Strategic ejaculation in the egg parasitoid Trichogramma turkestanica (Hymenoptera: Trichogrammatidae)

1. Several parameters influence sperm allocation by males, including their size and sperm stock, intra-specific variability, quality of females', as well as the risk and intensity of sperm competition. 2. Models predict that males should invest the maximum ejaculate size when sperm competition intensity is low. As sperm competition intensity increases, males should decrease the number of sperm transferred during mating. 3. This decrease in sperm transfer to females occurs because the benefits gained by males with each extra unit of expenditure on sperm decrease. When sperm supply is not unlimited, males could expect a better return by keeping some or all sperm for mating under lower competition intensity. 4. In this study, the ejaculate size of males that were kept in groups of one, five or 10 males prior to mating, has been investigated in the haplodiploid egg parasitoid Trichogramma turkestanica Meyer (Hymenoptera: Trichogrammatidae). 5. As predicted by theory, the number of sperm transferred decreased significantly with an increase in the number of rivals. 6. This is the first study showing strategic sperm allocation depending on sperm competition intensity in a parasitoid.