What are host plants and why do we need them?

Asters (Symphyotrichum spp.) are attractive larval host plants that can brighten central Pennsylvania landscapes.

“What are host plants?” was a question I received while teaching a recent Master Gardener workshop. It’s a very good question and those who are interested in helping pollinators, such as bees, moths, and butterflies, may find the answer particularly useful.

Let’s use butterflies as an example to answer the question. Most people think of butterflies as the beautiful, delicate creatures we see flying around our gardens and stopping to sip nectar from flowers. Butterfly youngsters, however, look quite different and many have very specialized food requirements which can only be met by host plants.

Butterflies reproduce through a process known as metamorphosis. Females lay tiny eggs that become larvae, often called caterpillars. The caterpillars eat, grow, and shed their skins, which is called molting. The caterpillars of some butterflies molt several times, growing larger each time. The last molt, or skin shedding by the caterpillar results in the formation of a pupa or chrysalis, which hangs from a leaf or branch as the body inside changes to form the legs, wings, and antennae of a butterfly. It is from this chrysalis that the butterfly emerges.

Many butterfly caterpillars are very selective about the food they eat. For example, monarch butterfly caterpillars are only able to feed on milkweed (asclepias, spp.). No other plants are suitable. Female monarchs lay their eggs on milkweed plants so that, when eggs become caterpillars, they have a readily available food source.

Another beautiful butterfly, the great spangled fritillary, also has very specific dietary requirements. Caterpillars that will become great spangled fritillaries rely on violets as their only source of food.

PHOTOS PROVIDED At left, Redbud (Cercis canadensis) is a larval host tree that provides a beautiful spring display.

Other butterfly larvae, while still selective, have a wider variety of food sources on which to depend. For example, eastern tiger swallowtail caterpillars can feed on tuliptree (Liriodendron tulipifera), wild black cherry (Prunus serotina), and ash (Fraxinus spp.).

Caterpillars that will become Henry’s Elfin butterflies feed on redbud (Cercis canadensis), American holly (Ilex opaca), and blueberries (Vaccinium spp.).

Plants that meet the specific dietary requirements of these caterpillars are called larval host plants, or simply host plants. They are essential to the butterfly life cycle, and therefore an important part of a landscape designed to support butterflies.

It is for this reason that Master Gardeners encourage those who want to help pollinators to include host plants in their yards and gardens. A list of larval host plants for butterflies native to Pennsylvania is available at https://extension.psu.edu/programs/master-gardener/counties/york/maescapes/maescapes-blog/butterfly-larval-host-plant-list.

In addition to meeting the needs of caterpillars, many host plants can also be attractive additions to a landscape.

Asclepias tuberosa, commonly called butterfly weed, can provide a bright yellow-orange blanket of color when massed in a garden bed.

In spring, violets provide a carpet of purple in the pastures around my home.

Liriodendron tulipfera, or tulip trees, become tall shade trees and bear pale yellow flowers with orange centers when mature.

Black cherries (Prunus serotina), are covered in white blossoms in spring and produce fruit for wildlife to enjoy.

A redbud (Cercis canadensis) with its branches covered in tiny rose-pink flowers makes a striking springtime specimen.

These are just a few of the many host plants we can add to our landscapes and, in so doing, provide food for the caterpillars which will become the butterflies we enjoy.

For additional information, please contact the Penn State Extension office in Clinton County at 570-726-0022.

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Debra Burrows, PhD, is a retired Penn State Extension educator. She can be reached at dcb3@psu.edu.

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