Spongy white wood is characteristic of white rot. It has already gone through the process of being broken down and is sufficiently decayed for most stag beetle larvae. It's not longer as nutritient-rich as wood that hasn't gone through decomposition and is mostly cellulose. In order to break it down further, you will need organisms that are capable of breaking cellulose down like fungi that produce brown rot.
If you're feeding stag beetle larvae, white rot is preferrable for most species. If you're feeding rhinoceros beetle larvae, you'll want to break it into fine pieces and compost it with a large volume of fallen leaves. Composting may take a relatively long period of time depending on how good you are at maintaining temperature, moisture levels, and the level of decomposition that suits the beetles you wish to raise.
You will achieve much faster results through fermentation of wood using additions such as sugar, flour, or various types of grain bran, but you will need to know what you're doing or you risk producing a poor quality substrate that starves or kills your larvae. I do not recommend adding leaves in fermentation as you do not know what kinds of things you'll be introducing into a nutrient-rich situation. Leaves should be composted separately or decayed leaves should be collected and processed to eliminate potentially harmful organisms prior to mixing with the finished product of fermentation.