Wooden tongue depressors as oviposition substrate for the detection of container-breeding Aedes species in ovitraps – Do we miss something?

Ovitraps are easy-to-use and inexpensive traps for detecting potentially invasive container-breeding Aedes mosquitoes, particularly the Asian tiger mosquito ( Aedes albopictus ). These traps consist only of a black container filled with water, and an oviposition substrate. Wooden tongue depressors are commonly used oviposition substrates, but they leave the inner surface of the container largely uncovered, which could be used for egg laying as well. The aim of this study is to investigate whether observations of container-breeding Aedes species can be missed if only eggs laid on the oviposition substrate are examined. In 2022, five ovitraps were set up at each of six locations in the province of Salzburg in Austria. Wooden tongue depressors, roughened with sandpaper were used as oviposition substrate. The water and the oviposition substrate were replaced with a fresh one at each of the weekly samplings during the observation period, and the inner surface of the container was wiped with a paper tissue. The eggs on the stick and tissue were counted and the species of the eggs determined (using morphological characteristics and multiplex PCR). We found eggs of three Aedes species, mainly from Ae. japonicus , but also from Ae. albopictus and Ae. geniculatus . All of those laid their eggs on the wooden tongue depressors as well as on the container wall. However, the tongue depressor was clearly preferred to the container wall. Only in 2.9% of the sampling events eggs would have been missed if only the tongue depressors had been investigated. However, this proportion decreased with an increasing number of eggs per sampling event. We can therefore conclude that, although the container wall is also regularly used by the mosquitoes to oviposit, the probability of overlooking the presence of container-breeding Aedes mosquitoes by examining only the eggs laid on the wooden tongue depressor is very low.