Impacts of climate variability on fruit fly (Ceratitis spp.) performance and population dynamics

Ceratitis capitata and Ceratitis rosa are fruit flies of major economic pests status in South Africa and the world. They cause widespread damage through puncturing fruit during egg laying and larvae subsequently developing within the fruit. Through the use of laboratory-based tolerance estimates, it has become apparent that the performance of these flies is strongly influenced by the thermal environment. However, it is currently unknown whether the patterns observed in laboratory studies reflect performance under natural environments. Gaining a better understanding of the field fitness in these flies is not only important for predicting when they are likely to be the most destructive, but also for the tailoring management strategies such as the sterile insect technique (SIT). It is therefore vital to translate laboratory estimates of performance to real world conditions.

Another strategy for the prediction of Ceratitis population phenology is the use of day-degree (DD) models. Associations between climatic variables and species phenology are used to predict peak abundance and plan management strategies accordingly. However, there is often significant mismatch between model predictions and realised abundance measures, indicating an absence of important information. Temperature is often used as a proxy for climate in general; however, abiotic variables occur in concert and the relationship between may not always be linear. Understanding the impact of multiple climate variables, such as temperature and humidity, may significantly improve our understanding of development in Ceratitis and improve DD models substantially.

We found a vast improvement in thermal tolerance when the thermal history of the flies matched the conditions of the assay (e.g. prior cold acclimation before cold stress) across laboratory, semi- and field performance assays. This also corresponded with a significant decrease in tolerance and performance when there was a mismatch between thermal history and environment. We recommend that future SIT practices maintain flies at thermal conditions similar to those of the intended site in order to maximise performance in these conditions. As adult acclimation generally resulted in similar or better acclimation effects compared to that during developmental life stages, this acclimation phase could adequately occur at the end of the production process and not disturb normal insect husbandry practices.

The development time assays in both C. capitata and C. rosa highlight the strong effect that temperature and humidity have on successful recruitment. For C. rosa in particular, it was not possible to find conditions appropriate for development under our experimental conditions, indicating a high level of sensitivity to sub-optimal conditions in this species. Due to the non-linear association between temperature and humidity on development, particularly at high temperatures, we recommend that day-degree models should be expanded to incorporate the interaction between high temperatures and humidity on development in C. capitata. This may greatly improve predictions made from these models in natural systems.