Pollinators are essential for our vegetable gardens, boosting the production of a number of our favorite fruits and vegetables in a significant way. Pollinator nesting boxes can contribute to an increase in the local population of certain useful species and are a good way of introducing the family to these unsung allies.
A diversified community
A number of groups of solitary bees are excellent pollinators and can be attracted to a vegetable garden. Those insects are essentially looking for two elements: a source of food, meaning flowers, and an appropriate place to build a nest. Many species of solitary bees use hollow stems or tunnels dug into dead wood by beetle larvae for laying their eggs. The female builds a series of niches along the tunnel. In each of these niches she’ll lay an egg and accumulate reserves that will serve the larvae as food. The different groups of solitary bees are distinguished by among other things the substrates used to make a nest and to create these niches. In pollinator nesting boxes we’ll find mostly mason bees (Osmia), leafcutters (Megachile) carpenters (Xylocopa) and potters (Anthidium).
Materials easy to find around the house
Materials that can be used to construct a pollinator nesting box are easy to find at home. Many plants produce hollow stems that once dried out can be cut into segments about 15 centimeters in length. That’s the case with various grasses, including the common reed, as well as the flowering stalks of day lilies or the bamboo sometimes used as stakes. Raspberry bush stalks, which are not hollow, but whose center consists of low-density marrow, can also be used. Other bee species prefer to make their nests in dead wood. So you can cut branches or trunks into segments and drill holes in them. The diameter of the stalks or the holes has a direct influence on the type of bee that will use it. Diameters ranging from 3 to 12 millimeters suit the great majority of species that visit our vegetable gardens. Those stalks can be integrated in a structure made of wood or, for example, in a tin can, which will protect everything from bad weather.
Precautions to take
Some studies have been done on the use of these structures by solitary bees and their impact on populations. Pollinator nesting boxes may lead to an increase in the local population of solitary bees that is beneficial for the garden. However, a concentration of nests makes for a much higher density than what we observe in nature, which increases the risk of infection or parasitism. To maximize the beneficial effect of the nesting box, it’s important to replace the old stems with new ones every year, whether they’ve been used or not. Also, the structure housing the stems must be kept clean. Finally, the choice of nesting box location is crucial: sunny spots should be chosen, sheltered from prevailing winds.
Although the beneficial effects of pollinator nesting boxes for the garden are well documented, their impacts on local and regional biodiversity are still to be determined. Some studies suggest that introduced species, being more generalist, are the first to benefit from these installations. The Insectarium de Montréal will be launching a project aimed at shedding more light on species benefiting from these structures during the summer of 2020.