"Improved Grab (Ex): To use this ability, an Siphonut must hit with two of its arms. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can use its Stinger, but never beforehand. " (emphasis mine)

So are you saying that it can't use its stinger without grappling with the foe first? If so, why is the stinger listed as the primary attack?