How to Protect Your Flowers From Harmful Garden Caterpillars

Swallowtail Caterpillar, Japan

TOSHIAKI ONO / Getty Images

Butterflies are always welcome in the flower garden. We plant nectar-rich butterfly flowers, hang up butterfly décor in the garden, and snap photos of butterflies on our flowers to look up in field guides later. However, when butterfly babies (i.e. garden caterpillars) show up in the garden, they’re likely to be on the receiving end of a spray can full of insect killer.

Some caterpillars turn into moths while others turn into butterflies. Some caterpillars are pests; others are butterflies-to-be. How can flower gardeners discern one caterpillar type from another? How can we accommodate controlled munching from butterfly babies while ridding our gardens of uninvited gluttons?

Signs of Caterpillar Activity

Although we’ve all seen caterpillars in the garden, there’s usually more activity going on than meets the eye. Many caterpillars sport camouflage that helps them blend in with their host plant, but some even look like bird droppings, making them easy to overlook. You don’t have to wait until your plants are completely defoliated to determine the presence of caterpillars at work. Look for:

  • Eggs: The best way to distinguish caterpillar eggs from other insect eggs is to watch a butterfly crawling around on a host plant. As she moves across the host plant, look for the tiny specks she leaves behind. They may be laid singly or in clusters, and colors vary from white to yellow to green or brown.
  • Leaf holes: Chewing insect pests like beetles usually leave holes in the middle of foliage, but caterpillars start at the leaf’s edge and work inward. Look for ragged or scalloped leaf edges.
  • Frass: This is the fancy term for “caterpillar poop.” Frass looks like pepper grains, deposited on the foliage adjacent to actively feeding caterpillars.

Garden Caterpillar Identification

Gardeners interested in attracting butterflies to the garden must know how to identify their larvae or else risk decimating the next generation of butterflies with a round of pesticides. If you’re unfamiliar with the larval visitors in your landscape, you should consider purchasing a caterpillar field guide. A good guide for beginners is the Peterson First Guide to Caterpillars of North America from the Peterson First Guides series, which isn’t comprehensive for North America but can certainly expand your knowledge beyond the tomato hornworm and the “pretty stripy one.” The illustrations of 120 caterpillars include the adult form, and the text includes information about habitat, host plants, and defense mechanisms.

Most gardeners are familiar with and interested in the Monarch butterfly, which is a migratory species in decline due to habitat loss, pesticide use, and other factors. Look for Monarch caterpillars on milkweed plants, which caterpillars feed on to infuse their bodies with plant toxins as a protective mechanism. Two-inch mature caterpillars are conspicuous with their yellow, white, and black stripes.

Fun Fact

Milkweed provides nurseries for monarch butterflies, and it's important to keep supporting monarch butterfly habitats as they are in decline.

Host Plants of Egg-Laying Butterflies

Fortunately for flower gardeners, caterpillars are very picky about what they eat. This can help you to identify the “good” caterpillars according to the plant they’re feeding on. You can also plant-specific host plants to attract caterpillars that turn into butterflies. Find a place for these flowering host plants, and look for these caterpillars in action:

  • Butterfly weed: Monarch
  • California lilac: California Hairstreak
  • Dogwood: Spring Azure
  • Hollyhock: Painted Lady and Gray Hairstreak
  • Lupine: Boisdulval’s Blue
  • Mallow: Painted Lady and Skippers
  • Spicebush: Spicebush Swallowtail
  • Violet: Great Spangled Fritillary

Caterpillar Pest Control

Perhaps you have both the invited and uninvited species of caterpillars in your garden. How can you get rid of one without harming the other? Even organic caterpillar control methods, like Bacillus thuringiensis, are harmful to butterfly larvae.

The easiest way to get rid of unwanted caterpillars is to handpick them, wearing gloves for the “ick” factor. Drop the pests into a bucket of soapy water as you pluck.

You'll know if cutworm caterpillars are an issue if your seedlings are felled as if by tiny lumberjacks overnight. Prevent cutworm pests from feasting on your seedlings by surrounding them with cardboard toilet paper tubes or plastic cups with the bottoms cut out.

For vegetable crops, you want to protect from caterpillars, use floating row covers. This is a great way to protect part of a crop you don’t want to share, like fennel or parsley, while sacrificing the rest to a desirable caterpillar species that will later be fluttering around your flowers.