They could be termites (crawling insects) or flying insects ranging from small wasps to hornets or Carpenter bees.
If the holes are small, say 1/8th", likely to be termites or such as classic woodworm. If larger, say 1/4", probably wood wasps. If big, say 1/2" neat round holes, you may have Carpenter bees. These are really bad wood destroyers!
The best spray with a tube is 'Pyrethrum/Permethrin', widely available from Supermarkets such as Lemar/Supreme etc in NC. Normally sold as a Cockroach killer, 'Sertex' being widely available and cheap.
Sertex will kill almost anything from ants to spiders, Carpenter bees and cockroaches,. maybe even neighbours if they are a pest!
Feed tube into holes as deeply as possible and give a short blast, (remember this may bring out any 'residents' who may not be happy!) leave to dry, and if possible fill holes with wood filler (widely available in small ready mixed tubs from Yapi's such as Marshalls.) Then redecorate with stain/preservartive as required.
If you think you have Hornets or Carpenter bees, act quickly! They return to enlarge the same nest year after year and can destroy a 6" timber in just a few seasons. Male Carpenter's are large and black, can't sting,. females are slightly smaller and usually brown, look a bit like a large moth, but CAN sting if provoked. Hornets can also sting.
We had some 6" dia rustic poles outside which Carpenter bees reduced to 'balsa wood' in 4 years. They still return to the same site each year. It's an ongoing war! ;-(
Carpenter bees:
http://ento.psu.edu/extension/factsheets/carpenter-bees