Is Aedes albopictus development influenced by density dependent constraints?

Aedes albopictus is a competent vector of at least 22 arboviruses, including dengue and chikungunya viruses (Gratz, 2004, Med. Vet. Entomol., 18:215-227). The potential for pathogen transmission by Ae. albopictus has increased the need to understand its ecology and population dynamics. Previous studies have shown that density-dependence can affect larval development (Lord, 1998, J. Med. Entomol., 35:825-829). Density-dependent competition for the available food resources has often been argued as the factor influencing larval development (Walsh et al., 2012, Plos One, 7:1-6). Aim of the study was to assess if larval development time is influenced by the density of laid eggs and the possible consequence of development delay on the efficacy of the larvicidal compound diflubenzuron (DFB), extensively used on a monthly basis in the national territory for tiger mosquitoes control. Ovitraps were positioned in August 2013 for the collection of Ae. albopictus eggs; eggs were counted and 48 ovitraps (24 with high density of eggs: mean 51.6, range 30-93; 24 with low density of eggs: mean 11.9; range 9-15) were placed in tanks with 0.5 liter of water. Within each group, 3 subgroups consisting of 8 ovitraps were formed: 1 control without DFB and 2 with DFB at the concentration of 0.5 ppm and 0.25 ppm, respectively. Eight (4+4) controls without eggs were also added to assess the degradation over time of both DFB concentrations in absence of larvae. The level of water was maintained constant for the duration of the trial. The larvae were fed ad libitum with dried Gammarus pulex. Hatchings, development times, mortality and emerging adults were monitored for 8 weeks. In this period, DFB concentration was assessed every ten days by HPLC UV-DAD method after liquid-liquid extraction with diethyl ether. The number of eggs is significantly related with the last-hatching time and the time of last adult emergence (Negative binomial regression, p<0.01). DFB showed powerful larvicidal action and long persistence. In treated tanks adult emergence occurred 2 months later in only three tanks with 0.25 ppm (9 adults overall, 8 of them from tanks with high egg density). In 18 tanks DFB was still detectable after 50 days from the contamination with concentrations higher than the DFB LC90 for Ae. albopictus (Arshad et al, 1995, J. Am. Mosquito Contr., 11:72-76). Larval density did not influence significantly DFB degradation. After an initial sharp reduction of DFB concentration it remained almost stable, until a final intense reduction in the last two weeks. However, in the controls with no larvae the latter reduction did not occur and the concentration remained stable for a longer period. The presence of high egg densities determines a slowed down development whose duration potentially exceeds the period of effectiveness of DFB. Ae. albopictus seems to undergo density-dependent self-regulatory mechanisms independent from food resource availability that can stabilize mosquito population dynamics and possibly affect control measures.