Impact of mustards (Brassicaceae) grown as cover crops on non-target arthropod communities

The effects of mustard varieties grown as cover crops on non-target arthropod communities and diversity were investigated in 2008. The abundance of carabid beetles, collembolans, and mites were monitored before, during, and after the seeding, growth, and incorporation of two mustard varieties (‘Tilney’ and ‘Florida Broadleaf’) individually or in combination, and in plots where no cover crops were grown. Species richness and diversity indices and principal response curves also were used to analyze and compare communities over time across the selected cover crop treatments. Insects were collected from pitfall traps during four sampling periods in 2008: 06-09 April, 10-12 June, 24-27 June, and 17-19 July. Soil samples were collected in 2008 on 18 April, 02 June (1 week after planting of mustards), 29 June (shortly before incorporation of mustards), 18 July (2 weeks after incorporation of mustards), and 11 September and in 2009 on 25 May. A total of 528 carabid beetles representing 8 genera were recovered from pitfall traps in all plots combined in 2008. Anisodactylus santaecrucis Fabricius accounted for 54 percent of all carabids recovered, and Poecilus chalcites Say accounted for 32 percent. In general, total abundance of carabids and abundance of dominant species of carabids were not affected by the cover crops during growth or after incorporation. A total of 32,036 collembolans representing 10 genera from five families were recovered from soil cores from all plots and sample dates combined. One species, Desoria trispinata (MacGillivray) (Isotomidae), accounted for 82 percent of all individuals. Overall abundance of collembolans and the abundance of each of three dominant species did not differ significantly among the cover crop treatments on any of the sampling dates. Diversity indices indicated significant differences in collembolan communities among treatments on three dates, but the impacts of the mustard cover crops on collembolan diversity were inconsistent. A total of 11, 275 mites were identified from 31,798 mites recovered from soil cores in 2008 and 2009. On three dates in midsummer, 2008, overall mite density was significantly greater in plots where Tilney mustard was planted than in plots where no cover crop was planted. On 29 June, 2008, mite diversity was greater in plots where Tilney mustard was planted than plots where no cover crop was planted. On 18 July, 2008, densities of cunaxid mites were greater in plots where Tilney mustard was planted than plots where no cover crop was planted. Population peaks of predatory mites (Mesostigmata, Prostigmata: Cunaxidae) coincided with population peaks of collembolans. Overall, results indicate that the mustard cover crops had little significant effect on the abundance, diversity, species richness, or overall community dynamics of the taxa investigated. These data suggest that mustards grown as cover crops may be valuable alternatives to synthetic inputs in that they seem to have little effect on non-target arthropods. Further studies would be appropriate to determine the effects of seasonality and locality that contribute to the variability in the data