Abstracts Biological Sciences

A study of the factors governing the sex ratio in Chelonus Annulipes Wesm., a braconid parasite of the European corn borer.

by George. Wishart

In the introduction and attempted establishment of imported insect parasites of insect pests, it becomes necessary or desirable, at certain timesand with certain species, to breed numbers of them in the laboratory. In practically all cases where breeding of hymenopterous parasites has beenattempted, 'one of the chief difficulties has been that of securing a ratio of females to males comparable to that occurring in nature, where inthis order the females are usually in excess of the males. Since, in most of the parasitic Hymenoptera so far studied, males will fertilize morethan one female, the success of laboratory breeding operations is in proportion to the number of females which can be reared. The desirability ofinvestigating the factors governing the production of females at once becomes apparent. Any contribution to the knowledge of the operation of suchfactors would be of considerable value to biological control work. [...]The experiments carried out indicate that many of the environmental facto~s which it was felt were important in their relation to sex ratio inChelonus have no significance. It appears also that the reduced number of females secured in the laboratory is the result of lack of fertilization of a sufficientnumber of the eggs laid by mated females and not of any selective eliminationo The practice of rearing the parasitized host larvae at hightemperatures is shown to reduce the number of females in the offspring. In the laboratory many successive generations of this species have beenreared without any new introductions of stock. It is very probable that considerable inbreeding has occurred. This is also shown to affect the sexratio adversely. It is the writer's ,opinion that these two factors have sufficient significance to explain the low number of females obtained inthe laboratory.