Host Plants – What Are They & Why Are They So Important?

If you are new to native gardening you may have heard the term “host plant” thrown around or spoken about with importance. And, well, they are important. But what are they? Well, to put it simply a host plant is any plant that an insect lays it’s eggs on, or one that is eaten by it’s larvae.

You see, all the butterflies, moths, and various other insects have evolved to only lay their eggs on certain plants. And these plants are what their babies, i.e. caterpillars will eat. The most famous example of this in North America would be milkweed, and it’s job as a host plant for Monarch Butterflies. Monarch butterflies will only lay their eggs on Milkweed plants. If there is no milkweed, then there are no Monarch Butterflies.

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Why do butterflies only lay eggs on certain plants? Why not all?

Via the process of evolution butterflies learned that when their caterpillars ate certain plants, predators were less likely to eat the caterpillar. This happens because the baby caterpillar absorbs some quality from the chemicals in the leaves. This behavior became instinct, and overtime led to their species survival.

Often these are chemicals that give the caterpillar a strong or bitter aroma/taste, or in some cases a toxin that dissuades other insects or birds from eating the caterpillar. Like the aforementioned relationship between Monarch Butterflies and Milkweed….the Monarch Caterpillars eat milkweed leaves, and they absorb the toxins from the milkweed sap, making them less palatable or toxic to predators. Thus, it increases the chances of the caterpillar’s survival into the next generation of butterfly.

This evolutionary relationship is so strong that it has been shown in testing that lab-raised butterflies, who have never seen a host plant since pupating from a caterpillar will alite when exposed to the synthesized chemicals from their host plants. This has been repeatedly done on Black Swallowtails for the chemicals of their host plants of wild carrots and Poison Hemlock^[3][4][5][6].

Why host plants are important

To put all this information plainly, host plants are critical for a species survival. Without host plants many of butterflies, moths, as well as numerous other plant-eating insects (grass hoppers, many beetles) would have no way to feed their caterpillars and thus the species would die-out.

Most insect larvae can only eat a few host plants. And fewer still are tied to a single plant species. But as the research I’ve already mentioned shows is that it is the chemicals produced within the leaves that will determine what is a suitable host plant for a given species. And sometimes this means butterflies can be hosted by plants that aren’t even from their native range. The Black Swallowtail is probably the best illustration of this in that they are from North America and are hosted by natives such as Golden Alexander (Zizia aurea) and Mock Bishop’s Weed (Ptilimnium capillaceum). However, they can also be hosted by other exotic species such as Fennel (Foeniculum vulgare), Dill (Anethum graveolens), Poison Hemlock (Conium maculatum), and Wild Parsnip (Pastinaca sativa)^[5][6]. In fact these exotics are so easy to germinate that they are primarily what is used for larval/host research on Black Swallowtails in the lab rather than their native host plants!

And the proof? Well, I took the picture you see here of a Black Swallowtail caterpillar on Wild Parsnip I just happen to see in a roadside ditch a few years ago.

But beyond the caterpillars, we need to think of the next level of our complex food chain. Without the caterpillars, the birds would not have anything to feed their babies! Now you may be thinking, don’t they mainly eat worms? And well, yes, Robins and others will primarily eat worms. However, most are feeding their babies caterpillars. And in addition to that, the birds will actually feed on caterpillars all winter!

Believe it or not, many caterpillars will overwinter nestled in the bark of trees. And birds will walk up and down those tree trunks looking for dormant caterpillars to supplement their winter diet.

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What are the best host plants?

The best host plants are known as Keystone Species. First coined in the 1960’s, Keystone species are defined as plants that support a large portion of the ecosystem. When adapted to plants, it would be the plants that support the most pollinators within their type of plant (tree/shrub/vine/flower). And this is usually extended beyond pollinators to include other ways a plant can support the environment. For example, the acorns produced by Oaks also support squirrels, deer, and believe it or not, birds!

The most powerful host plants are trees, with the mighty oak trees hosting more than 500 caterpillars! Below is a listing of host plants and the number of insects they host:

Type	Common Name	Botanical name	Approximate number of insects hosted
Tree	Oak trees	Quercus sp.	500+
Tree	Black Cherry Tree	Prunus serotina	400+
Tree	Birch Trees	Betula sp.	200+
Tree	Maple Trees	Acer sp	200+
Shrub	White Meadowsweet (shrub)	Spirea alba	40+
Shrub	Common Snowberry(shrub)	Symphoricarpos albus	40+
Shrub	Blueberry	Vaccinium sp.	100+
Shrub	Spicebush	Lindera benzoin	10+
Flower	Sunflowers	Helianthus sp	60+
Flower	New York Ironweed	Vernonia noveboracensis	10
Flower	Black-eyed Susan	Rudbeckia hirta	8
Flower	Common Milkweed	Asclepias syriaca	5
Grass	Little Bluestem	Schizachyrium scoparium	14
Grass	Big Bluestem	Andropogon gerardii	9
Grass	Bottlebrush Grass	Elymus hystrix	7
Grass	Side-oats Grama	Bouteloua curtipendula	7

How come trees and shrubs host so many more species than flowers?

The answer to this question, as to why trees and shrubs can host so many species is a bit of a mystery. But one component may be as simple as available food supply. A mature tree will produce hundreds of pounds of leaves while it generally takes at least 3-5 flowers to support a few caterpillars. So, a single Oak can feed hundreds of caterpillars for hundreds of years, while a colony of flowers may only survive for a few years or decades before being overtaken. But the truth is we can’t know concretely, only speculate.

When it comes to Oaks and the amount they host, the answer to that question is a mystery. Doug Tallemy believes it might be due to just how long Oaks live and have been around from an evolutionary perspective^[7]. A single tree can live for hundreds of years, but the species has been around from thousands of years[dt], which provides a long time scale for an insect to adapt to the chemical defenses available in the leaves. Compare this with a colony of flowers that may only exist for some years (a decade?) before being overtaken by shrubs or trees in a general forest succession.

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How many host plants do I need to plant to have an impact?

The answer to this question will heavily depend on the type of host plant you are considering – tree, shrub, flower. If you are planting a single tree, that will make many leaves that can support many caterpillars. And the same would go for shrubs. Just one large shrub can easily support many caterpillars as well (but 2 or 3 would be better).

==>Related – The Best Oak Trees for Small Yards

When it comes to flowers though, such as Milkweeds, you will need more than a few to have an impact. While it is possible to get caterpillars on just a couple of milkweed plants, you will have many more if you plant 5-10. So, try to plant 5 plants close together to make sure you can support your local caterpillar population.

What are the best flowering host plants?

While trees and shrubs have the ability to host large numbers of species, most people can only manage a few of those in a regular yard. This begs the question, what are some of the best host plants that are flowers? The answer to that depends on where you live, but the table below should guide you in the correct direction (if you are in the United States/Canada).

Please note that where no specific botanical name is given, the number of species hosted is approximate for the whole family of plants. This is most notable for Wild Rose and Asters, which contain many plant species and do host many pollinators. However, any given species may only host 1-20 depending on your location.

Common name	Botanical Name	Number of species hosted	Region
Wild Rose	Rosa sp.	100+	North America
Asters	Symphyotrichum	100+	Central and Eastern North America
Wild Strawberry*	Fragaria sp	65+	North America
Wild Sunflower	Helianthus annus	50+	North America
Western Brackenfern	Pteridium aquilinum	28	Western and Central North America
Violets	Viola sp	20+	North America
Fireweed	Chamerion angustifolium	20	North America
Western Pearly Everlasting	Anaphalis margaritacea	20
Common Yarrow*	Achillea millefolium	15	North America
Evening Primrose	Oenothera biennis	15	Eastern North America
Joe Pye Weed	Eutrochium sp	10-15	Eastern North America
Black Eyed Susan	Rudbeckia sp	10-15	Central and Eastern North America
Yellow Wild Indigo	Baptisia tinctoria	7	Eastern North America

Conclusion

If one of your gardening goals is to improve your local environment, then planting host plants is just about the best thing you can do! As I stated, a single tree or shrub can have a huge impact in supporting the local caterpillars, but for flowers you will want to plant 5-10 close together to ensure you provide enough food. Nonetheless, when you are gardening selecting and planting host plants, try to make it visually appealing to inspire your neighbors to do the same.

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