Evidence of indirect effects of pesticide seed treatments on weed seed banks in corn and soybean agroecosystems

Background/Question/Methods

Use of pesticide seed treatments (coating crop seeds with insecticide and/or fungicides) is a common practice in conventional corn and soybean production. Despite its widespread use, however, little is known about how this practice may affect soil food webs and their associated ecosystem services. One potentially important agroecosystem service provided by soil food webs is regulation of weed populations in the soil seed bank. We conducted a two-year field experiment in which identical genotypes of corn and soybean were planted with and without pesticide seed treatments in a CRD with five replications. We sampled the seed bank during both the corn (fall 2013) and soybean (late summer 2014) phases of the rotation. Soil samples were transported to a greenhouse facility and weed seed bank composition and density was assessed over a period of four months via the direct germination method. Seed bank data were analyzed with ANOVA and several multivariate techniques to assess the effect of seed treatments on seed bank density, species composition, and diversity.

Results/Conclusions

We hypothesized that pesticide seed treatments reduce the abundance of natural enemies (e.g., seed predators and soil borne pathogens) that damage or destroy weed seeds in the soil and that seed banks exposed to treated seeds would therefore be larger and less diverse than those exposed to corn and soybean seeds without pesticide seed treatments. In accordance with our hypothesis, weed seed banks were less diverse in treated compared to untreated plots (Shannon diversity, H; p = 0.007). While not statistically significant (p > 0.05), differences in mean weed seed bank density and species richness were also in the direction that we hypothesized. These data provide the first evidence that we are aware of that agricultural weed seed banks, and hence weed populations, may be indirectly affected by pesticide seed treatments. Additional research will be necessary to determine the generality of these responses and their underlying mechanisms.