The 'sawdust' you see makes me think it might be squash borers(Google it)moths lay eggs on the plant then those worms tunnel into stalks.they love hollow ones and will make whole branches droop and die.you can stab them with guitar wire or wire thin enough to chase up the punture hole.dont use sevin,that shit is horrible,and I would use bt very sparingly in flower and not very far into it.good luck
I would cut the branch off or u will lose the whole plant,i fought them last yr u can try to spilt stem an dig it out not so easy, but bt works get keeping them out little late once they bore.once it bores it way up an gets bigger they bore back down thur main stem to the ground.