Environmental factors that influence the association of an earthworm (Lumbricus terrestris L.) and an annual weed (Ambrosia trifida L.) in no-till agricultural fields across the eastern U.S. Corn Belt

The earthworm, Lumbricus terrestris L., caches seeds of the agricultural weed Ambrosia trifida L. in its burrow, providing seeds with a protected overwintering site. Seedlings subsequently emerge from the burrows, resulting in an association of the two species (hereafter “LtAt association”). Although populations of these species frequently co-exist in no-till agricultural fields in the eastern U.S. Corn Belt, an association is not always evident. To identify environmental influences on the LtAt association, 30 no-till agricultural fields were surveyed across the eastern U.S. Corn Belt during spring 2007, 2008 and 2009. The LtAt association occurred across states and soil types, but the strength of the association varied with climate differences during the previous September through March. The strongest environmental driver of LtAt association was frequency of “moderate rain day” (MRD; day that received 12.8–25.3 mm of precipitation), with a 1-day increase in MRD frequency increasing the odds of LtAt association by a factor of 1.42. Thus, the potential for L. terrestris to cache seeds and facilitate seedling recruitment is increased by precipitation frequency and amount during September through March. These results highlight the importance of climate variation within a region in driving trophic interactions that regulate weed population dynamics.