Recently I read a book that has been making waves in gardening circles, Douglas Tallamy’s Bringing Nature Home. I believe I can summarize his main points thus: Wildlife, including insects and birds, are dependent on suburban gardens for habitat because development and other disturbances have resulted in widespread habitat loss and degradation; and that typical garden plants do not provide food for the insects and other arthropods that form the foundation of the food web for the larger animals like birds and reptiles. It was an interesting read with some caveats. One, it’s written almost entirely for gardeners in the Mid-Atlantic states, relying heavily on his personal observations in his own gardens in Delaware and Pennsylvania, and discussing the non-native garden plants and invasives common in that area. He treats Douglas-fir as a nonnative! Which it is there, but not for us here. Two, his uses of statistics don’t always ring true-his assumptions aren’t necessarily valid, and his numbers don’t always add up. For instance, he cites the USDA statistic that 41.4% of the land area in the US is in agriculture, and jumps to the conclusion that therefore the rest is urban. I’m pretty sure there are other land use categories besides agriculture and urban! And there’s different degrees of urbanization, from the density of Manhattan to typical Washington rural residential. He also cites ‘ecologists’ as estimating that only 3-5% of the land area in the US is undisturbed, which will depend of course on your definition of undisturbed.

Tallamy wants us all to plant our gardens with native plants, because native plants are more likely to support robust populations of insects and other invertebrates, and most of the animals we think of as ‘wildlife’ depend on these insects and other invertebrates for their food supply, either directly, as with birds feeding their nestlings (most all birds eat bugs, primarily caterpillars, as nestlings, whatever the adults may eat), or indirectly, with other animals eating insectivores. Basically, insects turn plants into bird and reptile and fish food. He cites anecdotal evidence from his own observations, and research by his college students, that insects etc. are declining overall (not just butterflies or bees) and believes this is a result of habitat loss through widespread planting of nonnative plants, and the invasion of more natural areas by nonnative plants. By and large, most insects don’t eat nonnative plants because they are not co-evolved; the plants have phytochemicals to deter herbivores and only co-evolved herbivores will be able to eat them. Monarchs and milkweeds are a well-known example of this; few animals can eat milkweed other than Monarchs. This is, of course, a generalization and we will cover some exceptions in a later issue. Speaking strictly for the Mid-Atlantic area, he states that although the Norway Maple has been in this country for over 300 years, there are still essentially no insects that eat it here, while native maples support 285 species of Lepidoptera; 300 years is too short for co-evolution to have occurred. And Clematis vitalba, a nonnative weedy clematis that is increasingly common here in Washington, supports 40 species of insects in its native range in Eurasia, but only 1 species in North America (he doesn’t say what that one species is though, or how long C. vitalba has been here). You can see the problem, even if his numbers are not 100% correct. He goes so far, in fact, as to compare nonnative plants to statuary-ornamental, but sterile and lifeless.

What is meant by insect/plant co-evolution? Most all plants create phytochemicals of one kind or another to deter herbivores, for self-defense. Phytochemicals are what give plants their flavors and fragrances. These phytochemicals have a variety of effects. Some taste bitter, some bind up the nutrients in your gut so you starve, some poison the nervous system or heart. Digitalis in foxgloves, pyrethrin in chrysanthemums, nicotine in tobacco, salicylic acid in willows, tannins in oaks and conifers, are all phytochemicals to deter herbivores. In response, herbivores like insects evolve to be able to tolerate or detoxify the particular phytochemicals in a particular group of plants. Many mammalian herbivores have the ability to at least partially detoxify many different phytochemicals, and rely on eating a varied diet to avoid overloading on any one toxic phytochemical. Most insect herbivores, on the other hand, have evolved the ability to more thoroughly detoxify or sequester a limited range of phytochemicals, so they specialize on plants with those chemicals. Makes sense, since most mammals can move around a lot more than caterpillars, and thus sample a larger variety of plants than caterpillars can. Besides Monarchs on milkweeds, there’s also checkerspots on paintbrush and Mourning Cloaks on willows. A few insect species, like the various Tiger Swallowtails, can eat a wide variety of plants, but only about 10% of insect species are considered to be generalists. Other species that appear to eat a variety of plants are eating plants that share the same phytochemicals. But just being in the same genus isn’t enough to make different species of plants edible, as in the Norway Maple example above-the nonnative garden sweet violet, Viola odorata, is inedible to greater fritillaries, which rely on native violets; and the California Sister, which eats the oaks common in California and Oregon, can’t eat our only native oak. And as we see from the Norway Maple example, co-evolution takes a long time.

So, to follow David James’ exhortation above to Support your Local Caterpillars, you should plant more native plants in your garden. To that end I have put together a short list of Washington native plants that are known to be good host plants for our more-common butterfly caterpillars, drawn from the book Landscaping for Wildlife in the Pacific North- west, by Russel Link. Keep in mind that not all butterflies will occur in your area-no Monarchs west of the Cascades (sorry, Seattle), and not all native plants occur in all parts of the state-no Bigleaf Maples east of the Cascades (sorry, Spokane). Before you go launching into a total reworking of your landscape, you might want to take a look at Eileen Starks’ Real Gardens Grow Natives, which was reviewed by Joyce Bergen in our March 2015 issue. The book covers ecological landscaping principles, designing for wildlife, soil preparation and planting techniques, and wildlife-friendly garden maintenance. Also, recently out from Xerces, is Gardening for Butterflies. It’s a beautiful and inspiring book, and while not written exclusively for the PNW, it does cover our area very well. And, it is exclusive to butterflies, not wildlife in general like the above books, with information on garden design, plant selection, and garden maintenance- turns out most of our traditional garden maintenance activities are devastating to butterflies and other insects.

Keep in mind this list is for plants that caterpillars eat, not nectar sources, which will be covered in a later issue.

Maple: bigleaf, vine, Douglas – Pale and Western Tiger Swallowtail, Mourning Cloak

Hawthorne: black, Columbian – Pale Tiger Swallowtail, Mourning Cloak, Gray Hairstreak

Cherry: bitter, choke – Pale and Two-Tail Tiger Swallowtail, Spring Azure, Lorquin’s Admiral

Willow: many species – Tiger Swallowtail, Lorquin’s Admiral, Mourning Cloak, Viceroy

Ceanothus: redstem, snowbrush – Pale Tiger Swallowtail, California Tortoiseshell, Brown Elfin

Oceanspray – Pale Tiger Swallowtail, Gray Hairstreak, Spring Azure, Lorquin’s Admiral

Pearly Everlasting – Painted Lady

Western Bleeding Heart – Clodius Parnassian

Lomatium: many species – Anise and Indra Swallowtails

Eriogonum: different species – Euphilotes blues, green hairstreaks

Lupine: many species – Silvery and Boisduval’s Blues, Western Sulphur

Stinging Nettle – Red Admiral, Painted Lady, Milbert’s Tortoiseshell, Satyr Anglewing

Kinnikinnick – Brown Elfin

Salal – Brown Elfin, Spring Azure

Bitterbrush – Brown Elfin

Milkweeds – Monarchs

Violets: early blue, stream – Greater fritillaries, Western Meadow Fritillary

Thistles: any – Painted Lady, Mylitta Crescent

Grasses: most any – Common Ringlet, wood nymphs, grass skippers

Pine: lodgepole/shore western white, Ponderosa – Pine White

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