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Robert is right about plastic boxes, and about regular checking. Theoretically, any plastic intended for food use should be inert for practical purposes. In reality, we found that this was not totally reliable, but Tupperware proved suitable and also Lakeland Plastics, who I expect you have heard of, based in Kemble, are very obliging if you ask them what their products are made of - go for polyethelene. Lakeland quite like being able to advertise that their products are "museum standard" or "as used by the National Trust" and are always interested issues of collections care (although may be a bit more atuned to the needs of 18th century furniture and antique textiles than natural history collections). Remember that lids and bases are usually of a different chemical makeup with plastic boxes, because of the practicalities of rigid bases and flexible lids. We were concerned with the storage of all sorts of objects, not just natural history collections, and I dont recall ever looking at the composition of specialist entomology boxes - I guess you just need to ask the suppliers or manufacturers.

I think you do need to clean the boxes if controlling the RH is not effective, as they very likely have spores that may cause trouble. You probably need to clean the pins as well if problems continue. Use alcohol, acetone (test first in case you melt the plastic) or a non-ionic detergent like one of the Synperonic products (originally developed to clean out the insides of oil tankers and such like).

The big advantage of plastic boxes over wooden is cost. Wooden collections boxes are significantly more expensive. Also, my line about wood off-gassing was a bit throw away - its true up to a point, but wood may also give off gases that could be just as problematic as plastics and it is an incredibly complicated subject - each wood is different. If the boxes are lined with a textile, the dye in the textile may off-gas, as might the glue joining it all together. You also need to know if the wood is treated with anything. Untreated wood can be an advantage because it is porous, so you can control relative humidity externally ie the RH of the room, not the box, and this is most practically achieved by controlling the heating. Controlling RH is one of the black arts of heritage conservation, and if you want to know more about the technicalities, I can give you some references that go into eye-crossing detail.

I have just been given some lightweight aluminium boxes, complete with little aluminium specimen pots with glass lids, and will be putting my rather haphazard personal collection in these - if anything hideous happens to them, I will let you know, but it strikes me that aluminium boxes might be a very reasonable solution, so long as RH is controlled.