Woltz, J. M. 2013. Relationships between landscape structure, coccinellid activity density, and aphid suppression in soybean. Dissertation, Michigan State University, East Lansing, MI USA.
Arthropod natural enemies provide valuable pest suppression in agricultural landscapes, and numerous studies have demonstrated the effects of landscape structure on natural enemy abundance and diversity. Coccinellid beetles are important generalist predators in a variety of agricultural landscapes, and have been found to be particularly important for suppression of the soybean aphid, Aphis glycines, Matsumura. I assessed the relationship between multiple landscape metrics and coccinellid abundances in soybean fields using Partial Least Squares regression. Coccinellid abundances were lowest in landscapes dominated by annual crops grown in large fields and highest in landscapes dominated by semi-natural habitat, with a higher diversity of crops grown in smaller fields. In addition to landscape-scale drivers, a large body of research examines the ability of local habitat management to enhance natural enemy efficacy. I examined the influence of landscape structure, local habitat management, and their potential interaction, on coccinellid abundances and A. glycines suppression in soybean. I selected pairs of soybean fields in landscapes of varying composition and planted buckwheat, Fagopyrum esculentum, strips adjacent to one field in each pair. A. glycines suppression was high in all contexts. Coccinellid abundance was higher in buckwheat than in control margins in all landscapes. Additionally, coccinellid abundance in soybean was positively related to amount of semi-natural vegetation in the landscape. I found no evidence of an interaction between landscape and local variables, suggesting that landscape factors drive coccinellid potential to suppress soybean aphid.
Aphis glycines colonies are patchily distributed within soybean fields. To examine how coccinellid arrival rate at aphid-infested patches influences A. glycines control, I experimentally varied coccinellid immigration to 1m2 soybean patches using selective barriers. In a year with low ambient aphid pressure, naturally-occurring levels of coccinellid immigration to host patches were sufficient to suppress aphid population growth. In contrast, A. glycines populations increased dramatically in patches with reduced coccinellid immigration. In a year with higher ambient aphid colonization, aphid suppression was incomplete. In a system in which natural enemy populations cannot track pest populations through reproduction alone, natural enemy immigration to infested patches can result in pest control. Finally, nocturnal predators have been shown to be responsible for significant predation of soybean herbivores in some regions, yet research on A. glycines natural enemies has focused on members of the diurnal predator community. I assessed diel variation of the predator community and A. glycines predation events. Anthocorids were responsible for the majority of observed predation, which occurred during daylight hours. Anthocorids and nabids were more active in the afternoon and carabids were more active at night. Different taxa were observed most frequently on video and within vacuum samples. Vacuum samples may represent predator abundance more accurately, while video data can be used to better assess relative time spent foraging. Overall, A. glycines population dynamics are influenced by a combination of local and landscape effects that structure predator communities and influence the spatial and temporal interactions between predators this aphid. Factors that influence early season coccinellid immigration into infested patches show the greatest potential to improve A. glycines suppression.
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