Future challenges in integrated pest management require the development of selective and environmentally safe pesticides along with new strategies to apply them. The more specifically these insecticides act, and the less are their adverse side effects on beneficial insects and the environment, the more they are appropriated to control arthropod pests. Insecticides with novel modes of action such as chitin synthesis inhibitors disrupt cuticle formation [38]; suppression of chitin deposition in treated insect often causes high mortality during molting [39].
The survivorship of mosquitoes is very important for production of progeny, development, and transmission of pathogens among the hosts. Several studies reported the effects of sublethal exposure of IGRs on survival, fecundity, fertility and blood intake capacity of mosquito females [27–30, 40–43]. However, very few studies have evaluated the direct effect of IGRs, mainly pyriproxyfen, on mosquito adults [44–47].
Among the new strategies that are trying to be implemented to control vector mosquitoes is the approach called "auto-dissemination”, based on the possibility that wild adult mosquitoes exposed to artificial resting sites contaminated with IGRs (so far pyriproxyfen, a juvenile hormone analogue), can disseminate insecticide to larval breeding sites, thus preventing adult emergence [27, 29, 31, 32]. This strategy is facilitated by the oviposition behaviour of Ae. aegypti, that typically distribute the eggs from a single gonothrophic cycle among several temporary sites. The ‘‘auto-dissemination’’ approach can be proposed as a ‘pull’ (i.e attraction of wild mosquitoes to contaminated sites) and ‘push’ (i.e. dispersal of contaminated mosquitoes and dissemination of IGR to larval habitats) control strategy with the potential to target the myriad of cryptic larval breeding sites that cannot be reached by traditional larvicidal applications.
The objective of our work was to evaluate for the first time if an IGR belonging to the chitin synthesis inhibitors group can be transferred by mosquito females who contact a treated surface to larval breeding sites. On the other hand, it was also studied whether the contact with a lufenuron-treated surface can have effects on female’s fertility, fecundity and their blood intake capacity.
In our work, lufenuron proved to be highly effective on Ae. aegypti larvae with an EI50 of 0.164 ppb and an EI95 of 1.20 ppb. These values were significantly lower than those found by Salokhe and colleagues [48], they obtained a value of 6 ppb; although they do not specify if used lufenuron technical grade or a formulation, so we cannot make a direct comparison with our results. The values obtained in our work are similar to those obtained for other IGRs of this group such as triflumuron [22] with a value of EI50 and EI90 of 0.8 and 1.8 ppb, respectively. Also for diflubenzuron a value of 0.5 ppb and 3.5 ppb was obtained for the EI50 and EI90 [42]. Although the EI50 values for pyriproxyfen, the IGR used in the “auto-dissemination” assays, are approximately 10 times smaller (EI50 = 0.011 ppb, [26]), we believe that the values obtained for lufenuron indicate that it would be a good candidate to explore in future field trials.
Regarding the ‘‘auto-dissemination’’ or horizontal transfer of lufenuron, our work shows that mosquito females contaminated with lufenuron can transfer enough material to water containers to exert a significant lethal effect on larvae developing there. This effect was dependent on the concentration applied on the paper and also the number of females added to each cage. This is the first work where it is demonstrated that an IGR of the BPU group can be transferred to larval breeding sites by mosquito females. On the other hand there is numerous evidence of this phenomenon for pyriproxyfen [23], [20], [29], [30] were mortality is almost exclusively limited to the pupal stage [26]. However, the use of IGR of the BPU group could mean an advantage since mortality would occur earlier in development, that is, between larval molts or molting from larvae to pupa.
The doses of lufenuron used in our work were slightly higher than those used in other works with pyriproxyfen. This is due on one hand to the difference in effectiveness between both active ingredients (measured as EI50), and on the other hand because the surfaces used in the works using pyriproxyfen were different from the paper used in our study. For example, in the work of Itoh and colleagues [26] a film of polyethylene terephthalat was used while Chism and Apperson [31] used seed germination papers. These surfaces could allow a greater bioavailability of the IGR therefore it would be interesting to repeat our work using different surfaces and evaluate if this could have any differential effect.
Itoh and colleagues [26] reported equivalent effects were achieved using 1, 3 or 5 Ae. aegypti females, however Chism and Apperson [31], using Ae. albopictus, found that percentage mortality was significantly lower at a density of one female per cage and increased to higher and equivalent levels of mortality at 3 and 5 females, they argue this difference to a variation in sensitivity between both species. In the case of our work, the results were similar to those of Chism's work, probably because as lufenuron is less potent as a larvicide than pyrirproxyfen, the amount of females that are involved in the transfer of the IGR is important.
In the last part of our work the effect of lufenuron on fertility, fecundity and blood intake capacity of adult Ae. aegypti females directly exposed to a surface treated with lufenuron was evaluated. When females were exposed to lufenuron 24 hours after blood meal (ABM) a reduction in the hatching percentage close to 40% was observed of those females that had contacted a surface with 1 mg/cm2. Also, when the contact with the surface treated with the IGR was before blood feeding (BBM), a 40% reduction in egg hatching was observed at the dose of 1 mg/cm2. This would indicate that the moment at which contact with lufenuron occurs (before or after blood feeding) is not decisive in the effect on fertility and fecundity but the dose used does; since the same results were obtained with both ABM and BBM regimes. On the other hand, the studies developed with pyriproxyfen indicate that the moment of contact with this IGR is decisive for its effect on the fertility and fertility of the adult’s mosquitoes. Aiku and colleagues [49] found significant effect on egg hatching when An. stephensi females were exposed to bednet treated with 2% pyriproxyfen 24 hours after blood feeding. Also Gaugler and colleagues [33] found the same results with Ae. albopictus, suggesting that mosquitoes are most susceptible when bloodfed one day prior to pyriproxyfen exposure and therefore exposed while developing their eggs. Itoh and colleagues [26] reported that tarsal contact of Ae. aegypti with 0.1 mg/cm2 pyriproxyfen before a bloodmeal induced a large decrease in the number of eggs hatch compared with contact after a bloodmeal.
Finally, our work shows a reduction in the percentage of females that fed on blood for the dose of 1 mg/cm2 of lufenuron, from 97% in the control group to less than 60% in the treated one. However, there was no effect of previous exposure to lufenuron on female mortality. There are no other works that evaluate the direct effect of IGRs on the blood intake capacity of mosquito females. Only the work of Vasuki [43] found that Ae. aegypti larvae and pupae exposed to sublethal doses of the IGR hexaflumuron significantly reduced the quantity of blood ingested as adult females with a corresponding reduction in egg laying.
During the past decades, a larger number of chemically unrelated insecticidal compounds have been developed and commercialized that interfere with chitin synthesis, which is essential to reproduction, growth, and development of insects [50]. This paper introduces an innovation by first exploring the possibility that an IGR belonging to the group of BPUs, such as lufenuron, can be transferred by gravid females to breeding sites and that at the same time can have an effect on fertility, fertility and blood intake capacity of adult mosquitoes. Although lufenuron is effective against many insects, the proposed approach targets container breeding species with such tiny amounts of compound disseminated exclusively to their breeding sites, that impacts on non-target species are likely to be minimal. In the future it might be interesting to explore lufenuron ability to replicate the results found here in field conditions.