There is a meaningful distinction between a nectar source and a host plant. Nectar sources feed adult insects. Host plants feed larvae — the eggs and caterpillars that grow into the next generation of pollinators. A garden that offers only nectar without host plants functions as a diner but not a nursery. For pollinator populations to be sustained at a local level, both are necessary.
Why Host Plant Specificity Matters
Many native bee species are oligolectic, meaning they are nutritionally dependent on pollen from a single plant family or genus. Research by entomologists including Laurence Packer at York University has documented that Ontario alone supports over 400 native bee species, a substantial proportion of which have narrow pollen requirements. When the plants they depend on are absent from a landscape, those bee populations decline locally regardless of how many ornamental flowers are present.
Butterflies and moths are similarly constrained at the larval stage. Unlike adult butterflies, which visit a broad range of flowers for nectar, caterpillars are typically restricted to a small set of host plant genera. The monarch butterfly's dependency on milkweed is the most widely cited example, but many other species share similar constraints.
Milkweeds and the Monarch Butterfly
The monarch butterfly (Danaus plexippus) is found across southern Canada from British Columbia to Nova Scotia during its summer breeding season. Its caterpillars feed exclusively on plants in the genus Asclepias (milkweeds). In Canada, three milkweed species are particularly relevant:
| Species | Common Name | Range in Canada | Notes |
|---|---|---|---|
| Asclepias syriaca | Common milkweed | ON, QC, NB, NS, MB (scattered) | Most widespread; spreads by rhizome; drought tolerant |
| Asclepias tuberosa | Butterfly weed | ON (southern), QC (limited) | Does not exude milky sap; needs well-drained soil; orange flowers |
| Asclepias incarnata | Swamp milkweed | ON, QC, MB, SK, NS | Tolerates wet conditions; pink flowers; preferred by monarchs near wetlands |
Note on tropical milkweed (Asclepias curassavica): This non-native species is widely sold in Canadian garden centres and should be avoided. Research has shown it can interfere with monarch migratory cues and may harbour the parasite Ophryocystis elektroscirrha at higher rates than native species. Native milkweeds go dormant in autumn, which triggers migration behaviour. Tropical milkweed does not go dormant and may cause monarchs to remain north past safe migration windows. See Xerces Society guidance on tropical milkweed for further detail.
Host Plants for Native Bees
For ground-nesting bees (which account for roughly 70% of native bee species), host plants must be paired with suitable nesting substrate. Bare or sparsely vegetated sandy soil adjacent to flower patches significantly increases bee habitat value. The following table summarizes some of the better-documented host plant relationships among Canadian native bees.
| Bee Genus | Host Plant (Pollen Source) | Plant Common Name | Notes |
|---|---|---|---|
| Andrena spp. | Rudbeckia, Helianthus | Black-eyed Susan, sunflower | Many species are Asteraceae specialists |
| Colletes spp. | Symphyotrichum, Solidago | Native asters, goldenrod | Late-season specialists; emergence timed to aster bloom |
| Megachile spp. | Various Fabaceae | Wild lupine, vetches | Leafcutter bees; also use Rudbeckia and Echinacea |
| Bombus spp. | Broad diet; prefer Monarda, Trifolium | Wild bergamot, clovers | Long-tongued; access tubular flowers inaccessible to smaller bees |
| Xylocopa spp. | Broad diet | Various | Carpenter bees; need dead wood or thick pithy stems for nesting |
Host Plants for Lepidoptera Other Than Monarchs
Several other butterfly and moth species that breed in or migrate through Canada have specific larval host plant requirements.
Eastern Tiger Swallowtail (Papilio glaucus)
Larvae of the eastern tiger swallowtail feed on a range of native trees and shrubs including wild black cherry (Prunus serotina), tulip poplar (outside Canadian range in most areas), and various willows (Salix spp.). For Canadian yard contexts, including even one native cherry or serviceberry adds significant larval habitat value.
Painted Lady (Vanessa cardui)
A wide-ranging migrant butterfly, the painted lady can use thistles (Cirsium spp.) as larval host plants. Native thistle species should be distinguished from invasive bull thistle and Canada thistle — tall thistle (Cirsium altissimum) is a native species that supports painted ladies in central Canada.
Sphingid Moths (Sphinx moths)
Several species in the family Sphingidae, including the hummingbird clearwing moth, use native viburnums, hawthorns, and cherries as larval hosts. These moths are important pollinators of deep-throated flowers such as wild columbine and some native orchids.
Stem and Cavity Nesters
Not all bees nest in the ground. Mason bees, leafcutter bees, and small carpenter bees use hollow stems, beetle borings in dead wood, and other pre-existing cavities. Leaving pithy-stemmed plants like elderberry, joe-pye weed, and ironweed standing through winter — rather than cutting them back in autumn — provides nesting material. Research by the Xerces Society documents that simply leaving plant stems standing over winter can increase cavity-nesting bee populations in adjacent areas.
Practical note: The practice of leaving garden beds uncut through the winter may conflict with municipal bylaw requirements in some Canadian cities that specify maximum plant heights or require "tidy" yard maintenance. Checking local bylaws before establishing a habitat planting is advisable. Several municipalities, including Ottawa, have established exemptions or habitat yard programs that allow unmowed areas with native plants.
What to Avoid
Cultivars with doubled flowers (labelled as "double" or "filled" in nursery catalogues) often produce little or no pollen and may have reduced nectar. Echinacea cultivars with dramatically altered petal shapes, and Rudbeckia hybrids with fully doubled disk flowers, should be avoided in favour of straight species or minimally altered cultivars. The seed heads of some non-native ornamental grasses can also produce seed that spreads invasively, displacing the native plants that host insects depend on.
References
- Packer, L. (2010). Keeping the Bees: Why All Bees Are at Risk and What We Can Do to Save Them. HarperCollins Canada.
- Xerces Society. Gardening for Butterflies. Timber Press, 2016.
- Environment and Climate Change Canada. COSEWIC Assessment: Monarch Butterfly. 2016.
- Pleasants, J. M., & Oberhauser, K. S. (2013). Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity, 6(2), 135–144.