Slugs have recently been observed feeding on milkweed plants in a local pollinator garden, resulting in significant leaf damage. The pests stripped the leaves from one young plant and were found eating two others before being removed by hand.
The gardener stated, “Sorry, slugs. We’re saving the leaves for the monarchs!” This highlights an ongoing effort to preserve milkweed for monarch butterflies, which rely on these plants for survival.
Despite milkweed containing toxic compounds known as cardenolides—named from the Greek word for heart and referring to their chemical structure—slugs appear unaffected by these toxins. Monarch caterpillars use cardenolides as a defense against predators, deterring most birds except for species like the black-headed grosbeak that can consume them without ill effects.
Eric Lee-Mäder, co-director of the Pollinator Program for the Xerces Society for Invertebrate Conservation and author of “The Milkweed Lands: An Epic Story of One Plant, Its Nature and Ecology,” noted that planting milkweed attracts a wide range of insects beyond just monarchs. He wrote, “If you welcome this plant (milkweed), a diverse, living fan base will follow close behind–and not only monarch butterflies.”
Gardeners who cultivate milkweed often see various insects such as aphids, lady beetles, honey bees, bumble bees, soldier beetles, longhorned beetles, tussock moth caterpillars, cabbage white butterflies, earwigs, spiders, assassin bugs, praying mantises, snails—and slugs—feeding on or around the plants. Slugs are typically nocturnal feeders and leave slimy trails along with chewed leaves or holes.
According to the UC Statewide Integrated Pest Management Program (UC IPM), snails and slugs are common garden pests that emerge at night to feed on many types of garden plants. The program explains that effective management requires removing moisture and hiding places in addition to trapping or handpicking; baits alone are often insufficient if shelter and food are abundant. UC IPM also lists several slug species that commonly harm landscape plants—including gray garden slug (Deroceras reticulatum), banded slug (Lehmannia poirieri), three-band garden slug (L. valentiana), tawny slug (Limacus flavus), and greenhouse slug (Milax gagates).
UC IPM further notes that slugs reach maturity in about three to six months depending on species and lay eggs beneath leaves or in soil cracks.
Some gardeners attempt chemical-free control using beer-baited traps because slugs are attracted to yeast odors; however, in this instance the gardener reported manually removing seven slugs instead: “Sorry, slugs. We’re saving the leaves for the monarchs!”
