The importance of subterranean mosquito habitat to arbovirus vector control strategies in north Queensland Australia

In north Queensland, 14 localities were surveyed for mosquito lan ae (third and fourth instar) during summer/autumn and winter from 1996 to 1999. Abso lute population numbers in subterranean habitats, mainly service manholes a nd pits (97%) but also some wells, septic tanks, storm drains, and sumps, w ere expressed as a proportion of total numbers in these sites plus surface sites within a 100-m radius. When correction factors were applied to subter ranean samples, the 472,477 larvae mainly of Aedes tremulus (Theobald) grou p, Aedes notoscriptus (Skuse), and Aedes aegypti (L.) comprised 78% of the total population. In relation to the proportion of the overall immature mos quito population from subterranean habitats (propsub), linear regression co efficients for minimum temperature, relative humidity, and Mesocyclops cope pod prevalence were significant for winter data; but for summer, only relat ive humidity was significant. Linear regression coefficients for Mesocyclop s prevalence approached significance (P = 0.061) in summer. When multiple l inear regression was used to model propsub, 68% of the variation was accoun ted for by relative humidity and the prevalence of Mesocyclops. In the drie r and cooler towns, increased use of subterranean sites during winter was c aused by reduced availability of surface oviposition sites because of the d ry season. In the wetter coastal towns, no such restrictions applied and am bient conditions remained more equitable all year round. Mesocyclops were s urprisingly common, particularly in these coastal towns. Release of known n umbers of Mesocyclops indicated that 3-sweep netting in service manholes wa s sensitive down to densities of one Mesocyclops per 10 liters, and overall recovery varied from 1 to 4%. In relation to control, service manholes rep resent a stable habitat for mosquito (7% positive overall) and Mesocyclops populations If they remained wet, service manholes positive for mosquito im matures or Mesocyclops during summer/autumn had 96% and 85% chance, respect ively, of being positive the following winter. Even allowing for the effect of drying, a mosquito-positive manhole had a 79% chance of remaining posit ive the following winter. In view of the importance of these sites as refug es from adverse ambient conditions, it is proposed that a winter control st rategy using Mesocyclops presents a cost-effective control option to reduce the recolonization of surface sites when conditions become more suitable.