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Wolbachia endosymbionts are known to affect the fitness of their hosts, but most of this information is from laboratory studies. In Drosophila melanogaster, Wolbachia frequencies vary clinically in frequency in Australia and may confound climatic adaptation. Here we use field cages in a reciprocal exchange design to test for Wolbachia effects in D. melanogaster in winter at temperate and tropical sites. Infected flies of both populations had a lower fecundity in tropical north Queensland, whilst in temperate southern Victoria Wolbachia effects depended on the nuclear population background. Here infected flies from Victoria were more fecund. Wolbachia also influenced larval/pupal viability in the tropics but this was dependent on population background. In comparisons of the populations, there was no evidence for local adaptation for total fecundity, viability or survival over winter. However, in Victoria, a local population had a higher late-life fecundity than a tropical population from Queensland that had higher early-life fecundity. At a tropical site, local Queensland flies had a higher early fecundity than Victorian flies. In contrast to many laboratory studies, mortality rates in the field cages increased only slightly over time. Both the Wolbachia effects and population differences have not been previously detected in laboratory studies with D. melanogaster and highlight the utility of Drosophila field studies in fitness experiments.

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Bollgard II cotton (which expresses two Bt insecticidal genes cry1Ac/cry2Ab) and conventional cotton, grown in the laboratory or field and sampled at different stages, was exposed to Helicoverpa armigera (Hübner) larvae of three genotypes: homozygous for resistance to Cry2Ab; homozygous for susceptibility to Cry2Ab, and heterozygous for resistance. Survival of all genotypes was limited on Bollgard II but increased as plants aged. This was particularly the case for homozygous resistant individuals, with 8.5% of this genotype surviving to pupation on mature cotton. The increasing survival is assumed to be caused by the decline in the titer of Cry1Ac toxin after flowering in Bollgard II because Cry2Ab homozygous resistant larvae can tolerate high levels of Cry2Ab toxin. Larvae heterozygous for resistance performed no better on Bollgard II than homozygous susceptible larvae. Survivors on Bollgard II grew more slowly and produced smaller pupae that yielded adults with reduced longevity and fecundity. When reared on conventional cotton, all genotypes generally performed equally, indicating an absence of fitness costs associated with Cry2Ab resistance under the conditions examined.