Phil,
Your question has generated an interesting conversation. The latest post provided by Jonathan provides a very good context. I will add a few lines to expand on some aspects.
No need for anyone without interest in insect biology, entomology or phytosanitary measures to read the rest of this post.
Heat treatment protocols were developed in the context of phytosanitary measures, particularly those pertaining to forest products in international trade. The ? 56 degrees C for 30 minutes at the core ? is the internationally accepted standard. For products such as lumber the treatment is carried out in large industrial kilns and the treatment times are calculated on the basis that every single piece of wood contained in any bundle will reach an internal core temperature of 56 degrees C (or 133 degrees F) for 30 minutes. The intent is to neutralize the live insects that may be present in the wood. Lethality is the result of drastic changes in temperature and reduction of moisture levels around the larvae.
I am unclear about the type of insects you are dealing with. In a lot (most?) of cases the insects will be some form of wood borer (the introduced Asian Longhorn Beetle being the main culprit for the last two decades or so for Ash species). The larvae of these insects do not like low moisture content levels during their development. The lumber you are using is well below saturation point and the larvae are no longer protected by bark. Not a good prognostic for them ?.
Lumber without bark that has been further processed (resawn; jointed; planed) will usually present low levels of live larvae. If they are present, one will either see the larvae inside galleries that have been sliced open or will see galleries filled with sawdust. The larvae exposed to air will be facing an uphill battle to complete their life cycle (ie become an adult wood borer). Those still protected inside a gallery may have a higher chance of survival. The risk of an adult wood borer emerging from a piece of wood found in our woodworking shops is virtually zero. They need bark and high moisture content levels in adjacent wood, a combination not met in our shops.
All that being said (or written) the answer to your original question depends to some extent on the status of the wood you are working with. Since you lumber has been cut for a certain time, it is likely that most larvae that may have been present to start with have already completed their life cycle. Some pictures of your material would be useful.
I hope this long answer will nonetheless be useful. Please feel free to reach out through DM if you want to discuss further.
Regards,
Jacques
