Management of balsam twig aphids in Christmas trees

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Management of

Balsam Twig Aphids
in Christmas Trees

BY RICHARD S. COWLES

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Management of Balsam Twig Aphids in Christmas Trees

By Richard S. Cowles

The balsam twig aphid, Mindarus abietinus Koch, can be
a serious pest of true firs {Abies spp.) grown for use as
Christmas trees. The feeding of these aphids causes rapidly
expanding shoots and their associated needles to curl.
Besides the unsightliness of twisted foliage, large colonies of
aphids developing in late spring and early summer excrete
copious honeydew, which drips over the foliage and
supports the growth of sooty mold fijngi. Sooty mold is not a
plant pathogen, but is an unsightly black coating, and can
physically block sunlight and thereby interfere with plant
functions such as photosynthesis. The current practice of
Christmas tree growers is to spray true firs one or two times
with insecticide, typically chlorpyrifos (Lorsban), each
growing season to prevent this injury. These sprays, in turn,
can adversely impact naturally occurring beneficial
organisms that help to maintain low populations of other
pests, such as spruce spider mites. Therefore, effective
management of balsam twig aphid needs to take into account
the effectiveness of insecticide treatments and the side
effects of these sprays on non-target beneficial arthropods.

Life cycle. Small black eggs clothed in rods of a white
wax overwinter on the bark of branches. Stem mothers, also
called the fundatrices (fundatrix is the singular), hatch firom
these eggs and develop into wingless adults. The adult is
pear-shaped and about as long as a fir needle is wide. The
bluish-gray body has four indistinct longitudinal stripes of
powdery white material, which is especially prominent on
the head and thorax. The stem mother gives live birth to her
young, which develop into either wingless ftindagenae or
winged sexuparae, which are all parthenogenic clones of the
mother. The feeding of the stem mother and her offspring on
succulent new growth causes the young needles to curl as
they expand. This twisted growth forms a pseudogall in
which the inwardly curving needles protect the rapidly
reproducing colony of aphids from being dislodged by rain
or wind. Sexuparae, which are all females, complete three
molts and are winged as adults, allowing them to disperse

from the tree on which they developed. Sexuparae have a
distinctive arrangement of five dark spots on the thorax and
four hardened sections of cuticle (sclerites) on the upper
surface of their abdomens. Fundagenae and sexuparae
produce the third generation, called sexuales, which consist
of females and males. The sexuales physically resemble the
sexuparae. This last generation of aphids, produced in the
first half of June, mates and lays the overwintering eggs on
the trees (Johnson and Lyon 1988).

Injury to Christmas trees principally is due to the
distortion of needles resulting from the feeding of stem
mothers and their offspring on rapidly expanding terminal
growth. Development of overwintered balsam twig aphid
eggs is closely synchronized with the growth of their
principal host: nearly all of these eggs hatch by the time that
50% of balsam fir trees have broken new growth. So,
although balsam twig aphids can develop on other hosts (all
firs, white and Colorado spruce, and juniper), the close
synchrony of this aphid's development with balsam fir bud
break makes it especially damaging to this species.
However, poor confrol of aphid stem mothers on balsam fir
can lead to damage on nearby Fraser and Canaan firs. In this
scenario, the rapid dispersal of the winged sexuparae from
balsam fir can coincide with bud break on Fraser or Canaan
firs, leading to aphid colony establishment and damage.

The economic loss to Christmas tree growers from
balsam twig aphids arises from several factors. First,
aesthetic damage, both from twisting of terminals and from
sooty mold, may reduce the value of trees. This is
particularly true for trees being produced for wholesale
marketing, where purchasers are wary of aesthetic injury.
Surprisingly, choose-and-cut customers do not usually object
to (or even notice) balsam twig aphid injury, and therefore
insecticide treatments may not be warranted for this clientele
(Kleintjes et al. 1999). Secondly, economic loss can result
from the expenses of applying insecticides to confrol this
pest and all the additional costs associated with secondary

Connecticut Agricultural Experiment Station

Bulletin 988

pest outbreaks. These pest outbreaks happen when the
insecticides used to control balsam twig aphids kill
beneficial insects or mites, which may have been keeping
other pests in check. Outbreaks of spruce spider mite can be
especially costly to control.

MANAGEMENT OPTIONS

Cultural control. One approach to minimize damage by
this insect is to grow other species of fir, such as Fraser fir or
the balsam-Fraser hybrid, the Canaan fir. It is unknown
whether balsam twig aphids avoid laying eggs on these
species, or whether the long time between aphid hatch and
availability of new growth on these late-breaking firs causes
high mortality in stem mothers. In any case, there is
generally much less damage to firs with late bud break
characteristics than to balsam fir. However, as mentioned
previously, winged aphids can migrate from balsam fir to
cause severe infestations on nearby plantings of other firs.

Biological control. Many arthropods are avid predators
of balsam twig aphids. For example, surveys of balsam twig
aphid populations in 2000-2002 revealed very high
populations of the muhicolored Asian ladybeetle {Harmonia
axyridis), lacewing larvae, syrphid larvae, whirlygig mites
{Anystis spp.), and parasitic wasps feeding on these aphids.
Although these predators and parasites may not build up fast
enough to prevent twisting of foliage, their activity reduces
the overall aphid population and decreases the number of
overwintering eggs laid later in the summer. Therefore, an
important objective for managing balsam twig aphid should
be to conserve predators and parasites. Healthy populations
of beneficial arthropods provide a natural mechanism for
aphid population reduction, which is especially needed
where chemical control has been inadequate.

Chemical control. Use of insecticides, especially
chlorpyrifos (Lorsban), has been the mainstay for managing
balsam twig aphid populations in Christmas trees for several
years. Chlorpyrifos has the unusual property of converting
directly from a solid to a gas, allowing it to fumigate the
plantation (Fritt 1993). Its ability to penetrate areas not
covered by spray deposit has made it the material of choice
for use with mist blower sprayers, permitting rapid spraying
of plantings. However, chlorpyrifos is an organophosphate
insecticide and is moderately to highly toxic to humans,
birds, fish, and beneficial arthropods (ace.orst.edu). As an
organophosphate insecticide, it continues to undergo review
by the U.S. Environmental Protection Agency and has
already had some uses removed from its label. Growers in
the South Windsor area experienced poor control of balsam
twig aphid following two applications of chlorpyrifos to
plantings in 2002, suggesting that local populations of these
aphids may be resistant to this insecticide. Therefore,
alternatives to the use of chlorpyrifos need to be tested to
determine how well they can perform in suppressing balsam

twig aphid populations in Christmas tree plantations.

Aphid control field experiments. These studies were
conducted at cooperating growers' fields to determine the
most effective insecticides for control of balsam twig aphid.
High and low volume spray application was used to
determine which insecticides are appropriate for different
kinds of spray equipment. Because there were many aphid
predators observed during the test in 2000, the predator
populations were measured along with the twig aphids in the
second year to determine which insecticides selectively
killed the aphids rather than their predators.

High-volume spray and soil-applied systemic
experiment This experiment was conducted at a cooperating
grower's field in Somers, with balsam fir planted in a 5 x 6
foot pattern. Plots consisted of a 4 x 4 array of trees, plus a
buffer row along all sides. There were four replicates in a
randomized complete block design. Materials used in foliar
sprays included pymefrozine (Endeavor SOW, Syngenta,
Greensboro, NC), thiamethoxam (Flagship 25W, Syngenta),
horticultural oil (Lesco Horticultural Oil, Lesco,
Sfrongsville, OH), chlorpyrifos (Lorsban 4E, Dow
AgroSciences, Indianapolis, IN), insecticidal soap (Olympic,
Bradenton, FL), acephate (Orthene 70S, Valent, Walnut
Creek, CA), and endosulfan (Thiodan 3E, FMC,
Philadelphia, PA). For products needing a surfactant, Silwet
L-77 (Helena, Memphis, TN) was added, and it was also
included by itself as a control. Imidacloprid (Provado 1.6F,
Bayer, St. Louis, MO) was applied in the fall or spring to the
soil for systemic uptake, or in the spring as a foliar spray. A
backpack sprayer equipped with a TK-2 floodjet nozzle
(Spraying Systems, Wheaton, IL) was used on October 20,

1999, for the liquid broadcast application in a spray volume
of 40 gallons per acre. To guarantee soil incorporafion, this
application was made during a rain event of 0.5 inches. Due
to dry spring conditions, a Kioritz injector was used for the
March 31, 2000, soil incorporation. Six 5-ml injections were
placed along the dripline for each tree at a depth of 2 inches.
Foliar sprays were applied on two days, April 25 and 26,

2000, when balsam fir had reached 50% bud break. A
hydraulic sprayer applied 435 gallons per acre. Conditions
on April 25 were 60 F, sunny and dry. Conditions on April
26 were 40 F, with wet snow changing to rain. Populations
of stem mothers were assessed on May 3, with four beating
samples from each of four trees in each plot. The beating
tray was a pad (8.5 x 1 1 inches) of white paper on a
clipboard. At this date, stem mothers had begun producing
young, which were too small to count. Plots were evaluated
again on June 13-14 by counting alate (winged) aphids with
beating samples and counting the percent of damaged
terminals (from a sample of 50 terminals on each of four
trees per plot). Data were subjected to analysis of variance.

Chlorpyrifos, insecticidal soap, thiamethoxam, the
imidacloprid foliar spray, oil, and acephate all gave excellent
early-season control (> 95% mortality) of fundatrix aphids

Management of Balsam Twig Aphids in Christmas Trees

Table I . Control of balsam twig aphid with a high-volume spray (435 gallons per acre) and soil applied systemic insecticide
application, n = 4.

Aphid

count

Control %

Damaged

Product

Chemical name

Use rate

Application type

5/3

6/13-14

5/3

terminals %

Endeavor SOW

pymetrozine

0.1 Iba. i. /Ac

spring foliar spray

6.3d

20ab

89

15bc

Flagship 25W

thiamethoxam

0.1 Iba. i. /Ac

spring foliar spray

l.Od

3.8ab

98.3

3.1c

+ Silwet L-77

Lesco Oil

oil

2%

spring foliar spray

1.3d

21a

98

llbc

Lorsban 4E

chlorpyrifos

8floz/100gal

spring foliar spray

0.3d

21a

99.6

6.3c

Olympic Insecticidal

Soap

insecticidal soap

2%

spring foliar spray

0.3d

21a

99.6

7.4c

Orthene 75 S

acephate

1 lb a.i./Ac

spring foliar spray

3.0d

lOab

95

21bc

Provado I.6F

imidacloprid

0.4 lb a. i./Ac

fall broadcast

45ab

4.8ab

23

3Ibc

Provado 1.6F

imidacloprid

0.4 lb a. i./Ac

spring soil injection

30bc

0.8b

50

21bc

Provado 1.6F

imidacloprid

8 fl oz/Ac

spring foliar spray

l.Od

llab

98.3

2.4c

+ Silwet L-77

Thiodan 3E

endosulfan

21 floz/lOOgal

spring foliar spray

24b

9.3ab

59

37b

Silwet L-77

surfactant

4floz/100gal

spring foliar spray

27bc

5.5ab

54

37b

Untreated check

59a

3.5 ab

_

57a

' Means in each column followed by the same letter are not significantly different, P = 0.05, Newman-Keul's test applied to square root
(.x+0.5) transformed counts of aphids.

(Table 1). The later evaluation of aphid populations and
percent terminal curling revealed some interesting
phenomena. First, curling and twisting of foliage occurs only
in response to feeding from the early-season aphid
populations. This is evident from the strongly correlated
(r = 0.74) May 3 aphid counts and percent shoot damage.
Secondly, the population of the aphids in June was
approximately inversely proportional to the populations
observed in early May. In other words, treatments with high
counts in May tended to have low counts in June and vice
versa. This is probably explained by (I) the exploitation of
aphids by predators and parasites, and (2) migration of
winged aphids from heavily infested trees. It is striking that
trees did not maintain high populations of aphids throughout
the trial. Predators and parasites can be credited with
responding to aphid colonies efficiently enough to prevent
trees from remaining heavily infested for more than a few
weeks. For example, on the untreated check trees, the
population fell from averages of 59 to 3.5 aphids over
40 days. Unfortunately, predators did not reduce aphid
populations quickly enough to prevent damage during the
critical few weeks while shoots were rapidly growing.
Treatments resulting in a shift from low counts in May to
high counts in June (Endeavor, oil, Lorsban, and insecticidal
soap), demonstrate the ability of colonizing winged
sexuparae to generate new colonies on trees from which

aphids had previously been virtually eliminated (because of
the earlier insecticide spray). In these instances, predators
hadn't yet had time to find these colonies and reduce the
aphid population.

Thiamethoxam was exceptional in its residual protection
of trees. Besides providing early season control and
protection from shoot distortion, thiamethoxam also
generally prevented sexuparae from establishing new
colonies. The excellent overall control results with
thiamethoxam and imidacloprid suggest that other highly
systemic and selective experimental aphicides, such as
acetamiprid (Assail) and flonicamid, may be of value for
balsam twig aphid management.

Preventing colony development after the period of shoot
elongation may be indirectly beneficial by reducing the
numbers of overwintering eggs being laid, which would
influence the pest pressure in the following year. Endeavor,
oil, Lorsban, and insecticidal soap permitted resurgence in
the pest population. Pest resurgence in June could actually
lead to worsening aphid populations from one year to the
next. This scenario would only be likely where there are
insecticide resistant aphids, populations are left untreated in
the Christmas tree plantation, or populations on wild trees
surrounding the farm re-infest the planting soon after
spraying. In this experiment, use of small plots in a
randomized complete block design virtually guaranteed

Connecticut Agricultural Experiment Station

Bulletin 988

rapid recolonization of trees by winged aphids emigrating
from the untreated checks and ineffective treatments.

Imidacloprid applied to the soil in the spring was initially
ineffective at reducing fundatrix populations, but was the
only treatment that appeared to reduce the numbers of
sexuparae and their progeny on Christmas trees in June
relative to the untreated check. Therefore, imidacloprid
applied as a soil treatment to control white grubs or balsam
woolly adelgids might provide partial protection from the
next year's population of fundatrices by limiting the late-
season population growth of aphids and by preventing egg
laying on treated trees. This strategy, however, would have
little value in providing reduction in shoot distortion during
the year of application.

Low volume spray experiment. The most effective
materials from the 2000 trials were included in a further
low-volume application test using a backpack mist blower
sprayer. Insecticidal soap and oil were not included because
these materials are known to require a high spray volume.
There were six treatments in a randomized complete block
design with six replicates. The block of balsam firs in
Somers had not been sprayed with experimental insecticides
in 2000, but had been sprayed with chlorpyrifos. An
additional insecticide, triazamate (Aphistar SOW, Dow
AgroSciences), was included in the test because it was being
investigated for use on true firs for control of conifer root
aphids, and was expected to have some activity against
balsam twig aphids. Insecticides were applied in 20 gallons
per acre of spray with a Solo backpack mist blower on
May 3, 2001, under sunny 90 F conditions, and 0-10 mph
wind. Aphid counts were taken with beating samples
conducted on May 10 and June 7, using the same method as
in 2000. Populations of syrphid larvae, lacewing larvae,
ladybeetle larvae and adults, and whirlygig mites (Anystis
spp.) were counted from additional June 7 beating samples,
using a 28 inch square canvas beating sheet (BioQuip
Products, Gardena, CA). The beating sample was a
composite from four trees, and four subsamples per free.
Damage ratings were taken on June 7 by visually rating the
percent of terminal shoots that were damaged as category 0
(no damage), 1 (1-25% curled), 2 (26-50% curied),
3 (51-75% curled) or 4 (76-100% curied). Four samples of
20 shoots were evaluated and averaged for each plot.

Materials tested with low-volume spray application were
not as effective as when they had been applied with the
hydraulic sprayer. For example, Lorsban, Provado, Flagship
and Endeavor provided 80, 85, 85 and 27% control
(Table 2), respectively, compared with 99.6, 98, 98 and 89%
control in the previous year. The poorer efficacy with the
low-volume spray may also have been affected by the later
spray timing. Trees and aphids grew very rapidly in 2001 in
response to unseasonably hot weather, and the optimal
timing for balsam twig aphid control had passed before these

sprays were applied. Therefore, the spray in 2001 was
directed at controlling colonies of stem mothers with young,
rather than just the stem mothers. Surprisingly, this later
timing was very effective in reducing foliar distortion,
suggesting that spraying later than 50% bud break on balsam
firs may be acceptable for managing these aphids. The most
impressive result was the protection of Christmas trees from
shoot damage with Aphistar, which gave significantly better
control (95% reduction on May 10) than the other
treatments. On the other hand. Endeavor did not provide any
significant benefit with the low volume spray.

Aphistar had the greatest impact on predator populations.
Use of Aphistar significantly reduced the numbers of
ladybeetles, lacewings and syrphid larvae (Table 3). As
these are all aphid predators and there were few aphids
remaining following use of this product, the predators may
simply have starved or left due to the lack of food. Overall,
the total numbers of predators did not differ among any of
the treatments, because the increased numbers of Anystis
(whirlygig mites) compensated for the reduction in the aphid
specialist complex (Table 3). While not significantly
different, there were numerically the most predators in the
Aphistar-treated plots and the fewest in the Lorsban plots.

Balsam twig aphid management guidelines. Alternatives
to chlorpyrifos with excellent activity against balsam twig
aphid include insecticidal soap, horticultural oil, Aphistar,
and Provado. The optimal choice of an insecticide will
depend to a large extent on each grower's spraying
equipment. Horticultural oil and insecticidal soap both
require high volume application, so these materials are only
compatible with hydraulic sprayers or small airblast sprayers
that can fit between rows of Christmas trees. Insecticidal
soap may be a less desirable choice because it is more costly
than horticultural oil (1.15-3 times the price) and controls
fewer pests and fewer stages. For example, fewer spider mite
eggs are killed by soap than by oil (Osborne and Pettit 1985,
Cowles and Abbey 1999). Some growers have been satisfied
with horticultural oil, but this material can cause injury to
balsam firs, characterized by browning and needle drop from
the previous years' growth. Additional research will be
necessary to determine whether oil's phytotoxicity to balsam
firs can be avoided by adjusting the use rate, application
timing, or by including adjuvants that will encourage better
spreading of spray droplets and faster drying. Horticultural
oil is especially valuable for use on Eraser firs, because they
are more tolerant of oil and are also more susceptible to
other pests (spruce spider mites and elongate hemlock scale)
that can be controlled with oil. Horticultural oil can damage
any plant material if it comes out of suspension in the spray
tank, which can result in spraying undiluted oil. Mechanical
agitation or addition of surfactants may be necessary to
prevent separation.

The remaining insecticides, Aphistar and Provado, are

Management of Balsam Twig Aphids in Christmas Trees

Table 2. Control of balsam twig aphid with a low-volume spray applied with a backpack mist blower (20 gallons per acre),
« = 6.

Aphid

count"

Control

(%)

Damagi

Product

Chemical name

Rate/Acre

May 10

June 7

May

10"

Rating'

Lorsban 4E

chlorpyrifos

16fl. oz.

21b

49a

80

1.0b

Provado 1.6F

imidacloprid

8 fl. oz.

16b

40ab

85

1.2b

+ Silwet L-77

4 fl. oz.

Flagship 25W

thiamethoxam

3oz.

16b

34ab

85

1.5b

+ Silwet L-77

4 fl. oz.

Aphistar 50W

triazamate

8oz.

5c

lie

95

0.0c

+ Silwet L-77

4fl. oz

Endeavor SOW

pymetrozine

10 oz.

77a

22bc

27

3.2a

+ Silwet L-77

4 fl. oz.

Untreated check

106a

25bc

_

3.2a

' Means in each column followed by the same letter are not significantly different, P = 0.05, Newman-Keul's test applied to
square root (x+0.5) transformed counts of aphids.

" Percent control is relative to the population in the untreated check.

' Damage rating is based on the percent damaged terminals: 0, 0%; 1,1- 25%; 2, 26 - 50%; 3,51- 75%; 4, 76 - 100%.

Table 3. Survival in percent of balsam twig aphid pregators with a low-volume spray applied with a backpack mist blower
(20 gallons per acre), « = 6.

Lady

Lace-

Whirlygig

Product

Chemical name

Rate/Acre

beetle

wing

Syrphids

mites

Lorsban 4E

chlorpyrifos

16fl. Oz.

31a

1.5bc

12 ab

6.3a

Provado 1.6F

imidacloprid

8 fl. Oz.

13b

4.3ab

10 ab

17a

+ Silwet L-77

4 fl. Oz.

Flagship 25W

thiamethoxam

3 0z .

25a

1.8bc

8.2 b

23a

+ Silwet L-77

4 fl. Oz.

Aphistar 50W

triazamate

8oz.

4.5c

0.2c

3.0 c

28a

+ Silwet L-77

4 fl. Oz.

Endeavor 50W

pymetrozine

10 fl. Oz.

6.7a

15a

7.5a

+ Silwet L-77

4 fl. Oz.

Untreated check

26 a

2.8ab

13ab

18a

"Means in each column followed by the same letter are not significantly different, P = 0.05, Newman-Keul's test applied to
square root (jc+0.5) transformed counts of predators.

appropriate for use with either low or high-volume spray
equipment. Research in 2001 demonstrated that Provado +
Silwet and Aphistar were effective with a low volume spray
volumes (20 gallons per acre). Aphistar is only available

under a Section 1 8, or Emergency Exemption, registration.
Although this product is systemic, thorough spray coverage
is still required to be effective. I have observed conspicuous
failure of aphid control with Aphistar on the side of trees

Connecticut Agricultural Experiment Station

Bulletin 988

opposite the mist blower sprayer, a phenomenon called a
"spray shadow." Combining Provado with an organosilicone
surfactant (Silwet L-77, Kinetic, or CapSil 30) improves its
effectiveness. For large Christmas tree plantations, the only
practical way to quickly apply an even distribution of spray
may be to use Aphistar or Provado + surfactant with a boom
sprayer, with the spray boom passing over the tops of the
trees. Other alternatives for improving spray distribution
may be to remove the trees from the center row in each
block to provide another drive row, to use a small sprayer
that can fit between every row, or to adjust the space
between rows at the time of planting to give additional drive
rows for sprayers. A backpack mistblower could also be
used to walk through and spot spray just those areas where
coverage with the tractor-driven mistblower is inadequate.

Two areas need further research for improving
management of balsam twig aphids: investigation into
management of the egg laying sexuales and improved
integration of chemical, cultural and biological control.
Interfering with the egg laying activity of sexuales, either
with insecticides or repellents, could provide a new control
strategy. Until recently, the residual activity of most
systemic insecticides was too short to affect multiple
generations of aphids. Neonicotinyl compounds, however,
can control sucking insect pests over several months when
used as a soil-applied systemic (Varela et al. 1996, Cowles
and Cheah 2002). It is especially intriguing that spring
application of imidacloprid reduced the populations of
sexuparae on trees (Table 1). Application of this systemic
insecticide (or similar materials, like thiamethoxam) to the
soil for control of white grubs or balsam woolly adelgid may
control the sexuparae and sexuales generations, and may
prevent balsam twig aphid injury in the following year by
reducing the numbers of overwintering eggs (Kleintjes
1997). Dramatic reduction in egg laying could lead to a
reduced need for a spring application of insecticides and
improved chances that biological control would be sufficient
for springtime aphid population suppression.

The common practice of yearly application of
insecticides to manage balsam twig aphids in all true fir
plantings is not warranted by customer preferences and
economic risk. Scouting and threshold-based treatment
guidelines could go far toward reducing unnecessary
spraying for this pest (Kleintjes et al. 1999). The percent of
damaged shoots can sometimes be correlated with counts of
stem mothers determined from beating samples taken at bud
break (Kleintjes et al. 1999). If the estimated population of
stem mothers can be used to predict the percent shoot
damage, then treatment decisions can be based on this
information, whether the threshold used is relatively low (for
wholesale-marketed trees) or higher (for choose-and-cut
marketing).

Further advances in balsam twig aphid management will
require integration of biological, cultural, and chemical
control to stabilize aphid populations at non-economic
population levels. Key to this effort will be clarification of
the seasonal population dynamics of the aphid predators and
parasitoids, with a special focus on improving the
establishment and success of the beneficial species active at
the same time as the fundatrix generation. Hannonia
axyridis, the multicolored Asian ladybeetle, and Anystis
mites were especially common and active, and would be
good prospects. The balsam twig aphids have a short season
of activity, so ground covers that support other species of
aphids could in turn support these predators further through
the growing season and increase their overwintering
populations. Further investigations should optimize the use
of selective aphicides so that they would only be used at
times when there is a risk that aphid populations are great
enough relative to predator populations to risk economic
damage.

REFERENCES

Cowles, R.S. and T.M. Abbey. 1999. Of mites and men.
Amer. Nurseryman 190(4): 68-77.

Cowles, R.S. and CAS-J Cheah. 2002. Systemic control
of hemlock woolly adelgid, 1999. Arthropod Management

Tests 27- G47.

Fritt, J.R. 1993. The benefit of fumigant action of
chloropyrifos for control of cotton aphid. Down to Earth
48(2): 8-12.

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