Aphids, including soybean aphid, are known for for their association with an obligate symbiont, Buchnera aphidicola. The Buchnera are housed in special aphid cells and transmitted from female aphids to her offspring. Aphids do not survive without the essential amino acids and other nutrients contributed by Buchnera.
The soybean aphid houses additional bacteria including Arsenophonus which is not required but has been found in an overwhelming majority of soybean aphids tested*. Arsenophonus is an obligate symbiont in some insect species, a male killing parasitoid in others and there is some evidence that Arsenophonus provides protection against parasitoid wasps. Its function in soybean aphid is unknown.
In tests with soybean aphids and parasitoids, the presence of Arsenophonus had no effect on three parasitoids tested.* Other possibilities must be considered. It may be parasitic, taking nutrients from the aphid or competing with Buchnera. It could be opportunistic, with no clear effect on the aphid. It could serve a similar function to Buchnera by delivering essential nutrients.
Recent advances in molecular biology have simplified the identification of many bacteria that live inside animals including insects. These techniques provide a lot of information but leave questions of role and function unanswered.
*Wulff JA, Buckman KA, Wu K, Heimpel GE, White JA (2013) The Endosymbiont Arsenophonus Is Widespread in Soybean Aphid, Aphis glycines, but Does Not Provide Protection from Parasitoids or a Fungal Pathogen. PLoS ONE 8(4): e62145.