Managing cold tolerance to faciliate biocontrol of Drosophila suzukii through IIT and SIT

The recent invasion of fruit production areas by the Spotted Wing Drosophila fly (SWD), Drosophila suzukii, is a major concern for Europes soft fruit production sector. Chemical control proved to be ineffective and inadvisable due to regulation, pest resistance, and environmental and human health concerns. The classical biological control approach based on natural enemies might be challenging due to behavioural or physiological inadequacies. A promising alternative approach to fight against insect pests is the use of innovative biotechnology, such as mass-release of sterile insects. Sterile insects can be generated by irradiation (Sterile Insect Technique, SIT) or by Wolbachia-induced cytoplasmic incompatibility (Incompatible Insect Technique, IIT). These innovative methods represent a major breakthrough in pest management and have proved to be successful in controlling various invasive and/or pest species. However, these environmentally-friendly technologies require the development of mass-rearing of competitive insects at an industrial scale, as well as the ability to stockpile, handle and ship insects usually under stressing low temperature. The objective of the SUZUKILL project is to develop an alternative avenue for controlling SWD, initially in greenhouses, through the release of sterile insects produced via SIT and IIT. Protocols for mass-rearing and sterilization of SWD, as well as the adaptation of current quality control procedures for tephritid fruit flies, will be developed and validated in this project. We will identify critical elements of cold tolerance to better control this trait and develop quality management (including assessment of the genetic diversity and mating competiveness) in order to facilitate the implementation of these environmentally-friendly control methods. The specific expertise of French (UMR CNRS 6553 ECOBIO, Université de Rennes 1) and Austrian (Universität für Bodenkultur Wien, BOKU) partners will generate the fundamental and applied knowledge required for the management of cold tolerance and for an appropriate genetic/quality control strategy of the domesticated populations in the mass- rearing. The project will benefit from expertise and infrastructure of Insect Pest Control (IPC) Section from the Joint FAO/IAEA Division of Nuclear Techniques in Food & Agriculture for the implementation of a full SIT package. SUZUKILL will also benefit from recent developments made by the second French partner (UMR CNRS 5558 LBBE, Université de Lyon 1) in the field of IIT for this insect pest. By developing a common strategic research agenda, SUZUKILL project partners will coordinate transnational research program that will provide innovative, practical and biological solutions against the threat of SWD in Europe, and consequently, it will support and contribute to Europes phytosanitary scientific capability.

There is an increasing demand for the replacement of the intensive use of chemicals by environmentally-friendly sustainable methods. Classical biological control approaches based on natural enemies might be challenging for the control of the invasive Spotted Wing Drosophila (SWD). A promising alternative could be through the release of sterile insects. Sterile insects can be generated through irradiation-based Sterile Insect Technique (SIT) or Wolbachia-induced Incompatible Insect Technique (IIT). A common requirement of SIT and IIT is the necessity to produce, manage and handle huge number of mass-reared insects at low cost. Low temperature storage or treatment is an integral part of the mass-rearing or release protocols. In addition, under laboratory conditions and continuous mass-rearing, strains can lose their ability to survive and compete under natural conditions. In this project we identified a specific and cost-effective artificial diet that produced high quality and quantity of SWD adults. The egg-laying system consisted of a novel developed "wax-panel" that allowed the production of large number of eggs. A suitable range of radiation doses was adapted and here we could show that irradiation does could be lowered under hypoxia. No negative effects on the adult's fitness was found compared to the non-irradiated flies. Two Wolbachia strains, wHa and wTei induced almost complete CI regardless of the presence of the natural occurring wSuz strain. However, a decrease of CI intensity with increasing age of males was detected. Complete sterility of infected and irradiated adults was observed and none of the quality control parameters tested was negatively affected by these treatments. Thermal limits of SWD flies were defined and the primary metabolites were analyzed by Omics methods. Different metabolic signatures revealed that cold tolerance was associated with major remodeling of membranes. Starvation promoted cold tolerance and we also found that dietary restriction derived from increasing larval density in diets positively affected cold tolerance. Because fluctuating thermal regimes can enhance cold tolerance of insects this was applied to SWD. We could almost offset the negative effect of cold storage. We concluded that fluctuating thermal regimes could thus be used as very efficient method for short-term cold storage before application of SIT and IIT. We assessed genetic changes occurring during laboratory domestication. Data reveal selection processes from generation 0 to 10. These data revealed that lab adaption occurs relatively quickly and this should be considered, i.e. wild population should be used to refresh and enrich the laboratory strain.