Relationship between age and colour of hatched eggshells of Ochlerotatus vigilax (Skuse) (Diptera: Culicidae) on an Australian saltmarsh

Abstract The presence of aedine mosquito eggshells in soil has recently been used to delineate oviposition patterns in coastal wetlands. However, the colour of eggshells may reveal information about the oviposition activity of mosquitoes that is not evident from the density and distribution of eggshells alone. This study describes the rate of colour change of newly hatched eggshells of Ochlerotatus (Ochlerotatus) vigilax , the common saltmarsh mosquito. In March 1996, eggshells were buried in soil of five mosquito-breeding sites on Kooragang Island, NSW. Eggshells were graded by colour using Munsell^® soil-colour charts. Seventy-five percent of newly hatched eggshells were black and 25% were dark brown in colour. Black eggshells faded quickly to lighter shades of brown, so that none were recovered 20 months after commencement of the study. Eggshells faded to a median of dark brown after 13 months and medium brown after 2 years. In another study, significant differences in colour frequency between existing saltmarsh and two recently created oviposition sites were found; the latter site being characterised by numerous black and dark-brown eggshells and very few light-brown eggshells. The results suggest that active and inactive oviposition sites may be identified using the frequency distribution of eggshell colours based on large numbers of eggshells, whereas the colour of individual eggshells only coarsely reflects age since eclosion.