Divinópolis, August 26, 2015
Dear Editor,
The adult females of Culex quinquefasciatus are vectors responsible for
transmission of different arboviruses and lymphatic filariasis. The mosquito has
anthropophilic and endophilics habits, their larvae develop in almost all habitats
modified
by humans3. Different insecticides groups such as
organophosphates, carbamates, and pyrethroids have been widely used in mosquito control,
but in some places, the temephos has shown ineffectiveness for effective control of
C. quinquefasciatus
7 as a result of resistance development6. So, there is a need of monitoring the populations
through periodic studies for the detection of susceptibility.
The aim of this study was to establish the response level of C.
quinquefasciatus larvae to organophosphate, pyrethroid, and ivermectin. The
sub-lethal doses were determined.
C. quinquefasciatus larvae were obtained from a laboratory colony as
previously described by GERBERG5. Selected
4th instars larvae of C. quinquefasciatus were exposed for
one hour to 40, 8, 1.6, and 0.32 ppb (parts per billion) of the organophosphate, temephos
(Fersol 500CE); pyrethroid, deltamethrin (Fersol 25CE), and ivermectin (Ivomec 1%
w/v,
Merial). For each insecticide tested, the larvae were divided into groups consisting
of 20
specimens as well as the control group, three replications for each treatment (360
larvae
per group). Control groups were placed in recipients containing just dechlorinated
water.
After the exposure time, larvae were washed and transferred to other plastic recipients
containing dechlorinated water and food (murine chow, Labina - Purina(r)). The temperature
was maintained at 26 °C throughout all the tests, and they were conducted by adapting
WHO8 methodology. The LC50 was
determined using the probit program DL50 (1987).
Table 1 presents that temephos showed low
LC50 among the insecticides tested (1.37 ppb), with a confidence interval of
95% from 1.22 to 1.53. In contrast to the study done in Brazil by ALVES et
al.
1 in 2011 that showed temephos LC50 was
over 50 times higher than that presented in the actual study, while deltamethrin
LC50 was four times higher. However, ivermectin LC50 was lower
than 1/4 previous LC50 and the curve presented lowest inclination, demonstrating
higher heterogeneity in the response to this insecticide.
Table 1
- Lethal concentration (LC) for insecticides in ppb, for
Culex quinquefasciatus larvae after 1h of exposure
Insecticides
LC50 (IC 95%)
p-value
c
2
Inclination
Temephos
1.37 (1.22 - 1.53)
< 0.005*
17.53
0.13
Deltametrine
14.18 (12.20 - 16.48)
< 0.005*
16.93
0.68
Ivermectin
1.01 (0.82 - 1.26)
< 0.005*
27.47
0.06
IC = Confidence Interval; X2 = Chi-square.
However, as also observed by ALVES et al.
1 in 2011, probit analysis demonstrated that the
highest concentration used was just the one used for deltamethrin. It is also observed
that
there are differences between insecticides, but temephos has a greater effect on mosquito
larvae in lower concentrations, which for local populations of culicids meant no detection
of insecticide resistance. However, lower genetic variability may explain the responses
of
populations to the tested insecticides. CHEN et al.
2 in 2009 evaluated in laboratory the
bioavailability of nine commercial formulations of temephos at a dose of 1 mg/L, all
of
them exhibited varying levels of toxicity against larvae of C.
quinquefasciatus, Aedes aegypti, and A.
albopictus.
Currently there are thousands of dengue cases in Brazil and tonnes of insecticides
are
being used to combat mosquitoes, however, few studies have shown resistance of these
insects. Aedes and Culex are part of the same subfamily
of dipterans and not always the way to combat each one is distinguished. Thus, the
types of
insecticides and the doses to be employed against these dipterans should be corroborated
for efficacy, in addition, some studies point out that the ivermectin can be an insecticide
to be used against these insects1
,
4.