Targeted treatment tactics for slugs

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Targeted treatment of the most mollusc-riddled parts of a field has taken a step closer, thanks to an innovative research programme that tags and tracks grey field slug populations.

AHDB-funded PhD student Emily Forbes has been hot on the slime trail of these major crop pests for the past three years. Based at Harper Adams University (HAU), she uses radio-frequency identification (RFID) tag technology, originally developed to track cattle, to monitor slug movement in commercial fields and explore options for more precise control.

Emily said: “Slugs need to shelter from the sun; it’s why they prefer to feed at night and shelter underground during the day. RFID tags reveal what slugs get up to, no matter where they hide.”

The tags, which are about the size of a grain of rice, were injected into slugs. To make sure tagging did not change the slugs’ behaviour, a laboratory experiment was set up to monitor their movements. Video tracking found that the slugs’ routines did not change, compared with non-tagged slugs. This finding gave the green light for the approach to be used to record natural slug activity in the field.

The PhD focused on the distribution of slugs across a field. In particular, Emily looked to define what causes slug populations to develop in distinct patches. An RFID scanner, which operates in a similar way to a metal detector, was swept over the ground regularly to map the whereabouts of tagged slugs.

It turns out that grey field slugs are not particularly adventurous. Over four nights of monitoring, on average, slugs travelled less than a metre and stuck to a defined ‘home range’. Individuals also used slime trails, created by other slugs, to navigate around their patch.

Through precise monitoring of slug movements, Emily established that patches were able to form in relatively small areas, much smaller than originally anticipated. The use of mathematical models also found that patches were relatively stable over time, which opens up the potential for targeted treatment.

As the lowest-cost chemical option, metaldehyde-based slug pellets have dominated the molluscicide scene. It looked like this control option would be lost in 2020. However, in July, the High Court ruled that the planned withdrawal was unlawful. Although the products remain on the market, their authorisation is fragile. The industry needs to look more closely at other options, including ferric phosphate and biological control. As these options are associated with higher treatment costs, any information to help target treatment is in high demand.

The same HAU research group also uses RFID tags to establish how slugs respond to specific treatments. Metaldehyde-treated slugs tend to produce lots of slime and die on the soil surface. However, those treated with ferric phosphate stop producing slime, retreat underground and die a few days later.

The integrated pest management (IPM) of slugs can be investigated in far more detail, thanks to RFID tags. IPM options, which require further investigation, include the use of companion crops to provide a ‘preferred’ food source for slugs, the potential of less attractive/more repellent crops, the role of slug-hunting beneficials and the influence of various cultivation techniques.

The studentship project, which is due to report later this year, has also improved understanding of the soil characteristics associated with patch formation. AHDB plans to invest in follow-up work to establish how soil maps can help with the precision application of slug treatments.

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