• Dr. Eric McCloud

Farm Pollinators: Mason Bees and Squash Bees

Many flowering plants have the neat trick of manipulating insects, or birds, or even mammals to help them accomplish sexual reproduction. The process that animals help with, pollination, is also of vital importance for cultivated plants and their humans because it is often the fruits or seeds of these plants that are harvested. Here are two little stories of pollinators that are important on the farm.

Mason Bees

There are about 300 species of mason bees spread across the northern hemisphere and all use mud or macerated plant materials to build nests between cracks in stones, in hollow stems, or in the holes left by wood-boring insects. These small, solitary bees lay eggs in sectioned chambers of their mud tube nests and provision each with a ball of pollen stuck together by nectar and saliva. Many species of mason bees are broad specialists in the types of plants they pollinate (they are termed polylectic), while others are more specialized (oligolectic). In the wild, mason bees often pollinate a variety of plants in the rose family and related families. Mason bee species are generally excellent pollinators of cultivated fruit-bearing shrubs and trees. Mason bees’ high pollination efficiency and a large measure of their desirability for orchardists and farmers come from several traits. Mason bees can forage in cool spring weather early in the season and are not as bothered by poor weather as honeybees. Unlike honeybees and other farm pollinators, mason bees do not forage over great distances so, pollination of a particular crop can be achieved with more certainty from nests that are near that crop on the farm.

*Photos by Deb Goedde*

Also, mason bees are not so good at packing and storing pollen and this inefficiency works to the benefit of plants and farmers seeking pollination. While many kinds of bees, including honeybees, pack the pollen into tight balls carried on their hind legs, mason bees hold pollen more loosely on the undersides of their abdomens. This messiness is the plant’s gain – mason bees spread pollen from plant to plant more readily than other bees with tidy pollen baskets on their legs. For these reasons, semi-domesticated and even commercial mason bee colonies are sometimes used on farms, orchards, and berry operations. It’s thought that this may help to promote and conserve the diversity of native mason bee species.

Nevertheless, the use of mason bees as an alternative and an augment to honeybees is not without its difficulties and these very efforts at conservation via semi-domestication may be contributing to the decline in population numbers of some mason bee species. In North America, two species of non-native mason bees have been widely introduced and, in some cases, managed, for fruit pollination. These introductions have been implicated in the decline of native species because the introduced species present competition for wild native species and because they have brought along diseases to which native species are not resistant. In one study, six native species of mason bees were shown to be declining after a non-native species had been introduced. Further complicating this is that augmentation of pollination services by wild mason bees through the provision of “bee hotels” may simply increase rates of parasitism and disease. So, the mason bee boxes that are popularly sold to homeowners, gardeners, and farmers could be contributing to bee decline rather than being the conservation tool and fun nature study hobby that it is advertised to be. We just don’t know at this point and more research is needed. For now, the best conservation strategies for mason bees may be to find ways to provide more natural habitats for these valuable farm species that are also an important part of the natural setting of insect pollinators for wild plants.

Squash Bees

Early in the morning, squash bees – specifically the males – can be seen in squash, melon, gourd, and pumpkin fields buzzing from flower to flower. Oligolectic for plants in the squash family and preferring new world species that match their native distribution, squash bees forage before dawn, moving from male to female blossoms before curling up to sleep inside blossoms shortly after dawn. Like other insects, squash bees have two kinds of eyes, compound

eyes composed of many individual facets, and light gathering ocelli with a single lens. Larger ocelli in squash bees help them navigate in the dim pre-dawn air. Males stay up a little bit later to find females already nestled into blossoms that will soon be closing. Small aggregations of males and females pass the heat of the day sleeping in closed squash blossoms.

Squash bees are ground nesting solitary bees although in some cases, individual nests may be clustered. Females build vertical tunnels with brood chambers on the sides. Grubs in these chambers are provisioned with squash pollen.

Squash bees are highly efficient pollinators of plants in the squash family, so much so that the use of commercial honeybees hives for pollination in squash fields is generally not necessary when squash bees are present. The presence of managed honeybees has also been shown to be detrimental to squash bee populations. In addition to being wild native species (there are 20 species in two genera), squash bees have some other advantages over managed honeybee populations. Varroa mite is serious parasitic pest of honeybees and squash bee species are resistant to it. Given these observations, some vegetable crops productions specialists recommend avoiding the use of honeybees in squash fields. Further, squash bees appear to be resistant to several common honeybee virus diseases. The typical foraging radius for squash bees is also quite small – they tend not to travel more than about a quarter of a kilometer (a bit less than 290 yards) from their burrows. This concentrates their activity in the squash fields where they live.

Farming practices beyond the use of honeybees can affect squash bee populations. Squash bees overwinter in their burrows and tillage reduces overwintering success. No till practices generally increase spring squash bee densities several fold. Also, squash bees are susceptible to soil applications of neonicitinoid insecticides. Both nest initiation rates as well as reproductive success are strongly affected. Seed treatment with these types of insecticides has not been shown to have a significant detrimental effect but this cannot be ruled out.

Conservation and encouragement of squash bees then, will depend on careful utilization of farm management and honeybee management practices.

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