Wolbachia pipientis is a rather peculiar bacterium. It's an intracellular organism, and is found in a wide variety of tax, including nematodes, crustacea, and arachnids. About 20% of insect species are thought to have Wolbachia. Wolbachia has evolved a number of mechanisms to ensure transmission, which is generally maternal. There are a number of consequences of infection with these intracellular bacteria observed in different species - including reproductive isolation between infected and uninfected strains of the mosquito Culex pipiens. I also recall seeing a old paper (though I've long-since lost the citation) which claimes a growth advantage of infected Drosophila larvae in crowded culture, though I think this must have predated the identification of Wolbachia.
So some laboratory Drosophila strains are infected with Wolbachia, while some are not. This can lead to artefacts in many experiments. For example, in my own lab, we had some perplexing results concerning the effects of particular transgene on viability of some combinations - the involvement of Wolbachia became clear firstly when we noticed the effects depended on which which strain was paternal and which maternal in the cross, and finally the observation that this effect vanished after treating the transgenic stocks with antibiotic. These kind of artefacts are particularly important to consider when working with complex characteristics such as lifespan (as we do in my lab).