Is the white powder coming out of small holes? If so, it is most
likely mature powder post beetles drilling holes to lay their eggs.
Timbor is the treatment for powder post beetles. If this is the case,
any eggs already layed may not be affected by the Timbor treatment...
some eggs may be sealed away..., which means the larvae will emerge in
the spring (they will dig out of their holes) and a similar powder
will be seen as they emerge. Powder post beetles will return to the
same boards (same area), each year, to lay their eggs.