Stability of Patches of Higher Population Density within the Heterogenous Distribution of the Gray Field Slug Deroceras reticulatum in Arable Fields in the UK

Abstract

Exploitation of heterogenous distributions of Deroceras reticulatum, in arable fields by targeting molluscicide applications toward areas with higher slug densities, relies on these patches displaying sufficient spatio-temporal stability. Regular sampling of slug activity/distribution was undertaken using 1 ha rectangular grids of 100 refuge traps established in 22 commercial arable field crops. Activity varied significantly between the three years of the study, and the degree of aggregation (Taylor’s Power Law) was higher in fields with higher mean trap catches. Hot spot analysis detected statistically significant spatial clusters in all fields, and in 162 of the 167 individual assessment visits. The five assessment visits in which no clusters were detected coincided with low slug activity (≤0.07 per trap). Generalized Linear Models showed significant spatial stability of patches in 11 fields, with non-significant fields also characterized by low slug activity (≤1.2 per trap). Mantel’s permutation tests revealed a high degree of correlation between location of individual patches between sampling dates. It was concluded that patches of higher slug density were spatio-temporally stable, but detection using surface refuge traps (which rely on slug activity on the soil surface) was less reliable when adverse environmental conditions resulted in slugs retreating into the upper soil horizons.

A predictive model and a field study on heterogeneous slug distribution in arable fields arising from density dependent movement

“…In order to collect the data on slug abundance, square grids of 10 × 10 refuge traps (with 10 m distance between neighbouring traps) were set up in various commercial crop fields across England. Traps were checked and the counts recorded on a regular basis (every 2–3 weeks) 18 . Analysis of the field data collected in 167 sampling visits to the sites where traps were installed has shown that, apart from rather rare exceptions (linked to cases of very low average population density) slug distribution is distinctly heterogeneous, usually consisting of patches of high density separated by areas with very low slug density 18 .…”

“…Traps were checked and the counts recorded on a regular basis (every 2–3 weeks) 18 . Analysis of the field data collected in 167 sampling visits to the sites where traps were installed has shown that, apart from rather rare exceptions (linked to cases of very low average population density) slug distribution is distinctly heterogeneous, usually consisting of patches of high density separated by areas with very low slug density 18 . A typical example is shown in Fig.…”

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Figure 1 Example of a heterogeneous slug distribution. ( a ) Trap counts on the grid of traps recorded on 02 September 2016 in a crop field in Shropshire UK (see 18 for full details), and ( b ) a proxy of the corresponding density distribution reconstructed from the trap counts using linear interpolation.

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“…The species is also known to be a common pest in agricultural environments in many global regions, where it can cause considerable economic damage. Evidence for the grey field slug population distribution in natural environments is meagre, but in farm fields its distribution is known to exhibit a distinctly patchy structure on the subfield scale of 10–50 m 18 . Moreover, patches of high slug density were shown to be relatively stable within a season (with the spatial location of most patches showing little or no change) but not necessarily between seasons 18 , 19 , although precise reasons for the latter are not well understood.…”

“…Evidence for the grey field slug population distribution in natural environments is meagre, but in farm fields its distribution is known to exhibit a distinctly patchy structure on the subfield scale of 10–50 m 18 . Moreover, patches of high slug density were shown to be relatively stable within a season (with the spatial location of most patches showing little or no change) but not necessarily between seasons 18 , 19 , although precise reasons for the latter are not well understood. Earlier studies 20 , 21 detected correlations between the slug density and a number of edaphic characteristics such as soil organic matter content, soil texture and others.…”