Wikipedia writes this about ISPM 15;
International Standards For Phytosanitary Measures No. 15 (ISPM 15) is an International Phytosanitary Measure developed by the International Plant Protection Convention (IPPC) that directly addresses the need to treat wood materials of a thickness greater than 6mm, used to ship products between countries. Its main purpose is to prevent the international transport and spread of disease and insects that could negatively affect plants or ecosystems. ISPM 15 affects all wood packaging material (pallets, crates, dunnages, etc.) requiring that they be debarked and then heat treated or fumigated with methyl bromide and stamped or branded, with a mark of compliance. This mark of compliance is colloquially known as the "wheat stamp". Products exempt from the ISPM 15 are made from alternative material, like paper, plastic or wood panel products (i.e. OSB, hardboard, and plywood).
It seems there are two ways to make sure the pallet is treated;
- HT is the code for heat treatment to a minimum of 56°C (133°F) for a minimum of 30 minutes
- MB is the code for methyl bromide fumigation.
I have checked the pallets I got and they have the mark HT which means Heat Treated and is safe to use for building a bee hive in my opinion.
I couldnt find any pallets with MB (Methyl Bromide) so far and it could be Sweden which has regulations about this sort of pesticide. This could be on an EU level too. Check on the wheat stamp before using the pallet timber.
A gorgeous Pallet Wood Top Bar Hive;
How to dismantle a pallet - video