model

Penelope A. Hancock presents recently published work on ‘Predicting the spatial dynamics of Wolbachia infections in Aedes aegypti arbovirus vector populations in heterogeneous landscapes‘.

Uncertainty surrounding density-dependent mosquito population growth rates prevents us from predicting the outcome of mosquito control interventions. A timely example is the introduction of Wolbachia bacterial infections into wild Aedes aegypti populations, the major vector of the dengue, Zika and chikungunya viruses. Wolbachia suppresses the ability of mosquitoes to transmit these viruses to humans. Once introduced, it spreads via a driving mechanism that allows the bacteria to infect a higher proportion of mosquitoes over successive generations of the mosquito population.

Field releases of Wolbachia into the wild Aedes aegypti populations in Cairns, northeast Australia, resulted in surprising patterns of spatial spread that were highly variable and difficult to predict. We developed a new mathematical model incorporating the results of experimental studies of density-dependent dynamics in this mosquito species. The model produces patterns of spatial spread that show similar features to those observed in the natural populations of Cairns (see animation below). Spatial spread is slow and amorphous, with the Wolbachia advancing further in some directions than others.

Variability is the norm

High variability in fundamental demographic traits, such as survival and fecundity, is characteristic in Aedes aegypti mosquito populations. Body size, as measured by wing length, shows a wide range of values over a typical sample of individuals collected from the field (Figure. 1A). This variation is important because body size is closely associated with fecundity in female mosquitoes, an essential parameter in models of mosquito populations and how they respond to interventions. Experiments conducted in field-caged mosquito populations show a close relationship between female body size, fecundity and the level of density-dependent competition that the mosquito experiences during its larval development stage (Fig. 1B). In addition, the time it takes for larvae to develop into adults is also strongly density dependent. This means that density-dependent dynamics need to be accounted for when predicting mosquito generation times, and in modelling the spread of Wolbachia from generation to generation.

Figure 1. A. Wing lengths of female mosquitoes sampled from the field and the field cage. B. Female fecundity (red) and larval development time (blue) resulting from different larval densities.Putting it all together

Our experimental studies of density-dependent demographic relationships have enabled the development of a mathematical model that represents variation in mosquito numbers across space and time. The model was able to produce patterns of spatial variation in mosquito abundance that are similar to those seen in field populations (Figure. 2A). We also found that the rates of spatial spread of Wolbachia predicted by the model were similar to those observed following the field releases conducted in northeast Australia (Figure. 2B). Our models can help interpret Wolbachia field release dynamics by allowing the effects of environmental and demographic heterogeneity to be considered.

Figure 2. A. The number of pupae per house, observed in the field and predicted by the model. B. The observed spread of Wolbachia at two sites in Cairns in comparison to the model prediction.

Figure 2. A. The number of pupae per house, observed in the field and predicted by the model. B. The observed spread of Wolbachia at two sites in Cairns in comparison to the model prediction.

Read the full article, ‘Predicting the spatial dynamics of Wolbachia infections in Aedes aegypti arbovirus vector populations in heterogeneous landscapes‘ in Journal of Applied Ecology.

Photo by Kmaluhia