84 1993

7 40.1 15.68 2325

8 50.1 28.86 2989

9 54.2 37.06 3321

MFR

10 65.0 71.61 3985

a

 Treatments consisted of imidacloprid dosages scaled as a linear function of stem diameter (recommended 

by the manufacturer, MFR) or as an exponential function of stem diameter (based on a xylem transport 

model). The mass of active ingredient (AI) in the latter dosage was estimated to achieve xylem sap 

concentrations required for 100 percent HWA mortality based on controlled laboratory experiments (Cowles 

and others 2006).

b Diameter at breast height, 1.3 m above ground height.

c

 Expected daily water use was modeled using tree diameter and long-term climate data (Model 1 in Ford 

and others 2007) for October 2007. 2
on trees receiving imidacloprid compared to those not 

receiving imidacloprid (Doccola and others 2003, 2005). 

Less is known about optimal dosages, especially in the 

Southern Appalachians where the mild climate promotes 

year-round water use in eastern hemlock. Hence, our 

approach was to compare efficacy results on trees receiving 

different dosages as opposed to results from comparisons 

with untreated controls. Specifically, our objectives were 

to compare HWA infestation and new shoot production on 

trees experiencing the same climate but receiving different 

imidacloprid dosages: those based on a linear scalar of 

tree diameter (e.g., consistent with the MFR dosage), or an 

exponential scalar of tree diameter (e.g., based on xylem 

sap transport models). We hypothesized that (1) live HWA 

population will be lower on trees receiving dosages that 

account for variation in xylem sap transport compared with 

trees receiving the linearly scaled dosage treatments, and 

(2) new shoot production (a proxy for tree health) will be 

greater on trees receiving the exponentially scaled dosage 

treatment compared with trees receiving the linearly scaled 

dosage treatment. 

Methods

Study Site 

The study site was located in a cove habitat in a third-order 

treatment catchment (WS28) in the Coweeta basin in the 

Nantahala Mountain Range of western North Carolina, 

USA. WS28 is a 144-ha watershed ranging from 900 to 

1500 m in elevation with a moist valley or cove at its center 

(39 ha) wherein slopes average 27 percent. This watershed 

was a multiple-use demonstration treatment watershed, with 

the cove area thinned in 1963–64 (then a 40-year-old stand) 

to promote growth of yellow-poplar (Liriodendron tulipifera

L.), eastern hemlock, and flowering dogwood (Cornus

florida L.) (Douglass and Swank 1976, Swank 1998). 

Climate in the Coweeta basin is classified as marine, humid 

temperate (Swift and others 1988). Long-term average 

annual precipitation and temperature near WS28 are 1948 

mm and 11.6 °C, respectively (Swift and others 1988).

Within the cove, we selected 20 eastern hemlock trees 

spanning a d.b.h. range of 10 to 65 cm (table 1). No visible 

differences in health among selected trees existed. Based 

on visual inspection, all trees were infested with HWA; 

however, tree crowns were still in good condition (i.e., < 30 

percent needle loss) and appeared healthy at the beginning 

of the study. None of the trees had been previously treated 

by chemicals or biological control organisms. 

Field Treatments 

We ordered the 20 trees by diameter and divided the 

diameter range into 10 diameter categories. Two trees in 

each diameter category were assigned to receive one of 

two treatment dosages of imidacloprid. Thus, for each 

treatment, 10 trees spanning a range of diameters were 

used as replicates (n = 10 per treatment). Although the 10 

replicates were not the same size diameter, the same amount 

of diameter variation existed in the 10 replicates of both 

treatment categories. We used a range of tree diameters for 

two reasons. First, we wanted to encapsulate the natural 

variation in tree replicates that is evident in a second-growth 

Southern Appalachian forest. Second, we expected to find an 

effect of diameter with the two treatments (described below), 

and using a range of tree diameter replicates would allow 

the effect of diameter to be explored though correlation and 

regression techniques. 

The secondary peak of water use of eastern hemlock trees 

in the Southern Appalachians occurs in October (Ford 

and Vose 2007), which is also the timing of peak HWA 

feeding activity after breaking diapause (Ward and others 

2004). We applied a water use model (Model 1 in Ford and 

others 2007) to the 20 eastern hemlock trees to estimate 

average daily water use for October 2007 (table 1) based 

on tree diameter and long-term climate data. We combined 

the model-based predictions of daily water use (and thus 

potential xylem sap dilution and transport of the insecticide) 

with the HWA mortality versus imidacloprid xylem sap 

concentration relationship of Cowles and others (2006) 

extrapolated to 100 percent HWA mortality (e.g., 100 ppm 

imidacloprid concentration). We applied this dosage to half 

the trees (xylem transport treatment, n = 10, table 1 shows 

actual treatment dosages). The remaining half of the trees 

were treated with the MFR dosage (n = 10): 56–67 mg 

AI cm-1 d.b.h., following the manufacturer-recommended 

dosages, except for the three trees < 25 cm, which we dosed 

at the same rate recommended for trees > 25 cm, so that 

this group of trees would receive about the same dosage 

(table 1 shows actual treatment dosages). All trees were 

treated during the first week in October 2007. Dosages were 

applied using the appropriate number of 2 or 3 ml Mauget®

Imicide® capsules (10 percent imidacloprid, 110.7 mg ml-1, 

J. J. Mauget Co., Arcadia CA). The contents of each capsule 

were introduced into the xylem stream by drilling holes 2 

to 3 cm deep, equally spaced around the circumference of 

the tree at about 1 m above ground height. Each hole was 

angled slightly downward, the capsule nozzle was inserted 

into the hole, air bubbles were eliminated, and the contents 

of the capsule effused into the xylem stream. The contents 

of most capsules emptied within 24 hours after installation. 

If the contents of a capsule did not empty within 24 hours, 

3
the capsules were primed again by squeezing repeatedly to 

remove any air bubbles. All capsules had emptied after 72 

hours and were thus removed from each tree.

Sampling

At 4 and 56 weeks after the treatment application, we 

sampled branches from all trees by dividing the canopy into 

12 sections: vertically the canopy was divided into 3 sections 

based on height of the live crown; radially the canopy was 

divided into quarters based on cardinal direction. From each 

tree section, we excised a main branch and clipped four to 

five subsample branches and placed them into a bag with 

a towel saturated with deionized water. All samples were 

refrigerated until HWA population assessment. Counts of 

live HWA population were assessed on all 12 subsampled 

branches by randomly choosing 3 of the possible 4 to 5 

subsampled branches, and then choosing 5 sub-subsamples 

per branch. From each sub-subsample, we removed the 

terminal 4 cm of growth and counted all live HWA nymphs 

and adults on this length using a stereo microscope and a 

dissecting probe to assess HWA viability. No eggs or ovisacs 

were included in these counts. The presence or absence of 

new tree growth was also noted on each sample. Therefore, 

for each tree, 7.2 m of terminal branch growth was analyzed 

for HWA populations and the presence of new growth 

(4 cm x 12 tree sections x 3 subsample branches x 5 subsubsamples = 7.2 m). All HWA population assessments were 

completed within 6 weeks of sampling.

Statistical Analyses

To determine if significant differences in live HWA 

populations existed among trees with differing treatments, 

we used a generalized linear model (McCullagh and 

Nelder 1989) to analyze the counts of live individuals 

with a Poisson probability density function and a log link. 

We modeled HWA counts per unit length as a function 

of treatment, sampling time, and their interaction using 

analysis of deviance (a generalization of analysis of 

variance, McCullagh and Nelder 1989) (PROC GENMOD, 

SAS Institute, Inc. Cary, NC, USA). 

To determine if significant differences in the presence or 

absence of new shoot growth existed among trees with 

differing treatments, we again used a generalized linear 

model with a binomial probability density function and a 

logit link. We modeled the proportion of samples with the 

presence of new growth as a function of treatment, sampling 

time, and their interaction using analysis of deviance (PROC 

GENMOD). We used the Type III approach to both analyses 

of deviance and reported the estimated least-squares means 

where appropriate. All statistical tests were performed at 

α = 0.05 level.

We computed simple correlation coefficients (Pearson’s R) 

between the difference in proportion of new shoot growth 

in 2008 from 2007 and d.b.h. for each tree. Because we 

expected this relationship to be negative for the trees with 

linearly scaled dosage treatments, we utilized a one-tailed 

test of the transformed Fisher’s z statistic. Finally, we 

computed a simple nonparameteric correlation coefficient 

(Spearman’s RS

) between the presence of new shoot growth 

and live HWA population density across both sampling 

times and treatments. 

Results and Discussion

Live HWA population density differed significantly between 

treatments ( c

2

0.05,1 = 23.75, P < 0.001) and sampling times 

( c

2

0.05,1 = 6.18, P = 0.01); however, the relationship between 

treatments was not consistent between sampling times 

(interaction c

2

0.05,1 = 7.62, P = 0.006; fig. 1). At 4 weeks after 

Figure 1—Live hemlock woolly adelgid (HWA) population density 

on trees assessed 4 and 56 weeks after treatment. Treatments 

are imidacloprid dosages based on linear (MFR, manufacturerrecommended) or exponential (xylem transport model) function 

of stem diameter. Vertical bars are the least-squares means 

(LSMEAN) and error bars are one standard error of the LSMEAN. 

Vertical bars with different lowercase letters are statistically 

different between times and treatments at α = 0.05.

Weeks after treatment

4 56

Live HWA density (nymphs & adults cm-1) 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1 Xylem transport

MFR

a a

b

c

4
treatment application, live HWA population density was 32 

percent lower (LSMEANS) on trees receiving the xylem 

transport dosage treatment compared with trees that received 

the MFR dosage treatment. Hence, HWA control was 

more effective when dosage was adjusted for differences 

in xylem sap transport to provide a lethal concentration 

across all tree sizes, thus supporting our first hypothesis. 

At 56 weeks after treatment, live HWA population density 

remained lower on trees that received the xylem transport 

dosage treatment compared with trees that received the MFR 

dosage treatment; however, it was only 10 percent lower 

(LSMEANS). The live HWA population density did not 

change over time on trees that received the MFR 

dosage treatment.

The proportion of new shoot growth also differed 

significantly between treatments ( c

2

0.05,1 = 9.30, P = 

0.002) and sampling times ( c

2

0.05,1 = 82.26, P < 0.001). 

The increase in new shoot growth between sampling times 

was significantly greater in trees that received the xylem 

transport dosage treatment compared with trees that received 

the MFR dosage treatment (interaction c

2

0.05,1 = 10.37, 

P = 0.001; table 2), confirming our second hypothesis. 

Differences in new shoot growth assessed at 4 weeks 

after treatment reflected conditions before treatments, 

as this shoot growth occurred several months before our 

experimental treatments began. New shoot growth assessed 

at 56 weeks reflected the growth that was put on the spring 

after imidacloprid exposure, and this increased for both 

treatment groups. On average, trees from both treatments 

had 95 percent of samples showing new shoot production; 

Table 2—Proportion of sample branches on each tree with new shoot growth at 4 and 56 

weeks after treatment, and the factor of increase between sampling times

Proportion of sample branches 

with new growthb

Treatmenta 4 weeks 56 weeks

Factor of increase in 

LSMEANS estimates 

of proportion of new 

growth from pre- to 

post-treatment

Xylem transport 

LSMEAN

(Upper–Lower)

0.01

(0.003–0.03) c

0.61

(0.51–0.74) a

66.2

MFR

LSMEAN

(Upper–Lower)

0.12

(0.09–0.16) b

0.59

(0.48–0.69) a

4.8

a Treatments consisted of imidacloprid dosages scaled as a linear function of stem diameter (recommended by 

the manufacturer, MFR) or as an exponential function of stem diameter (based on a xylem transport model).

b

 Least-squares means (LSMEAN) and upper and lower standard error plus LSMEAN given. Different 

lowercase letters beside treatment LSMEANS are statistically different between times and treatments at 

a=0.05.

Figure 2—Change in new shoot growth among trees spanning 

a range of stem diameters (d.b.h.), i.e., the difference in 

proportion of sample branches showing presence of new growth 

from pretreatment to 56 weeks post-treatment. Treatments are 

imidacloprid dosages based on linear (MFR, manufacturerrecommended) or exponential (xylem transport model) function 

of stem diameter. Line shows an ordinary least–squares regression 

fit to MFR data (R2

 = 0.54, P = 0.02; y = 0.59 – 0.007x). One tree 

from each treatment group showed no new shoot production after 

treatment, thus n = 9 for both treatments.

DBH (cm)

0 10 20 30 40 50 60 70

Change in new shoot growth 

(proportion of samples showing presence of new growth) 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1 Xylem transport

MFR

5
however, from one growing season to the next, trees 

receiving the xylem transport dosage treatment had a greater 

increase in new shoot growth (i.e., a 66-fold increase from 

4 to 56 weeks for xylem transport versus a 5-fold increase 

for MFR) compared with trees receiving the MFR dosage 

treatments (table 2). The difference in proportion of new 

shoot growth from 2007 to 2008 was negatively correlated 

with d.b.h. in the trees receiving the MFR treatment 

(R = 0.74, z = 0.95, P = 0.01; fig. 2), indicating that the 

larger-sized trees did not respond as well as smaller-sized 

trees in this treatment. In contrast, there was no correlation 

between tree size and new shoot growth in xylem transport 

dosage treatments (R = 0.28, z = 0.29, P = 0.74; fig. 2).

Using dosages based on xylem transport models compared 

to manufacturer-recommended dosages resulted in a lower 

amount of imidacloprid applied to small diameter trees, but 

a greater amount applied to large diameter trees. The tree 

diameter where the amount of imidacloprid applied using 

a dosage based on the xylem transport model equaled the 

amount applied using the MFR dosage was ~48 cm. Live 

HWA population densities showed that for small diameter 

trees, the xylem transport and MFR dosages were equally 

effective. In contrast, in the larger-diameter trees, xylem 

transport dosages were more effective than the MFR 

dosages. These results confirm those of Cowles (2009), who 

found that low dosages of soil-applied imidacloprid were 

effective at reducing HWA populations on small diameter 

trees (0.125 g AI 2.5 cm-1 d.b.h.), but that the effective 

dosage required to reduce HWA populations by the same 

amount on large diameter trees was fourfold greater (0.5 g 

AI 2.5 cm-1 d.b.h.). Results from current and previous studies 

(Ford and Vose 2007, Ford and others 2007) suggest that the 

primary mechanism of systemic imidacloprid effectiveness 

against HWA is linked to the volume and velocity of xylem 

water movement, and thus concentration of the active 

ingredient in the xylem sap. By extension, we would expect 

efficacy of other systemic insecticides (e.g., dinotefuran) to 

require an exponentially scaled dosage of d.b.h., including 

those that are soil applied and absorbed through the root 

system (e.g., soil drench or soil injection). Previous studies 

show some indication that soil-applied imidacloprid is 

more effective at reducing HWA population density than 

stem-injected imidacloprid treatment because it increases in 

concentration in the foliar tissue over time (Tattar and others 

1998) and remains effective at reducing HWA populations 

for many years (Cowles 2009). A direction for future work 

would be to use a similar approach as presented here to 

optimize soil imidacloprid applications across tree sizes and 

climates with information on tree water use. 

Numerous studies have shown that imidacloprid is an 

effective chemical treatment against HWA. Although 

we did not sample HWA populations on our trees before 

treatment, live HWA population density in non-treated 

eastern hemlock trees sampled in the Coweeta basin during 

Table 3—Example of tree size and density in Tsuga canadensis-dominated (comprising at 

least 50 percent of the basal area) riparian areas in the Coweeta basin

Treatmenta

Replicate plot MFR

Xylem 

transport

Hemlock 

diameter range

Hemlock 

density 

---------- g AI ha-1 ----------- -------- cm -------- trees ha-1

1 537.78 320.62 2.6 44.4 375

2 789.25 603.09 7.2 60.5 475

3 674.68 531.43 6.5 63.3 450

4 527.70 243.13 4.0 34.3 575

5 1027.70 662.46 2.8 61.0 875

6 748.16 391.99 1.9 50.3 1,150

7 592.20 302.98 1.8 46.1 950

8 485.63 788.75 2.8 70.5 375

Average 

(SE)

672.89 

(63.53)

480.56 

(69.05)

a

 Potential amount of imidacloprid (active ingredient, AI) applied per hectare if all hemlock trees were treated 

with dosages calculated according to the manufacturer’s recommendations (MFR) or the xylem transport model.

6
the study period (fall 2008) averaged 2.1 (±0.5 SE) nymphs 

and adults cm-1.

1 Compared with our experimentally treated 

trees, these untreated trees had live HWA populations that 

were about threefold higher, confirming the effectiveness of 

imidacloprid for controlling HWA. While our exponentially 

scaled treatment dosage was intended to achieve a 

concentration in the tree that would result in 100 percent 

HWA mortality, live HWA nymphs and adults were still 

evident on our treated trees at 4 and 56 weeks following 

treatment. Previous studies have shown that a single 

application of soil-applied imidacloprid can remain effective 

against HWA for at least 3 years (Cowles 2009); however, 

less is known about the long-term effectiveness of steminjected imidacloprid.

Management Implications

Trees receiving dosages based on a xylem transport model 

were significantly healthier compared with dosages based 

on a linear increase in AI with tree diameter—they had 

significantly lower densities of live HWA populations 

and a greater proportion of new shoot growth. Hence, our 

results clearly indicate that improved control is possible 

when dosages are adjusted for climate and tree size-based 

variation in xylem sap flow. Because of the effectiveness of 

imidacloprid as an insecticide, land managers are limited to 

a maximum application of 0.45 kg AI ha-1 in the Southern 

Appalachians to prevent nontarget and offsite impacts. Our 

xylem transport-based dosage application resulted in less AI 

applied (19 705 mg for xylem transport versus 21 145 mg 

for MFR) to the same number of similar-sized trees (table 

1) and resulted in greater HWA control and tree recovery. 

To examine this outcome further, we used tree diameter and 

density data from eight randomly located plots in riparian 

areas within the Coweeta basin that contained at least 

50 percent T. canadensis (Nuckolls and others 2009) to 

calculate dosage. Using those data, dosage determined with 

the xylem transport model would require 29 percent less 

AI applied per ha compared with the MFR dosage (table 

3). The combination of increased effectiveness and lower 

application rates suggests that our xylem transport dosages 

have the potential to significantly improve the control of 

HWA in the Southern Appalachians. Chemical treatment 

of eastern hemlock for HWA infestation is a short-term 

solution, and treatments are often reapplied after a few 

years. In riparian trees that may warrant systemic injection 

of imidacloprid to protect against contamination of surface 

water sources, dosages based on xylem transport would 

likely result in increased tree health and fewer live HWA 

infesting large-diameter trees compared with MFR dosages. 

In nonriparian areas, improving treatment dosages would 

not only reduce cost and nontargeted effects (Dilling and 

others 2009, Kreutzweiser and others 2009, Raupp and 

others 2004), but would potentially allow land managers to 

treat a greater number of trees in most Southern Appalachian 

stands. Finally, a spreadsheet application that may be used 

to estimate tree water use for a given tree diameter and 

treatment window, as well as the dosage (g AI) based on the 

xylem transport of the systemic insecticide, can be found at

http://www.srs.fs.usda.gov/pubs/ja/31601-Model_application.xls.

Acknowledgements

This study was supported by the Forest Service, U.S. 

Department of Agriculture, Southern Research Station 

and NSF grant NSF-DEB #0218001 to the Coweeta LTER 

program. We acknowledge the field and laboratory support 

of R. McCollum, S. Cladis, M. Hutchins, K. Poorman, C. 

R. Maier, and L. Wear. We are grateful to K. Klepzig, M.E. 

Montgomery, and S. Salom for providing helpful comments 

on a previous version of this manuscript. 

Literature Cited

Byrne, F.J.; Toscano, N.C. 2006. Uptake and persistence of imidacloprid in 

grapevines treated by chemigation. Crop Protection. 25: 831–834.

Castle, S.J.; Byrne, F.J.; Bi, J.L.; Toscano, N.C. 2005. Spatial and temporal 

distribution of imidacloprid and thiamethoxam in citrus and impact on 

Homalodisca coagulata populations. Pest Management Science. 61: 75–84.

Cobb, R.C.; Orwig, D.A.; Currie, S. 2006. Decomposition of green foliage 

in eastern hemlock forests of southern New England impacted by hemlock 

woolly adelgid infestations. Canadian Journal of Forest Research. 36(5): 

1331–1341.

Cowles, R.S. 2009. Optimizing dosage and preventing leaching of imidacloprid 

for management of hemlock woolly adelgid in forests. Forest Ecology and 

Management. 257: 1026–1033.

Cowles, R.S.; Montgomery, M.E.; Cheah, C.A.S.-J. 2006. Activity and residues 

of imidacloprid applied to soil and tree trunks to control hemlock woolly 

adelgid (Hemiptera: Adelgidae) in forests. Journal of Economic Entomology. 

99(4): 1258–1267.

Dilling, C.; Lambdin, P.; Grant, J.; Rhea, R. 2009. Community response of 

insects associated with eastern hemlock to imidacloprid and horticultural oil 

treatments. Environmental Entomology. 38(1): 53–66.

Doccola, J.J.; Ramasamy, I.; Castillo, P. [and others]. 2005. Erratum: Efficacy 

of Arborjet VIPER microinjections in the management of hemlock woolly 

adelgid (Adelges tsugae). Journal of Arboriculture. 31(4): 203–206.

1

B. Reynolds, 2009. Summary report. Effects of Imidacloprid application on soil microarthropods and hemlock woolly adelgids at Coweeta Hydrologic Laboratory, SRS 07-CA11330140_064. On file with: Barbara C. Reynolds and Laura Wear, UNC-Asheville, Department of Environmental Studies, Asheville, NC 28804, and Chelcy Ford, U.S. Department of 

Agriculture Forest Service, Southern Research Station, Coweeta Hydrologic Lab, Otto, NC 28763.

7
Doccola, J.J.; Wild, P.; Ramasamy, I. [and others]. 2003. Efficacy of Arborjt 

VIPER microinjections in the management of hemlock woolly adelgid 

(Adelges tsugae). Journal of Arboriculture. 29(6): 327–330.

Douglass, J.E.; Swank, W.T. 1976. Multiple use in Southern Appalachian 

hardwoods: A 10-year case history. In: Proceedings of XVI IUFRO World 

Congress Division 1. Distributed by IUFRO Secretariat, Sch’dnbrunn - 

Tirolergarten, A-1131 Vienna, Austria: 425–436.

Elbert, A.; Becker, B.; Hartwig, J.; Erdelen, C. 1991. Imidacloprid: A new 

systemic insecticide. Pflanzenschutz-Nachrichten. 44(2): 113–136.

Eschtruth, A.K.; Cleavitt, N.L.; Battles, J.J. [and others]. 2006. Vegetation 

dynamics in declining eastern hemlock stands: 9 years of forest response to 

hemlock woolly adelgid infestation. Canadian Journal of Forest Research. 

36(6): 1435–1450.

Fidgen, J.G.; McClellan, Q.C.; Salom, S.M. 2002. Efficacy and residual 

activity of two systemic insecticides for control of hemlock woolly adelgid 

on young eastern hemlocks. In: Onken, B.; Reardon R.; Lashomb, J, eds. 

Hemlock Woolly Adelgid in the Eastern United States Symposium. East 

Brunswick, NJ: U.S. Department of Agriculture Forest Service: 329-334. 

Ford, C.R.; Vose, J.M. 2007. Tsuga canadensis (L.) Carr. mortality will impact 

hydrologic processes in southern Appalachian forest ecosystems. Ecological 

Applications. 17(4): 1156–1167.

Ford, C.R., Vose, J.M.; Daley, M.; Phillips, N. 2007. Use of water by eastern 

hemlock: implications for systemic insecticide application. Arboriculture 

and Urban Forestry. 33(6): 421–427.

Jenkins, J.C.; Aber, J.D.; Canham, C.D. 1999. Hemlock woolly adelgid 

impacts on community structure and N cycling rates in eastern hemlock 

forests. Canadian Journal of Forest Research. 29: 630–645.

Kreutzweiser, D.P.; Thompson, D.G.; Scarr, T.A. 2009. Imidacloprid in leaves 

from systemically treated trees may inhibit litter breakdown by non-target 

invertebrates. Ecotoxicology and Environmental Safety. 72: 1053–1057.

McAvoy, T.; Mays, W.T.; Salom, S.M.; Kok, L.T. 2005. Impact of imidacloprid 

on hemlock woolly adelgid and water quality at Mt. Lake, Virginia. In: 

Onken, B.; Reardon, R., eds. Third Symposium on hemlock woolly adelgid 

in the Eastern United States. U.S. Department of Agriculture Forest Service, 

Asheville, NC. 324–334.

McClure, M.S. 1991. Density-dependent feedback and population cycles 

in Adelges tsugae (Homoptera: Adelgidae) on Tsuga canadensis. 

Environmental Entomology. 20: 258–264.

McClure, M.S.; Cheah, C.A.S.-J. 1999. Reshaping the ecology of invading 

populations of hemlock woolly adelgid, Adelges tsugae (Homoptera: 

Adelgidae), in eastern North America. Biological Invasions. 1: 247–254.

McCullagh, P.; Nelder, J.A. 1989. Generalized linear models. London: 

Chapman and Hall. 532 p.

Nauen, R.; Elbert, A. 1994. Effect of imidacloprid on aphids after 

seed treatment of cotton in laboratory and greenhouse experiments. 

Pflanzenschutz-Nachrichten. 47: 177–210.

Nuckolls, A.; Wurzburger, N.; Ford, C.R. [and others]. 2009. Hemlock declines 

rapidly with hemlock woolly adelgid infestation: impacts on the carbon 

cycle of Southern Appalachian forests. Ecosystems. 12(2): 179–190.

Orwig, D.A.; Foster, D.R. 1998. Forest response to the introduced hemlock 

woolly adelgid in southern New England, USA. Journal of the Torrey 

Botanical Society. 125(1): 60–73.

Orwig, D.A.; Foster, D.R.; Mausel, D.L. 2002. Landscape patterns of hemlock 

decline in New England due to the introduced hemlock woolly adelgid. 

Journal of Biogeography. 29(10–11): 1475–1487.

Raupp, M.J.; Webb, R.E.; Szczepaniec, A. [and others]. 2004. Incidence, 

abundance, and severity of mites on hemlocks following applications of 

imidacloprid. Journal of Arboriculture. 30(2): 108–113.

Skinner, M.; Parker, B.L.; Gouli, S.; Ashikaga, T. 2003. Regional responses 

of hemlock woolly adelgid (Homoptera: Adelgidae) to low temperatures. 

Environmental Entomology. 32(3): 523–528.

Stadler, B.; Müller, T.; Orwig, D.; Cobb, R. 2005. Hemlock woolly adelgid in 

New England forests: Canopy impacts transforming ecosystem processes 

and landscapes. Ecosystems. 8(3): 233–247.

Steward, V.B.; Horner, T.A. 1994. Control of hemlock woolly adelgid using 

soil injections of systemic insecticides. Journal of Arboriculture. 20: 

287–288.

Swank, W.T. 1998. Multiple use forest management in a catchment context. In: 

Cresser, M.; Pugh, K., eds. Multiple land use and catchment management: 

Proceedings of an international conference. Aberdeen, Scotland UK: The 

Macaulay Land Use Research Institute: 27–37.

Swift, L.W.; Cunningham, G.B.; Douglass, J.E. 1988. Climate and hydrology. 

In: Swank, W.T.; Crossley, D.A., eds. Forest hydrology and ecology at 

Coweeta. Ecological studies. NY: Springer-Verlag: 35–55. Vol. 66.

Tattar, T.A.; Dotson, J.A.; Ruizzo, M.S.; Steward, V.B. 1998. Translocation of 

imidacloprid in three tree species when trunk- and soil-injected. Journal of 

Arboriculture. 24(1): 54–56.

U.S. Department of Agriculture. 2006. Hemlock Woolly Adelgid Infestation 

Map-2005. Durham, NH: U.S. Department of Agriculture Forest Service 

GIS Group.

Ward, J.S.; Cheah, C.A.S.-J.; Montgomery, M.E. [and others]. 2004. Eastern 

hemlock forests: Guidelines to minimize the impacts of hemlock woolly 

adelgid. NA TP 03 04: 34. Morgantown, WV: U.S. Department of 

Agriculture Forest Service, Northeastern Area State & Private Forestry. 28 p.

Webb, R.E.; Frank, J.R.; Raupp, M.J. 2003. Eastern hemlock recovery from 

hemlock woolly adelgid damage following imidacloprid therapy. Journal of 

Arboriculture. 29(5): 298–302.

8


Ford, Chelcy R. ; Reynolds, Barbara C. ; Vose, James, M. 2010. Xylem 

transport models optimize effectiveness of systemic insecticide applications 

for controlling hemlock woolly adelgid (Adelges tsugae). Gen. Tech. Rep. 

SRS–120. Asheville, NC: U.S. Department of Agriculture Forest Service, 

Southern Research Station. 8 p.

The hemlock woolly adelgid (HWA, Adelges tsugae Annand) is causing 

widespread decline and mortality of eastern hemlock trees (Tsuga canadensis

(L.) Carr.). Stem injection of insecticide is widely used as a control measure, 

but its effectiveness depends on individual tree hydraulic characteristics. 

Recent work has shown that eastern hemlock daily water use is exponentially 

related to tree diameter, with smaller-diameter trees using significantly less 

water than larger-diameter trees. In this study we modeled daily water use 

for 20 eastern hemlock trees across a range of diameters. Based on expected 

daily water use and, thus, potential xylem transport of insecticide, we applied 

a dosage estimated to achieve a lethal and uniform xylem sap concentration 

of imidacloprid to half the trees (xylem transport treatment), and treated 

the remaining half based on the manufacturer-recommended dosage (MFR 

treatment), which is a linear function of tree diameter. At 4 and 56 weeks 

after treatment, we assessed all trees for the presence or absence of new shoot 

growth and live HWA population density. We found that both treatment dosages 

significantly reduced live HWA populations; however, 4 weeks after treatment, 

live HWA population density was 32 percent lower (LSMEANS) on xylem 

transport treatment trees compared with MFR treatment trees (treatment by time 

interaction, P = 0.006). Both treatment dosages also significantly increased the 

proportion of new shoot growth; however, over time xylem transport treatment 

trees had a significantly greater increase in new shoot production compared 

with MFR treatment trees. We conclude that dosages based on a xylem transport 

model not only significantly improved tree health more than the trees receiving 

the current recommended dosage, but also caused greater reductions in the live 

HWA population. Improving treatment dosages would reduce treatment cost, 

nontargeted effects, and would allow land managers to treat a greater number 

of trees.

Keywords: Eastern hemlock, hemlock woolly adelgid, imidacloprid, insecticide 

efficacy, sap flow, systemic insecticide, transpiration.
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