SCIENTIFIC NOTE

Journal of the American Mosquito Control AssociaTion. 24( 1): 123-126, 2008
Copyright (02008 by The American MosquIto Cootrol Association, Ine.
SCIENTIFIC NOTE
LABORATORY EVALUATION OF CYROMAZINE AGAINST
INSECTICIDE-SUSCEPTIBLE AND -RESISTANT MOSQUITO LARVAE
FREDERIC DARRIET,' MORTEZA ZAlM'
AND
VINCENT CORBEL'
ABSTRACT. In this sludy, the aetivily of eyromazine was evaluated following WHO standard
procedures against susceptible and rcsistant mosquito strains of /J.nopl1eles gambiae, Culex quinquefascifllus,
and Acdes aegypti. The dose for 50% and 90% inhibItion of adull emergence (IE~o and I~o) ranged from
0.028 mg/liler lo 0.17 mglliter and from 0.075 mg/liter ta 0.42 mgllitcr, respedively. The e/TecLs of
eyrom:nine weœ closer LO that of chi tin synthesis ;nhlbitors rather than that of juvenlle hormone analogues,
with onl 1 10-20% PUjl:tI mortalily. The toxicity of cyromazine was not sn'ongly aITected by the presenç' of
eommon resistance mlOchanlsm, Le., J(dr mutalion and Ace.I Il (resistance ratio from 0.5 102.3) The absence
of cross resislanee with eommon insecticides (pyrethroids, carbamates, organophosphales) m(lkes
cyromazine a potential candidate for disease vector control, especlally for the managemenL of 1D5e{;licide
resistance.
KEY WORDS
resislance
Cyromazine, IGR, Aedes aegypri, Anopheles gambiae, Culex quinquefasciarus, inseeticide
Because of the deve]opment of pesticide
resistance in mosquitoes, there is an urgent need
to search for safe and cost-effective insecticides
for veetor control. Cyromazine [N-cycJopropyl­
l ),5-tria7ine-2,4,6-triamine] is an insect growth
regulator belonging to the aminotriazine insecLi­
cides. lt affects larval and pupal cuLicles in
dipterans and sorne other insects. Although the
exact mode of acLion of cyromazine is not weU
understood, it has beeo shown to disturb some
steps of sclerotization of tbe cuticle (Bel et al.
2000). This molecule may also interfere with
DN/\ synthesis by disturbiog the integration of
cytosine and adenosine into the cell (Binnington
and Retnakaran 1991). Cyromazine also estab­
lishes a oew standard for low environmental
(Nelson et al. 1986), and human risk (acute oral
LD 0 for rates = 3,387 mglkg [Tomlin 2000)) and
ofrers new approaches to insecticide resistance
management. ln most cases, cyromazine is use<l
against pest species of agrîcultural (ieaf miners,
Mediterranean fruit Dy) and veterinary impor­
tance (Deas, houseDies) (Friedel 1986, Tomli..o
2000).
For mosquito control, cyromazine has heen
evaluated in the laboratory against different
strains of Aedes aegypli L., Anopheles garnbiae
Gîles, and Culex quinquefasciatus Say, which
present none, l, or several rcsistance mechanisms
to insecticides. This study, carried out i..o colla b­
'Institut de Recherche pour le Développement,
Laboratoire de LUIte Contre les Insectes Nuisibles,
91 t Avenue Agropolis, BP 64501, 34934 Montpellier
Cedex S, France.
2 WHOICDS/PVC, World HealÙl Organlzation, 1211
Geneva 27, Switzerland.
oration with the WHO Pesticide Evaluation
Scheme (WHOPES), examined the potential
larvicidal effeet of cyroma:z-ine and ilS cross­
resistance with commonly used insecLicides.
Two laboratory strains of An. ga.rnbiac, 3 of
CX. quinquefascialus, and 2 of Ae aegypli were
used in this study. The susceptible reference
straios (SS) of An. gambiae (KIS), Cx quinque­
fascialus (S-LAB), and Ae. aegypli (BORA) have
been reared for many years i..o the laboratory and
are free of any detectable insecticide resislance
mechanism. Tbe pyrethroid-resistant strains of
An. gambiae (VKPR), Ae. aegypli (LHP), and CX.
quinquefascialus (BC) arc homozygous for the
knockdown resistance (Kdr) gene (Darrîet et al.
1997, Chandre et al. 1998, Brengues et al. 2003).
The car bama te/or gan opboso pha te- resistan t
strain of CX. quinqucfasciatus (SR) TS homozygote
for the gene encoding for an insensi live acetyl­
cbolinesterase (Ace. Ill), but remained fuUy sus­
ceptible to pyrethroids and DDT. The S-LAB,
BC, and SR strains share the same gcnetic
background but ooly di.ffered by the presence of
susceptible or resistant" alleles at the Kdr and
Ace.I locus (Berticat et al. 2002).
The technical grade (96.3%) of cyromazine
used in our study was provided by Syngenta,
Basci, Switzerland. Larval bioassays were carried
out with the use of technical-grade cyromazine
dissolved in ethanol. Each hioassay was repeated
3 limes with 3rd instars. Groups of 25 larvae were
placed in 99 ml of distilled water, with ] ml of
i..osecticide solution. FOlU lots per concentration
(Il. = 100) and a minimum of 4 concentrations per
replicate, providingmortality within a range ofû­
100%, were used for each replicate. The long
duration of the test required that larvae were
123
124
JOURNAL OF THE AMERICAN MOSQUITO CONTROL ASSOCLADON
VOL. 24, No. l
v
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O"I\OV"'INVVl~
t'fI~t""l",,"-f-.-
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o
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provided every day with a smail amount of food
(dry cat food [100 mglliter] for Cx. quinquefascia­
tus and Ae. aegyplÎ and dry fish food (Tetramîu®;
50 mg/liter] for An. gambiae) until the 1st pupae
occUfred in the cups. Larvae in the control
batches were fed in the same manner as those of
the treated batches. The temperatme was main­
tained at 27°C throughout the experiment.
According to the WHO guidelines for laboratory
testing of mosquito larvicides (WHO 2005), the
results were expressed lU terms of emergence
inhibition rates (lE) based on tbe number of
larvae that did not develop successfully iuto
viable adults. In recording daily the IE%,
moribund, dead larvae and pupae, as weil as
adultmosquitoes that did not completely separate
from the pupal case were considered as affected.
The experirnent was stopped wheu all larvae or
pupae in the control batches died or emerged in
adults. Data were anatyzed according to the
method of Finney (1971) by using Probit®
software (Raymond et al. 1997). This software
uses an iterative method of maximum Iikelihood
to fit a regression between logarithm of concen­
tration and the probit emergence inhibition. This
software then provided an estimation of IE so and
IE 90 with their 95% confidence intervals. Two
mosquito strains of the same species were
considered as having the same susceptibility to
cyromazïne wben the ratio between their IE so
(resistance ration RR so ) or IE90 (RR 90 ) had
confidence limits including the value 1.
Results from larval bioassays are sununarized
in Table 1 and Fig. 1. Few differences in suscep­
tibility were noted between the 3 mosquito species
(Table 1). With the susceptible strains, the IE 50
ranged from 0.028 to 0.17 mg/liter and the IE 90
from 0.075 to 0.42 mglliter. We did not observe
lower efficacy of cyroroazine against Kdr- and
Ace.l-resistant mosq uitoes coropared to the
susceptible ones (the resistance ratios RR so or
RR 90 had confidence liroits including the value l,
except at the IE so level on the Kdr-resistant strain
of An. gambiae). From the mortality profIles of
cyroroazine on the 3 mosquito species (Fig. 1), we
noted tbat this chemîcal acted mainly by killing
the larvae. The effect on pupae accounted for
about 10-20% mortality, depending on the straiu
and the doses tested. Consequently, the effects of
cyromazine were closer to that of chitin synthesls
inhibitors (i.e., diflubenzuron) rather than that of
juvenile honnone analogues (i.e., pyriproxyfen).
However, cyromazine dîd not greatly affect the
time of development of treated larvae compared
to untreated ones.
In a previous study, Phonchevin et al. (1985)
reporled lower efficacy of cyromazine iu terms of
LC so against An. dirus (0.0042 mg/liter) and Cx.
quinquefascialUs (0.0068 mg/liter), whereas its
activity remained in a similar range of concen­
trations against Ae. aegypti (0.23 mgfliter). In the
Tl:
LHP suain (R.R KJ:JI ~
Bora str4'lj. (55)
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M oflalilY profiles of cyromazllle againsl susceptible and resislunl strains of Aedes aegypl'. Anopheles
;;WII la • and Culex quillqueJasciaius.
126
JOURNAL OF THE AMI'RrCAN MosQUITO CONTROL ASSOCIATION
USA (Oklahoma), SaJeh and Wright (1989) report­
ed an LC so of 0.17 mglliter against Ae. epacticus
larvae collected from stagnant water. A lï.eld trial
carried out in Israël with cyroroazine (2%
granular) at 0.5 rogfliter showed complete inhibi­
tion of emergence of ex. pipiens L. for 40 days
(Cohen 1986). To date, !l0 specific resistance to
cyromazioe was found in mosquitoes, whereas it
has been aJready observed in Musca domestica L.
in Brazil (Pinto anù Prado 2001), tbe USA (Tang
et al. 2002) and Denmark (Kristensen and
Jespersen 2003). Cross resislance between cyro­
mazine and diOubenzuron (ecdysoid) has also
been reporled in the common houseOy (Shen and
Plapp 1990). Despite thar.-cyromazine is somewhat
less tox.ic to mosquiloes than other lORs, its low
mammau.an toxicity and good activity against
pYTetbroid- and carbamate-resistant mosquitoes
makes il potenLial candidate for further study for
control of mosquito-borne disease.
We tbank the World Health Orgaoization
Pesticide Evaluation Scheme (WHOPES) pro­
gram for funding this work and Syngenta (Basel,
Switzerland) for providing the technical grade of
cyromazine.
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