by Pamela Sleightholm and Jeanne McRight
Many Lepidoptera (the family of butterflies and moths) are specialists, dependent on specific plants for nutrition and habitat. The plants and insects evolved together for hundreds of thousands of years, developing symbiotic relationships where each offers essential benefits and services to the other.
Above: Monarch butterfly sipping nectar and pollinating a swamp milkweed. Photo ©2021 Peeter Poldre.
A familiar example is monarch/ milkweed symbiosis. Monarch adults are butterflies that enjoy nectar from many flowers but deposit eggs on milkweeds and help pollinate the plants. In return, milkweed is the sole host of the monarch caterpillars. Milkweed contains toxic latex which many species avoid, but monarch caterpillars sequester the chemical in its tissues. These chemicals are what make the adult monarch distasteful and toxic to predators. In return, pollination allows the milkweed to reproduce and multiply, providing more food for late summer monarch caterpillars who become the adults who migrate. Loss of their milkweed hosts is a major factor in the decline of monarch butterflies in North America.
There are many other native plant hosts suitable for butterfly gardens that feed specialist butterfly caterpillars:
American Painted Lady: pussytoes, pearly everlasting
Black Swallowtail: golden alexanders, as well as non-native dill, parsley, fennel
Eastern Tiger Swallowtail: wild cherry, wild plum
Mourning Cloak: pussy willow
Red Admiral, Question Mark: nettles
Fritillaries: violets
skippers: little bluestem and other native grasses
Mourning cloak butterfly, black swallowtail caterpillar, monarch caterpillar, silver spotted skipper and eastern tiger swallowtail are all examples of specialist butterflies in Ontario. They need specific native host plants for various life stages. All photos ©2021 Peeter Poldre.
Plant keystone species to support biodiversity
Keystone species are host plants that feed the young caterpillars of approximately 90% of butterflies and moths.
Scientist Douglas Tallamy says "increasing plant diversity with the intentional inclusion of keystone plants is the most efficient and successful approach to support Lepidopteran diversity."
Try to include one or more keystone tree and shrub species, such as native oaks, maples, birch, cherries and plums, willows, raspberries and blueberries.
References:
To learn more about butterflies and the host and nectar plants they need, check out this page from Carolinian Canada: Native Butterfly Host and Nectar Plants.
Top Keystone Plant Genera in Eastern Temperate Forests - Ecoregion 8. National Wildlife Federation, Gardening for Wildlife.
Few keystone plant genera support the majority of Lepidoptera species. Tallamy et al, Tallamy et al, Nature Communications volume 11, Article number: 5751 (2020).
The City of Toronto’s online PDF, Butterflies of Toronto, is informative and packed with photos of butterflies that are found in Ontario.
The Butterflies and Moths of North America (BAMONA) web site can help with identification, and has information on host plants.