Weed seed predators - weed management while you sleep
by Dr. Charles Geddes
As you sleep, tucked away in your bed at night, little critters are out in your fields saving you money on weed control. Aside from germination, weed seed predation causes the largest seedbank decline, resulting in mortality rates of 50% to 90% of annual weed seedbank inputs.
Ground beetles, crickets, ants, earthworms, slugs, birds, mice, and other rodents consume a large amount of weed seeds that typically enter the seedbank around harvest time. Annual weed species tend to thrive in annual crop production. These weeds drop seed prior to or during crop harvest in late summer or early fall. At this time, a smorgasbord of weed seeds lay on the soil surface, often resulting in a spike of weed seed predator activity. These critters, however, are picky and at the buffet they tend to fill their bellies with seeds from certain weed species; if resources allow.
Experts don't understand perfectly the relationship of weed seed preference for each predator species, but recent research led by Dr. Chris Willenborg at the University of Saskatchewan suggests that the predator may like the smell of their chosen dinner. What experts do know is that promoting a wide range of beneficial weed seed predators in your field can reduce the weed seedbank before the growing season.
While research in Canada is in its infancy, reports from other countries in Europe and the United States suggest that land owners can optimize the benefits with certain cropping systems. For example, zero or delayed tillage leaves weed seeds on the soil surface following harvest, where predators can get them. Seed predation tends to decline when seeds are buried deeper and deeper in the soil profile. Designing cropping systems to provide canopy cover for as long as possible throughout the year can maintain preferred habitat for the predators; many that seem to like canopy cover and light to medium field residues. Farmers can prolong canopy cover by cover cropping or by integrating winter annuals or perennials into crop rotations.
Rates of weed seed predation often vary among crops with different maturity and canopy cover. Unfortunately, little research exists for the wide range of crops grown in western Canada because most recent studies happened in cropping systems dominated by soybean, corn and wheat. In Canada, a large knowledge gap remains regarding the benefits of weed seed predators and the potential impact of broad-spectrum insecticide application without first considering these beneficial predators. Could insecticide application result in greater weed problems in subsequent years? Unfortunately, we don't know.
Several knowledge gaps remain in Canadian agroecosystems, but with such large benefits of weed seed predators observed elsewhere (generally 50 to 90% of annual weed seedbank inputs), predation could be a valuable tool for Canadian growers. Other forms of harvest weed seed control (think chaff carts or the Harrington seed destructor) appear to be effective for weed species that retain their seed until harvest. However, weed seed predation could be a valuable tool to help manage seedbanks of weed species, like wild oat, that tend to lose their seed prior to harvest.