Recent jumping worm outbreaks have caused a lot of anxiety and worries in New England in the 2020-2021 seasons. Yet another invasive worm species, hammerhead worms, was sighted in New England in 2021.
Hammerhead worms (AKA arrowhead worms) belong to phylum Platyhelminthes (flat worms), class Turbellaria (non-parasitic flat worms), family Geoplanidae (land or terrestrial flat worms). Flatworms (Platyhelmintes) is a relatively diverse group of organisms which for the most part live in aquatic habitats. Some of them have adapted to survive in the terrestrial environment (family Geoplanidae), but still prefer areas with high soil moisture, not exposed to sun. Land flatworms are predators with a relatively broad host range. They can feed on slugs, snails, soil dwelling insect larvae, and earthworms. They have been reported as a pest in earthworm farms/production facilities because of accidental infestation and destruction of the colonies.
Among terrestrial flatworms, hammerhead worms (subfamily Bipaliinae) are the most spectacular, attracting attention by their size, sometimes bright colors and the flattened, half-moon shape of their head. Hammerhead worms are known in many parts of the world, but predominately they are native to tropical and subtropical regions (predominately Asia). Several species of the genus Bipalium were introduced to the United States, most likely with horticultural plants and soil media.
Bipalium kewense (Fig. 1 at left), the largest species, can reach up to 10 inches, has a pale yellow body and five longitudinal lines on its back. Sometimes the worm coloration is brown, which can mask the appearance of the stripe pattern. This species has been established in at least 12 southern states since the 1950's and can thrive in outdoor habitats (Ogren 1984). Mild winters, warm summers and an abundance of rainfall are necessary conditions for species survival. Bipalium kewense is not cold-tolerant and cannot survive harsh winters. Even though, the species has been recorded in some northern states (Pennsylvania, Maine, and Massachusetts); however, it can only survive in greenhouses, botanical gardens, and conservation facilities where it can be protected from harsh winter temperatures.
Bipalium pennsylvanicum has been found and described from Pennsylvania (Ogren 1987, Ogren and Sheldon 1991). This species is capable overwintering outside, is more cold-tolerant than B. kewense, and has more potential to invade other northeastern states. This species has three distinct black stripes on the back.
A smaller species, Bipalium adventitium (Fig. 2 at left), can reach about 6 inches and has a yellow to light brown body with one line on the back. This species was introduced into the U.S. presumably from Japan (based on a similar species present in Japan) and transported to California with soil and horticultural plants (Ogren 1984). It has become widespread across the country and is commonly sighted. It has been found in some Northeastern states (New York, Massachusetts, and Connecticut) and even in Canada (Klots, 1960, Ogren, 1984, Ducey and Noce 1998, Justine 2019).
The wet summer of 2021 and several relatively mild winters in the eastern U.S. were likely the reasons for the increased frequency of sightings of these hammerhead worms on the East coast, including New England (O. Kostromytska, personal observation). The specimens collected in Massachusetts are most likely Bipalium adventitium based on visual identification. According to weather patterns and the Bipalium range spread, it is likely that in the future sightings of hammerhead worms will be more frequent. This might be concerning from an ecological perspective. These worms are invasive and can potentially be harmful for local ecosystems. Hammerhead worms are predators with a somewhat wide range, but they prefer earthworms as prey. In the southern states, where many native earthworm species are present, the expansion of land flat worm populations is bad news. In New England, where most of the earthworm species we have are introduced and the jumping worm population are thriving, these predatory worms are less of a concern. The hope is that the prey-predator cycle will balance the population of both species in the end. This might be true for the European earthworms so prevalent in our landscapes; however, so far, the efficiency of the hammerhead worms as a predator of jumping worms (Agrestis and Methaphire spp.) is questionable. A study by Gorsach and Owen (2014) showed that Bipalium adventitium was not an efficient predator of Amyntas agrestis. The reason might be ability of the jumping worms to defend themselves by active movements. When Bipalium spp. attacks an earthworm, it attaches itself to the earthworm body, which the earthworm can dislodge by active thrashing. The predatory worms then release a toxin to subdue their prey (Ducey et al. 2009). European earthworms might be less effective in this self defense strategy and therefore might be a preferred food for Bipalium spp. compared to jumping worms. The role of the Bipalium spp. in ecosystems and their efficiency in reducing jumping worm population densities is still to be determined.
The ability of Bipalium worms to produce the neurotoxin tetrodotoxin raises concern about their potential hazard to humans and pets. According to recent studies, these worms use the toxin as a defense and to subdue their prey which can sometimes be larger in size. The toxin is distributed around the worm body with disproportionally higher concentration in their head (Stokes et al 2014). Tetrodotoxin is one of the most potent toxins and acts on the nervous system. These worms do not produce a high enough concentration to be lethal to humans, but it can cause skin irritation and make pets sick if they eat these worms. Therefore, caution is advised when handling these hammerhead worms (should be avoided), and hands should be washed with soap after contact.
References
Ducey, P. K., & Noce, S. (1998). Successful Invasion of New York State by the Terrestrial Flatworm, Bipalium adventitium. Northeastern Naturalist, 5: 199–206.
Ducey, P., Messere, M., Lapoint, K. & Noce, S. (2009). Lumbricid Prey and Potential Herpetofaunal Predators of the Invading Terrestrial Flatworm Bipalium adventitium (Turbellaria: Tricladida: Terricola). The American Midland Naturalist. 141. 305-314.
Gorsuch, J. P., & Owen, P. C. (2014). Potential Edaphic and Aquatic Predators of a Nonindigenous Asian Earthworm (Amynthas agrestis) in the Eastern United States. Northeastern Naturalist, 21: 652–661.
Klots, A. B. (1960) A Terrestrial Flatworm Well Established Outdoors in the Northeastern United States, Systematic Biology, 9: 33–34.
Ogren, R. E. (1984). Exotic Land Planarians of the Genus Bipalium (Platyhelminthes: Turbellaria) from Pennsylvania and the Academy of Aatural Aciences, Philadelphia. Proceedings of the Pennsylvania Academy of Science, 58: 193–201.
Ogren, R. E. (1987). Description of a New Three-Lined Land Planarian of the Genus Bipalium (Turbellaria: Tricladida) from Pennsylvania, U.S.A. Transactions of the American Microscopical Society, 106(1), 21–30.
Ogren, R. E., & Sheldon, J. K. (1991). Ecological Observations on the Land Planarian Bipalium pennsylvanicum Ogren, with References to Phenology, Reproduction, Growth Rate and Food Niche. Journal of the Pennsylvania Academy of Science, 65: 3–9.
Stokes, A. N., Ducey, P. K., Neuman-Lee, L., Hanifin, C. T., French, S. S., Pfrender, M. E., Brodie, E. D., 3rd, & Brodie, E. D., Jr (2014). Confirmation and distribution of tetrodotoxin for the first time in terrestrial invertebrates: two terrestrial flatworm species (Bipalium adventitium and Bipalium kewense). PloS one: 9, e100718. https://doi.org/10.1371/journal.pone.0100718
Olga Kostromytska, Extension Assistant Professor of Turf Entomology, UMass Amherst
