Solitary Bees: Specialist Pollinators for Your Garden

Solitary bees, such as the digger and squash bee, nest in the soil, making them great pollinators in your garden. Learn which flowers they prefer and how they can help your garden grow.
By Eric Grissell
May 6, 2013
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Many species within the group of solitary bees are specialist pollinators of certain flowers.
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Bees are one of the most important insects to us. Not only are they great garden pollinators, they maintain biological balance and recycle soil nutrients. Learn all about bees — from why solitary bees make for specialist pollinators to which flowers they prefer in Bees, Wasps, and Ants (Timber Press, 2010) by Eric Grissell. The following excerpt was taken from chapter 8, "The Garden’s Pollinators: Bees."

You can purchase this book from the MOTHER EARTH NEWS store: Bees, Wasps, and Ants

Digger Bees and Squash Bees

Prior to being placed in Apidae, the digger bees and squash bees were once all placed in the family Anthophoridae, which shows their close relatedness to each other. As might be expected from the common name digger bee, most members of this group nest in the soil, excavating nests by use of their mandibles. These are primarily solitary bees that do not gather external building materials for nesting. Instead their cell walls are coated with secretions from an abdominal gland, which produces somewhat of a waxy or varnish-like final appearance. Females nest in vertical soil surfaces such as earthen banks, cliff faces, or adobe walls as well as in flat areas. Nest aggregations are common, and occasionally females may use the same burrow entrance, but each maintains her own cell (that is, they can be communal nesters). Many species create small mounds of earth or mud turrets resembling earthen chimneys at the entrance of their nests. Digger bees construct from one to several cells per nest, depending on the species.

Many species within the group are specialist pollinators of certain flowers. For example some Melissodes species specialize in thistles (Cirsium) or Callirhoe. Species of Diadasia are dedicated in their foraging habits, some visiting only sunflowers (Helianthus), others preferring cactus flowers (Cactaceae), and others flowers of the morning glory family (Convolvulaceae), mallows (Malvaceae), or evening primrose family (Onagraceae). The wonderfully named shaggy fuzzyfoot bee (Anthophora pilipes villosula) is a specialist pollinator of blueberries, although it is also useful for pollinating spring-flowering crops such as apples. It was introduced into the United States from Japan. The shaggy fuzzyfoot is especially desirable as a pollinator of crops because females work on rainy days, are fast flyers, and cover much territory when other bees fear to fly. Some Anthophora adorn the entrance to their nest with mud turrets.

Among the specialist pollinators within the digger bees must be listed the squash and gourd bees (Peponapis, Xenoglossa). These robust bees range in length from about 1/2 to 3/4 inch (12 to 18 mm). Although the body is basically black, the head and thorax are abundantly covered in yellowish hairs, and the abdomen has hair bands of similar coloration. Squashes, pumpkins, and gourds (Cucurbita) are New World endemics with both male and female flowers on the same plant, and they require a pollinator to move pollen from male to female flowers. Thus, in geological times, cucurbits would have required the services of a pollinator endemic to the area in which they grew. Squash and gourd bees coevolved to do this job in nature, but they were largely co-opted by honey bees as these cucurbits were hybridized and grown in agricultural monocultures. Here in the western United States where I live, there are still several native cucurbits that are pollinated by local squash bees. These bees arrive early in the morning, before the honey bees, and thus do the job they evolved to do.

Squash and gourd bees are the subject of a Squash Pollinators of the Americas Survey that was established by the U.S. Department of Agriculture’s Pollinating Insect Research Unit in Logan, Utah, in an attempt to determine just how important they are. As stated at their website, if squash bees “prove to be ubiquitous, prevalent, abundant and effective, then this would be the first case for unmanaged, native nonsocial bees playing a key role for production of any agricultural crop.” Thus, squash bees would seem to be potentially of great importance. Based upon the first year’s survey this proved to be true: “It appears that an unmanaged group of nonsocial native bees — the specialist squash bees — are largely responsible for the production of cultivated squashes across North America, and by extrapolation, to much of the Americas” (Cane 2005).

Nomadine or Cuckoo Bees

The cuckoo, or cleptoparasitic, habit is found randomly dispersed in several families of bees (that is, Halictidae, Megachilidae, and Colletidae), but nomadine bees (Apidae, subfamily Nomadinae) exclusively exhibit this behavior. There are more than 1200 known species of nomadine bees in the world, by far the largest number of cleptoparasitoids of any bee family. Nearly half of all nomadines occur in North America. Although related to digger bees, they appear completely unlike their cousins. These are essentially hairless, wasp-like bees in varying shades of black, white, yellow, and rust. They range in size from about 1/8 to 5/8 inch (3 to 15 mm) and have given up their pollen-collecting ways to live as thieves. These bees are cleptoparasitoids because they enter the nests of other bees and highjack the provisions for their own offspring. This is basically accomplished in two ways. One is for the cuckoo to enter the host bee’s nest as it is being provisioned and lay its own egg on the pollen provisions. The other is for the cuckoo to await the nest’s completion, then chew its way into the host cell, lay an egg, then patch the cell up when it leaves. Species of one common genus of cuckoo bee (Nomada) are known to parasitize selected bees in all the other bee families, including its own, whereas the related genus Triepeolus attacks only a few families, and Epeolus attacks only members of the family Colletidae (yellow-faced bees).

What's the Buzz? Learn more about bee families in All About Bees: The Great Garden Pollinators.


Reprinted with permission from Bees, Wasps, and Ants by Eric Grissell and published by Timber Press, Inc, 2010. Buy this book from our store: Bees, Wasps, and Ants.


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