Effect of transient coccinellid immigration on aphid suppression

Background/Question/Methods

Landscape structure could affect immigration of organisms to specific habitats to modulate rates of ecosystem services they provide. However, the impacts of changes in immigration remain little-explored. Coccinellids provide an important service by suppressing populations of soybean aphid in soybeans. Critical interactions between coccinellids and aphids occur in early summer when aphid densities in soybean are too low to support local reproduction of coccinellids.  Thus, the ability of coccinellids to suppress aphids likely depends on their immigration rates to infested patches. Our goals were: 1) determine how changes in coccinellid immigration affect aphid suppression and 2) calibrate easy-to-use collection methods with direct measures of immigration.

We manipulated coccinellid immigration in soybean by covering 1m³ PVC frames with mesh on 5, 2 or no sides, and monitored resulting changes in aphid populations. Within plots, we measured coccinellid activity-density with sticky traps while using video cameras to directly measure coccinellid arrival, allowing us to calibrate sticky trap data to a rate.

Results/Conclusions

Treatments altered immigration, nearly eliminating coccinellids in 5-sided treatments and reducing captures by 50% in 2-sided (~1 ladybug/card/wk) compared to open treatment (~2/card/wk). In a year with low aphid pressure, near elimination of coccinellids in 5-sided treatment resulted in substantial increases in soybean aphid populations, while coccinellids in 2-sided treatment suppressed aphid populations to extremely low levels comparable to open treatment. In the low-aphid year, resident predator densities and coccinellid egg-laying were low, indicating that predation was primarily due to transient predators. In a high aphid year, aphid counts did not vary significantly by treatment but were maintained below economic threshold levels. Populations of resident predators, including coccinellid larvae, were higher in the high-aphid year, suggesting that a combination of resident and transient predators held aphid populations in check. Video data is being reviewed.

Results show that altering immigration of coccinellids affected aphid suppression, indicating that immigration is an important target for landscape management, particularly in a year with low aphid pressure. In a year with high aphid pressure, resident predators may also contribute to suppression. Calibrating sticky traps will allow us measure immigration in multiple landscapes and determine the landscape characteristics that support sufficient immigration to control aphids. Ultimately, the goal of our work is to facilitate the design of agricultural landscapes that maximize biological control.