Jesse, to answer your question - I don't know that there is a clear general answer as to the numbers of wood-boring beetles today as compared to the 1930's across the historic ivory-bill range. There was much more bottomland forest at that time but much of this was immature, having been recently logged. Despite Tanner's assertions it is not at all clear that cerambycid numbers are generally much lower in immature forests than in old-growth, although there may be particular species that favor old-growth. What few studies I have seen seem to indicate a surprising ability of cerambycids to endure and recover from considerable canopy loss.
What I can say is that dead wood volumes in the Big Woods vary greatly but are generally lower than those reported from the Singer Tract in the 1930's. Pileated woodpecker densities, which are quite definitely correlated with dead wood volumes, apparently reach or exceed those reported by Tanner in numerous mature bottomland forests in the South today. I have seen plenty of beetle sign, including cerambycid sign, in bottomland hardwoods in Arkansas and Louisiana, less in cypress/tupelo. It is hard to imagine that food is really a limitation in some areas.
It is hard for me to believe that ivory-bills were generally increasing prior to 1990. In most areas forest acreage continued to decline right up to the 1990's. Although the existing forests were slowly becoming more mature, I think the continued deforestation would have more than
countered this. Not only did it eliminate critical habitat but it increased the fragmentation of key areas. Now the acreage is slowly increasing, but I think the situation is very tenuous right now. Still I have great hope and I think the bird is far more adaptable in its feeding than Tanner believed.