**6. Conclusions**

evaluation of the fitness costs in resistant insects under stress conditions (in terms of nutrition, temperature, and larval density, for instance) can bring forth relevant data related to the evolution of resistance in the field. However, such investigations are still scarce [60-62].

**5. Possible changes on development and reproduction of insecticide-**

present some examples of resistance side effects in vector mosquitoes.

the resistance alleles *ace-1R* (modified AChE), *Ester1*

time [70].

As previously discussed, resistance genes may cause changes or even dysfunctions upon direct physiological process and indirect life history traits. The knowledge of the insecticide resist‐ ance costs and which parameters are altered are important to better design strategies of insect control, especially considering vectors of pathogens, once general developmental and repro‐ ductive life-traits are strongly associated to their vectorial capacity. In the following, we

The longevity of insects is generally evaluated in fitness investigations as a key parameter of vector/parasite relationship. Decreased longevity has been detected in species resistant to different classes of insecticides. Both *Culex pipiens pallens* and *A. aegypti* selected for PY resistance in laboratory presented decreased longevity [63-65]. Pyrethroid resistance also induced similar effects on the longevity of *A. gambiae* females, in this case presumably due to affected energy metabolism and oxidative stress [66]. Defenses to non neurotoxic compounds can also affect longevity, as observed in one *A. aegypti* lineage selected in the laboratory for diflubenzuron (a chitin synthesis inhibitor) resistance [67]. As resistance mechanisms vary among species and populations, especially when metabolic, the life span of the resistant insects is not always affected, even when high resistance ratios are observed. This was the case of two

Brazilian field populations of *A. aegypti* resistant to both OP and PY insecticides [68].

The time to complete the larval development is also of particular interest, since the longer it takes the higher is the exposure to adverse conditions of the breeding site and to natural predators and pathogens. Likewise longevity, resistance to several insecticides can affect this parameter. Increased developmental time was observed in *Culex quinquefasciatus* and *A. aegypti* selected in the laboratory for PY resistance [64, 65], and also to an *A. aegypti* field population with high resistance level to OP [65]. Natural populations of *C. pipiens* harboring

presented a longer larval developmental time [69]. The *kdr* mutation was also the prime cause for a delay in the larval development of *A. aegypti*, especially when mutant and PY susceptible larvae were reared together and under more stringent conditions [57]. Again, impacts on this parameter were not restricted to neurotoxic insecticides, as demonstrated for an *A. aegypti* laboratory strain resistant to *Bti* toxins, which presented impairment on the larval development

Some behavioral aspects can also be affected by resistance, as the ability to detect a potential host. Under laboratory conditions, for example, fewer OP resistant *A. aegypti* females respond‐ ed to the blood meal stimuli, compared to their susceptible counterparts [68]. Similar results were observed in lineages of the same vector selected for resistance to a chitin synthesis

and *Ester4*

(overproduction of EST) also

**resistant insects**

252 Insecticides Resistance

The idea of "evolution-proof insecticides" is a challenge for the introduction of new com‐ pounds. A possible strategy proposed to slow the evolution of insecticide resistance would be to apply compounds with action over older mosquitos, i.e., when females have already laid most of their eggs. In this direction, there would be a very weak selection pressure over resistance genes, once practically all the offspring of susceptible and resistant individuals have emerged at each generation [76]. This is particularly interesting to the control of vector-borne diseases, because several pathogens have an intrinsic incubation time of their life cycle inside the insect organism. where the insects are able to feed on blood and lay their eggs several times before become infective. Nonetheless, they cannot live long enough to have the opportunity of a infective blood feeding. Mathematical models have shown that this kind of approach against old insects would dramatically affect the course of insecticide resistance [77].

New strategies are currently being tested in the field, like the release of genetically modified mosquitoes that suppress the natural population [78, 79] and of a strain carrying endosymbiont bacteria that diminishes the mosquito vectorial capacity [80, 81]. However, until these tools are not available for a high-scale application and considering distinct vectors, the use of insecticides must continue to play a central role, especially during epidemic outbreaks. In this sense, physiological, molecular, and evolutionary aspects of insecticide resistance need to be further studied and discussed with the aim to better improve the control of undesired insect populations.
