RESPONSE OF Culex quinquefasciatus LARVAE TO THREE INSECTICIDES

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. quinquefasciatus⁷ 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 4^th 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	c2	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 insects^1,4.

REFERENCES

• ¹
  Alves SN, Tibúrcio JD, Melo AL. Suscetibilidade de larvas de Culex quinquefasciatus a diferentes inseticidas. Rev Soc Bras Med Trop. 2011;44:486-9.
• ²
  Chen CD, Lee HL, Chan CK, Ang CL, Azahari AH, Lau KW, et al. Laboratory bioefficacy of nine commercial formulations of temephos against larvae of Aedes aegypti (L.), Aedes albopictus Skuse and Culex quinquefasciatus Say. Trop Biomed. 2009;26:360-5.
• ³
  Forattini OP. Culicidologia médica. São Paulo: EDUSP; 2002. 2v.
• ⁴
  Freitas RMC, Faria MA, Alves SN, Melo AL. Effects of Ivermectin on Culex quinquefasciatus larvae. Rev Inst Med Trop Sao Paulo. 1996;38:293-7.
• ⁵
  Gerberg EJ. Manual for mosquito rearing and experimental techniques. Am Mosq Control Assoc. 1979;5:1-124.
• ⁶
  Kozaki T, Brady SG, Scott JG. Frequencies and evolution of organophosphate insensitive acetylcholinesterase alleles in laboratory and field populations of the house fly, Musca domestica L. Pestic Biochem Physiol. 2009;95:6-11.
• ⁷
  Sathantriphop S, Ketavan C, Prabaripai A, Visetson S, Bangs MJ, Akratanakul P, et al. Susceptibility and avoidance behavior by Culex quinquefasciatus Say to three classes of residual insecticides. J Vector Ecol. 2006;31:266-74.
• ⁸
  World Health Organization. Instructions for determining the susceptibility or resistance of mosquito larvae to insecticides. Geneva:WHO;1981. (WHO/VBC/81.807).

Publication Dates

• Publication in this collection
  Nov-Dec 2015