How Did Hammerhead Worms Get to the US?

Hammerhead worms are predatory land flatworms recognized by their spade-shaped heads. These elongated, unsegmented creatures can vary in size, sometimes growing over a foot long, and are typically light brown with one or more dark stripes. They are an invasive species established in various regions across the United States.

Native Origins and Global Spread

Hammerhead worms are native to tropical and subtropical regions of Southeast Asia, with some species like Bipalium kewense originating from areas such as Vietnam and Cambodia. From their native range, these flatworms spread globally through human-assisted means. They frequently hitchhiked across continents via the international trade of goods, particularly in the soil of potted plants and nursery stock. This allowed them to establish populations worldwide, including Europe, prior to their widespread presence in the United States.

Pathways to the US

The primary method by which hammerhead worms arrived in the United States is through the horticultural trade. They were likely introduced unintentionally in the soil or root systems of imported potted plants and nursery stock. Historical records suggest their introduction to the U.S. occurred as early as 1891 or 1901. Once introduced, their spread within the country continued through similar means, such as the movement of potted plants, mulch, and other gardening materials.

Current Presence and Ecological Impact

Hammerhead worms are currently found in numerous U.S. states, particularly thriving in warmer, humid environments of the Southeast and along the East Coast, though they have also been reported on the Pacific Coast. They prefer moist soil habitats with organic matter, such as gardens, leaf litter, and under rocks or logs. Their presence raises concern due to their predatory nature; they prey on earthworms, slugs, and snails. Earthworms are important for maintaining healthy soil structure, fertility, and nutrient cycling, so their predation can impact ecosystems.

Some species produce tetrodotoxin, a neurotoxin also found in pufferfish, which they use to subdue their prey. While this toxin is not typically harmful to humans through touch, it can cause skin irritation, and ingestion can lead to discomfort or illness in pets. These worms are also known for their remarkable regenerative abilities; if cut into pieces, each fragment can regenerate into a new, complete worm.

Managing Their Spread and Reporting

If a hammerhead worm is encountered, avoid cutting it, as this can lead to regeneration and an increase in their population. Recommended methods for disposal include placing the worm in a sealed container with salt, rubbing alcohol, hand sanitizer, or soapy water, or freezing it. Wearing gloves is advised when handling them to prevent skin irritation from their mucus. Reporting sightings to local extension offices, state departments of agriculture, or citizen science platforms like iNaturalist helps researchers monitor their spread and distribution.