'Crop Pollination By Bees' by Delaplane and Mayer (CABI Publishing) has a
chapter on soybeans. The authors report that self-pollination is the norm,
before the flower is open. The benefits of bee pollination are inconclusive.
Some work shows improved fruit-set and yield, but it varies greatly with
locality and variety. It is reported that bees are rarely used for
pollinating soybeans, but they may become more useful with the development of
hybrid soybeans.
Matthew J Allan