Spatiotemporal distancing of crops reduces pest pressure while maintaining conservation biocontrol in oilseed rape

Abstract

BACKGROUNDAgricultural landscapes provide resources for arthropod pests as well as their natural enemies. To develop integrated pest management (IPM) practices, it is important to understand how spatiotemporal location influences crop colonization and damage severity. We performed a 3‐year (2016–2018) field experiment in winter oilseed rape (OSR, Brassica napus) fields in Estonia, where half of the fields were within 500 m of the location of the previous year's winter OSR field and half were outside this zone. We investigated how distance from the previous year's OSR crop influences the infestation and parasitism rates of two of its most important pests: the pollen beetle (Brassicogethes aeneus) and the cabbage seed weevil (Ceutorhynchus obstrictus).RESULTSWhen the distance from the previous year's OSR crop was >500 m, we recorded significantly reduced pest pressure by both B. aeneus and C. obstrictus in the study fields. Biocontrol of both pests, provided by parasitic wasps, was high in each study year and commonly not affected by distance. Mean parasitism rates of B. aeneus were >31%, occasionally reaching >70%; for C. obstrictus, mean parasitism was >46%, reaching up to 79%, thereby providing effective biocontrol for both pest species.CONCLUSIONSpatiotemporal separation of OSR fields can reduce pest pressure without resulting in reduced parasitism of OSR pests. This supports a spatiotemporal field separation concept as an effective and sustainable technique for IPM in OSR.

Parasitoids of the cabbage seed weevil deliver high and consistent parasitism in variable landscapes: A showcase of conservation biocontrol

“…S4). Like Sulg et al, 11 we found the distance to the previous year's closest OSR field to influence cabbage CSW infestation rate (Table 4). In 2 out of 3 years Sulg et al found pod damage in fields with the previous year's closest field located more than 500 m from the current year's OSR field to be smaller than in fields with closest field located within 500 m (0.88% vs 3.28% and 14.18% vs 20.58%, respectively).…”

“…The parasitoids of OSR insect pests have been the object of intense scientific work over the last five decades, resulting in a wealth of detailed knowledge of their biology and ecology, especially of pollen beetle (Brassicogethes aeneus (Fabricius)) [4][5][6] and cabbage stem flea beetle (Psylliodes chrysocephala (Linnaeus), 7,8 but also of the cabbage seed weevil (CSW; Ceutorhynchus obstrictus (Marsham). [9][10][11] However, although farmers are becoming more aware of parasitoid presence and activity 12,13 the concept of protecting the parasitoids naturally present in the field still does not seem to play a decisive role in farmers' spraying decisions, mainly because of the complexity of OSR pest management. There are at least five important OSR pest species, each with its specific parasitoid species.…”

“…Landscape factors (field size, field location, field management, changes in OSR area) have been studied extensively for parasitoids of pollen beetles. 11,[19][20][21] The main parasitoids of pollen beetle spend most of their life in OSR fields, overwintering in the harvested OSR field and moving to the following year's OSR field immediately after emerging. 5 For both pollen beetles and their parasitoids, the area of, the distance to and the management (e.g., tillage) of last year's OSR fields in the local area have been suggested as factors of importance for their occurrence in a specific OSR field.…”

“…21 However, the different temporal and spatial dynamics of the OSR resource seen in different contexts result in inconsistent results, and several studies have added interannual changes in OSR area to this list of landscape factors affecting pest and parasitoid densities. 11,19,22 In this study, we aimed to assess the occurrence and activity of CSW parasitoids in Danish OSR fields and identify the local landscape factors influencing them. Specifically, we hypothesized that (i) parasitoids of the cabbage seed weevil commonly occur in OSR fields in Denmark, (ii) parasitism is comparable in magnitude to what has been found in other European OSR-producing regions, (iii) parasitoid occurrence and parasitism in the current OSR crop are associated with landscape elements offering food resources and overwintering sites present in the local landscape, and (iv) parasitoid occurrence and parasitism in OSR fields are influenced by the temporal and spatial dynamics of OSR in the local landscape (current and previous year's area and proximity) as well as by insecticide use in previous year's OSR fields.…”

Spatiotemporal isolation of oilseed rape fields reduces insect pest pressure and crop damage

“…Distance decay effects of pest dispersal between growing seasons are thus likely a general phenomenon that could be used for regulation of host specialised pest species. While distance decay effects could also be assumed for top‐down control by specialised parasitoids, parasitisation remained stable independent of distance to the host crop in the previous season in a recent assessment in winter oilseed rape (Sulg et al, 2023).…”