Chemosterilization of Oncopeltus fasciatus. 3. Competition Between Normal and Tretamine-Sterilized Insects1

Abstract

Sterilization of 90% of the males in a population of the large milkweed bug, Oncopellus fasciatus (Dallas) caused a nearly proportionate reduction in the production of viable eggs during the 1st 10 days, but the effect subsequently diminished and became negligible after 20 days because of higher mortality and reduced mating activity and sperm transfer by the sterile males. For these reasons the reduction in the production of viable eggs during the entire lifespan was only about half that caused by killing 99% of the males. Indirect evidence indicated that sterilization of 90% of the females would also not be much more efficient than an initial 90% mortality. When sterile males were added to a normal population (at a 3:1 or 4:1 ratio to normal males), mating pressure on the normal females was greatly increased. The sterile males were roughly half as active (in proportion to their numbers) as normals in mating, but their matings were fully competitive in sperm transfer during the 1st 10 days. Subsequently both mating activity and sperm transfer efficiency of the surviving sterile males declined, However, there was evidence that a small fraction of the sterile males was both long lived and highly competitive. Extrapolation suggested that a ratio of 100 or more sterile males per normal male would be required to cause a population decrease, and that sterile males would have to be released more than once during a life cycle. When both sterile females and sterile males were added to a normal population, a large fraction of the male mating activity was transferred to the sterile females, and mating pressure on normal females was lowered. The lifespan of the sterile (but not the normal) males was shorter than in experiments without added sterile females. The reduction in the production of viable eggs by the normal females was similar to that produced by adding only sterile males.