Impacts of Dryland Cropping Systems on Ground Beetle Communities (Coleoptera: Carabidae) in the Northern Great Plains

Abstract

Ground beetles are natural predators of insect pests and small seeds in agroecosystems. In semiarid cropping systems of the Northern Great Plains, there is a lack of knowledge to how ground beetles are affected by diversified cover crop rotations. In a 2-yr study (2018 and 2019), our experiment was a restricted-randomization strip-plot design, comprising summer fallow, an early-season cover crop mixture (five species), and a mid-season cover crop mixture (seven species), with three cover crop termination methods (i.e., herbicide, grazing, and haying). Using pitfall traps, we sampled ground beetles in five 48-h intervals throughout the growing season (n = 135 per year) using growing degree day (GDD) accumulations to better understand changes to ground beetle communities. Data analysis included the use of linear mixed-effects models, perMANOVA, and non-metric multidimensional scaling ordinations. We did not observe differences among cover crop termination methods; however, activity density in the early-season cover crop mixture decreased and in summer fallow increased throughout the growing season, whereas the mid-season cover crop mixture peaked in the middle of the summer. Ground beetle richness and evenness showed a nonlinear tendency, peaking in the middle of the growing season, with marginal differences between cover crops or fallow after the termination events. Also, differences in ground beetle composition were greatest in the early- and mid-season cover crop mixtures earlier in the growing season. Our study supports the use of cover crop mixtures to enhance ground beetle communities, with potential implications for pest management in dryland cropping systems.

Keywords: cover crop mixture; cover crop termination; generalist predator; growing degree day.

Fig. 1.

Experimental design representing the fallow/cover crop phase (top), where a summer fallow, an early-season cover crop mixture, or a mid-season cover crop mixture were established (top), and the winter wheat phase (bottom). Three alternative methods were used to terminate cover crops (top). Three pitfall traps (circles) were placed within each 8 × 13 m strip plot of the fallow/cover crop phase of each year and were homogenized after each 48-h sample period (n = 27 traps/sample period).

Fig. 2.

Mean ground beetle activity density recorded in 2018 (A) and 2019 (B) among treatments with different presence and composition of cover crops (fallow, early-season, and mid-season cover crop mixtures), and sample intervals (growing degree days, GDD °C). Error bars indicate the SEM. Termination of cover crops occurred at approximately 1300 GDD °C each year.

Fig. 3.

Mean ground beetle species richness recorded in 2018 (A) and 2019 (B) among treatments with different presence and composition of a cover crop (fallow, early-season, and mid-season cover crop mixtures) and sample interval (growing degree days, GDD °C). Error bars indicate the SEM. Termination of cover crops occurred at approximately 1300 GDD °C each year.

Fig. 4.

Mean ground beetle species evenness recorded in 2018 (A) and 2019 (B) among treatments with different presence and composition of a cover crop (fallow, early-season, and mid-season cover crop mixtures) and sample interval (growing degree days, GDD °C). Error bars indicate the SEM. Termination of cover crops occurred at approximately 1300 GDD °C each year.

Fig. 5.

Variation in ground beetle communities in 2018 (A) and 2019 (B) in response to the presence and composition of a cover crop and sample intervals. Error bars represent the SE of plots within the presence and composition of a cover crop using different shapes (fallow, early-season, and mid-season) and sample intervals using different colors (growing degree days, GDD °C). Termination of cover crops occurred at approximately 1300 GDD °C each year.