Summer of slugs: digging deeper

Lucy Drummond - Summer Student with the Plant Health team in 2017 - ran a project into the effects of slug management techniques on soil invertebrates

Lucy at work in the labLast summer was a busy time for the RHS Plant Health team. Alongside the regular enquiries - received through RHS Gardening Advice - we also played host to several summer students, including Lizzie - of we’re going on a slug hunt fame - and Lucy Drummond.

During her summer studentship Lucy collected data from RHS Entomologist, Hayley Jones's, Integrated Gastropod Management project. Rather than looking at the effects of the slug management treatments on their slimy targets, Lucy was interested in the unintended effects they might have been having on other soil invertebrates.

Slug treatments assessed

Lucy collected samples from a sub-set of the gastropod management treatments used in Hayley's study:

  • metaldehyde pellets – the most commonly used type of slug pellets
  • ferric phosphate pellets – these slug pellets are certified as organic
  • nematode biological controlP. hermaphrodita
  • no treatment – this is important to compare with the other strategies

Berlese-Tullgren funnelsThe samples were taken from both RHS Garden Wisley and RHS Garden Harlow Carr (32 plots total).

How the samples were collected

To collect a sample Lucy used a special tool, called a soil auger, to remove a cylinder of soil (a soil core) from the centre of each plot. Once the soil cores were collected the soil fauna were extracted using modified Berlese-Tullgren funnels. These work by placing the soil core below a light bulb, which provides a heat and light source that the soil fauna move away from, leading them into a collection vessel below.

Assessing the samples

Lucy patiently spent hours looking down a microscope sorting and identifying the soil fauna to species level - where possible -  focussing on springtails (Collembola).

Springtails are a major part of the soil mesofauna (organisms with a width between 100 μm and 2 mm). Many soil biodiversity studies explore this group of invertebrates as they are numerous in soils whilst still being relatively easy to identify compared to the even smaller microfauna (widths less than 100 μm). Aside from springtails the soil mesofauna grouping also includes: mites, waterbears and pot worms.

Many of the soil dwelling springtails are very small and lack strong colours or patterns, making identification a sometimes tricky and time-consuming process!

Springtails Hypogastrura denticulataFindings from the study

Lucy collected over 350 springtails - which belonged to at least 27 different species. She found that there was a higher density of springtails (number of springtails per unit of soil) at Harlow Carr than at Wisley, but that the densities did not differ significantly between the treatments. 

Lucy also looked at the springtail diversity (number of different species) under each of the treatments and found the greatest number of species of springtail under ‘no treatment’ and the ‘nematode biological control’, with the fewest springtail species being retrieved from the ‘metaldehyde pellets’ treated plots, however, these differences were not statistically significant when the diversity indices were compared. 

For now Lucy has gone back to Cambridge Univeristy for her final year study. However, she has left us with more knowledge and data on what effects gardener’s gastropod management choices may be having on the often overlooked wildlife beneath our feet.

Read more about Plant Health summer studentships 2018 at RHS Garden Wisley (pdf 686kB)

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