The term "host plant" has two related but distinct meanings in pollinator gardening. For butterflies and moths, a host plant is the specific species on which females lay eggs and caterpillars feed. For bees, host plants refer to the floral resources — pollen and nectar — that a bee species requires, sometimes exclusively. Understanding both types of host relationships is necessary for designing a garden that genuinely supports pollinator reproduction, not just foraging visits.
Providing nectar without larval host plants supports adult pollinators visiting the garden but does not enable breeding. A complete pollinator habitat includes both food resources and the specific plants required for reproduction.
Butterfly Host Plants in Ontario
Butterfly species vary considerably in how specific their larval host plant requirements are. Some species — called generalists — can develop on plants from multiple families. Others are monophagous, developing on a single plant genus or even species. The monarchs and fritillaries fall into this more specialist category.
Monarch Butterfly (Danaus plexippus)
The monarch is strictly dependent on milkweed species (Asclepias spp.) for larval development. In Ontario, the most commonly available native milkweed species are:
- Common milkweed (Asclepias syriaca) — the most widespread Ontario species. Grows in full sun on well-drained to average soil. Spreads aggressively by underground rhizome. Best suited to naturalized areas or large plots.
- Swamp milkweed (Asclepias incarnata) — grows in moist to wet conditions, making it suitable for rain gardens and pond edges. Less aggressive than common milkweed. Pink flower clusters bloom July–August.
- Butterfly weed (Asclepias tuberosa) — orange-flowered species that tolerates dry, sandy soil. Does not spread by rhizome. Suitable for rock gardens and sunny, well-drained sites. Less common in southern Ontario naturally but widely available commercially.
Tropical milkweed (Asclepias curassavica), a non-native ornamental, is widely sold but is not recommended for Ontario gardens. It does not die back reliably in Canadian winters and may disrupt migration cues.
Great Spangled Fritillary (Speyeria cybele)
A large, orange butterfly common in southern Ontario. Females lay eggs in late summer near — but not always on — native violet species. Caterpillars hatch in fall, overwinter, and begin feeding on violet leaves the following spring. Native violets to include in the garden:
- Viola sororia (common blue violet) — extremely adaptable, tolerates sun to shade, moist to dry conditions
- Viola pubescens (downy yellow violet) — woodland species, part shade
- Viola canadensis (Canada violet) — moist woodland, shade tolerant
Black Swallowtail (Papilio polyxenes)
Larvae feed on plants in the carrot family (Apiaceae). In Ontario gardens, suitable native and naturalized host plants include wild parsnip, Queen Anne's lace, and golden alexanders (Zizia aurea). Golden alexanders is the most garden-appropriate choice — it blooms in May with yellow umbels, tolerates part shade, and is genuinely native to Ontario.
Cabbage White and Other Generalists
Several common Ontario butterflies — including the cabbage white, clouded sulphur, and painted lady — are generalist feeders whose larvae develop on plants from multiple families. These species do not require specific native plant hosts and are less conservation-critical than specialist species.
Native Bee Host Plants in Ontario
Canada has over 800 species of native bees, with Ontario hosting a significant portion of that diversity. Many are oligolectic — they collect pollen from a restricted range of plant families or genera. The following plant groups support specialist bee populations.
Goldenrod (Solidago spp.) — Critical for Specialist Bees
Goldenrod species collectively support a documented diversity of specialist bees, including several species in the genera Colletes, Halictus, and Andrena. These bees time their flight periods to match goldenrod bloom and provision their nests exclusively with goldenrod pollen. Without goldenrod in the landscape, these specialist species cannot reproduce.
Native goldenrod species for Ontario gardens:
- Canada goldenrod (Solidago canadensis) — spreads vigorously; best in naturalized areas
- Stiff goldenrod (Solidago rigida) — more compact, better for garden beds, large yellow flower clusters
- Zigzag goldenrod (Solidago flexicaulis) — shade tolerant; useful for woodland gardens
Asters (Symphyotrichum spp.) — Late-Season Specialist Hosts
Several native bee species in the genus Colletes are strict aster specialists, flying only in September and October and collecting pollen exclusively from aster family plants. New England aster (Symphyotrichum novae-angliae) and smooth blue aster (Symphyotrichum laeve) are the most productive Ontario species for these bees.
Sunflowers and Coneflowers — Helianthus Specialists
Several Melissodes and Svastra bee species are sunflower specialists. Woodland sunflower (Helianthus divaricatus) and pale-leaf sunflower (Helianthus strumosus) are native to Ontario and support both specialist foragers and a range of generalist bees.
| Insect | Host / Larval Plant | Relationship Type |
|---|---|---|
| Monarch butterfly | Asclepias spp. (milkweed) | Obligate larval host |
| Great spangled fritillary | Viola spp. (native violets) | Obligate larval host |
| Black swallowtail | Apiaceae (Zizia aurea, others) | Larval host family |
| Colletes (cellophane bees) | Symphyotrichum spp. (asters) | Pollen specialist |
| Andrena (mining bees) | Solidago spp. (goldenrods) | Pollen specialist |
| Tri-colored bumblebee | Generalist forager | Wide range of natives |
Bumblebee Species in Ontario
Ontario is home to several bumblebee species, including the common eastern bumblebee (Bombus impatiens), the tri-colored bumblebee (Bombus ternarius), and others. Unlike solitary bees, bumblebees are generalist foragers and visit a wide range of native flowers. However, they show clear preferences for tubular flowers that match their tongue length.
Long-tongued bumblebee species (such as Bombus pensylvanicus) prefer deep-tubed flowers including wild bergamot, native clovers, and monkshood. Short-tongued species are more commonly found on open flowers like goldenrod, coneflowers, and asters.
Bumblebee queens emerge in early spring and require early-blooming native plants to establish their colonies. Bloodroot, wild columbine, and native willows provide the first pollen and nectar of the season. Without adequate early resources, queen survival decreases and colony establishment is delayed.
Nesting Habitat Alongside Host Plants
Most Ontario native bees nest in the ground (approximately 70% of species) or in hollow plant stems. Host plants alone are insufficient if nesting habitat is absent. Ground-nesting bees require areas of bare or sparsely vegetated, undisturbed soil — a sunny, south-facing slope is ideal. Mulching beds too heavily eliminates ground-nesting sites.
Stem-nesting species overwinter in the hollow or pithy stems of plants such as goldenrod, Joe-Pye weed, and elderberry. Leaving these stems standing through winter — cutting them down no earlier than late April — allows nesting bees to emerge naturally in spring.