Quantitative evaluation of funnel traps for sampling immature Aedes aegypti from water storage jars

The trend is increasing to incorporate assessments of abundance into surveys for immature Aedes aegypti to identify the most important types of containers that should be targeted for control. In this study, we examined whether funnel traps could be used to sample immature Ae. aegypti from water storage jars ranging in size from 0.28-m diameter (30 liters) to 0.52-m diameter (150 liters). The effects of jar size and duration of funnel trap sampling were investigated and a set of calibration factors was developed to convert funnel trap numbers to absolute population estimates (0.28-m diameter = 2.5, 0.38-m diameter = 3.0, 0.48-m diameter = 4.6, and 0.52-m diameter = 7.4). Although the funnel traps were highly sensitive (90-100%) for detecting immature Ae. aegypti at densities as low as 25 3rd and 4th instars per jar, the large variation in funnel trap recapture rates meant that absolute population estimates based on a single funnel trap sample were inaccurate. However, by using a computer simulation, estimates of the total overall numbers of larvae from multiple jars were reasonably accurate (+/- 20%), if more than 50 positive jars were surveyed. For example, 95% confidence intervals for the percentage error in estimated numbers of immatures from a series of 50 0.38-m-diameter and 50 0.52-m-diameter jars, were -10.0% to +10.2% and -19.9% to +17.8%, respectively. Although we generally recommend the use of nets to sample immature Ae. aegypti in jars, under some conditions funnel traps may be more acceptable than nets, because some householders object to the increased turbidity associated with net sampling in jars.