**5. Insect growth regulators**

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

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 resistance [87, 88].

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 in mosquitoes [92].

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

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 integrity of cuticle, and causes leakage of body fluid and ultimately mortality of target organisms. Diflubenzuron acts on all stages of the mosquito life cycle, larval stages in partic‐ ular, younger larvae show higher susceptibility. Up to date, studies on resistance management are limited to laboratory populations. For instance, *Cx. pipiens* developed 7-fold resistance to diflubenzuron in response to selection for 5 generations [85]. *Culex quinquefasciatus* collected from the east coastal area in Dar es Salaam, Tanzania, developed 2.4- to 6.6-fold resistance after 10 generations of selection [86]. *Aedes aegypti* developed 3.3-fold resistance after 10 generations of selection, of which the resistance level increased to 8- to 20-fold after this population was hybridized with a mixing collection from 35 locations and then selected for 5 generations [93]. In general, the risk of resistance development to diflubenzuron in mosquitoes is relatively low.
