Intraspecific Variation in Tsuga canadensis Foliar
Chemistry
Laura Ingwell, Joseph Brady, Matthew Fitzpatrick, Brian Maynard,
Richard Casagrande, and Evan Preisser
Northeastern Naturalist, Volume 16, Issue 4 (2009): 585–594
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2009 NORTHEASTERN NATURALIST 16(4):585–594
Intraspecific Variation in Tsuga canadensis Foliar
Chemistry
Laura Ingwell1, Joseph Brady2, Matthew Fitzpatrick1, Brian Maynard3,
Richard Casagrande3, and Evan Preisser1,*
Abstract - Three groups of Tsuga canadensis (Eastern Hemlock) trees were
analyzed to compare their chemical composition and the potential for naturally
occurring resistance to Adelges tsugae (Hemlock Woolly Adelgid [HWA]). Potentially
resistant “parent” trees located in southern Connecticut were compared with
rooted propagules from those same trees and control trees located in northern Vermont,
outside of the current HWA range. For trees in each group, we quantified Ca,
P, K, C, and N and developed terpenoid profiles using solid-phase microextraction
(SPME) and gas chromatography-mass spectrometry (GC/MS). There was no significant
variation in terpenoid profiles between the three groups of hemlock trees.
Propagules retained elevated levels of Ca and N from fertilization during propagation,
suggesting that their chemical composition does not mirror the parent trees.
The potentially resistant “parent” trees had higher levels of K compared to control
trees. This may impart some level of tolerance/resistance to HWA and explain their
persistence in hemlock forests that have otherwise been decimated by HWA. Comparison
to regional foliar chemistry databases suggest that while rare, such elevated
K levels do exist in natural hemlock populations. Such individuals may persist as
HWA continues to spread across the region.
Introduction
The invasive Adelges tsugae Annand (Hemlock Woolly Adelgid [HWA])
is a major threat to Tsuga canadensis (L.) Carr. (Eastern Hemlock) and Tsuga
caroliniana Engelm. (Carolina Hemlock), leading to massive mortality
within hemlock forests throughout the eastern United States. Since its introduction
from Asia to Virginia in the 1950s, HWA has spread north and east
across the United States. It is thought to be limited in the northern portion
of its range by colder climates (Parker et al. 1999). There are nine species of
Tsuga occurring worldwide: the two eastern US species mentioned above,
two that occur in western North America, and five Asian species (Farjon
1990). Of these nine species, mortality resulting from HWA infestation appears
to occur primarily in Eastern and Carolina Hemlock (McClure 1992,
McClure et al. 2001).
The mechanistic basis for hemlock resistance to adelgid infestations has
not yet been identified. However, HWA induces tree mortality only after a
prolonged period of heavy infestation, depending on geographic locality.
1Department of Biological Sciences, University of Rhode Island, Kingston, RI
02881. 2Department of Chemistry, University of Rhode Island, Kingston, RI 02881.
3Department of Plant Sciences, University of Rhode Island, Kingston, RI 02881.
*Corresponding - preisser@uri.edu.
586 Northeastern Naturalist Vol. 16, No. 4
HWA uses a long stylet bundle to feed on ray parenchyma cells, depleting
stored nutrients and resulting in reduced growth, needle loss, and, ultimately,
mortality (McClure 1991, Young et al. 1995). In its native range, HWA occurs
in very low densities and appears to have little detrimental effect on
infested hemlocks (McClure 1999, McClure and Cheah 1999). One explanation
for the low densities and minimal effects of HWA on hemlocks in
their native range may be that the native hemlock species possess chemical
defenses that limit HWA infestation and subsequent damage.
Several studies have examined the foliar chemistry of both HWAsusceptible
and -resistant hemlock species. Many studies have focused on
terpenoids, foliar chemicals that are an important nutritional element for
herbivores (e.g., McClure and Hare 1984). Terpenes vary greatly within and
among a plant species and are commonly used to characterize resistance
or tolerance of host species. Lagalante and Montgomery (2003) characterized
the terpenoid profiles of seven Tsuga species: Eastern Hemlock,
Carolina Hemlock, and five others that are known to be resistant to HWA.
Their research identified five key terpenes (germacrene D, α-humulene,
β-caryophyllene, isobornyl acetate, and α-pinene) whose concentrations
differed markedly in susceptible versus resistant hemlock species. They suggest
that these terpenes may play a role in determining the degree of hemlock
susceptibility/resistance to HWA infestation. Lagalante et al. (2006) also documented
seasonal and spatial variation in Eastern Hemlock terpenoids. HWA
aestivation coincided with periods of high terpenoid content, suggesting that
adelgids may avoid increased concentrations of these chemical compounds.
In related research, Pontius et al. (2002) examined the foliar chemistry of
Eastern Hemlock and four HWA-resistant hemlock species and identified
four cations (potassium, calcium, phosphorus, and nitrogen) that may play
a role in hemlock susceptibility to HWA infestations; their study also linked
HWA-infestation densities and decline rates to the foliar cations measured.
While there is no published evidence of HWA resistance and/or tolerance
in either Eastern or Carolina Hemlocks, a few healthy looking Eastern
Hemlock trees persisting amidst devastated stands have been found during
landscape-level surveys of HWA and Fiornia externa Ferris (Elongate Hemlock
Scale [EHS]) in New England forests (Preisser et al. 2008). The healthy
looking trees are described as having little or no needle loss, deep green
color, intact canopy, and full, thick branches (Ingwell 2007).
As part of a research project examining the potential for naturally occurring
HWA resistance in Eastern Hemlocks, we report the results of a study
examining the chemical characteristics of Eastern Hemlocks. Specifically,
we examined the terpenoid profiles and cation concentrations of three groups
of Eastern Hemlock trees. The first “parent” group was comprised of potentially
HWA-resistant trees located in Connecticut. The second “control”
group was comprised of trees located to the north of HWA’s current range.
The third “propagule” group consisted of cuttings taken from the potentially
resistant “parent” trees in Connecticut and grown under controlled conditions
at greenhouse facilities at the University of Rhode Island.
2009 L. Ingwell, J. Brady, M. Fitzpatrick, B. Maynard, R. Casagrande, and E. Preisser 587
Materials and Methods
We have initiated a study to locate, propagate, and experimentally evaluate
the foliar chemistry in these rare individual surviving trees. The criteria
used to identify potentially resistant Eastern Hemlock trees have been covered
in detail elsewhere (Caswell et al. 2008). Briefly, candidate trees must
be mature (>10 m in height), healthy (deep green needles, full and thick
branches) trees, and little colonized by HWA. These trees must be growing
in stands that have >95% mortality of neighboring hemlocks and have not
been treated by pesticides or horticultural oils.
Plant material
Foliage from mature Eastern Hemlock was gathered from forest stands
in Connecticut, Vermont, and from trees grown at the University of Rhode
Island’s East Farm (Kingston, RI). Six trees that have been identified as potentially
resistant to HWA infestation, located in three different forests near
the towns of East Haddam, Madison, and Old Lyme, CT, were sampled and
are hereafter referred to as the “parent” group. Cuttings were taken from these
potentially HWA-resistant “parent” individuals in 2005 and 2006 and grown
under controlled conditions at URI (Caswell et al. 2008). We sampled foliage
from the propagules (= rooted cuttings in cultivation at East Farm) of five of
these Connecticut trees, which are hereafter referred to as the “propagule”
group. Foliage from five mature Eastern Hemlocks from HWA-free areas near
Springfield, VT served as the “control” group. Samples from all three groups
were collected in November 2007. Two branch cuttings were taken from each
of the cardinal directions, for a total of eight cuttings per tree. Cuttings were
placed in hydrated floral foam and returned to the lab for analysis.
Cation analysis
To minimize idiosyncratic variation in foliar chemistry, we followed the
protocol of Lagalante et al. (2006) and only analyzed foliage from the previous
year’s growth. Prior to needle collection, any scale insects, adelgids, or
other organisms on the samples were manually removed using a stainless
steel forceps or spatula. Two 20-mL disposable scintillation vials were filled
with excised needles for each sample and sent to the University of Georgia
Stable Isotope/Soil Biology Laboratory (www.uga.edu/sisbl) for quantification
of C, N, Ca, P, and K following the standard methods employed by UGA
and described in Allen (UGA) (1974) and Jones et al. (1990).
Terpenoid analysis
Two samples were collected from each tree; each sample consisted of one
needle from each cardinal direction (four needles per sample). Prior to needle
collection, any scale insects, adelgids, or other organisms on the samples
were removed. Samples were prepared following methods in Lagalante and
Montgomery (2003), with the exception of using a 10-mL headspace vial,
and stored at -20 °C. All samples were processed within one month of collection
date.
588 Northeastern Naturalist Vol. 16, No. 4
Volatile compounds were equilibrated in the headspace vial and loaded
onto the solid-phase microextraction (SPME) following Lagalante and
Montgomery (2003), with the exception of using a 65-μm PDMS fiber and
equilibration times from one to three hours. Samples were analyzed using
an Agilent 6890N GC coupled with an Agilent 5973i mass selective detector
(MSD). The inlet temperature was 275 °C with a 2:1 split ratio. The SPME
was inserted into the injection port for two minutes for sample desorption.
Separation was accomplished with a 7.0 m HP-5MS column with a nominal
diameter of 250 μm and film thickness of 0.25 μm, with a constant flow of
3.0 mL/min. The oven was programmed to begin at 40 °C for two minutes
before ramping at a rate of 3 °C/min to a final temperature of 94 °C followed
by a two-minute post-run at 280 °C. The transfer line between the GC
and the MS was held at 300 °C. The MSD was tuned daily using Agilent's
STUNE.U program. The electron multiplier's voltage was set to zero relative
to the optimized voltage (1200–1400 eV).
On the basis of previous work showing significant variation between the
terpenoid profiles of susceptible and resistant hemlock species, (Lagalante
and Montgomery 2003, Lagalante et al. 2006) we concentrated on a subset
of seven hemlock terpenoids: α-pinene, myrcene, piperitone, isobornyl
acetate, β-caryophyllene, α-humulene, and δ-cadinene. These were identified using both a mass spectrum database search (NIST MS Library 2002)
and compared retention indices reported on a DB-5 column (Adams 2007).
Authentic samples (Wilkem Scientific, Pawtucket, RI) for isobornyl acetate,
α-humulene, α-pinene, and β-caryophyllene were compared to retention
indices for experimental samples. The chromatogram peaks were integrated
and relative quantity determined following Lagalante and Montgomery
(2003) for each of the seven compounds.
Statistical analysis
Terpenoid and cation concentrations were compared among the parent
and control groups using one-way ANOVA. Because the parent and
propagule groups came from the same individuals, they were compared
using a matched-pairs analysis. This analysis allowed us to evaluate the
effect of propagation techniques on the foliar chemistry of the trees. All Pvalues
were corrected for multiple comparisons at α = 0.05 using a step-up
FDR correction (Benjamini and Hochberg 1995). All data were checked for
normality prior to analysis, and all analyses were performed using JMP v.7
(SAS Institute, Inc., Cary, NC).
Results
Cations
The three groups differed in their relative abundance of K, P, N, Ca,
and C (Fig. 1). Parent trees had significantly more K than did control trees
(F1,9=10.11, P < 0.05 after step-up FDR adjustment), but did not differ in other
cations (all adjusted P > 0.05). Compared to the parent group, propagules
2009 L. Ingwell, J. Brady, M. Fitzpatrick, B. Maynard, R. Casagrande, and E. Preisser 589
had significantly more Ca (paired t-test with 4 d.f. = -7.82, adjusted P less than
0.05) and N (paired t-test with 4 d.f. = -5.64, adjusted P < 0.05). Parents and
propagules did not differ in their concentrations of K, P, and C.
Figure 1. Mean cation concentrations ± standard error of five measured cations for each
of the three tested groups. Parent trees have significantly more K than control trees (P less than
0.05). Propagules have significantly more Ca and N than parent trees (P less than 0.05).
590 Northeastern Naturalist Vol. 16, No. 4
Terpenoids
The relative concentrations of the seven quantified terpenoids did not differ
between the parent and control groups (all F1,9, adjusted P > 0.05; Fig. 2).
When compared to the parent group, propagules had significantly more piperitone
(paired t-test with 4 d.f. = 3.44, adjusted P < 0.05), but did not differ in
the relative concentrations of the other terpenoids (all adjusted P > 0.05).
Figure 2. Mean
relative area percent
± standard
error of seven
terpenoids for
each of the three
tested groups.
Parent trees and
control trees
did not differ in
their terpenoid
concentrations,
while parent
trees have significantly
more
piperitone than
propagules (P less than
0.05).
2009 L. Ingwell, J. Brady, M. Fitzpatrick, B. Maynard, R. Casagrande, and E. Preisser 591
Discussion
Intraspecific variation in Eastern Hemlock chemistry both between and
among groups supports the idea that foliar chemistry may contribute to HWA
tolerance or resistance. Such chemical variation reflects the nutritional and
defensive chemistry of the host tree, a critical component in determining
whether phytophagous insects such as HWA can successfully establish and
persist (Montgomery and Lagalante 2008).
We found that the potentially HWA-resistant “parent” trees and the
“control” trees from outside of the current range of HWA differed in some
aspects of their foliar chemistry. Although it would have been ideal to compare
the foliar chemistry of the potentially resistant parent trees to other
trees in the immediate area, the massive HWA-related hemlock mortality in
these forests prevented us from doing so. As a result, we cannot reject the
possibility that site, climatic, and regional factors may account for some of
the differences between these individuals.
While there were no apparent differences in terpenoid composition, parent
trees had elevated levels of potassium relative to the control trees. Potassium
is a key chemical required by plants to activate enzymes involved in growth,
regulate the opening and closing of stomates, and carry on photosynthesis.
Pontius et al. (2006) found lower concentrations of potassium among HWAresistant
species as well as a positive correlation between potassium concentrations
and HWA-infestation levels and hemlock decline symptoms. Trees
that had high levels of potassium supported larger populations of HWA and
displayed increased symptoms of decline, leading the authors to suggest that
HWA alters the chemistry of the host tree. They concluded that this chemical
may be a limiting factor to HWA population growth. The increased level of
potassium in the parent trees could be a result of HWA infestations; however,
the trees sampled in the parent group do not support HWA populations, as reflected by their persistence in such a devastated environment.
Other research, however, has found a negative correlation between potassium
and insect population growth on herbivore-resistant alfalfa plants
tested with Therioaphis maculate Buckton (Spotted Alfalfa Aphid) (Kindler
and Staples 1970) as well as in resistant sorghum plants attacked by
Schizaphis graminum Rondani (Greenbug) (Schweissing and Wilde 1979).
Potassium is very important for the overall fitness of a plant, and increased
levels in the plant produce higher resistance to pathogens and increased crop
yield in some species, including citrus and nut trees (Armstrong 1998, Rosecrance
et al. 1996, Sumith and Bandara 2002). It is possible that elevated
potassium levels in the parent trees make them unsuitable for HWA development,
explaining their persistence in otherwise devastated areas.
Two foliar chemistry databases are available which report potassium
levels in Eastern Hemlock trees that were measured for other research. The
Foliar Chemistry Database of the Northeastern Ecosystem Research Cooperative
(NERC 2009) reports potassium concentrations for 132 hemlocks
across Maine, Massachusetts, New York, and New Hampshire. Only two
592 Northeastern Naturalist Vol. 16, No. 4
percent of the trees in this database had levels of potassium comparable to
the reported values for the parent group. The Tree Chemistry database (Pardo
et al. 2005) published by the US Forest Service Northeastern Research
Station reports potassium levels in an additional 23 trees across nine sites.
Two of these sites in central New York had levels comparable to the parent
trees reported here. Both of these databases suggest that elevated levels described
in this research occur in nature, but are not common. Trees reported
by the historical database as exhibiting high levels of potassium should be
monitored for potential resistance to HWA.
Differences between the cation concentrations of the propagule and parent
groups are most likely explained by the fact that propagules from the
parent trees were grown in a greenhouse environment that included a liquid
application of 20-20-20 fertilizer (Caswell et al. 2008). Specifically, the
propagules possessed much higher concentrations of nitrogen and calcium
than did the parent group. This result suggests that fertilized rooted cuttings
are not an appropriate substitute to the parent trees in regards to foliar chemistry
and implies that care should be taken in using propagules to assess the
HWA resistance of their parent trees.
Conclusions
While there are a number of variables (biological, temporal, regional,
etc.) which contribute to the susceptibility of a host tree, our study reveals
substantial intraspecific variation in Eastern Hemlock foliar chemistry and
shows that the chemical variation occurring in Eastern Hemlock cultivars
(Lagalante et al. 2007) can also be found in field-collected specimens.
Since foliar chemistry plays a critical role in determining plant resistance
or tolerance to HWA, this suggests the possibility that naturally occurring
variation in foliar chemistry may produce at least some individuals of Eastern
Hemlock whose chemical makeup renders them relatively less vulnerable
to HWA infestations. If so, the adelgid's role as a natural “selective filter”
that kills non-resistant hemlock trees may inadvertently make it easier to
identify these potentially resistant individuals. Similar to the evidence of
selective-pressures from Dendroctonus brevicomis LeConte (Western Pine
Beetle) enhancing frequencies of chemically distinct, less suitable Pinus ponderosa
P. & C. Lawson (Ponderosa Pine) trees (Sturgeon 1979), the potential
for chemically unique, potentially resistant Eastern Hemlock populations to
survive infestation is promising. Further examination of these individuals
may enable the identification of key chemicals linked to their survival. Such
knowledge would facilitate the use of techniques and technologies intended
to enhance these characteristics, as well as breeding programs aimed at producing
resistant/tolerant Eastern Hemlock trees for use in conservation and
reforestation efforts.
Efforts to remove hemlock trees prior to infestations in order to maximize
economic value, such as pre-emptive logging, will limit our ability to detect
naturally occurring resistance. We have seen the impacts of pre-emptive
logging with the Chestnut Blight tragedy, resulting in the loss of naturally
2009 L. Ingwell, J. Brady, M. Fitzpatrick, B. Maynard, R. Casagrande, and E. Preisser 593
occurring blight-resistant individuals (Schlarbaum et al. 1997). These preemptive
logging methods should be avoided until some alternative resistance
screening is available to test trees prior to their removal.
Acknowledgments
Comments from D. Orwig, M. Montgomery, A. Lagalante, and C. Thornber
improved this manuscript. This work was partially funded by RI-AES Hatch Grant
RH-00175 to ELP and USFS Grant 06-JV-11242300-126.
Literature Cited
Adams, R.P. 2007. Identification of Essential Oil Components by Gas Chromatography/
Mass Spectrometry. 4th Edition. Allured Publishing Corporation, Carol
Stream, IL.
Allen, S.E. (Ed.). 1974. Chemical Analysis of Ecological Materials. John Wiley and
Sons, New York, NY.
Armstrong, D.L. (Ed.). 1998. Potassium for agriculture. Better Crops with Plant Food
82:4–5.
Benjamini, Y., and Y. Hochberg. 1995. Controlling the false discovery rate: A practical
and powerful approach to multiple testing. Journal of the Royal Statistical Society
B 57:289–300.
Caswell, T., R. Casagrande, B. Maynard, and E. Preisser. 2008. Production and evaluation
of Eastern Hemlocks potentially resistant to the Hemlock Woolly Adelgid.
Pp. 124–134, In B. Onken and R. Reardon (Eds.). Fourth Symposium on Hemlock
Woolly Adelgid in the Eastern United States. USDA Forest Service, Hartford, CT.
Farjon, A. 1990. Pinacea. Drawings and Descriptions of the Genera Abies, Cedrus,
Pseudolarix, Keteleeria, Nothotsuga, Tsuga, Cathaya, Pseudotsuga, Larix, and
Picea. Koeltz Scientific Books, Konigstein, Germany.
Ingwell, L. 2007. Have you seen this hemlock? The College of Environment and Life
Sciences, The University of Rhode Island, Kingston, RI. Brochure. 6 pp.
Jones, J.B.J., B. Wolf, and H.A. Mills. 1990. Organic matter destruction procedures. Pp.
195–196, In Plant Analysis Handbook. Micro-Macro Publishing, Inc., Athens, GA.
Kindler, S.D., and R. Staples. 1970. Nutrients and the reaction of two alfalfa clones to
the Spotted Alfalfa Aphid. Journal of Economic Entomology 63:938–940.
Lagalante, A., and M. Montgomery. 2003. Analysis of terpenoids from hemlock
(Tsuga) species by solid-phase microextraction/gas chromatography/ion-trap mass
spectrometry. Journal of Agricultural and Food Chemistry 51:2115–2120.
Lagalante, A.F., N. Lewis, M.E. Montgomery, and K.S. Shields. 2006. Temporal and
spatial variation of terpenoids in Eastern Hemlock (Tsuga canadensis) in relation
to feeding by Adelges tsugae. Journal of Chemical Ecology 32:2389–2403.
Lagalante, A.F., M.E. Montgomery, F.C. Calvosa, and M.N. Mirzabeigi. 2007. Characterization
of terpenoid volatiles from cultivars of Eastern Hemlock (Tsuga
canadensis). Journal of Agriculture and Food Chemistry 55:10850–10856.
McClure, M. 1991. Density-dependent feedback and population cycles in Adelges
tsugae (Homoptera: Adelgidae) on Tsuga canadensis. Environmental Entomology
20:258–264.
McClure, M. 1992. Hemlock Woolly Adelgid. American Nurseryman
3/15/1992:82–89.
McClure, M. 1999. University of Rhode Island GreenShare factsheets: Hemlock
Woolly Adelgid. URI, Kingston, RI.
594 Northeastern Naturalist Vol. 16, No. 4
McClure, M., and C. Cheah. 1999. Reshaping the ecology of invading populations
of Hemlock Woolly Adelgid, Adelges tsugae (Homoptera: Adelgidae), in Eastern
North America. Biological Invasions 1:241–254.
McClure, M.S., and J.D. Hare. 1984. Foliar terpenoids in Tsuga species and the fecundity
of scale insects. Oecologia 63:185–193.
McClure, M.S., S.M. Salom, and K.S. Shields. 2001. Hemlock Woolly Adelgid. USDA
Forest Service, Morgantown, WV. FHTET-2001-03:1–19.
Montgomery, M.E., and A.F. Lagalante. 2008. The role of volatile terpenoids in the
relationship of the Hemlock Woolly Adelgid and its host plants.Pp. 118–123, In B.
Onken and R. Reardon (Eds.). Fourth Symposium on Hemlock Woolly Adelgid in
the Eastern United States. US Forest Service, Hartford, CT.
Northeastern Ecosystem Research Cooperative (NERC). 2009. Northeastern Ecosystem
Research Cooperative foliar chemistry database. USDA Forest Service Northeastern
Research Station and University of New Hampshire Complex Systems
Research Center. Available at http://www.folchem.sr.unh.edu. Accessed February
19, 2009.
Pardo, L.H., M. Robin-Abbott, N. Duarte, and E.K. Miller. 2005. Tree chemistry database
(version 1.0). General Technical Report NE-324. US Department of Agriculture,
Forest Service, Northeastern Research Station, Newton Square, PA.
Parker, B., M. Skinner, S. Gouli, T. Ashikaga, and H. Teillon. 1999. Low lethal temperature
for Hemlock Woolly Adelgid (Homoptera: Adelgidae). Environmental
Entomology 28:1085–1091.
Pontius, J., R. Hallett, and M. Martin. 2002. Examining the role of foliar chemistry in
Hemlock Woolly Adelgid infestation and hemlock decline. Pp. 86–99, In R. Reardon,
B. Onken, and J. Lashomb (Eds.). Symposium on the Hemlock Woolly Adelgid
in Eastern North America. US Forest Service, New Brunswick, NJ.
Pontius, J., R. Hallett, and J. Jenkins. 2006. Foliar chemistry linked to infestation and
susceptibility to Hemlock Woolly Adelgid (Homoptera: Adelgidae). Environmental
Entomology 35:112–120.
Preisser, E., A. Lodge, D. Orwig, and J. Elkinton. 2008. Range expansion and population
dynamics of co-occurring invasive herbivores. Biological Invasions 10:201–213.
Rosecrance, R.C., S.A. Weinbaum, and P.H. Brown. 1996. Assessment of nitrogen,
phosphorus, and potassium uptake capacity and root growth in mature alternatebearing
Pistachio (Pistacia vera) trees. Tree Physiology 16:949–956.
Schlarbaum, S.E., F. Hebard, P.C. Spaine, and J.C. Kamalay. 1997. Three American
tragedies: Chestnut Blight, Butternut Canker, and Dutch Elm Disease. Pp. 45–54,
In K.O. Britton (Ed.). Proceedings of the Exotic Pests of Eastern Forests. Tennessee
Exotic Pest Plant Council, Nashville, TN.
Schweissing, F.C., and G. Wilde. 1979. Temperature and plant-nutrient effects on resistance
of seedling Sorghum to the Greenbug. Journal of Economic Entomology
72:20–23.
Stein, S., Y. Mirokhin, D. Tchekhovskoi, and G. Mallard. 1987–2002. The NIST MASS
Spectral Search Program. Standard Reference Data Program of the National Institute
of Standards and Technology, Gaithersburg, MD
Sturgeon, K.B. 1979. Monoterpene variation in Ponderosa Pine xylem resin related to
Western Pine Beetle predation. Evolution 33:803–814.
Sumith, J.A. and J.M.R.S. Bandara. 2002. Effect of potassium on the development
and severity of damping-off in Tobacco (Nicotiana tabacum L.). Annals of the Sri
Lanka Department of Agriculture 4:327–335.
Young, R., K. Shields, and G. Berlyn. 1995. Hemlock Woolly Adelgid (Homoptera:
Adelgidae): Stylet bundle insertion and feeding sites. Annals of the Entomological
Society of America 88:827–835.