The term "host plant" in the context of pollinator habitat refers to a plant that a specific insect depends on for larval development — the plant on which eggs are laid and that caterpillars or larvae feed on. This is distinct from a "nectar plant," which provides adult feeding. Many native bees also show preferences for pollen from specific plant families, a relationship sometimes called oligolecty.
Understanding host plant relationships helps prioritize planting decisions. A garden that offers many nectar sources but lacks the plants required for larval development is functionally incomplete for those species.
Monarch butterfly and milkweed
The monarch (Danaus plexippus) is the most widely known example of a host-plant-dependent insect in Canada. Monarch larvae feed exclusively on plants in the genus Asclepias (milkweeds). In Canada, the most commonly available and ecologically relevant milkweed species are:
- Common milkweed (Asclepias syriaca) — native across southern Ontario, Quebec, and parts of the Maritimes; spreads aggressively via rhizomes; found in zones 3–8
- Swamp milkweed (Asclepias incarnata) — native to wetland edges; more compact than common milkweed; zones 3–8; pink flowers
- Butterfly weed (Asclepias tuberosa) — native to parts of southern Ontario; orange flowers; drought-tolerant once established; zones 3–9
Note on tropical milkweed (Asclepias curassavica): This non-native species is widely sold in garden centres but is not recommended for Canadian conditions. In warmer climates it does not die back in winter and can disrupt monarch migration patterns. It does not establish outdoors in most Canadian zones anyway, but seed packets and potted plants should be identified carefully.
Native butterflies and their host plants
Several butterfly species regularly present in Canadian gardens have specific native host plant requirements beyond milkweed:
| Butterfly | Primary host plants | Notes |
|---|---|---|
| Monarch (Danaus plexippus) | Asclepias spp. | Obligate milkweed host |
| Black swallowtail (Papilio polyxenes) | Wild carrot, Queen Anne's lace, golden alexanders (Zizia aurea) | Also uses garden dill and parsley |
| Tiger swallowtail (Papilio canadensis/glaucus) | Wild cherry (Prunus spp.), trembling aspen, birch | Widespread across Canada |
| Painted lady (Vanessa cardui) | Thistles (Cirsium spp.), pearly everlasting (Anaphalis margaritacea) | Migratory; uses a wide variety of hosts |
| Red admiral (Vanessa atalanta) | Stinging nettle (Urtica dioica) | Nettle patches, even small ones, support larvae |
| Pearl crescent (Phyciodes tharos) | Smooth aster (Symphyotrichum laeve), other asters | Common in eastern Canada |
Native bees and pollen specialist relationships
While most native bees are generalists — collecting pollen from a wide range of flowering plants — some species are oligolectic, meaning they collect pollen almost exclusively from one plant genus or family. These relationships are more common than widely appreciated:
- Andrena (mining bees): several species are specialists on willows (Salix), goldenrods (Solidago), or sunflowers (Helianthus)
- Colletes inaequalis: a specialist on early-blooming plants in the rose family, particularly serviceberry and wild plum
- Sunflower bees (Svastra obliqua): specialists on Helianthus species native to Canada's eastern and central regions
- Squash bees (Peponapis pruinosa): highly specialized on plants in the gourd family; present where squash is grown
Native trees and shrubs as host plants
Woody plants are often underestimated as host plants. Several native Canadian trees and shrubs support a large number of Lepidoptera (moths and butterflies) species in their larval stages:
- Native oaks (Quercus spp.): documented as hosts for hundreds of moth and butterfly species in North America
- Willows (Salix spp.): among the most important woody hosts; also provide early pollen for bees
- Wild cherries and plums (Prunus spp.): host to tiger swallowtails, several silkmoths, and dozens of moth species
- Trembling aspen (Populus tremuloides): the most widely distributed tree in Canada; supports tiger swallowtails and numerous moth species
- Native birches (Betula spp.): hosts for tiger swallowtails and many moth species
Where yard size permits, even a single native tree provides host plant value that many square metres of herbaceous planting cannot replicate. The value of large native trees as insect habitat is well documented in the ecological literature.
Nest sites alongside host plants
Host plants support larvae; nest sites determine whether those larvae ever exist. Most native bees nest in the ground. Some nest in hollow stems or holes in wood. Recommendations for host plant planting are more effective when combined with conditions that support nesting:
- Leave areas of undisturbed, sparsely vegetated bare or sandy soil for ground-nesters
- Leave dead standing stems over winter — many species overwinter as pupae inside dried plant stems
- Avoid excessive mulching in planting beds used by ground-nesting bees
For further detail on this, the Wild About Gardening resource produced by the BC Society for Ecological Restoration includes nesting habitat guidance for British Columbia specifically.
Related topics
See also: Building a Bloom Calendar for Canadian Pollinators for timing information on many of the plants mentioned here, and How to Create a Pesticide-Free Yard Zone for reducing chemical risks to larvae and adult insects.