Full steam ahead: direct steam exposure to inhibit spread of invasive aquatic macrophytes

Biosecurity protocols designed to prevent invader spread have become integral to invasive species management strategies. However, application of many proposed spread-prevention practices is inhibited due to low practicality, high expense, undesirable non-target effects and a lack of known efficacy. Here, we examine the use of direct steam exposure to induce substantial fragment (i.e. propagule stage) degradation of seven invasive macrophytes: Ceratophyllum demersum, Crassula helmsii, Egeria densa, Elodea canadensis, Elodea nuttallii, Lagarosiphon major and Potamogeton crispus. Each species was independently exposed to steam treatments in loose clumps of three fragments, steamed at a distance of 2–3 cm from the source, for varied exposure times: 10 s, 30 s, 1 min, 2 min, and 5 min. Furthermore, we develop and apply a novel degradation scale describing visual tissue biodegradation stages and/or resumption of growth for fragmentary propagules. Steam treatments were observed to be highly efficacious, with total degradation being induced by 10 s of direct steam exposure. This was apparent for all species following a seven day recovery period, except C. demersum, which took until 21 days. Conversely, control specimens displayed excellent survival and/or viability (i.e. resumption of growth). Therefore, we argue that this innovative, yet simple technique can be used to improve biosecurity practices to inhibit the spread of invasive macrophytes.