Radiation biology of mosquitoes

Abstract

There is currently renewed interest in assessing the feasibility of the sterile insect technique (SIT) to control African malaria vectors in designated areas. The SIT relies on the sterilization of males before mass release, with sterilization currently being achieved through the use of ionizing radiation. This paper reviews previous work on radiation sterilization of Anopheles mosquitoes. In general, the pupal stage was irradiated due to ease of handling compared to the adult stage. The dose-response curve between the induced sterility and log (dose) was shown to be sigmoid, and there was a marked species difference in radiation sensitivity. Mating competitiveness studies have generally been performed under laboratory conditions. The competitiveness of males irradiated at high doses was relatively poor, but with increasing ratios of sterile males, egg hatch could be lowered effectively. Males irradiated as pupae had a lower competitiveness compared to males irradiated as adults, but the use of partially-sterilizing doses has not been studied extensively. Methods to reduce somatic damage during the irradiation process as well as the use of other agents or techniques to induce sterility are discussed. It is concluded that the optimal radiation dose chosen for insects that are to be released during an SIT programme should ensure a balance between induced sterility of males and their field competitiveness, with competitiveness being determined under (semi-) field conditions. Self-contained 60Co research irradiators remain the most practical irradiators but these are likely to be replaced in the future by a new generation of high output X ray irradiators.

Figure 1

Cobalt⁶⁰ irradiator. The Gammacell 220 ^® (MDS Nordion, Ottawa, Canada), an example of a conventional self-shielded irradiator. In the irradiate position the sample chamber is surrounded by several rods or "pencils" of the isotope. The dose rate of the cell is determined by the activity of the source, and the absorbed dose delivered to the insects controlled by adjusting the exposure time. The sample chamber is a vertical cylinder, approximately 150 mm diameter by 200 mm tall (3.7 L) and has a typical dose uniformity ratio of about 1.7. Such self-shielded isotopic irradiators (⁶⁰Co or less commonly ¹³⁷Cs) are the main means of insect sterilization for SIT programmes worldwide.

Figure 2

Prototype X ray machine. An RS2400 self-shielded cabinet 150 kV X ray irradiator (Rad Source Technologies Inc, Alpharetta, Georgia, USA), with a working volume of about 18 L divided into 5 horizontal cylinders 176 mm diameter by 150 mm long (3.6 L each) with a dose uniformity ratio of 1.3. This unit requires a 10 kW, three phase electrical supply and is cooled by chilled water or a self contained water-to-air heat exchanger (seen in the background). High power X ray units have only recently become available and are not in general use yet.