Insecticide resistance is a growing problem that hampers mosquito control in different regions around the world. Nowadays, resistance to the main neurotoxic insecticide classes has spread among several populations of insect disease vectors. Therefore, biological control methods and the use of chemicals with distinct mechanisms of action have been increasingly employed. The utilization of chitin synthesis inhibitors, for example, represents one major current strategy for mosquito chemical control. For this reason, evaluation of the efficacy of such compounds on mosquito field populations is essential. This work deals with the CSI triflumuron efficacy against Ae. aegypti field populations, as well as laboratory colonies of Ae. albopictus and Cx. quinquefasciatus.
Our results indicate that triflumuron, under laboratory conditions, is effective against the three mosquito species evaluated. As expected, this compound induced adult emergence inhibition of Ae. albopictus and Cx. quinquefasciatus in a dose-dependent way. Previous results of our group have proven that this compound acts in a similar way on Ae. aegypti. Novaluron, another CSI, has the same effect on this mosquito . Moreover, other IGRs, such as methoprene (juvenile hormone mimic) and halofenozide (ecdysone agonist), exhibited similar results [33, 34].
Several authors have observed that high doses of CSIs caused more pronounced mortality of mosquito larvae, relative to pupae and adults [17, 18, 35]. These results corroborate the data presented here, performed with triflumuron, and confirm the direct relationship between CSI concentrations and the precocity of their effects.
Triflumuron EI50 and EI90 against Cx. quinquefasciatus (Table 1) were 3.3 and 4.7 times higher than the corresponding values in Ae. albopictus. In this latter species, in turn, triflumuron effective doses were around 1.5 - 1.8 times higher than in Ae. aegypti (EI50 and EI90 of 0.86 μg/L and 1.8 μg/L, respectively) . The triflumuron doses recommended by the manufacturer for Aedes and Culex control are around 0.12 g/L and 0.24 g/L, respectively , values much higher than the effective concentrations for these culicids.
Although there are some reports related to field simulated assays, to our knowledge this is the first evaluation of the triflumuron effect on Ae. albopictus under laboratory conditions. With respect to Cx. quinquefasciatus, our results slightly differ from other reports, which declared EI50 and EI90 values of 2.0 and 7.0 μg/L, respectively [37, 38]. Intraspecific variations of IGR effective doses among different laboratories are very common and are derived, in part, from the use of distinct protocols, as already discussed by Braga et al.. Additionally, variations in effective doses among different CSIs for a given species, or among different insect species for a given CSI, have also been reported. Diflubenzuron, the first chitin synthesis inhibitor commercially available, has been used in the control of various insect species, especially in agriculture . Concentrations of 0.5 μg/L of this compound resulted in 50% of Cx. quinquefasciatus larval mortality . Su et al. found for novaluron, another promising CSI, EI50 and EI90 of respectively 0.16 and 0.60 μg/L for Cx. quinquefasciatus. Ali et al. studied the effect of diflubenzuron in Ae. albopictus and encoutered an EI50 of 0.45 μg/L for this species, the EI90 being 0.84 μg/L . Hexaflumuron, another CSI, also proved very effective against Ae. albopictus, with the EI50 of 0.2 μg/L .
Although effective doses of triflumuron are higher when compared to other IGRs, Mian and Mulla emphasized the efficient activity of this CSI on mosquitoes, which is confirmed by its effectiveness in field trials . Sulaiman et al. verified that 14.0 mg/L of triflumuron inhibited Ae. albopictus emergence for 168 days under simulated field conditions . Likewise, Batra et al. found that triflumuron was effective against Cx. quinquefasciatus and Anopheles stephensi for seven weeks .
We also demonstrated that triflumuron is effective against Ae. aegypti field populations exhibiting different resistance levels to organophosphates and pyrethroids. In such cases, a small number of larvae always remained alive for many days, since the primary action of CSIs is not to induce mortality, but to interfere with development.
Aedes aegypti larval mortality was more pronounced when populations were treated with DD rather than EI99, confirming the direct relationship between CSI concentration and the precocity of its effects, likewise this has previously been shown for the Rockefeller strain . However, total mortality occurred later in populations exposed to DD. Empirical observations during CSI bioassays indicate that in many cases, a small number of larvae remain alive for long periods, regardless of the dosage. However, these larvae never progress to adulthood. Probably, this occurs due to natural variations among specimens of field populations.
The absence of adult emergence when field populations are exposed to triflumuron, repudiates cross-resistance between this CSI and neurotoxic inseticides. However, a positive correlation between pupal mortality rates and the temephos resistance ratios was noted, suggesting a potential triflumuron tolerance in populations resistant to this OP. This triflumuron tolerance could derive from the increased activity of enzymes related to metabolic resistance, already detected in several Brazilian Ae. aegypti populations [10, 34]. In contrast, correlation between deltamethrin mortality rates and triflumuron induced pupae mortality in these Ae. aegypti populations were not detected. This result may in part be associated with the qualitative nature of the PY bioassay, consisting of a single dose. Although there are few reports concerning IGR resistance, diflubenzuron tolerance associated with mixed-function oxidases was evidenced in blowflies . In mosquitoes however, to date IGR resistance has not yet been directly associated with increased activity of detoxifying enzymes.