A reader writes to ask whether glowing green nightcrawlers are safe for turtles to eat. We’ve written previously about glowing green nightcrawlers, also known as glowworms, but this reader’s specific query, which we can surmise relates to a desire to expand a beloved pet turtle’s palate, has not been previously addressed. To that end, we’ve prepared a refresher on the nature of growing green nightcrawlers, as well as their suitability for animal consumption (we assume the possibility of human glowworm consumption is not of widespread interest). As you’ll see, some glowworms can probably be fed to pets and some cannot.

To begin, it’s useful to clarify what is meant when one refers to glowing green nightcrawlers. Glowworms are commonly simple brown worms (either common earthworms or their cousin, the larger European nightcrawler) that have been treated with a special dye. Invented several years ago, this chemical can be placed in the nutrients that worms consume or absorbed through the skin. Once the worms ingest the dye, they glow a bright fluorescent shade of green about 24 hours later.

These types of glowing green nightcrawlers are often marketed as fisherman’s bait; the dye is designed to be non-toxic so that it can be fed to a pet turtle or any other animal that eats earthworms. Interestingly, the remarkable feat was not in making the dye non-toxic to fish, humans, or other animals, but rather to the glowing green nightcrawlers themselves; these worms are highly sensitive to chemicals and even artificial food coloring will kill them. In summary, earthworms that have been treated with artificial dyes should be safe for animal consumption, but we do caution that some dyes used in the past may have been mildly toxic, so if the origin of the worms is in question, it’s best to ask for clarification.

Glowing green nightcrawlers have been around far longer than human-made dyes, however. Traditionally, “glowing green nightcrawler” refers to any of a number of species of insect larvae (which are not worms, at least on the standard definition of “worm”) that exhibit a quality called bioluminescence. Bioluminescent glowworms produce light through a series of natural chemical reactions involving the chemical luciferin. Glowing green nightcrawlers of this sort are native to Europe, North and South America, Asia, and Australia and New Zealand, although in each instance they are the larvae of different kinds of insects (most commonly fireflies and beetles). The firefly larvae type of glowworms are found worldwide and make up dozens of species in the family Lampyridae.

In contrast to their artificial counterparts (which in general should be safe to eat by any animal), bioluminescent glowing green nightcrawlers are not usually suitable for use as pet food. The Lampyridae larvae are believed to use their characteristic green glow as a warning signal to predators (a concept known as aposematism), indicating their mildly toxic nature to these animals. (Interestingly, the adult fireflies probably use the green glow for an entirely different purpose: as a mating signal.) Although it isn’t certain that ingesting a bioluminescent glowworm would be toxic to a turtle or other pet, we recommend not making the attempt as a glowing green nightcrawler’s green glow could well serve as a warning to predators just as a poisonous toad’s spots do.

The bottom line: glowing green nightcrawlers should be safe for turtles and other animals to eat as long as they are merely earthworms treated with non-toxic artificial dye, and as long as that dye is not one of the older, toxic ones. Glowing green nightcrawlers made with previously available dyes, as well as those that exhibit bioluminescence, are likely not safe to eat. When in doubt, play it safe and stick to regular old brown worms.