Boletín de la AeE

In animal species subjected to local mate competition, the precisión of the sex ratio deposited within a patch is of evolutionary importance. Precise sex ratio increases the average fítness of females by decreasing the probability of producing broods without males. Differential mortality between sexes will affect the resulting tertiary sex ratio and it is expected that females should respond by changing their sex allocation strategy.

We have evaluated the developmental mortality of each sex and the sequence and precisión of sex allocation in Trichogramma principium and T. prefiosum on two host eggs, Epheslia kuehniella (Ek) and Pluíella xylosteüa (Px). The eggs of Ek are larger than those of Px and are therefore expected to be of higher quality. Individual females were observed ovipositing on 15 consecutive hosts and each egg was individually incubated.

The developmental mortality was lower on Ek than Px for both T. principium (9.23% vs 32.0%) and T. pretiositm (16.67% vs 38.4%). In all cases, larval mortality resulted in a decrease in sex ratio ranging from 12 to 25% and the mortality of males was significantly higher than expected in both hosts for T. principium and in Ek for T. preliosnm. The sequence of sex allocation (primary) showed a male second strategy for both hosts in T, prelkmtm and on Ek for T. principium. This species changed to a male flrst strategy, with 90% males on the first oviposition, on Px. When cumulaíive oviposition was analyzed for precisión, precise sex ratio was allocated in the first two hosts for Ek, the higher quality host while in Px, a host with higher developmental mortality, precise sex ratio was found throughout the oviposition sequence.

The underlying mechanisms are either male first or second strategies and the strategy used changed according to the host encountered. In hosts where larval mortality is low, the differential mortality of males is probably of little signiñcance and a male second strategy is used. When larval mortality is high but equally distributed between sexes, as in T. pretiosiim on Px, the sex allocation sequence remains unchanged. A high and differential mortality results in a change toward a male first strategy as in T. principium on Px. High differential mortality can reduce the valué of precise sex allocation if the female strategy remains unchanged. However when a female adjusts her strategy, as T. principiíim did, the female can optimize her fitness by maximizing the probabilities of havíng at least one male per clutch.