Sunflower Bees

Species in the genera Diadasia and Svastra are collectively called sunflower bees because, along with several other groups of plants, they are sunflower (Helianthus annuus) specialists. In addition to sunflowers, these bees specialize on mallows (Malvaceae), cacti (Cactaceae), evening primroses and willow herbs (Onagraceae), and wild bindweeds (Convolvulaceae). In fact, several cacti and mallows rely on sunflower bees for pollination services. Diadasia species are restricted to the New World; most occur in the western United States, although some occur in Canada as well. Diadasia species are small to medium sized, ranging from 5 to 22 mm long. They are robust and hairy. Males have long antennae.

These bees are solitary ground-nesters, but they often nest in aggregations. Their nests are shallow and often have turreted (chimney-like) entrances. These bees usually fly in the morning and early afternoon. Each cell is provisioned with a nectar and pollen mixture and a single egg. Larvae hatch and turn into pupae. After a period of inactivity, adult bees emerge.

Sunflower crops are typically planted in rows - pollen-producing male plants in one row, nectar-producing female plants in another. In order for pollination to occur, pollen must be moved from male plants to female plants by pollinators, primarily bees. Honey bees (Apis mellifera) are often used to pollinate sunflower crops, but worker honey bees specialize in either collecting pollen or nectar and primarily visit only one type of row. However, the presence of native bees, like sunflower bees, causes honey bees to change their foraging activity - the native bees chase the honey bees between the rows of sunflowers, making them up to five times more efficient pollinators, increasing seed set from three to 15 seeds produced per honey bee visit. In fact, research by Sarah S. Greenleaf and Claire Kremen has shown that the only sunflower fields to achieve 100% pollination are those fields with abundant native bee populations. The most important of these native bee species are long-horned (Melissodes spp.) and sunflower (Diadasia and Svastra spp.) bees. In this study, pollination by native bees accounted for only 7% of total pollination, but they were indirectly responsible for an additional 40% of pollination by alternating honey bee behavior.

Diadasia is a genus of bees in family Apidae. Species of Diadasia are oligolectic, specialized on a relatively small number of plant species.

Their host plants include asters, bindweeds, cacti, mallows, and willowherbs, although mallows are the most common and likely ancestral host plant for the whole genus. Its tribe is Emphorini. In the Sonoran Desert, Diadasia rinconis is considered the "cactus bee" as it feeds almost exclusively on a number of Sonoran Desert cactus species, its life cycle revolving around the flowering of the native species of cacti.