2.7. Cross resistance to different insecticides
2.7.1. Insect growth regulator
The estimation of emergence inhibition (EI) concentrations
of pyriproxyfen, a juvenile hormone analogue that prevents larvae
from developing into adults, was also performed through
dose–response bioassays in RecR-F17 A. aegypti individuals (WHO,
2005; Andrighetti et al., 2008) using individuals of the Rockefeller
strain as the standard susceptible controls. The stock solution
of pyriproxyfen (250 mg/L) was prepared in deionized water and
stored at 4 ◦C. The working solution (2.5 mg/L) was prepared immediately
before each test. Three bioassays were performed using
720 L3 exposed to eight different concentrations of the insecticide
(90 larvae per dose) and 80 larvae serving as the control group (4
replicates with 20 larvae each). The number of dead larvae, pupae
and adults was recorded at 48 h post-exposure and after that at
every 24 h until all individuals were dead or had emerged. For each
concentration, the total number of dead individuals was used to
estimate EI concentrations using Polo-PC software. The RR based
on EI for 50 (EI50) and 90% (EI90) of larvae were calculated comparing
RecR individuals with the Rockefeller strain. According to
Mazzarri and Georghiou (1995),RR < 5 characterizes low level resistance,
5–10 characterizes moderate resistance, and RR > 10 a high
resistance level. RR90 was utilized to determine status of resistance
to pyriproxyfen.