SPECIALIST INSECTS

While the most famous insect specialist that visits our gardens is the monarch butterfly (Danaus plexippus), between 19% and 37% of native bee species that occur in central and eastern United States are pollen specialists. That is an astounding figure given there are more than 3700 species of native bees in the United States and Canada. The monarch butterfly is a fairly strict specialist, in its summer range, the larvae (caterpillars) feed only on host plants from the genus Asclepias (milkweed). Similarly, pollen specialist native bees have native host plants. 
If our landscapes were devoid of native plants, we would be failing to provide the critical nourishment that these pollen specialist bees need to complete their life’s work—specific pollen from specific native plants to rear the next generation of bees.
Perdita perpallida - a narrow
​oligolectic specialist of Dalea

bee host plant specializations

​Specialization can range from narrow to broad. For example, monolectic bees specialize on a single plant species, while narrow oligolectic bees specialize on plants belonging to one plant genus, and oligolectic bees on a few related plant genera.
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Andrena ziziae - an oligolectic specialist of Zizia and other Apiaceae genera
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Andrena geranii - a narrow oligolectic specialist of Hydrophyllum

​In many instances, these specializations can be mutually beneficial for the bee and the host plant: improved pollination for the plant and an increase in the abundance of host plants for the bee.

BEE DIET

Bees are essentially hairy, vegetarian wasps. Their carnivorous, predatory wasp ancestors feed their larvae insects or spiders. The radiation of flowering plants (angiosperms) during the Cretaceous period and the alternative food source that these pollen- and nectar-rich plants provided, is one reason bees likely evolved or diverged from their wasp cousins to feed their offspring an entirely plant-based diet. Bees are also equipped with numerous branched hairs that enable females to efficiently transport pollen loads back to their nests. While adult females consume pollen for nourishment (protein and lipids), the larvae are the primary consumer of the specialized or specific pollen diet.

SPECIALIST BEES AND CLIMATE CHANGE

Specializing on collecting only pollen from one genus or a handful of plant genera can be risky, particularly with the projected changes resulting from the climate warming—plants may flower earlier or later than historical averages, or have a shorter flowering period. The result is a very precarious situation for pollen specialist bees that are already at risk of becoming extirpated from the highly fragmented or degraded habitats where they live. Climate change may cause the mismatch or unpairing of the emergence of the specialist bees and the flowering phenology of the plants they depend upon. 
​Geranium Mining Bee (Andrena distans)
​a narrow oligolectic specialist of Geranium

HOW CAN YOU HELP SPECIALIST BEES?

There are simple things gardeners can do to help specialist bees. In addition to remnant and restored natural landscapes, common (and uncommon) specialist bees can thrive in pesticide-free, native plant-rich urban and suburban gardens, when their pollen host plants are present. By focusing on some specific plant families and genera, we can maximize the number of native bee specialist host plants.

Perennials
The plant family Asteraceae is by far the heavy hitter, supporting the most pollen specialist bee species in all regions of the United States. Summer- and autumn-blooming perennial plants in this family such as black-eyed Susan (Rudbeckia), sunflower (Helianthus), goldenrod (Solidago, Euthamia, Oligoneuron), and aster (Symphyotrichum, Eurybia) are already foundational plants in native plant gardens and restorations.
One of the most fascinating native bee host plants is native loosestrife (Lysimachia). The flowers are nectarless and in addition to producing pollen, glands on the flower petals and filaments produce floral oil. The pollen and the oil is simultaneously collected by oil-collecting, ground-nesting bees in the genus Macropis. Females have specialized brushes on their forelegs for combing the oil and, while doing so, use their other legs to brush the pollen. The females use the oil in two ways, as food (combined with the pollen) and as a nest waterproofing material. 
Dufourea monardae, a narrow
​oligolectic specialist of Monarda
Shrubs
A number of native woody shrubs also support specialists. Willow (Salix), already considered a keystone plant for the number of lepidopteran larvae (butterfly and moth caterpillars) it hosts, also provides pollen for one to two dozen (or more) specialist bees depending on the region.
Dogwood (Cornus), another common garden plant, also supports several oligolectic specialists. After planting a few species of dogwoods in my garden, the following growing season I documented one of the monolectic dogwood specialists  visiting the flowers—the short-haired dogwood mining bee (Andrena integra). 

Some additional things to do to support specialist bees:
  • advocate for the preservation and restoration of rare native plant communities,
  • plant more native bee host plants in your garden,
  • keep the garden pesticide-free, and
  • ask local native plant growers to grow more regionally-appropriate native bee host plants.

​​​Remember, if you plant it, they will come!
To learn more about what native plants support pollen specialists bees in your region, please refer to the comprehensive regional listings compiled by Jarrod Fowler at: http://jarrodfowler.com/specialist_bees.html