The Garden Detective: Caterpillar or Worm—Is It Good or Bad?

Q:  “Should I squish the caterpillars that are eating my mock orange shrub, or will they become butterflies that I want in my garden,” asked the HelpLine caller.

A:  After getting a description and searching lots of references, I told the caller she had sawfly larvae! And, no, she probably didn’t want to raise a new generation of sawflies. 

What is a caterpillar? What is a worm?
Typically, we think of caterpillars as larvae of butterflies, moths, and skippers (Lepidoptera), and worms as segmented legless critters that live in the ground (Annelida), but it’s complicated! Both terms are applied to many creatures that aren’t members of these particular parts of the animal kingdom. We call various other insect larvae—including sawflies (Hymenoptera)—caterpillars. We also include “worm” in the common names of lots of garden insects that are not worms, but actually in the order Lepidoptera, too: armyworms, bagworms, cabbageworms, cutworms, hornworms, inchworms, and webworms. Wireworms are click-beetle larvae (from the order Coleoptera).

Good or bad?
All of these critters are going to eat things in your garden, and many are considered garden pests. Identifying them is critical to knowing what they will eat and if you want to let them be—or not! Even the larvae of the butterflies that we encourage as pollinators eat their host plants. We want earthworms in our compost, but some worms are destructive! So, good or bad?

 Caterpillar ID
It’s mostly about the “legs.” Butterfly and moth larvae have 3 pairs of legs and 2 to 5 pairs of prolegs (leg-like parts). You know they aren’t wood-boring larvae if the prolegs are armed with hooks (crochets). Sawfly larvae have at least 6 pairs of prolegs, and no crochets.

You may need to provide host plants to feed the butterflies and moths you want. You might not typically grow what desirable caterpillars eat—or you might not grow enough to share!

• Monarch butterfly larvae (Danaus plexippus) eat milkweeds (Asclepias). Butterfly weed (Asclepias tuberosa) is one attractive milkweed that will fit easily into most landscapes.
• Swallowtail butterfly larvae eat a variety of plants, depending on the species.
• Black swallowtails (Papilio polyxenes) feed on many garden crops, such as parsley and dill, while others feed on tree, shrub, or vine leaves.
• Zebra swallowtails (Eurytides marcellus) feed on pawpaw trees (Asimina triloba).
• Eastern tiger swallowtails (Papilio glaucus) can eat the leaves of many trees, including tulip poplars (Liriodendron tulipifera) and willows (Salix).
• Spicebush swallowtails (Papilio troilus) need spicebush (Lindera benzoin) and/or sassafras (Sassafras albidum).
• Pipevine swallowtails (Battus philenor) feed on pipevines (Aristolochia).

  

  

  

  

  

  

Not all moth and butterfly larvae are our friends. Many are major vegetable garden pests:

• White cabbage moths (Mamestra brassicae) produce cabbageworms that eat all members of the Brassica family: broccoli, Brussels sprouts, cabbage, cauliflower, and many greens, such as bok choy, collards, kale, mustard, and the tops of turnips and rutabagas.
• Hawk/hummingbird/sphinx moths (Manduca) produce tomato and other hornworms, that eat tomatoes, tobacco, and their relatives.
• Sesid moths (Melittia cucurbitae) produce squash borers that can destroy your summer and winter squash crops, as well as pumpkins and some gourds.

Worm ID

• Most of the soil-dwelling worms gardeners see are light-colored European earthworms belonging to the genus Aporrectodea. Earthworms are great at decomposing plant wastes. Their own wastes and their tunneling add nutrients and help aerate and drain soils.
• Burrowing worms. “Nightcrawlers” (Lumbricus terrestris) are also European imports that burrow deeply, but clear areas near their burrow entrances at night, often creating noticeable mounds of food material.
• Litter-dwelling worms. These include the “red-wigglers” (Eisenia foetida) used by vermicomposters. Red wigglers are not “cold hardy” in our climate, but the litter-dwelling Asian jumping worms (Amynthas agrestis) can survive outdoors and are seriously damaging forest habitats—including Great Smoky Mountains National Park!
• Worm-like garden dwellers. Not all critters living in the soil are worms. Grubs (immature Japanese, June, and other beetles) and root maggots (fly larvae) are “bad” soil-dwelling pests that you may encounter in your garden. Grubs are usually white/gray, fat, C-shaped larvae, often found under turf. Various root maggots (Delia) attack the roots of Brassica crops, corn, and onions. Similarly, carrot rust fly larvae (Psila rosea) devour carrot roots, and sometimes celery, celeriac, coriander, dill, fennel, and parsley roots, too!

Control issues
A final note—if you choose to use pesticides to manage garden damage—be careful about using Bt (Bacillus thuringiensis) for controlling caterpillars! It will indiscriminately kill all your caterpillars, not just the undesirable ones.

Article written by Debbie Green, Extension Master Gardener^SM Volunteer.

More information
Field Guide to the Southern Piedmont
by Jonathan J. Storm, Briget C. Doyle, Rachel V. Furman, Julie M. Smoak, & Melissa A. Storm
Field guide from South Carolina that includes many insects and plants we have in Western North Carolina. Good photographs and descriptions.

Jumping Worm Field Guide
from the Wisconsin Dept. of Natural Resources
Compares the European nightcrawler with the jumping worm.

Garden Insects of North America (2^nd Edition)
by Cranshaw, W. and D. Shetlar (2018). Princeton, NJ: Princeton Press.