What's Eating Your Herbs?

Herbs are often regarded as iron clad where pests are concerned, because the fragrant volatile compounds in leaves make them unattractive to animals like rabbits and deer. Yet herbs do have insect enemies capable of causing serious damage. Here are five common herb-eating pests, including two that mature into beautiful butterflies, and the herbs they’re most likely to be found munching.

Mint – Four-Lined Plant Bugs

Members of the mint family are famous for their ability to thrive almost anywhere, but every plant has a hidden weakness. Appearing in May, four-lined plant bugs (Poecilocapsus lineatus) feed on a long list of plants, including dahlias, lavender, geranium and zinnia, but their favourites are the mints. Feeding by plant bugs causes numerous small round spots to form where the leaves were pierced by the insects’ sucking mouthparts.

Native to much of North America, the four-lined plant bug is most active in late spring when the tiny red nymphs are actively feeding on tender new growth. As the bugs mature, they become yellow with four black stripes. At all life stages, plant bugs are fast movers that drop to the ground or to lower branches when disturbed, making them impossible to hand pick. Four-lined plant bugs feed for only a few weeks, so most gardeners deal with them by trimming back damaged branches in early summer, which has the added benefit of encouraging new growth.

Parsley, Dill and Fennel – Parsleyworms

Another herb consumer in North American gardens, parsleyworms are eager eaters of parsley, dill, fennel, and carrot tops. They are the larvae of the beautiful Eastern black swallowtail butterfly (Papilio polyxenes), so most gardeners regard parsleywoms as more of a blessing than a curse. Various black swallowtail subspecies range from Canada to South America. It is the official butterfly of the states of Oklahoma and Missouri.

Should you have too many parsleyworms feeding on the dill you are growing for making pickles, it’s easy to move them to an alternate host plant such as fennel, parsley, or even wild carrot (Queen Anne’s lace). Clip off the branch with the caterpillar on it and quickly place it among foliage of its new host plant. The best time to move parsleyworms is midmorning, just as they are starting their day of nonstop eating.

Borage and Many More – Painted Lady Butterfly Caterpillars

Seen throughout much of the world, painted lady butterflies are famous for being seasonal migrants. Many travel from Africa to Europe in May, though they tend to fly so high that their highways in the sky are difficult to trace. In North America, painted ladies wait out winter along the US-Mexico border and fly north in late spring, often in a northeasterly direction.

Painted lady larvae make use of a long list of host plants, including thistles, hollyhocks, borage and other plants that are too hairy or prickly for other pests to consider. Feeding season is early summer, when the spiny black and yellow striped caterpillars cover themselves with a light silken web as a defence against birds, wasps, and other predators. Though they look scary, the caterpillars cannot sting. Should they appear on your prized hollyhocks, you can safely move them to a robust clump of borage or thistles where a few leaves won’t be missed.

Basil – Garden Earwigs

A couple of warm, wet summers is all it takes to help garden earwig populations build to the point where they occupy every nook and cranny of your deck, patio or raised beds. When basil shows random holes with smooth edges or young dill plants are skeletonised overnight, the culprits are likely earwigs, which feed at night and hide in sheltered spots during the day.

As explained in detail in this earwig article, earwigs are easy to trap in small containers baited with cooking oil and soy sauce, or you can try various habitat traps. Do check out the comments thread here if you’re icked out about earwigs and think you have a serious problem. You are not alone!

Rosemary, Lavender and Sage – Rosemary Beetles

Native to the southern Mediterranean, North Africa and the Middle East, green-and-purple striped rosemary leaf beetles (Chrysolina americana) made their first garden appearance in the UK in 1994, and have since spread throughout much of Europe, from Serbia to Cyprus.

Groups of this small metallic beetle can strip leaves from rosemary, lavender and sage in a few days, and they will eat thyme plants, too. Rosemary beetles can complete their life cycle right in the garden because they pupate in soil around plants. Shake beetles onto a ground cloth or into a shallow box for collection, or tap individuals into a bowl of soapy water on cool mornings. Replacing mulch at the end of the season can reduce the number of ground-dwelling rosemary beetle larvae present in the herb garden in winter.

Plants Related to this Article

Lavender Grow Guide

Mint Grow Guide

Lemon Balm Grow Guide

Bugs, Beneficial Insects and Plant Diseases

Earwig Guide

Rosemary Leaf Beetle Guide