Wooden pallets intended for international use are treated, either by heat or by gassing with Methyl Bromide, to kill any insects which may be hidden in cracks or other imperfections within the wood.
Methyl Bromide is an extremely reactive chemical, which means that even after a few days of the pallet's exposure to the atmosphere, it will no longer pose any risk to honey bees housed within a hive made from that material. In practice, pallets will not become available for beehive construction for many months - sometimes years - as they are often recycled or held in storage for long periods.
Plywood comes in many grades, which are determined both by the type of glue used during manufacture, and the quality of the wood laminates used - however, the manufacturing process itself is the same, with glued wooden laminates being subjected to high clamping pressure and heat which results in a dense, heavy material.
Interior grades are made with interior-quality glue and, like cheap forms of chipboard, will deteriorate quickly when wet.
Exterior grades of plywood range from those with a small number of thick plies - intended for uses such as cement forms and emergency shop window repairs - to those with many thin, uniform plies, such as high quality marine plywood. The glue used in exterior plywoods is usually thermal-setting urea-formaldehyde, which will off-gas slightly when the plywood is freshly cut, but soon stops. A layer of paint on the surfaces exposed to the bees can always be used to reduce this to a minimum, but any off-gassing will cease with age.
The difference between high quality marine plywood and exterior plywood is both the number, thickness and wood-species of the laminates used, and also the absence of 'voids' (spaces, gaps) in certified marine plywood.
It is true that timber treated with the insecticide 'copper arsenate' may sometimes be found amongst used timber - it can easily be spotted by it's green or greeny-blue colour. Copper arsenate is a poison which kills wood-boring insects. As honey-bees do not - in the normal course of events - eat the wood from which their hives are made, I consider it safe to use for beehives, especially for areas of transient contact such as feeder shells, roofs, stands etc., and perfectly safe when that wood is painted. I would not normally use it to make the bee boxes themselves however if alternative material was available.
Whe dealing with second-hand timber such as used floor boards, scaffold boards etc., there is absolutely no way of knowing what the provenance of that timber has been - whether it has been subjected to intentional or accidental chemical treatment at some point in it's life, such as wood-worm killer or other noxious substances. That is one of several reasons why I always paint the inside of my bee boxes.
LJ