The Sterile Insect Release Method and the Importance of Thermal Conditioning Before Release - Field-Cage Experiments With Dacus-Tryoni in Spring Weather

We present a rationale for the strategy of suppressing the fertility and hence the rate of increase of target populations in spring by the release of sterile insects. Insects for mass release are normally reared in warm conditions and are potentially unsuitable for use in cold weather. A comparison was made of the effectiveness of warm- and cold-conditioned sterile flies in experiments in large field-cages during three successive spring seasons. Sterile males and females, and wild males and females were released into the cages in the ratio 2:2:1:1, respectively. Warm-conditioned sterile flies survived poorly between release and mating. Their mating competitiveness was inferior to that of the wild flies, and they were relatively ineffective in reducing their fertility. Sterile flies released in a cold-conditioned state (equal to that of the wild flies), survived as well as wild flies, and suppressed their fertility to an extent which indicates that their mating competitiveness can be equal to that of the wild flies, if the weather is not too severe. The concept of total competitiveness is introduced. If competitiveness is calculated by means of the ratio of sterile to wild flies that exists at release (rather than at mating), the resulting value is a measure of the potential of the sterile flies to survive to mating age, and to mate in competition with the wild flies. This value is especially pertinent to situations such as a puparial release, when there is a delay between release and mating. In such circumstances, it is needed for calculating the required ratio of sterile to wild insects to achieve the desired reduction of fertility (and thus rate of increase) in the target population.