Role of parasitoids and landscape structure in aphid population dynamics in winter canola

Authors: Elliott, Norman - Norm; GILES, KRISOPHER - Oklahoma State University; BAUM, KRISTEN - Oklahoma State University; ELZAY, SARAH - Oklahoma State University; BACKOULOU, GEORGES - Langston University

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2023
Publication Date: 8/14/2023
Citation: Elliott, N.C., Giles, K.L., Baum, K.A., Elzay, S.M., Backoulou, G.F. 2023. Role of parasitoids and landscape structure in aphid population dynamics in winter canola. Biological Control. https://doi.org/10.1016/j.biocontrol.2023.105330 .
DOI: https://doi.org/10.1016/j.biocontrol.2023.105330

Interpretive Summary: Winter canola is typically planted in October in central Oklahoma and harvested in June. Three aphid species typically infest canola fields in central Oklahoma: the cabbage aphid, green peach aphid, and turnip aphid. We sought to determine whether the structure of the landscape surrounding a canola field at a spatial scale roughly corresponding to the dispersal capabilities of aphid parasitoids had an influence on parasitism levels of aphids in the canola field. To that end, we determined how composition and configuration of the landscape in a total of 23 canola fields sampled during three growing seasons was related to aphid density and parasitism. Parasitism was estimated using aphid infested sentinel plants stationed in fields. Two parasitoids, Diaeretiella rapae (M’Intosh) and Aphelinus nigritus Howard, parasitized turnip aphids on sentinel plants. Diaeretiella rapae was abundant and consistently present in fields, whereas A. nigritus occurred sporadically. Statistical analysis demonstrated that D. rapae abundance was positively related to landcover of wetlands and to decreasing patch size (high patch density). Aphelinus nigritus abundance was positively related to the abundance of grasslands and woodlands. Aphid density in canola fields was related to landscape factors, with density increasing with increasing landcover of wetlands and patch density. How habitat and/or host specialization by D. rapae interact with landscape structure to determine observed patterns of parasitism in canola is discussed. The results contribute to a growing body of evidence that the characteristics of the landscape surrounding agricultural fields affects levels of biological control of aphids in fields of agricultural crops. This knowledge will be useful in predicting and managing aphid infestations in agricultural fields.

Technical Abstract: Winter canola (Brassica napus L.) is typically planted during October in central Oklahoma and is harvested in June. Three aphid species typically infest the canola crop in central Oklahoma: the cabbage aphid, Brevicoryne brassicae L., green peach aphid, Myzus persicae (Sulzer), and turnip aphid, Lipaphis pseudobrassicae (Davis). We sought to determine whether landscape structure at a spatial scale roughly corresponding to the dispersal ability of aphid parasitoids influenced parasitism levels of aphids in canola fields. To that end, we evaluated how composition and configuration of the landscape surrounding each of 23 canola fields sampled during three growing seasons (7, 8, and 8 fields per growing season) was related to aphid relative density and parasitism rate. Parasitism rate was estimated using aphid infested sentinel canola plants stationed in fields in autumn and spring of each growing season. Two parasitoids, Diaeretiella rapae (M’Intosh) and Aphelinus nigritus (Howard), parasitized turnip aphids on sentinel plants. Diaeretiella rapae was consistently present in canola fields, whereas A. nigritus occurred sporadically. Significant correlations to landscape variables were observed for both parasitoid species and for aphids in autumn, but there were no significant correlations during spring. Parasitism rate by A. nigritus in autumn was positively correlated (P < 0.05) to the abundance of woodlands. Parasitism rate by D. rapae in autumn was positively correlated to landcover of wetlands and to high patch density (small patches). There were relatively large in magnitude but non-significant (0.05 < P < 0.15) correlations for parasitism rate by D. rapae to contagion and to landcover by grasslands and woodlands. Aphid relative density in canola fields during autumn was positively correlated to landcover of wetlands and patch density as well as to other landscape variables. Partial redundancy analysis demonstrated that parasitism rate by D. rapae was strongly positively related to landcover of wetlands and to patch density, negatively related to contagion, and nearly independent of landcover of grasslands and woodlands. Parasitism rate by A. nigritus was positively but not strongly correlated to landcover of grasslands and woodlands. How habitat and/or host specialization by D. rapae interact with landscape structure to determine observed patterns of parasitism in canola fields in Oklahoma is discussed.