Quantifying temporal variation in the benefits of aphid honeydew for biological control of alfalfa weevil (Coleoptera: Curculionidae)

Authors: Rand, Tatyana; LUNDGREN, JONATHAN - Ecdysis Foundation

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2018
Publication Date: 11/15/2018
Publication URL: http://handle.nal.usda.gov/10113/6471074
Citation: Rand, T.A., Lundgren, J.G. 2018. Quantifying temporal variation in the benefits of aphid honeydew for biological control of alfalfa weevil (Coleoptera: Curculionidae). Environmental Entomology. 48(1):141–146. https://doi.org/10.1093/ee/nvy165.
DOI: https://doi.org/10.1093/ee/nvy165

Interpretive Summary: Sugar feeding by biological control agents, such as parasitoid wasps, may enhance their ability to control important crop pests. Unfortunately, sugars are often limiting in highly simplified cropland monocultures. Restoring sugar resources within these simplified agricultural landscapes has thus become a focus of conservation biological control efforts. Although the importance of sugars in promoting biological control is likely to fluctuate greatly through space and time, studies are rarely conducted at more than one site or year. Here we measured how the importance of sugar resources associated with honeydew secreted by pea aphids influenced parasitism of the alfalfa weevil across irrigated alfalfa fields in Montana, USA over five years. A positive relationship between parasitism of alfalfa weevil and pea aphid numbers across alfalfa fields was observed in two of five years, with parasitism increasing 2-4 fold over gradients in aphid density. The relationship was strongest in the two years of lowest parasitoid densities, suggesting that sugar resources may be particularly important when wasps are rare. Pea aphid densities were also at their lowest densities in the two years where a significant relationship was found, suggesting that aphid densities may generally be sufficiently high that parasitoid are not limited by sugars in most years. Our results reinforce previous studies done as smaller scales which suggest that sugar resource addition, for example via sugar sprays, will only be effective when natural sources, such as aphid honeydew, are low. More studies that explore the environmental conditions under, and frequency with, which increasing sugar resource availability actually enhances parasitism levels in the field will be necessary in gauging the broader potential of sugar resource addition (i.e. through flowering strips, banker plants or sugar sprays) to bolster biological control of important crop pests.

Technical Abstract: Sugar feeding by biological control agents, such as parasitoid wasps, may enhance their ability to control important crop pests. Unfortunately, such non-host resources are often limiting in highly simplified cropland monocultures. Restoring sugar resources within these simplified agricultural landscapes has thus become a focus of conservation biological control efforts. Although the importance of sugars in promoting biological control is likely to fluctuate greatly through space and time, studies are rarely conducted over multi-site and multi-year scales. Here we quantified temporal variation in the potential importance of sugar resources associated with phloem feeding homopterans (the pea aphid, Acyrthosiphon pisum) in determining levels of parasitism of the alfalfa weevil (Hypera postica) by its dominant parasitoid, Bathyplectes curculionis, across irrigated alfalfa fields in Montana, USA over five years. A positive association between parasitism of H. postica and A. pisum densities at the site scale was observed in two of five years, with parasitism increasing 2-4 fold over gradients in A. pisum density. The relationship was strongest in the two years of lowest parasitoid densities, when increases in per capita impacts of individual parasitoids would be expected to be particularly important in augmenting parasitism rates. A pisum densities were also at their lowest densities in the two years where a significant relationship was found, suggesting that aphid densities may generally be sufficiently high that parasitoid are not limited by sugars in most years. Our results reinforce previous studies done as smaller scales which suggest that sugar resource addition, for example via sugar sprays, will only be effective when natural sources, such as aphid honeydew, are low. More studies that explore the environmental conditions under, and frequency with, which increasing sugar resource availability actually enhances parasitism levels in the field will be necessary in gauging the broader potential of sugar resource addition (i.e. through flowering strips, banker plants or sugar sprays) to bolster biological control.