**6. Conclusions**

Field-collected and laboratory-selected *Cx. quinquefasciatus* that were resistant to methoprene did not show cross-resistance to spinosad and spinetoram. The presence and absence of crossresistance to other pesticides in spinosad-resistant mosquitoes seemed to be related to their

Methoprene, hydroprene, kinoprene, and triprene were synthesized in 1960s. These insect growth regulators interrupt juvenile hormone balance during the transition from the late 4th instar larvae to pupae and adults. Most mortality occurs at pupal stage or incompletely emerged adults. Another juvenile hormone analog pyriproxyfen was synthesized in 1970s, the IRG activity of which is much higher than methoprene [80]. The earliest experimental studies on potential of resistance development in mosquitoes to juvenile hormone analogs were in 1973 [82]. The collective results indicated low risk of resistance development [82–86]. For example, the selection of *Cx. quinquefasciatus* by methoprene for 10 generations only lead 3.9 to 21.3-fold of resistance [86], while the selection of *Cx. pipiens* for 8 generations only resulted in 8- to 13-fold resistance to methoprene and cross-resistance to triprene [83]. Higher levels of resistance to methoprene did not necessarily occur in response to longer period of selection. *Culex tarsalis* Coquillett developed 86-fold resistance after 62 generations of selection [84], while 218-fold of resistance was achieved in *Cx. pipiens* after 40 generations of selection. In the latter case, selected mosquitoes were also cross-resistant to hydroprene and triprene, but not to diflubenzuron [85]. Rapid discharge and reduced detention of methoprene in mosquito tissue played an important role during entire process of resistance development, while metabolic detoxification seemed related to development and maintenance of high level

Data are meager with regard to resistance development in wild populations of mosquitoes. *Aedes taeniorhynchus* (Wiedemann) in Florida, USA, showed 15-fold resistance after applica‐ tions of methoprene product during 1989 to 1994 [89]. Methoprene tolerance in *Aedes nigro‐ maculis* (Ludlow) was discovered in central California, USA, after 20 years of treatment. Control failure was encountered during 1998–1999 [90], where resistance levels reached thousands of fold [91]. The documented resistance seemed not related to the metabolic detoxification by P450 monooxygenase and carboxylesterase, and treatments using *B.t.i*. partially restored the susceptibility to methoprene [91]. Another reports in wild populations showed that 4.7- to 16-fold in *Cx. pipiens* in Cypress [15] and 9- to 54-fold in *Cx. quinquefascia‐ tus* in southern California [81]. Limited data showed very low risk of resistance to pyriproxyfen

Diflubenzuron was synthesized in mid 1970s by Philips-Duphar B.V. This compound is a nonselective chitin synthesis inhibitor that interrupts formation of exoskeleton, interferes with

modes of actions [81].

144 Insecticides Resistance

resistance [87, 88].

in mosquitoes [92].

**5.2. Chitin synthesis inhibitor (diflubenzuron)**

**5. Insect growth regulators**

**5.1. Juvenile hormone analogs (methoprene and pyriproxyfen)**

This chapter reviewed and analyzed historical data of resistance and resistance management in mosquitoes to biorational larvicides with microbial and IGR origins. Bacterial larvicide *B.t.i*. possesses the lowest risk of resistance development, which depends on the intact endotoxin complex and synergism among individual toxins, particularly the presence of Cyt1A. More importantly, *B.t.i*. plays a critical role in mitigation of resistance development and susceptibility restoration and maintenance in other biorational larvicides. The binary toxins from *B. sphaericus* have numerous advantages in controlling mosquito larvae; the resistance development risk is somewhat difficult to determine, as many factors are attribut‐ able to the ultimate outcome of the scope and magnitude of resistance. Based on available data from laboratory and field studies worldwide, the combination of *B.t.i.* with *B. sphaericus*, through biofuse technology or genetic engineering, is the best choice to enhance the larvicidal activity and efficacy, to prevent resistance development, and to restore susceptibility to *B. sphaericus*. It seems that larval mosquitoes develop resistance to spinosad fairly fast if resistance management tactics are not implemented strategically, which can be attributed to the mode of action, i.e., the activation of nACh receptors in competition with acetylcholine, and chances of sublethal exposures. Strategies to prevent resistance development and to restore spinosad susceptibility after resistance development in mosquitoes should be developed and imple‐ mented urgently. As to the resistance development to IGRs, the overall risk is low. However, it must be pointed out that juvenile hormone analogs such as methoprene and pyriproxyfen act at the transition from the late 4th instar larvae to pupae and adults; the activity mostly depends on the internal juvenile hormone level. Individuals with lower internal juvenile hormone titer such as the late 4th instar larvae and pupae are more susceptible to the analogs. In wild immature mosquito populations, different instars coexist in the aquatic habitats, of which the internal juvenile hormone levels vary greatly. This phenomenon would lead to sublethal exposures and subsequently tolerance even resistance development.

There is no doubt about the consequence resulted from occurrence and spread of resistance, such as cost increase of control operations, outbreak of vector populations, and vector-borne diseases. On the other hand, there are some negative impacts of resistance development on mosquito biological fitness, such as shortened longevity and reduced fecundity [77, 78, 94], which may lower the vectorial capacity [95–97]. Therefore, evaluation on the exact impact of vector resistance to pesticides on the epidemiology of vector-borne diseases can be compli‐ cated. During the past decades, pesticide resistance development and spread promoted banning or limited applications of nonselective, long-lasting synthetic pesticides. At the same time, this situation also advanced toxicological studies and detection technology of resistance, as well as the research, development, and application of biorational pesticides, and other mosquito control techniques.

It must be emphasized that the occurrence of resistance to pesticides in mosquitoes has been on the rise, including cases to the biorational pesticides discussed in this chapter. For longterm benefits, susceptibility monitoring by standardized protocols must be implemented at the same time when a pesticide is introduced to the control operations. The collaboration among academic research, industrial development, and field application and evaluation is crucial to prolong the life and enhance efficacy of pesticides, as well as protect the environment and nontarget organisms.
