Biosecurity in action: How effective is hot water pressurised spray to prevent the spread of INNS?

May 2021

Freshwater Invasive Non-Native Species: what’s the problem?

Freshwater invasive non-native species (INNS) have been accidentally introduced in rivers, canals, and lakes, causing significant ecological and economic damage. For example, invasive plant species can take over a waterbody and out-compete native plant species. Invasive mussel species can attach to boats and infrastructure, causing blockages and issues with water treatment. INNS can also impact our enjoyment and engagement with the environment, for example preventing recreational activity and navigation.

How do INNS spread in the environment?

INNS can be accidentally moved from site to site, for example from one lake to another lake, attached to clothing and equipment used in the environment. This can lead to the accidental spread and the establishment of new populations of INNS. Stopping the spread of INNS is important and this is where biosecurity comes in. By cleaning equipment used in one location before moving it to another, this removes any INNS present and prevents INNS spread. Soaking equipment in hot water (45oC for 15 minutes) is recommended to remove INNS, however not all equipment can be cleaned this way. Hot water pressurised spray machines are often used to clean large equipment such as vehicles and boats. Whilst known to be effective against certain INNS, there are clear knowledge gaps on the use of hot water spray in field conditions which need to be addressed to inform biosecurity guidance.

Biosecurity: Hot water Pressurised Spray

We tested the effectiveness of hot water sprays in killing freshwater INNS in the field.

We looked at two invasive animals and two invasive plant that are considered a high threat in the UK:

How hot is hot?

We first measured the maximum on-contact temperature when spraying from difference distances and machine-set temperatures. We used a metal backboard to mimic large piece of equipment, such as vehicles and boats that would be cleaned using hot water spray. When spray was applied for 15 seconds, relatively low on-contact temperatures were achieved even when spraying from a short distance (Table 1).

Is hot water spray effective?

High-pressure hot water spray applied from 10cm for 15 seconds killed all the zebra mussels and killer shrimp (Table 2). However, when we sprayed from longer distances or shorter durations, we found high but not complete mortality of these two INNS.

In contrast, hot water spray was ineffective in causing mortality in crassula, even after 90 seconds of exposure. Fragmentation and complete mortality of floating pennywort was seen following both hot and cold spray treatments, therefore the pressure of the spray likely caused this mortality.

All INNS mortality results are shown in Table 2 below.

Study Summary

• Hot water pressurised spray killed zebra mussel and killer shrimp when applied at a high temperature from close distance and for a long duration.
• However, Crassula did not die in the treatments tested here.
• The pressure of the hot water spray killed floating pennywort.
• Hot water pressurised spray machines must be set to their highest temperature setting (below 100oC), here 90oC, to achieve mortality of INNS
• Both distance and duration of spray application are important to achieve high on-contact temperatures.
• Spray must be continuous applied to one contact point before continuing onto another area. Therefore, the behaviour of the spray-machine operator is important to ensure high temperatures are achieved on-contact; this must be considered when spraying a large piece of equipment and time needed to clean it.
• Containment and safe disposal of treatment water is vital to prevent INNS spread, given that not all INNS died when sprayed with hot water.

The study is published in Management of Biological Invasions, open access here.

This study was funded by a NERC Case Partner PhD with the Environment Agency and South West Water. All photo credit: Stephanie Bradbeer. May 2021

Steph Bradbeer is a PhD Student at the Faculty of Biological Sciences, University of Leeds.

The opinions expressed in this blog are the author's and not necessarily those of the wider Link membership.