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Forest Entomology
Evaluations of Insecticides and Fungicides for Reducing
Attack Rates of a new invasive ambrosia beetle
(EuwallaceaSp., Coleoptera: Curculionidae: Scolytinae) in
Infested Landscape Trees in California
Michele Eatough Jones,1,2John Kabashima,3Akif Eskalen,4Monica Dimson,3
Joey S. Mayorquin,4Joseph D. Carrillo,4Christopher C. Hanlon,1and Timothy D. Paine1
1Department of Entomology, University of California Riverside, Riverside, CA 92521 (michele.eatough@ucr.edu; christopher.hanlon@
ucr.edu; timothy.paine@ucr.edu),2Corresponding author, e-mail: michele.eatough@ucr.edu,3University of California Cooperative
Extension, 7601 Irvine Boulevard, Irvine, CA 92618 (jnkabashima@ucanr.edu; mjdimson@ucanr.edu), and4Department of Plant
Pathology and Microbiology, University of California Riverside, Riverside, CA 92521 (akif.eskalen@ucr.edu; jmayo001@ucr.edu;
jcarr022@ucr.edu)
Subject Editor: Timothy Schowalter
Received 21 March 2017; Editorial decision 22 May 2017
Abstract
A recently discovered ambrosia beetle with the proposed common name of polyphagous shot hole borer
(Euwallaceasp., Coleoptera: Curculionidae: Scolytinae), is reported to attack>200 host tree species in southern
California, including many important native and urban landscape trees. This invasive beetle, along with its asso-
ciated fungi, causes branch dieback and tree mortality in a large variety of tree species including sycamore
(Platanus racemosaNutt.). Due to the severity of the impact of thisEuwallaceasp., short-term management
tools must include chemical control options for the arboriculture industry and private landowners to protect
trees. We examined the effectiveness of insecticides, fungicides, and insecticide–fungicide combinations for
controlling continuedEuwallaceasp. attacks on previously infested sycamore trees which were monitored for
6 mo after treatment. Pesticide combinations were generally more effective than single pesticide treatments.
The combination of a systemic insecticide (emamectin benzoate), a contact insecticide (bifenthrin), and a fungi-
cide (metconazole) provided some level of control when applied on moderate and heavily infested trees. The bi-
ological fungicideBacillus subtilisprovided short-term control. There was no difference in the performance of
the three triazole fungicides (propiconazole, tebuconazole, and metconazole) included in this study. Although
no pesticide combination provided substantial control over time, pesticide treatments may be more effective
when trees are treated during early stages of attack by this ambrosia beetle.
Key words:ambrosia beetle, bifenthrin, emamectin benzoate, triazole fungicide
The as yet undescribed beetle with the proposed common name of
polyphagous shot hole borer (Coleoptera: Curculionidae:
Scolytinae) vectors fungal pathogens that cause the disease known
as Fusarium Dieback. The beetle was first identified in southern
California in 2003 (Rabaglia et al. 2006;Eskalen et al. 2012,2013).
It has now been reported in Los Angeles, Orange, Ventura,
Riverside, and San Bernardino Counties (Eskalen 2016). The beetle,
hereafter referred to asEuwallaceasp., is an ambrosia beetle closely
related to and morphologically indistinguishable from the Southeast
Asian species,Euwallacea fornicatus. Females are black (1.8–
2.5 mm) and the much less abundant males are wingless and smaller
in size (1.5–1.67 mm). Newly eclosed adults mate with siblings
while still in the maternal gallery.Female beetles carry fungal spores in mandibular mycangia and
transmit them to host trees while boring into the tree (Fernando
1959). Fungal species carried byEuwallaceasp. includeFusarium
euwallaceae,Graphium euwallaceae, andParacremonium pembeum
(Freeman et al. 2012,Eskalen et al. 2013,Lynch et al. 2016).
Females initiate galleries, inoculate galleries with fungi, and con-
tinue to expand that gallery over time, laying clusters of eggs
(Umeda et al. 2016). The initial entry hole to the gallery is used as
both an entrance and an exit. Both adult beetles and developing lar-
vae feed on the fungi. Fusarium Dieback is a vascular disease in
plants caused by fungi associated with the beetle (Eskalen et al.
2012,Freeman et al. 2013,Lynch et al. 2016). The disease is a result
of fungal colonization in active xylem tissue, leading to interruption
VCThe Authors 2017. Published by Oxford University Press on behalf of Entomological Society of America.
All rights reserved. For Permissions, please email: journals.permissions@oup.com1611
Journal of Economic Entomology, 110(4), 2017, 1611–1618
doi: 10.1093/jee/tox163
Advance Access Publication Date: 5 July 2017
Research article

of nutrient and water transport, causing branch dieback, and in se-
vere cases, tree death.
In California,Euwallaceasp. has been recorded attacking>200
species of trees, with 49 currently known suitable reproductive hosts
from>20 plant families (Eskalen 2016). Reproductive hosts include
many California natives (e.g., California sycamore (Platanus racemosa
Nutt.), coast live oak (Quercus agrifoliaNe´e),andarroyoandblack
willows (Salix lasiolepisBenth. andS. gooddingiiC.R. Ball)), common
urban species (e.g., Japanese maple (Acer palmatumThunb.) and
Liquidambar styracifluaL.), and important commercial crops (avo-
cado,Persea americanaMill) (Eskalen et al. 2013). Due to its broad
host range,Euwallaceasp. causes dieback and tree mortality in trees in
parks, residential neighborhoods, other public landscapes, and riparian
areas. It also has the potential for ecological impacts in natural forests
and ecosystems, as well as economic impacts in avocado-growing re-
gions in southern California. Management tools are needed by home
owners, park managers, and arborists to help protect landscape trees
and minimize the aesthetic and economic impacts ofEuwallaceasp.
The range of options for the immediate term may include direct control
using contact insecticides, systemic insecticides, and fungicides to pre-
vent infestation or manage infesting beetles and their associated fungi
to limit spread of this ambrosia beetle (Cranham 1966,Paine et al.
2011). The efficacy of sanitation options, such as chipping or solariza-
tion of infested wood to prevent beetle emergence, has previously been
reported (Eatough Jones and Paine 2015).
Trunk sprays have conventionally been used to protect trees from
bark beetle attack (Fettig et al. 2013a). Laboratory tests with cut logs
showed bifenthrin was most effective for deterring attack byEuwallacea
sp. (Eatough Jones and Paine 2017). Additionally, fungicides have been
used to protect trees from a variety of beetle-vectored fungal diseases
(e.g.,Appel and Kurdyla 1992,Haugen and Stennes 1999,Mayfield
et al. 2008). Systemic insecticides that are injected into the lower bole of
the tree have been receiving increased attention as alternatives to bole
sprays for preventing bark beetle attacks (Fettig et al. 2013a).
The objective of this study was to test the effectiveness of insecti-
cides, fungicides, and insecticide–fungicide combinations for control-
lingEuwallaceasp. We chose one systemic insecticide, one contact
insecticide, three triazole fungicides, and one bacterial fungicide applied
individually and in combinations. To test the effectiveness of the pesti-
cide treatments, we treated mature, infested sycamore trees, and moni-
tored for increasing attacks over time.
Materials and Methods
Naturally infested, mature California sycamore (Platanus racemosa
Nutt.) trees were selected on the campus of University of California,
Irvine (Orange Co.). We initially selected 100 trees and ninetreatments. The nine treatments were as follows: 1) bifenthrin, 2)
emamectin benzoate, 3) tebuconazole, 4) propiconazole, 5) metco-
nazole, 6)Bacillus subtilis, 7) bifenthrinþtebuconazole, 8) emamec-
tin benzoateþtebuconazole, and 9) untreated controls.Table 1
shows application rates and manufacturer information for all pesti-
cides used. Metconazole was applied using a backpack sprayer
(Chapin Tree/TurfPro model 62000, Batavia, NY) to 2.4 m trunk
height. Bifenthrin andB. subtiliswere applied by spraying to run-off
up to 2.4 m trunk height with a 200-gallon truck-mounted sprayer.
Tebuconazole was injected with a ChemJet tree injector (Mauget,
Arcadia, CA). Emamectin benzoate and propiconazole were injected
using a Tree I.V. system (Arborjet, Woburn, MA).
We measured tree diameter at 1.5 m above ground (diameter at
breast height, DBH) for all selected trees and assessed infestation levels
as light, moderate, or heavy (categories based on a quick visual assess-
ment as approximately: light<50 attacks, moderate 50–200 attacks,
or heavy>200 attacks in1 m of trunk length centered at breast
height). All selected trees were GPS mapped and randomly assigned to
one of the nine treatments using arandom treatment assignment tool
(Urbaniak and Plous 2015), resulting in 11 trees per treatment (with
the extra tree going to the control group). Group assignments were
checked to confirm that there were no significant differences in DBH
and estimated infestation level. During the initial period of evaluation,
a 10th treatment was added: emamectin benzoateþpropiconazole.
These trees were scattered around the circumference of the area where
the initial nine treatments were located. In July 2015, one month after
other treatments were initiated, an 11th treatment was added: bifen-
thrinþemamectin benzoateþmetconazole. Mean DBH for selected
trees was 41.161.3 cm (mean and S.E.).
In June 2015, prior to beginning treatments, we quantified the
number of attacks on each tree. Pesticide treatments were applied in
late June 2015, after the initial attack data were collected, except for
treatment 11 (bifenthrinþemamectin benzoateþmetconazole), where
initial counts and pesticide application were performed in July 2015.
The number of attacks were assessed again 1 mo after pesticide applica-
tion (July 2015), and then at3 mo (September 2015) and 6 mo
(December 2015) after pesticide application. During the period of pesti-
cide application, several trees that had been selected were excluded
from the study due to tree removal or other unforeseen situations. Tree
removals (outside the control of the investigators) continued sporadi-
cally throughout the course of monitoring due to hazard conditions as
tree health declined. This resulted in an unbalanced number of trees as-
signed to each treatment.Data Collection
We counted allEuwallaceasp. entrance holes around the entire cir-
cumference of the main trunk within an area 0.9 m in length,
Table 1.Information for each chemical used
Active ingredient Class Rate (amount a.i.) Method Trade name Manufacturer
Bifenthrin Pyrethroid insecticide 240 g/liter Trunk spray Onyx FMC Corporation, Philadelphia, PA
Emamectin benzoate Avermectin insecticide 2.4 ml/cm DBH Injection TREE-€
age Arborjet, Woburn, MA
Tebuconazole Triazole fungicide 2.4 ml/cm DBH Injection Tebuject 16 Mauget, Arcadia, CA
Propiconazole Triazole fungicide 2.4 ml/cm DBH Injection Propizol Arborjet, Woburn, MA
Metconazole Triazole fungicide 18.14 g/cm DBH Trunk spray Tourneyþsurfactant Valent, Walnut Creek, CA
Surfactant for
metconazolePenetrating agent 2.9 ml/cm DBH Trunk spray Pentra-Bark AgBio Inc., Westminster, CO
Bacillus subtilis
QST 713 strainMicrobial fungicide 1% solution Trunk spray Ceaseþsurfactant Bayer Environmental Science,
Research Triangle Park, NC
Surfactant forB. subtilisPenetrating agent 2.9 ml/cm DBH Trunk spray Pentra-Bark AgBio Inc., Westminster, CO
1612Journal of Economic Entomology, 2017, Vol. 110, No. 4

starting at a trunk height of0.9 m from the ground. Each hole was
marked with a small dot to the side of the hole using a paint marker,
and counts were tallied with a hand-held tally counter. Entrance
holes were determined by their shape and size, a round hole approxi-
mately the size of the tip of a ball point pen (0.85 mm diameter). If
there was an accumulation of sap or boring dust, but it was unclear
if there was a hole underneath, the area was cleared with a plastic
putty knife or pen cap to verify the presence of a hole. At the end of
the count, the length of the count area was measured in two arbi-
trary locations around the trunk and recorded. Length of the count
area and DBH were used to calculate the total surface area counted
for each tree. Attack density for each tree and each sampling period
was calculated as total entrance holes/m
2.
During subsequent monitoring periods in July 2015, September
2015, and December 2015, all holes in the same area on the main
trunk were counted again. A different color of paint marker was
used each period, and both new holes and all previously marked
holes were marked with the new color and tallied. Marking and
counting all existing holes was necessary because bark regularly
peeled away from the trunk.
Additionally, in October 2015, due to the large number of peo-
ple participating in the counts, recorded data were audited against
paint marks visible on the trees. The audit was conducted by a
smaller group of graduate students and researchers that had been
working extensively with this ambrosia beetle, with three to four
people checking each tree. Some discrepancies, particularly a lower
number of paint marks visible than had been recorded, were ex-
pected due to shedding of bark, weathering, and public access to the
trees. In these cases, the originally recorded data were used. For
other unaccounted discrepancies between recorded data and paint
marks visible on the tree, data for that tree from that count period
were excluded from the dataset. This audit resulted in 25 of the 310
recorded tree counts between June and September being excluded
from the dataset due to probable counting errors. An additional 10
data points were missing due to tree removal after treatments were
begun. For the monitoring period in December, two or more people
checked each tree, each checking over the entire count area two or
three times. A list of all treatments, and the number of trees included
in each treatment at each count period, is given inTable 2.
Statistical Analysis
We analyzed the effectiveness of each pesticide treatment in two
ways. First, we compared the number of attacks within eachindividual treatment over time using a paired comparisont-test (SAS
Institute Inc. 2012). We compared the attacks/m
2for each tree for
pretreatment counts in June 2015 to subsequent counts at 1, 3, and
6 mo posttreatment. Since trees treated with bifenthrinþemamectin
benzoateþmetconazole were added in July 2015, paired compari-
sont-tests for this treatment were evaluated at 2 and 5 mo
posttreatment.
Second, we contrasted the increase in attacks for each pesticide
treatment to the increase in attacks for untreated control trees.
We have observed that newly emerging beetles are more likely to
initiate attacks on their natal host tree, rather than migrating to a
new host. Because of this, the number of new attacks recorded for
each tree was significantly correlated with the total number of at-
tacks already present on the tree (r¼0.66,P<0.001,n¼303).
We used % increase in attacks for each counting interval to com-
pare among treatments, to weight the new attacks by the total
number of attacks on the tree. This limited variation due to differ-
ences in the initial number of attacks among trees both within
and between treatments. The % increase in attacks for each tree
at each sampling period was calculated using attacks/m
2for each
count period as:
%new attacks¼
current periodprevious period
current period
Significant differences in the % new attacks for each pesticide
contrasted with untreated controls for each count period were as-
sessed by a nonparametric ANOVA and significant differences were
evaluated using the Kruskal–Wallisv
2ata¼0.10 (SAS Institute Inc.
2012).
We also compared the effectiveness of the three triazole fungi-
cides when each was applied individually. The fungicides compared
were tebuconazole, propiconazole, and metconazole. We used % in-
crease in attacks, as calculated above, for each counting interval to
compare among fungicide treatments. Significant differences in %
new attacks for the fungicides were assessed by nonparametric
ANOVA and significant differences were evaluated using the
Kruskal–Wallisv
2ata¼0.10 (SAS Institute Inc. 2012).
Results
For treatments assessed individually over time, only three treatments
had not had a significant increase in attacks after 1 mo (Table 3;
Figs. 1and2). These treatments includedBacillus subtilis,
Table 2.Number of trees included at each count period for each treatment
TreatmentNo. of trees in each treatment
June July Sept. Dec.
Untreated control 10 9 10 11
Bifenthrin 10 11 9 9
Emamectin benzoate 8 9 10 10
Tebuconazole 8 10 10 10
Propiconazole 8 9 9 8
Metconazole 8 8 7 8
Bacillus subtilis7777
Emamectin benzoateþPropiconazole 8 7 8 9
Emamectin benzoateþTebuconazole 9 8 9 9
BifenthrinþTebuconazole 11 11 10 10
Emamectin benzoateþBifenthrinþMetconazole NA 10 10 10
Data audits resulted in counts from a few trees being excluding during some months. Additionally, some trees were removed, as branch dieback lead to hazard
conditions in public spaces.
Journal of Economic Entomology, 2017, Vol. 110, No. 41613

emamectin benzoateþtebuconazole, and bifenthrinþtebuconazole.
Paired comparisons for total attacks comparing pretreatment counts
with 3 mo posttreatment and with 6 mo posttreatment counts
showed a significant increase in attacks for all treatments at
a¼0.10, indicating all treatments had a significant increase inattacks byEuwallaceasp. during later months in the monitoring pe-
riod (Table 3).
For each treatment compared to untreated control trees, trees
treated with the three pesticide combination of bifen-
thrinþemamectin benzoateþmetconazole had a significantly lower
Table 3.Statistical results for each treatment over time
Treatment 1 mo 3 mo 6 mo
t P tPtP
Untreated control2.22 0.062.59 0.033.05 0.01
Bifenthrin2.47 0.042.19 0.073.14 0.02
Emamectin benzoate2.97 0.033.63 0.014.13 0.004
Tebuconazole2.51 0.072.54 0.042.25 0.06
Propiconazole2.89 0.022.22 0.082.56 0.06
Metconazole3.57 0.013.20 0.022.95 0.03
Bacillus subtilis1.08 0.324.19 0.013.14 0.02
Emamectin benzoateþPropiconazole2.37 0.062.71 0.032.25 0.06
Emamectin benzoateþTebuconazole1.75 0.122.59 0.042.20 0.06
BifenthrinþTebuconazole1.81 0.102.30 0.053.13 0.01
Emamectin benzoateþBifenthrinþMetconazole NA NA4.57 0.0012.16 0.06
Paired comparisont-tests were performed comparing the initial pretreatment number of attacks/m2on each tree to attacks at 1, 3, and 6 mo posttreatment.
Treatments with signicant differences indicated that the number of attacks increased signicantly compared to initial attacks.
Fig. 1.Total attacks/m2for each treatment at each sampling period (mean and S.E.) for untreated controls and individual insecticides. For each treatment, bars
with different letters indicate that there was a signicant increase in attacks compared to pretreatment counts in June for pairedt-tests ata¼0.10. A signicant
difference indicated that there was a signicant increase in the number of attacks over time, which was associated with poorer performance for that treatment.
1614Journal of Economic Entomology, 2017, Vol. 110, No. 4

% new attacks compared to untreated control trees for the count in-
tervals ending in both September and December (Table 4;Fig. 3).
This treatment was not assessed for increased attacks in July, as thetreatment was applied in July. Trees treated withBacillus subtilis
had a significantly lower % new attacks compared to untreated con-
trols in July, but not in September or December (Fig. 3).
Fig. 2.Total attacks/m2for each treatment at each sampling period (mean and S.E.) for untreated controls,Bacillus subtilis, and insecticide combinations. For
each treatment, bars with different letters indicate that there was a signicant increase in attacks compared to pretreatment counts in June for pairedt-tests at
a¼0.10. A signicant difference indicated that there was a signicant increase in the number of attacks over time, which was associated with poorer performance
for that treatment.
Table 4.Statistical results for each pesticide treatment compared to untreated control
Treatment July Sept. Dec.
v
2Pv2Pv2P
Bifenthrin 0.11 0.74 0.78 0.38 0.01 0.93
Emamectin benzoate 0.14 0.71 3.78 0.05 0.09 0.76
Tebuconazole 0.39 0.53 0.24 0.62 2.06 0.15
Propiconazole 1.45 0.23 0.47 0.49 0.09 0.77
Metconazole 0.23 0.63 0.47 0.49 0.46 0.49
Bacillus subtilis2.69 0.10 0.47 0.49 0.09 0.77
Emamectin benzoateþPropiconazole 0.81 0.37 0.63 0.43 0.96 0.33
Emamectin benzoateþTebuconazole 1.56 0.21 0.75 0.39 3.08 0.08
BifenthrinþTebuconazole 0.09 0.76 0.14 0.71 0.97 0.33
Emamectin benzoateþBifenthrinþMetconazole NA NA 2.67 0.10 3.82 0.05
A nonparametric ANOVA compared the % new attacks for one treatment to untreated control trees for the sampling interval ending in the month indicated.
Journal of Economic Entomology, 2017, Vol. 110, No. 41615

Trees treated with the emamectin benzoateþtebuconazole com-
bination had a significantly lower % new attacks compared to
untreated control trees in December, but not in earlier months (Fig.
3). Trees treated with emamectin benzoate had a significantly lower
% new attacks compared to untreated control trees in September
only. No other treatments were significantly different from
untreated controls.
When comparing among the three triazole fungicides, there were
no significant differences in the % new attacks for trees treated with
the three triazole fungicides for any sampling interval (period ending
July,v
2¼0.16,P¼0.92; period ending September,v2¼0.46,
P¼0.80; period ending in December,v2¼1.09,P¼0.58).
Discussion
Pesticide Treatments Over Time
When comparing the total number of attacks over time within each
treatment, none of the individual pesticides or pesticide combina-
tions examined were effective at curbing the number of new attacks
for the entire 6 mo that treatments were tracked. Newly emerged
Euwallaceasp. tend to re-infest the maternal tree more often than
taking flight and finding new host material (personal observation).
It is likely that the majority of new attacks on each tree arose from
newly emerged beetles re-attacking the maternal tree, rather than
from new beetles colonizing the tree from other host material.
Therefore, the increase in attacks over time most likely indicates
that the pesticide treatments were unable to inhibit beetle larvae
from completing development and establishing new galleries.
However, three treatments did curtail new attacks on trees dur-
ing the first month posttreatment. Trees treated with the biological
fungicideB. subtilisand trees treated with the triazole fungicide
tebuconazole in combination with either bifenthrin or emamectin
benzoate did not have increased attacks during the first month after
treatment. AlthoughB. subtilishas not previously been studied as a
management option for ambrosia beetle management, it has been
successfully used to control root and foliar fungal diseases for a
wide variety of plants (Cawoy et al. 2011). Although none of the
pesticides tested provide complete control forEuwallaceasp. attacks
on infested trees, there are several options that may mitigate attack
rates compared to untreated trees.
Pesticide Treatments Compared to Untreated Control
Trees treated withB. subtilishad significantly fewer attacks than
untreated controls during the first month after treatment, but not at
later time periods. AlthoughB. subtilisonly provided short-term
control on previously infested trees, having a biopesticide option
available for home owners and land managers may provide an im-
portant tool for managing this invasive ambrosia beetle in areas
where chemical sprays are undesirable.
Only the three-agent pesticide combination with a combined
trunk spray, systemic insecticide, and a fungicide (bifenthrin, ema-
mectin benzoate, and metconazole) provided significant control
compared to untreated trees for the entire monitoring period. Some
two-agent combinations showed more limited periods of control.
These treatments all included the systemic insecticide emamectin
benzoate. Trees treated with emamectin benzoate in combination
with tebuconazole had accumulated fewer attacks than control trees
6 mo after treatment, but not during earlier time periods. Trees
treated with emamectin benzoate showed transitory control, with
fewer attacks than control trees at 3 mo, but not earlier or later.
Previous trials forEuwallaceasp. control indicated the systemic in-
secticide imidacloprid may also mitigate new attacks on previously
infested trees (Eatough Jones and Paine 2017). Although both have
shown some efficacy in separate trials, the systemic insecticides ema-
mectin benzoate and imidacloprid have not yet been directly com-
pared for control of this beetle.
Systemic insecticides that are injected into the lower bole of the
tree have been receiving increased attention as alternatives to bole
sprays for preventing bark beetle attacks (Fettig et al. 2013a).
Studies focusing on bark beetles have examined the efficacy of pre-
ventative sprays applied to uninfested trees that were subsequently
baited to attract bark beetles.Fettig et al. (2014)found that ema-
mectin benzoate in combination with propiconazole was more effec-
tive than emamectin benzoate alone for protecting pine trees from
Dendroctonus ponderosaeHopkins. Additionally, timing of the
Fig. 3.Each individual treatment (white bars) compared to the untreated con-
trol (gray bars) contrasting the % new attacks for each sampling interval
(mean and S.E.). The month listed on the graph is the end of the interval, for
the periods June to July, July to September, and September to December.
Within each treatment and time period, bars with different letters were signi-
cantly different ata¼0.10. Abbreviations are Bifen, bifentrhin; Met, metcona-
zole; Emamec, emamectin benzoate; Tebu, tebuconazole; and Prop,
propiconazole.
1616Journal of Economic Entomology, 2017, Vol. 110, No. 4

preventative treatment was important, with treatments applied in
the fall, 6 mo before beetle attack, being more efficacious than treat-
ments applied in the late spring, 1 mo before beetle attack (Fettig
et al. 2014). Similarly, abamectin and tebuconazole applied9mo
before beetle attack were effective for protecting trees fromD. pon-
derosae(Fettig et al. 2013b). Factors that influence chemical trans-
portation through the plant’s vascular system, such as low
temperatures and soil moisture, may affect the time it takes for ade-
quate distribution of the insecticides, and impact the effectiveness of
injected insecticides (Grosman et al. 2010;Fettig et al. 2013a,b,
2014).
However, unlike manyDendroctonusandIpsspecies, which
have a limited period of tree attack,Euwallaceasp. is actively laying
eggs year-round, and both adults and larvae may be found in an in-
fested tree at any time of year. Unlike bark beetles, which attack
phloem and cambium tissues, ambrosia beetles such asEuwallacea
sp. are active throughout the sapwood. These differences in behavior
may make timing and efficacy of systemic insecticides more difficult
to predict for this beetle. In the early stages of attack, trees are not
likely to exhibit symptoms associated with Fusarium Dieback, and
beetle entry holes are difficult to find, often making it unfeasible to
treat trees before beetles are present. Trees in this study were all in-
fested withEuwallaceasp. prior to treatment. However, emamectin
benzoate may still be effective when applied after wood-boring bee-
tles are present. Ash trees infested with emerald ash borer that were
injected with emamectin benzoate had significantly lower symptoms
of canopy decline than untreated trees (Flower et al. 2015).
Although emamectin benzoate alone did not reduce ambrosia beetle
attacks in this study, it may increase tree protection when used in
combination with fungicides and contact insecticides. Additionally,
long-term monitoring over several years, rather than for 6 mo, could
provide better information on the effectiveness of emamectin benzo-
ate alone, or in combination with other pesticides, for maintaining
or improving symptoms associated withEuwallaceasp. and
Fusarium Dieback.
We did not see any significant differences among the three tria-
zole fungicides tested in this study. None of the triazole fungicides
provided significant control when used alone. Our findings are simi-
lar toFettig et al. (2013b), indicating that insecticides in combina-
tion with fungicides will likely be more effective for controlling
wood-boring beetles. However, further research is needed to deter-
mine if a particular fungicide is more effective. Although fungicides
were tested in combination with insecticides, the limited number of
available infested trees did not allow us to test all combinations.
Pyrethroids such as bifenthrin are typically effective against bark
beetles for one or two field seasons (DeGomez et al. 2006;Fettig
et al. 2006,2013a). In this study, bifenthrin by itself did not provide
any significant control, but trees treated with bifenthrin in combina-
tion with emamectin benzoate and a fungicide had significantly
fewer attacks compared to control trees. Previous laboratory trials
with cut logs have shown bifenthrin was more effective for prevent-
ingEuwallaceasp. attack and reducing gallery formation than other
insecticides used as trunk sprays, including clothianidin, dinote-
furan, and fenpropathrin (Eatough Jones and Paine 2017).
In conclusion, ambrosia beetles can be difficult to control with
pesticides because of their cryptic habits. Ingestion of wood by po-
lyphagous shot hole borer is limited, and beetles spend little time on
the tree surface. This may limit the beetle’s interaction with pesti-
cides. However, due to the severity of the impact of this ambrosia
beetle on a wide variety of tree species in southern California, imme-
diate management options, including pesticides, are needed. We
found thatB. subtilisprovided short-term control (<3 mo) forEuwallaceasp. Pesticide combinations were generally more effective
than single pesticides. We used one contact insecticide applied as a
trunk spray (bifenthrin) and one systemic insecticide applied
through trunk injections (emamectin benzoate). Three triazole fungi-
cides were included in this study, propiconazole, tebuconazole, and
metconazole. There was no difference in performance among the
three fungicides, but limitations in available infested trees did not al-
low for a complete comparison of fungicide and insecticide combi-
nations. The combination of a systemic insecticide, a contact
insecticide, and a fungicide provided the best control; we used ema-
mectin benzoate, bifenthrin, and metconazole for this combination.
Testing of other three-agent combinations may also prove to be effi-
cacious. Ongoing research will continue to investigate the efficacy of
insecticide–fungicide combinations for controlling this ambrosia
beetle. Additionally, many of the trees included in this study were
heavily infested, with 51% of the trees having>100 attacks/m
2and
13% having>500 attacks/m2at the beginning of the trail. Several
of the trees exhibited symptoms of branch dieback, and some had to
be removed because heavy infestation created hazard conditions in
public spaces. Pesticide treatments may be more efficacious if trees
can be treated at the beginning of the infestation before symptoms
of Fusarium Dieback are evident while trees xylem vessels are still
active for the transportation of the systemic pesticides. Note that
this experiment was only done on previously infested sycamore
trees. Further studies need to be done on other hosts tree species for
better management options of this pest disease complex.
Acknowledgments
We wish to thank Robin Veasey, Colin Umeda, Gabby Martinez, Francis Na,
Kameron Y. Sugino, and Beth B. Peacock for assistance with lab and eld
work. We would like to thank the University of California Irvine Ofce of
Environmental Planning & Sustainability for providing resources and sup-
port. We would especially like to thank Matthew Deines and Richard
Demerjian at University of California Irvine. We thank Target Specialty
Products and Valent for providing resources. We would like to thank RPW
Services Inc., Great Scott Tree Service, Inc., Arborjet, and Mauget for valu-
able cooperative support. We would particularly like to acknowledge Donald
Grossman at Arborjet, Ann Hope at Mauget, Paul Webb at RPW, and Scott
Grifths at Great Scott Tree Service, Inc. for their assistance. This research
was funded by grants from the California Association of Nurseries and
Garden Centers, the Nursery Growers Association, and a California
Department of Food and Agriculture Specialty Crop Block Grant. This mate-
rial is based upon work that is supported by the National Institute of Food
and Agriculture, U.S. Department of Agriculture, Hatch project under CA-R-
ENT-7607-MS.
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