ANTS & EARWIGS can be controlled by placing a strip of rolled cotton (available from pharmacies) covered by a band of duct tape around the pole or tree and smearing a small amount of sticky Tanglefoot on the tape. Be very careful to place the Tanglefoot where the birds will not get into it. If the tree bark is rough, silicon sealer can be applied to fill in the gaps under the duct tape. The Tanglefoot needs to be renewed periodically.
WASPS & BEES may move into a nestbox. Wearing gloves, remove the paper wasp nest with a spatula in the evening or early morning when temperatures are cool and wasps are inactive. Bees are important pollinators of the wild foods that birds need to survive. Wild honeybees are also cavity-nesters and may move into a nesting box. In which case you can either try beekeeping or have them removed and relocated by a beekeeper. And if bumblebees move in, you may want to leave them be, as they will only use the box for a short while for nesting and are not aggressive. To prevent bees and wasps from colonizing a nestbox, apply a thin layer of Ivory bar soap onto the inside surface of the roof.
POISONS: many pesticides are toxic to birds and should not be used near nestboxes.
BIRDS OF PREY such as Sharp-Shinned or Cooper's Hawks, and American Kestrels have been observed preying on adult, fledgling and juvenile bluebirds. Having nearby dense cover is the bluebirds' best refuge. Avoid placing nestboxes near bird feeders or on tall utility poles where hawks often perch.
JAYS & CROWS can poke their heads into nestboxes and snatch eggs or nestlings. Using a box design with at least 6 inches from the bottom of the entry hole to the floor of the box will help keep young out of reach. If the wood of the box is very smooth, it is a good idea to cut grooves into the wood below the inside entrance hole to help nestlings get a foothold as they attempt to fledge.
HOUSE WRENS can enter bluebird nests and remove all the nesting material and destroy the bluebird eggs and young. House Wrens prefer boxes in dense shrubbery and trees. Placing your nestboxes out in the open, with only a few trees, will attract bluebirds, and be less attractive to wrens.
HOUSE SPARROWS are the most undesirable competition for bluebirds. House Sparrows are very aggressive birds and may destroy bluebird eggs and young in order to take over a nesting box. Because House Sparrows are not native, it is legal to control them by removing their eggs and nests. Don't feed House Sparrows. House Sparrows prefer small seeds, such as millet, cracked corn, and milo-- the stuff you find in common birdseed mixes. If you like to feed birds, try offering nyjer (thistle), as House Sparrows and Ground Squirrels are not attracted to it. Or a less expensive way to feed birds is to plant lots of seed and fruit bearing plants.
CATS, OPOSSUMS & RACCOONS can be deterred by mounting the nestbox on a pole. SNAKES are a common problem in the southeastern United States (and sometimes in SoCal), and can be deterred by mounting the box on a pole and adding a metal guard or a stovepipe baffle below the nestbox.