The term "host plant" refers to the specific plant species that an insect uses for egg-laying and larval development, rather than simply for feeding as an adult. For butterfly species, host plant relationships are often highly specific: a monarch caterpillar will only feed on milkweed (Asclepias species); a black swallowtail requires plants in the carrot family (Apiaceae). Many native bee species show similar but less absolute specialization, collecting pollen preferentially from particular plant genera.
Understanding host plant relationships helps explain why a garden planted with non-native ornamentals, even if it produces abundant nectar, may fail to support breeding populations of target species. Nectar sustains adults; host plants make reproduction possible.
Monarch Butterfly and Milkweed
The monarch butterfly (Danaus plexippus) is Canada's most discussed migratory species in the context of pollinator conservation. Its caterpillars feed exclusively on milkweed species, making milkweed availability directly connected to local reproduction rates before the southern migration.
Three milkweed species are native to Canada and broadly available through native plant nurseries:
- Butterfly weed (Asclepias tuberosa) — Zone 4. Dry, well-drained soil. Orange flowers that are also highly attractive to bumble bees and fritillary butterflies. Does not spread aggressively.
- Common milkweed (Asclepias syriaca) — Zone 3. Average to dry soil. Spreads by rhizome; best suited to larger areas or naturalized patches. The most common milkweed in Ontario and Quebec.
- Swamp milkweed (Asclepias incarnata) — Zone 3. Moist to wet soil. Well suited to rain gardens and wet meadows; less aggressive spreading than common milkweed.
The tropical milkweed (Asclepias curassavica), which is widely sold at garden centres, is not native to Canada and may interfere with migratory cues in southern areas if allowed to remain green late into the season. Its use is not recommended in Canadian gardens where monarch migration is a conservation priority.
Canadian Tiger Swallowtail
The Canadian tiger swallowtail (Papilio canadensis) is one of the most familiar large butterflies across most of Canada, ranging from boreal regions of BC and Alberta through Ontario and Quebec. Unlike some specialist species, it uses a fairly broad range of host plants from the willow, poplar, and cherry families.
Host plants for Canadian tiger swallowtail caterpillars include:
- Native willows (Salix species) — particularly pussy willow (Salix discolor)
- Trembling aspen (Populus tremuloides)
- Wild black cherry (Prunus serotina)
- Chokecherry (Prunus virginiana)
- White ash (Fraxinus americana)
Adding even a single native willow or chokecherry to a yard creates habitat that can support multiple generations of this species. Salix discolor is also among the earliest native pollen sources in spring, benefiting queen bumble bees and other early-emerging bees.
Native Bees and Specialist Pollen Relationships
Canada is home to roughly 800 native bee species, ranging from large bumble bees to tiny sweat bees under 5mm in length. Unlike the managed European honeybee, the majority of native bees are solitary: females construct individual nests and provision each cell with pollen for their offspring.
Many native bee species are oligolectic, meaning they collect pollen from a narrow range of plant genera. For these species, access to the appropriate plant is not just preferred — it is required for reproduction. Examples relevant to Canadian gardens include:
- Squash bees (Eucera (Peponapis) pruinosa) — Collect pollen exclusively from Cucurbita (squash and gourds). Nest in the ground near squash plants.
- Sunflower bees (Svastra and Diadasia species) — Highly associated with native and cultivated sunflowers (Helianthus species).
- Bellflower bees (Osmia and Chelostoma species) — Collect pollen from bellflowers (Campanula). Several species nest in hollow stems.
- Coneflower bees — Several Andrena and sweat bee species show strong preference for Echinacea and related composites.
Host Plants for Common Butterfly Species
| Butterfly | Host Plant(s) | Plant Family | Canadian Range |
|---|---|---|---|
| Monarch (Danaus plexippus) | Milkweeds (Asclepias spp.) | Apocynaceae | Southern ON, QC, MB, SK |
| Canadian Tiger Swallowtail (Papilio canadensis) | Willows, poplars, cherries | Salicaceae, Rosaceae | Widespread, boreal south |
| Black Swallowtail (Papilio polyxenes) | Wild parsnip, Queen Anne's lace, golden Alexanders | Apiaceae | Southern ON, QC, Atlantic |
| Eastern Tiger Swallowtail (Papilio glaucus) | Wild black cherry, tulip tree, sweetbay | Rosaceae, Magnoliaceae | Southern ON, Carolinian zone |
| Painted Lady (Vanessa cardui) | Thistles, hollyhocks, legumes | Asteraceae, Malvaceae | Widespread migratory |
| Red Admiral (Vanessa atalanta) | Stinging nettle (Urtica dioica) | Urticaceae | Widespread across southern Canada |
| Cabbage White (Pieris rapae) | Brassica species (introduced) | Brassicaceae | Widespread (non-native) |
Shrubs and Trees as Host Plants
Native shrubs and trees are among the most ecologically valuable additions to a pollinator garden because of the sheer number of insect species they support. Research by entomologist Doug Tallamy and others has documented the number of caterpillar species dependent on various native tree genera; in eastern North America, native oaks (Quercus) support several hundred species of Lepidoptera caterpillars, followed by willows, cherries, and birches.
Even in urban yards where space is limited, a single native shrub such as chokecherry, serviceberry (Amelanchier species), or native dogwood (Cornus species) contributes substantially to local insect diversity and provides fruit for birds. Serviceberry in particular offers early-spring nectar, summer fruit, and fall leaf color, making it one of the most compact multi-season choices for Canadian yards.