A friend who can be a nuisance – The violet carpenter bee

The violet carpenter bee can be considered a friend: by visiting flowers for nectar and pollen it provides pollination, fertilizing the visited flowers and enabling plants to produce seeds. Its diet is broad: Violet carpenter bees have been observed on almost 800 different plant species belonging to 85 different plant families. Still, they seem to prefer pollen from Fabaceae (especially Lathyrus) and Lamiaceae (especially Salvia and Lamium).

While carpenter bees can also be seen as foes, this is not because of their sting (that only females have): female bees are not aggressive and are mostly busy with visiting flowers, thus rarely sting. Instead, their nesting behavior can cause problems to people because of two reasons: 1) They prefer sunny and open areas to build their nests, such as gardens, parks, orchards, or forest edges. 2) Carpenter bees produce only one generation each year, but adult bees can live up to three years. Young bees sometimes inherit the nest they were born in, older bees reuse their previous nest or make a new nest nearby, which results in an accumulation of bees in one area. These nesting characteristics can lead to severe damage to wooden mountain cabins, wooden houses, or wooden structures such as decks, window frames, fences etc. 

So far, this does not seem to be a big problem in Switzerland. However, in the United States for example, and especially on the South and East coasts, heavy carpenter bee infestations occur from time to time. An explanation as to why this is not the case in Switzerland could lie in the climatic conditions and the excessive management of the environment, in particular the removal of dead wood. However, X. violaceae might become more common as climate change progresses and environmental management practices change, i.e. by leaving more dead wood around. This should not necessarily lead to problems, however. Infestations can be prevented by maintaining wooden structures, painting untreated or weathered wood with glazes or varnishes and avoiding damage to the wood surface. Existing nests can be covered in autumn before hibernation and while the bee is away by plugging the hole and painting it over. If nests are covered in spring/summer, there is a high probability the bees will reopen the holes, since they are marked with pheromones and bees have an excellent memory for the location of their nests. If nests are covered in winter, there is a risk that the bees will be trapped and killed, which should be avoided.