Prairies, once spanning the Upper Midwest, have now largely been replaced by agriculture. The lack of resources available in agricultural fields and the practices employed by farmers to maximize yield has led to a decline in insect and pollinator diversity. It is prudent to weigh the ecosystem services provided by a diverse insect community against current farming practices as they relate to crop yield. We sought to explain how landscape heterogeneity relates to insect diversity, as well as how insect diversity relates to crop yield across common farming practices. To evaluate how farming practices relate to yield and insect diversity we planted 35 single acre sites of Brassica carinata within 30 miles of Brookings, SD between 2017 and 2018. We randomly assigned each site with a combination of three treatments: tilling (yes/no), added honeybee hives (yes/no), and treatment with systemic neonicotinoids (yes/no). Using a sweep net we periodically collected insects from each site. To evaluate landscape heterogeneity we obtained landscape data surrounding each site and overlaid that data with buffers of 500m, 1000m, and 3000m from site center. Shannon Diversity of the landscape and insects were calculated for each site.
Results/Conclusions
There is a significant positive relationship between insect (and pollinator) diversity with yield in the absence of any farming practice (p=0.002 and p<0.0001, respectively). All farming practices will increase yield. However, farming practices alter the relationship between yield and diversity. The addition of seed treatment or tillage negates the relationship between insect (and pollinator) diversity with yield. Seed treatment alone results in a flat relationship between diversity and yield for all insects and a negative relationship for pollinators. Two or more farming practices coupled together will generally diminish any contribution of diversity.
Increased landscape heterogeneity results in a positive relationship between insect diversity at the 1000m scale (p=0.019) and pollinator diversity at the 3000m scale (p<0.001), suggesting large-scale heterogeneity contributes to overall insect diversity. Lastly, there is a positive relationship between carinata yield and landscape diversity at the 3000m scale (p<0.0001). Our results show that increasing large-scale landscape heterogeneity is a good way to increase diversity and that diversity can serve as a substitute for other farming practices. Increased heterogeneity could save farmers from the input cost of treatment or tillage, by way of increased insect diversity, while still providing similar yields.