Suppression of Soybean Aphid and movement of natural enemies in agricultural landscapes in Manitoba

Transient generalist predators are crucial for controlling the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), before populations reach economic injury levels. However, the sources of generalist predators in the landscape have not been studied directly. We conducted field studies to compare soybean aphid suppression in 27 soybean fields located across a gradient of landscape complexity in Manitoba, during 2013 and 2014. We infested soybean potted plants with 14 aphids / pot and treated them as 1) open to predation by ambient levels of natural enemies, versus 2) protected from predation using exclusion cages. This design was replicated five times in each field. In addition, we established bidirectional Malaise traps in at least one field border to monitor natural enemy movement from different adjacent habitats. Finally, we conducted two mark-release-recapture experiments to determine movement direction, travel distance and speed of Coccinella septempunctata L. (Coleoptera: Coccinellidae) moving between neighboring fields of soybean and alfalfa.

Results indicate that there was a strong to moderate suppression of soybean aphids across the province during both years, with levels of controls ranging from 3- to 22-fold reductions of aphid populations. Preliminary analysis of Malaise traps indicate a significant movement of natural enemies from alfalfa and natural vegetation to soybean, suggesting that these two habitats may act as sources of predators in soybean. For the mark-recapture experiments, we released 654 (2013) and 600 (2014) marked ladybeetles and recaptured around 5.5 %. Our results suggest that ladybeetles can move rapidly between both crops, suggesting that alfalfa may act as a source of natural enemies when they are infested by soybean aphid. Altogether these results show important pest suppression services provided by predators in Manitoba.