Direct measurement of ant predation of weed seeds in wheat cropping

The ecosystem service of predation of weed seeds by naturally occurring seed‐eating animals, including ants, in agricultural fields has been suggested to be a potentially important biocontrol option. Laboratory and field tests have found high levels of seed removal from experimentally placed seed; however, the effect of predation on naturally dispersed weed seeds is unknown. We measured the effect of invertebrate seed predators on natural weed seed dispersal and germination in a field experiment under commercial growing conditions. The two‐factor, large‐scale field experiment in a field used to grow wheat with conservation tillage used an insecticide to remove soil invertebrates (ants had been reduced by 85% relative to water controls) and shallow tillage for mechanical weed removal (no effect on ants). There was one natural weed seeding event, when a wind storm blew one single incursion of Salsola australis (Chenopodiaceae) tumbleweeds across the field. We measured the number of tumbleweeds after 2 months and found the removal of ants resulted in a doubling of tumbleweeds: 3383 ± 513 weeds ha⁻¹ in the insecticide treatment plots compared with 1768 ± 100 weeds ha⁻¹ in the water control treatment plots, and 1948 ± 227 weeds ha⁻¹ in the rest of the field. The difference in tumbleweed germination and growth was not due to growing conditions. We measured soil nutrients before the incursion and soil moisture during the weed growing period and found that there were higher levels of nitrogenous compounds in water control plots, but no other nutrient or moisture differences, than in insecticide exclusion plots. Synthesis and applications. Our results provide evidence that the ecosystem service of ant predation of naturally dispersed weed seeds limits weed abundance in commercial cereal fields in warmer climates. The fields were not managed to increase ants; alternative conservation agricultural management methods that promote agro‐ecology, including low or zero tillage and low insecticide use, could increase ant abundance and thereby reduce weeds. The use of seed predators as a type of biocontrol agent will need to be integrated with other weed management methods, including herbicides and shallow tillage, although the latter may conflict with ant survival. The use of seed predators may be particularly advantageous in organic systems, or locations where economic margins are low.