Worms that eat celery: caterpillars on celery plants are a pest

Would you be surprised to learn that celery worms are the caterpillars of the black butterfly that is the swallowtail? Gardeners often regret sending out butterfly caterpillars more than they regret exterminating garden insects or spiders. In this article, you will discover how to handle these interesting creatures in the garden.

What are celery worms?

Larvae of the eastern black butterfly ( Papillo polyxenes asterius ) are sometimes seen in the orchard chewing celery, parsnips and carrots. They can also be seen in the garden where they feed on dill, parsley and fennel. Their appearance changes according to their life stage. Young celery worms can look like bird droppings. As they grow older, they develop dark and light stripes with bright yellow spots.

One of its most striking features is a bright orange osmeter, which looks like a pair of horns or antennas. They keep the structure hidden behind their heads, but can bring it out into the open when they feel threatened. At the same time, they give off an unpleasant smell. If this is not enough to warn predators, they can throw balls of faeces with their jaws.

Controlling worms in celery or leaving it as a host plant?

Finding these «worms» that eat celery poses a dilemma for gardeners: should they leave them alone and risk losing their crop, or should they exterminate them? One thing that can reassure you is that, although many species of butterflies are endangered, the tail of the eastern purple martin is safe. Killing a few caterpillars in the garden won’t slow the species down.

On the other hand, caterpillars on celery plants may not indicate a serious problem. Swallowtails do not congregate in large numbers like some butterflies, so only a few larval worms may be found in celery. Why not keep a close eye on them to see if they do any real damage?

Whether they choose celery as the host plant or one of the other members of the carrot family, the control is the same. If there are only a few, you can pick them by hand. Use gloves and drop the caterpillars into a jar of soapy water to kill them.

If you find hand-picking particularly unpleasant, you can spray them with Bt (Bacillus thuringiensis), which kills the caterpillars by preventing them from digesting the food. It takes a few days for the caterpillars to die, but they no longer feed on your plants. This method is best used on young caterpillars. Try using a neem spray on older caterpillars.