Critical Evaluation of Quantitative Sampling Methods for Aedes aegypti (Diptera: Culicidae) Immatures in Water Storage Containers in Vietnam

In response to an identiÞed paucity of information on the size and composition of immature Aedes aegypti (L.) (Diptera: Culicidae) populations in large Þeld containers, we assessed net sampling and pumping/sieving methods for estimating and enumerating third (III)/fourth (IV) instar and pupal populations.Sweep net detection thresholds (number above which Ն90% chance of a positive sample) were Յ28 immatures for seven different container types (115Ð3000 liter jars and tanks) in the laboratory, and mean recovery percentages varied by container type (6.15Ð 41.29 and 7.39 Ð33.10% for III/IV instars and pupae, respectively).A pumping method or hand bailing was applied in the Þeld for the collection of III/IV instars and pupae from 406 receptacles, of which 343 had been previously sampled via a Þve-sweep netting technique.Larvae were 9.30 times more prevalent than pupae, and abundance varied by container type with means of 36 Ð537 III/IV instars and 6 Ð53 pupae per receptacle.Sweep netting for III/IV instars effectively identiÞed 86.2% of Ae. aegypti-positive containers, whereas sampling for pupae detected only 43.1% of positive containers.When conversion factors (inverse of laboratory recovery percentages) were applied to Þeld net sampling data, estimates of container populations were more accurate for III/IV instars than pupae (maximum R 2 ϭ 0.610 and 0.328, respectively); however, the relationship between immature abundance and emergent adult populations remains to be deÞned. KEY WORDS Aedes aegypti, abundance, net, sampling, surveillanceTraditional entomological surveillance for dengue focuses on the detection of immature stages of the principal global vector Aedes aegypti (L.) (Diptera: Culicidae).Surveys have usually involved visual inspection of all water-holding containers at selected premises, with immature (larvae, pupae, or both) samples taken by pipetting, dipping, or netting.Containers are scored as positive or negative for Ae.aegypti, and Breteau, Container, and House (or premise) indices are calculated (WHO 1997).However, these indices have largely been found to be inadequate for guiding vector control (Tun-Lin et al. 1996, Focks and Chadee 1997, Reiter and Gubler 1997, Focks 2002, Nathan et al. 2006), and in 1999, a World Health Organization expert body on strengthening dengue prevention and control recognized the need for "the reÞnement of existing indicators and/or the development of new indicators that better reßect transmission potential" (WHO 2000).