In a study of the quality of Trichogramma pretiosum Riley (Hymenoptera: Trichogrammidae), we compared female wasps emerging from natural hosts, parasitized in the laboratory or the field with those emerging from factitious hosts used for commercial mass production. Females from the natural hosts were larger, more fecund, and longer lived than those from the factitious hosts. Compared to small females, large female wasps are substantially more fecund when honey (carbohydrate) is available but marginally more fecund when honey is unavailable. The size of a female T. pretiosum depends on two factors: the size of the host egg from which it emerges even when the wasp was gregarious, and the number of conspecifics that emerge with it. The similarities in the size distribution of female wasps emerging from natural hosts, in conjunction with the mechanism by which Trichogramma measure host size and allocate eggs accordingly, suggests the hypothesis that size related components of fitness in female T. pretiosum are under strong selection in the field.
Abstract.
We tested the hypothesis that females of the egg parasitoid, Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae), could adjust their fecundity schedule according to host availability and that there was a negative correlation between reproduction and survival in these wasps.
Newly‐emerged females were provided with an unlimited or limited number of hosts in the first trial and with either unlimited, limited or zero hosts in the second trial.
When hosts were unlimited, wasps had the highest rate of reproduction in the first day, which decreased dramatically thereafter. When hosts were limited, wasps from the two trials differed in their response. In Trial I, females with limited hosts had lower first‐day fecundity than, and the same subsequent‐day fecundity as, those with unlimited hosts. However, in Trial II, females with limited host had a lower first‐day but a higher subsequent‐day fecundity than those with unlimited hosts. This indicates variation in Trichogramma's ability to shift its fecundity schedule in response to host availability.
There was a positive (rather than a negative) correlation between reproduction and survival. Wasps that oviposited (in host‐unlimited treatment) had greater longevity than those that could not (in host‐unavailable treatment).
The sex ratio of the progeny produced by wasps in both host‐unlimited and limited treatments shifted gradually from a female to a male bias as the wasps aged.
We consider the ability of parasitoids to adjust their fecundity schedule as an adaptation to changing host resources and discuss our findings with regard to theories of life history evolution.
1. The patterns of host-feeding and oviposition were examined in Aphelinus asychis Walker, which had been provided with second-instar pea aphids as hosts.2. Female wasps responded to increasing host density (between one and forty aphids for 24 h) with an increasing tendency to oviposit rather than to feed. Superparasitism occurred at all aphid densities, even when unparasitized aphids were available.3. Aphids intended for feeding were paralysed and died. Wasps did not feed on and oviposit in the same aphid.4. Feeding to satiation lasted between 4 min and 42 min. Females that had starved for s18 h generally deposited one or more eggs before feeding again, while the reverse was true in wasps that had starved for 2 2 1 h. 5 . The host-feeding behaviour of A.asychis is determined by a female's nutritional status. At low rates of host encounter, the anhydropic eggs may be resorbed. This reproductive strategy conforms to the destructive nonconcurrent type among the Hymenoptera.
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