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31
ENVIRONMENTAL HEALTH INSIGHTS 2016:10
Introduction
Stormwater catch basins are common sources of permanent
or semipermanent standing water that are routinely tar-
geted for mosquito larvicide applications by local mosquito
control programs to reduce local mosquito populations and
incidence of mosquito-borne diseases.1–16 A common prac-
tice is to treat the stagnant water in all or most of the tens
of thousands or sometimes hundreds of thousands of basins
that can exist in an agency’s operational area with at least one
application of an extended-release larvicide formulated to last
up to 150–180 days. Since the mid-1990s, extended-release
larvicides (eg, Altosid® XR, FourStar®, and Natular™ XRT)
have been used by the North Shore Mosquito Abatement Dis-
trict (NSMAD), a publicly funded mosquito control agency
serving the northeast Chicago metropolitan area, to reduce
mosquitoes in ∼50,000 catch basins each season. Extended-
release larvicide formulations are appealing logistically as a
single treatment could theoretically reduce mosquito larvae
and pupae for an entire season (ie, May through September).
However, there is growing evidence that these products do not
provide control for as long as expected.17–2 3 e cause of the
control failures is not well understood but is likely related to
the catch basins’ structure, hydrology, and characteristics of
the sediments and debris that they collect. Recently, analy-
ses from three years of small-scale larvicide eectiveness tri-
als conducted by NSMAD20 and a year of district-wide catch
basin monitoring21 revealed that extended-release larvicide
applications fail to control mosquitoes in a signicant portion
of basins. Because catch basins capture and retain runo and
debris, extended-release larvicides applied to these structures
may be susceptible to being ushed out of basins, dissolving
more rapidly than designed, or becoming buried in debris cap-
tured in the sump. All of these phenomena can inuence the
eectiveness of larvicides.
e objective of this project was to determine if additional
applications of extended-release larvicides could improve the
eectiveness of these products in basins in which ushing, rapid
dissolution, or burying may occur. To do this, three mosquito
larvicides, Natular™ XRT 180-day tablet (Clarke Mosquito
Control Products, Inc.), Natular™ T30 30-day tablet (Clarke
Mosquito Control Products, Inc.), and CocoBear™ larvicide
oil (Clarke Mosquito Control Products, Inc.), were evaluated
Small-Scale Trials Suggest Increasing Applications of
Natular™ XRT and Natular™ T30 Larvicide Tablets May
Not Improve Mosquito Reduction in Some Catch Basins
Justin E. Harbison1, 2, Marlon Henry2, Peter C. Corcoran2, Dave Zazra2 and Christopher Xamplas2
1Department of Public Health Sciences, Loyola University Medical Center, Loyola Universit y Chicago, Maywood, IL, USA. 2North Shore
Mosquito Abatement District, Nor thfield, IL, USA.
A BS TR A CT: Stormwater catch basins are commonly treated with lar vicides by mosquito control agencies to reduce local populations of mosquito species
capable of transmitting West Nile virus. Recent evidence suggests that extended-release lar vicides formulated to last up to 180 days in catch basins may
not be eective in some basins due to chronic ushing, rapid dissolution, or bur ying of treatment in sump debris. To investigate if increasing the number
of applications could improve eectiveness, a smal l study was performed over 13 weeks in 2015 to evaluate two extended-release larvicides (Natular™
XRT 180-day tablets and Natular™ T30 30-day tablets) and a lar vicide oil (CocoBear™). Over the course of 13 weeks, three groups of eight basins were
monitored for mosquitoes, each group receiving Natular™ XRT, Natular™ T30, or CocoBear™ larv icides. All basins received a single application at the
beginning of the study period. Once mosquitoes in a basin surpassed the treatment threshold during week ly monitoring, an additional application of the
associated lar vicide was given to that basin. e number of applications during the study period ranged from 1 to 10 for CocoBear™ basins, 1 to 7 for T30
basins, and 2 to 8 for XRT basins. Overall, the average number of applications and the cost of larvicide per basin were 4.4 applications at $0.66 per Coco-
Bear™ basin, 4.4 applications at $6.26 per T30 basin, and 4 applications at $16.56 per XRT basin. Basins treated with XRT and T30 needed reapplications
more often than expected, yet were no more eective than CocoBear™, suggesting that increasing the frequency of application of these lar vicide formula-
tions may not prov ide increased mosquito reduction in some basins.
KEYWORDS: larv icide, catch basins, mosquitoes, cost
CI TATI ON: Harbi son et al. Sm all-S cale Trials S uggest In creasi ng Appli cation s of
Natular™ XRT and Natular™ T30 Larvicide Tablets May Not Improve Mosquito
Reduction in Some Catch Basins. Environmental Health Insights 2016:10 31–34
doi: 10.4137/EHI.S36722.
TYPE: Sho rt Repo rt
RECEIVED: October 20, 2 015. RESUBMITTED: November 18, 2 015. ACCEPTE D FOR
PUBLICATION: Novemb er 20, 2015.
ACADEMI C EDITOR: Timothy Ke lley, Editor in C hief
PEER RE VIEW: Fiv e peer revie wers con tribute d to the pee r review rep ort. Rev iewers’
repor ts total ed 1009 wor ds, exclud ing any con dentia l commen ts to the ac ademic ed itor.
FUNDING: Aut hors dis close no ex ternal funding s ources.
COMPET ING INTE RESTS: Au thors di sclose n o potentia l conic ts of inter est.
CORRESPONDENCE: jharbison@luc.edu
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Harbison et al
32
ENVIRONMENTAL HEALTH INSIGHTS 2016:10
during the 2015 season in catch basins located in the same
small residential area used for previous larvicide trials.20
Methods
All trials were conducted between June and September
2015 in catch basins located within a 0.7 km2 residential
area near the center of the NSMAD operational area. As
the study area has been used in previous NSMAD catch
basin studies, there were several years of data on the present
study’s catch basins to inform the present trials. Prior analy-
ses found that extended-release larvicides failed to exhibit
any control of mosquitoes over three consecutive years for
12 individual catch basins in the study area. erefore,
each of the three larvicides, XRT, T30, and CocoBear™,
was randomly assigned to eight basins that included four of
these failure basins and four in which larvicides appeared to
be eective. XRT is formulated with 6.25% spinosad, T30
is formulated with 8.33% spinosad, and CocoBear™ is for-
mulated with 10% mineral oil. CocoBea r™ is not considered
as an extended-release larvicide but was included for control
purposes to compare the extended-release larvicides (one
and three applications expected, respectively, for XRT and
T30) with a larvicide in which weekly reapplications would
be expected. e use of untreated basins as a more strin-
gent control was not incorporated in this study over concerns
of that these untreated basins could pose a risk to residents
within and near the study area.
Beginning the rst week of June and ending the rst
week of September, each of the 24 study basins was monitored
for the presence of mosquito larvae and pupae by removing the
circular grate of each structure with a manhole hook and taking
two dip samples using a standard 350-mL dipper. Mosquitoes
collected in a dip sample were counted, with the number of
pupae and fourth instar larvae noted. All 24 basins received
their respective larvicide treatments in the second week of
monitoring. Each subsequent week, a basin was found to have
reached or surpassed an average of 12 mosquito larvae and/or
pupae per dip, and that basin received an additional treatment
of its respective larvicide. e 12-mosquito threshold was
found by dip samples taken from untreated basins in the study
area from 2011 to 2014 that showed that the average number
of mosquitoes from a total of 1,752 dip samples in untreated
basins was 13.7 (95% condence interval [CI] = 12.5–14.9).
e total number (sum of two dips) of mosquitoes in basins
across weeks was compared among the treatments using a
repeated measures analysis of variance. e number and the
total cost of larvicide applications were also recorded each
week. In 2015 a single XRT tablet cost NSMAD $4.14,
$1.43 for a T30 tablet, and estimated $0.15 per single basin
for CocoBear™. Precipitation data were downloaded from a
nearby weather station of the National Oceanic and Atmo-
spheric Administration’s National Weather Service Forecast
Oce located at the Chicago O’Hare Airport (http://www.
nws.noaa.gov/climate/index.php?wfo=lot).
Results
All basins except two (one treated with CocoBear™ [CBa]
and one with T30 [T30h]) surpassed the threshold for retreat-
ment at least once (Table 1). e number of applications to
a single basin during the study period ranged from 1 to 10
for CocoBear™ basins, 1 to 7 for T30 basins, and 2 to 8
for XRT basins (Table 1). Generally, there did not appear
to be a large dierence among the eectiveness of larvicide
treatments each week (Table 2). e mean total mosquitoes
(sum of two dips) and 95% CI were 17.13 (11.58–22.67) for
CocoBear™ basins, 21.38 (15.05–27.70) for T30 basins, and
21.70 (14.97–28.43) for XRT basins over 12 weeks with active
treatment. ere was no statistical signicance found between
treatments: F (2, 21) = 0.32, P = 0.73. e percentages of the
fourth instars and pupae observed from the total mosquitoes
found among the treatments each week ranged from 6.9% to
87.9% for CocoBear™, 9.0% to 78.2% for T30, and 9.2% to
75.3% for XRT. e total number of applications to the eight
basins was 35 for CocoBear™ and T30 basins and 32 for XRT
basins. Overall, the average number of applications and the
cost of larvicide per basin were 4.4 applications at $0.66 per
CocoBear™ basin, 4.4 applications at $6.26 per T30 basin,
and 4 applications at $16.56 per XRT basin.
Discussion
Over the monitoring period, basins appeared to hold similar
numbers of mosquitoes across the three larvicide treatments,
suggesting, at least in the study basins, approximately equal
eectiveness for the amount and type of larvicides used. Cer-
tainly, it appears that the use of the extended-release tablets
(both XRT and T30) may not be the most cost-eective way
to reduce mosquitoes in the study basins. Most extended-
release basins needed more than the expected applications
throughout the monitoring period, yet their eectiveness was
no better than that of the oil. e use of the active ingre-
dient (spinosad) of two extended-release pesticides for mos-
quito control, including the eect on nontarget species, has
been described at length elsewhere.24 –30 Although larvicide
oils are not formulated to be eective for longer durations, it
did appear to be more cost eective than tablets in the study
basins. e cost of labor for larvicide applications appears to
be about the same for the three products as the total appli-
cations were similar. However, the cost of product is signi-
cantly lower for CocoBear™ per application, making it the
most economical treatment.
Larvicide oils are designed to leave a thin lm on the
surface of treated water potentially killing larvae and pupae
via contact or suocation. However, because this lm tends
to break down quickly when the water surface is disturbed
(eg, from an inux of rain and runo ), this type of larvi-
cide is considered to be eective for a much shorter dura-
tion than the extended-release larvicides. Alternatively, the
XRT and T30 tablets are formulated to slowly release their
active ingredients over time, killing mosquito larvae and
Increasing applications of Natular™ XRT and Natular™ T30 lar vicide tablets
33
ENVIRONMENTAL HEALTH INSIGHTS 2016:10
pupae through contact or ingestion. Since the NSMAD was
established in 1927 and until the mid-1990s, the larvicide
oils were predominantly applied to the district’s catch basins.
Oils are somewhat easier to apply than the extended-release
larvicides because often the applicator does not need to leave
his or her vehicle to treat basins using a long nozzle to spray
the larvicide into basins. As such, all NSMAD basins could
usually receive three to four oil treatments in a season. How-
ever, our results suggest that three to four applications of oil,
as well as T30 and XRT, may not be enough for some basins.
Currently, it takes approximately two months or more for
NSMAD technicians to treat all basins with extended-release
larvicides, and often most basins do not receive an additional
treatment before the seasonal technicians become unavail-
able. Looking for longer lasting alternatives to extended-
release tablets and more clearly identifying areas with a
signicant number of basins in which larvicides fail will be
important next steps.
Acknowledgment
e authors thank Roger S. Nasci for review and insightful
comments on the initial draft of this article.
Table 1. A comparison of the number of larvicide applications made to eight CocoBear™ oil-treated, eight Natular™ T30-treated, and eight
Natular™ XRT-treated catch basins from June to September 2015. T indicates that the threshold for treatment ($12 mosquitoes per dip) was
surpassed and an application of the associated larvicide to the basin was made.
WEEK NUM BER
BASIN 23 24 25 26 27 28 29 30 31 32 33 34 35 TO TAL
CocoBear Oil
CBa T 1
CBb T T T T T T 6
CBc T T 2
CBd T T T 3
CBe T T T T T T T T T T 10
CBf T T 2
CBg T T T T T 5
CBh T T T T T T 6
T30 30-day Tablets*
T30a T T T T T T T 7
T30b T T T 3
T30c T T T 3
T30d T T T T T 5
T30e T T T T 4
T30f T T T T T T 6
T30g T T T T T T 6
T30h T 1
XRT 180- day Tablets
XR Ta T T 2
XRTb T T T 3
XR Tc T T T T T 5
XR Td T T T T 4
XR Te T T T T T 5
XRTf T T T T T T T T 8
XR Tg T T 2
XRTh T T T 3
Note: aThe duration of T3 0 tablets is formulated to last fou r weeks, and three treatments would be expected for 13 weeks on weeks 23, 28, and 33.
Harbison et al
34 ENVIRONMENTAL HEALTH INSIGHTS 2016:10
Author Contributions
Conceived, designed, and performed the experiments: JEH,
MH. Analyzed the data: JEH, MH. Wrote the rst draft of
the article: JEH, DZ, CX, PCC, MH. All authors reviewed
and approved the nal article.
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