Assessing the role of generalist predators in the biological control of alfalfa weevil (Coleoptera: Curculionidae)

Authors: Rand, Tatyana

Submitted to: The Canadian Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2017
Publication Date: 3/21/2017
Publication URL: https://handle.nal.usda.gov/10113/5763076
Citation: Rand, T.A. 2017. Assessing the role of generalist predators in the biological control of alfalfa weevil (Coleoptera: Curculionidae). The Canadian Entomologist. 149(4):525-533. doi:10.4039/tce.2017.9.

Interpretive Summary: The alfalfa weevil, Hypera postica (Gyllenhal), is a major and longstanding economic pest of alfalfa throughout much of the United States. While work on biological control of this species has disproportionately focused on introduced parasitoid was[s, generalist predators are also considered potentially important. However, experimental studies examining the potential impact of generalist on alfalfa weevil numbers and crop damage are lacking. In this study, I combined a cross-site predator survey with a predator exclusion experiment to: 1.) determine the dominant alfalfa weevil predators in eastern Montana –western North Dakota alfalfa fields; 2.) test whether abundant predator groups concentrate in patches of high alfalfa weevil or pea aphid numbers; and 3.) experimentally test whether generalist foliage dwelling vs. ground dwelling predators impact weevil larval survival and plant damage levels. Spiders numerically dominated the predator complex from sweep samples collected in first crop alfalfa in the region, followed by the damselbugs and ladybugs. No significant correlations were found between two dominant pest species (alfalfa weevil larvae or pea aphids), and damselbugs, spiders or ladybugs across the 10 sites in the survey. However, alfalfa weevil larval densities were significantly positively correlated with both total ladybug and damselbug density across transects at the experimental site. Thus, predator groups traditionally associated with, and often preferring, aphids can none the less show strong aggregation in areas of high weevil numbers in the field. The predator exclusion experiment revealed no significant predator impacts on alfalfa weevil larval surviving or damage to alfalfa. However, for pea aphids, which were inadvertently introduced into plots, final numbers were significantly higher in cages where all predators were excluded compared with cage controls, or treatments with ground predators excluded only. Thus, cumulatively our results suggest that even under conditions where predator numbers are sufficiently high to exert considerable pressure on the pea aphids, and predators aggregate in response to the alternative weevil prey, they may still have minimal impacts on alfalfa weevil densities. Additional experimental work done across sites and regions will be necessary to determine the broader potential of generalist predators as alfalfa weevil biological control agents.

Technical Abstract: The alfalfa weevil, Hypera postica (Gyllenhal), is a major and longstanding economic pest of alfalfa throughout much of the United States. While work on biological control of this species has disproportionately focused on introduced parasitoids, generalist predators are also considered potentially important. However, experimental studies examining the potential impact of generalist on alfalfa weevil numbers and crop damage are lacking. In this study, I combined a cross-site predator survey with a predator exclusion experiment to: 1.) determine the dominant alfalfa weevil predators in eastern Montana –western North Dakota alfalfa fields; 2.) test for aggregative responses of abundant predator groups to densities of alfalfa weevil and pea aphid pests within and across fields; and 3.) experimentally test whether generalist foliage dwelling vs. ground dwelling predators impact weevil larval survival and plant defoliation levels. Spiders numerically dominated the predator complex from sweep samples collected in first crop alfalfa in the region, followed by the Nabidae (Nabis spp.) and the Coccinellidae (98% Coccinella septempunctata). No significant correlations were found between two dominant pest species (alfalfa weevil larvae or pea aphids), and nabids, spiders or coccinelids across the 10 sites in the survey. However, alfalfa weevil larval densities were significantly positively correlated with both total coccinellid density and nabid density across replicate sweep transects at the experimental site. Thus, predator groups traditionally associated with, and often preferring, aphids can none the less show strong aggregative numerical responses to alfalfa weevil larvae in the field, particularly when weevil populations are high relative to the aphid primary prey as was found in this study. The predator exclusion experiment revealed no significant predator treatment effects on alfalfa weevil larval surviving or damage to alfalfa. However, for pea aphids, which were inadvertently introduced into plots, final densities were significantly higher in cages where all predators were excluded compared with cage controls, or treatments with ground predators excluded only. Thus, cumulatively our results suggest that even under conditions where predator densities are sufficiently high to exert considerable pressure on the pea aphids, and predators aggregate in response to the alternative weevil prey, they may still have minimal impacts on alfalfa weevil densities. Additional experimental work done across sites and regions will be necessary to determine the broader potential of generalist predators as alfalfa weevil biological control agents.