Sugiura, S. (2009). Seasonal fluctuation of invasive flatworm predation pressure on land snails: Implications for the range expansion and impacts of invasive species. Biological Conservation, 142, 3013-3019. https://www.doi.org/10.1016/j.biocon.2009.07.032

Summary

Platydemus manokwari is a large predatory flatworm that was originally discovered in New Guinea. It has been deliberately introduced as a biocontrol agent in some Pacific Islands in an attempt to control an invasion of the Giant East African Snail, and has accidentally been introduced to other Pacific countries potentially through the movement of live plant material or soil in potted plants or on machines. In the U.S. Pacific, this species has been found in the Federated States of Micronesia, Palau, Guam, and the Northern Mariana Islands. In Hawaiʻi the species was first reported on Oʻahu in 1992, and there are 30 observations on iNaturalist that have been posted since 2019.

Sugiura studied the effects of temperature and density on P. manokwari predatory activity and survival using field and lab experiments. The study took place in the western Pacific on the subtropical Chichijima island, the largest of the Ogasawara islands, which are a Japanese archipelago of over 30 islands located around 620 miles south of Tokyo. P. manokwari was introduced to this island in the 1990s, which is believed to have been accidental through the importation of contaminated soils. Since this introduction, there are areas of Chichijima in which local snail species have been extirpated by P. manokwari, yet the factors that influence flatworm predation pressure on land snails is unclear.

For the field study, Sugiura placed live snails of the introduced species Acusta despecta in nylon bags sized to exclude larger predators on the forest floor throughout the year (from November 2007 to September 2008), and then observed the level of P. manokwari predation by determining snail survival. This was coupled with lab experiments examining the impact of temperature and P. manokwari density on feeding activity. To examine temperature’s effect on P. manokwari, individual flatworms were housed in small containers and exposed to five different temperatures. Survival at each temperature was determined after a week, and then snails were added to the containers and snail survival was quantified. Sugiura also measured the predation rates on snails at three different densities of P. manokwari.

Results

The highest rates of snail mortality due to P. manokwari predation were observed in July, September, and November, while no predation was observed in February, the coldest month. Relative humidity and precipitation did not appear to impact predation rates. P. manokwari had high survival in all temperatures tested except for the 50 °F (10 °C) treatment, which was the coldest temperature tested, where only 23.3% survived. At 50 °F (10 °C) no snail predation was observed. In the ~57 °F (14 °C) treatment, however, half of the flatworms attacked snail prey, and for the other temperature treatments (64 °F/18 °C, 71 °F/22 °C, and 78 °F/25 °C), nearly all P. manokwari were observed hunting snails. Higher P. manokwari densities resulted in higher snail predation rates.

Take Home Points

• Introductions of P. manokwari are a serious concern because they can feed on any species of gastropod (i.e., slugs and snails) and they have caused local snail extinctions in areas they invade.
• Climate-change induced warming will likely expand the potential range of P. manokwari, and they may be active for longer periods throughout the year in a warmer climate.
• Rapid population growth of P. manokwari can be expected under warming conditions, and higher densities could also intensify their predation of native slugs and snails.
• Stringent quarantine procedures should be enacted for any imported materials that contain soil to prevent introduction; P. manokwari has been introduced via potted plants and other infected sources of soil in the past

Final Note: Except for upper elevations found in Hawai’i (e.g., see below), no place in the region has mean annual air temperatures below 57 °F (14 °C), so P. manokwari establishment is not likely to be limited by temperature anywhere except where the 57 °F (14 °C) isotherm occurs (i.e., on Maui and Hawai’i Island). These temperature isotherms are expected to shift due to climate change, which may lead to greater impacts of P. manokwari at higher elevations in the future.