Effects of host size experience on sex ratios in the parasitoid wasp Spalangia cameroni

The sex ratio response of a female Spalangia cameroni is affected both by the size of the host that she is parasitizing and by the size of hosts that she has previously encountered. When given small and large hosts simultaneously, Spalangia cameroni females oviposit a greater proportion of sons in the small hosts (King, 1988, Evolution, 42,1190-1198). This study examines how a female's offspring sex ratio is affected by her previous host size experience. First, van den Assem et al.'s life expectancy hypothesis was tested (Neth. J. Zool., 34,33-62). This hypothesis suggests that when females encounter only small hosts, the production of daughters will be inhibited initially and then, as no large hosts become available, production of daughters will increase. This hypothesis was not supported. The second hypothesis tested was that in their sex ratio responses, females will judge hosts as small or large relative to other hosts encountered. This hypothesis was supported under some conditions. Females judged hosts as small or large relative to previously encountered hosts when parasitizing large hosts and when the previous experience with hosts was immediately prior. When a female received only one host size, females that were given small hosts produced either a greater proportion of sons or the same sex ratio as females that were given large hosts.