Sampling to determine density of arthropods in intensively grazed grasslands

For arthropod population field studies, understanding the population dynamics of the target organism and assessing impacts, does rely on having a sampling method that provides an accurate measurement of population density. Unsuitable methods may provide underestimates or widely variable measures of population density. The suitability of three vacuum methods: Tellus vacuum cleaner and quadrats, Vortis, G-vac, along with collection of turves followed by heat extraction (Turf) were compared targeting adult Listronotus bonariensis (Argentine stem weevil, ASW), Sitona obsoletus (clover root weevil, CRW), ladybirds, lacewings, predatory beetles and spiders in intensively-grazed dairy pasture. The hypothesis was that all four methods were comparable in their efficacy to sample taxa and quantify density. Overall, for ASW, no one method was found to provide a consistently high mean density compared to the other methods, while for CRW, the Tellus and Turf/Berlese methods gave generally higher mean densities, even though their standard errors were large. For predatory beetles and lacewings, the turf method was the most effective for density measurements, while for ladybirds and spiders, the Vortis provided the best estimate of mean density. While the G-vac , provided the lowest density estimates, it had the highest detection value compared to the three other methods. Increasing pasture dry matter had a significant effect on sampling efficiency of CRW but not ASW. Density estimates varied with methodology and the taxa being targeted. There was also wide variation between methods in time and space in terms of relative density estimates, that makes valid comparisons between methods problematic. While no one method was consistently superior than another, for weevils, the Tellus or Turf methods would seem most appropriate, as they generally provide higher mean densities. However, for detection purposes, the G-vac is a convenient tool for mass collection, in part because it provides a larger collection area per unit effort. In summary, the hypothesis was that all four techniques were comparable in their efficacy to sample taxa and quantify density is disproved. Sampling method will depend on taxa being targeted, and that more than one method may be required when conducting research on specific taxa to accurately measure density and diversity in both natural and modified grasslands.