Including Host Plants in Your Garden

If you plant it, they will come. Butterfly weed got its name because it is a good food source for monarch butterfly caterpillars.

On a late summer visit to a local garden, I was treated to an unusual sight. An entire clump of flowering plants had lost almost every leaf and was standing barren! What was going on? I had to take a closer look.

The plant bed in question was a clump of swamp milkweed (Asclepias incarnata). Monarch butterflies lay their eggs on milkweed plants. Chewing on the milkweed were a number of brightly colored caterpillars, feasting on every leaf they could find. This is gardening for butterflies at its best!

Many of us think of including colorful blooms in our gardens, to provide nectar to adult butterflies. However, most gardeners forget that butterflies need food in another part of their life cycle, as larvae. Butterfly eggs are tiny, and the larvae that hatch out of these eggs cannot travel far in search of food. To ensure that the caterpillars have a chance to survive, female butterflies lay their eggs on “host plants,” which the caterpillar can eat as food. Butterflies, like most insects, do not actively care for their young, so it is critical that they lay their eggs in the best spot possible, where there is plenty of food, and shelter from predators. 

The tiny caterpillars are eating machines that leave a trail of frass (caterpillar droppings). As they grow, their exoskeletons become tight, and must be shed before they get larger. Each growth phase is called an instar. Caterpillars of many species, such as monarchs, swallowtails and luna moths, can grow quite large, almost the size of a pinky finger. 

Once they are done growing, the larvae find a suitable spot to turn into pupa. In this phase, the caterpillar transforms into a winged adult. Many types of butterflies have specific places where they pupate, from under leaf litter to a sheltered tree branch. In this phase of the butterflies’ life cycle they are immobile, so they must have an undisturbed location, free of foot traffic, mowing and other hazards. Many butterflies will emerge in a few weeks, but there are species, such as the iconic tiger swallowtail, that spend a whole winter as pupae, before emerging as adults in spring. The adult butterflies survive on nectar from flowers. 

Caterpillars are adapted to eat native vegetation, and most cannot survive on exotic plants introduced from other parts of the world. As gardeners, we can include native host plants in our gardens to help create habitat. Trees such as native oaks (Quercus sp.) are hosts to hundreds of butterflies and moths, and are some of the most productive plants in a garden. Herbs such as fennel (Foeniculum vulgare), dill (Anethum graveolens) and parsley  (Petroselinum crispum) are hosts to the large, beautiful swallowtail butterflies. 

To encourage monarchs, whose numbers are dwindling around the United States, be sure to include native milkweed in large clumps in the garden. Remember, the caterpillars eat fast, so make sure there is plenty of food! Pennsylvania is home to three native milkweed species. Butterfly weed, (Asclepias tuberosa) have delicate leaves and bright orange flowers, while swamp milkweed is taller, with umbels of pink flowers and wispy seed pods. Common milkweed (Asclepias syriaca) has robust stems and leaves, and pink flowers.  

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Photo by Sarada Sangameswaran