Effects of cereal and cereal-flower companion planting on cabbage pests, natural enemies and yield
by Anna Sophie Tergast née Köneke
Date of Examination:2024-12-17
Date of issue:2025-08-21
Advisor:Prof. Dr. Michael Rostás
Referee:Prof. Dr. Michael Rostás
Referee:Prof. Rainer Dr. apl. Meyhöfer
Persistent Address:
http://dx.doi.org/10.53846/goediss-11443
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Abstract
English
In many countries, including Germany, growing public concern about pesticide use and the political aim of its reduction has increased the importance of alternative pest control systems. In this context, crop diversification practices can enhance in-field biodiversity and decrease pesticide reliance by improving habitat structure for natural enemies and suppressing pest herbivores. One specific type of these diverse practices is companion planting, which we define as adding one or several plant species to a cropping system to benefit the focal crop. Brassica vegetables, such as cabbage, have been the focus of numerous studies examining pest regulation through companion planting and similar crop diversification systems. Notably, cabbage has often been undersown with various types of clover. Despite the wealth of published research and political initiatives advocating for more sustainable production practices, the implementation rate of diverse cropping systems in cabbage and other Brassica vegetables remains relatively low. Therefore, this thesis focuses on a companion planting system with currently low representation in the literature: adding cereal companion plants, such as wheat or barley, to cabbage for pest regulation purposes. The primary aim of this thesis is to evaluate the effects of cereal companion plants, both independently and in combination with flower plants, on cabbage, focusing on key pests, their natural enemies, and cabbage harvest weight. Cereal plants are not part of the host spectrum of the major cabbage pests but have been commonly studied as banker plants in greenhouse settings, providing alternative prey/host sources for natural enemies. In the trials included in this thesis, winter varieties of cereal plants were sown between crop rows several weeks prior to cabbage transplanting to maximise their effects from the beginning of the cultivation period. These cereal plants would wilt during the cabbage cultivation period, allowing only the cabbage to be harvested. To meet the thesis objectives, the publication chapters present results from several field trials conducted either solely in central Germany or simultaneously in two locations: central Germany and Morioka in Japan. Chapter 1 introduces companion planting and a detailed overview of the focal study system. Chapter 2 reports on a five-year field study evaluating the effects of wheat companion plants in white and Chinese cabbage in central Germany. We compared the effects on three pests (cabbage flea beetles, aphids, and cabbage root flies), natural enemies (ladybirds, hoverfly larvae, spiders, ground beetles, and rove beetles), and crop harvest weight between cabbage grown with wheat companion plants and untreated control plots. We observed an overall regulating effect on aphid and flea beetle populations, but not on cabbage root flies. While the abundance of most natural enemies was not consistently affected by companion planting, pitfall trap captures of ground beetles decreased, whereas rove beetle captures increased. Notably, the harvest weight of crops grown with wheat was significantly lower over the study period. However, all parameters exhibited substantial variability between years, influenced by pest abundance, climatic conditions, and vegetation development. Chapter 3 tests the wheat companion planting system in combination with the flowering plant sweet alyssum, focusing on white cabbage pests (cabbage flea beetles and aphids), natural enemies (ladybirds, hoverfly larvae, and spiders), and harvest weight. Wheat companion planting and the combination with sweet alyssum intercropping were each compared to untreated controls and insecticide controls in a 2020 field trial at both central Germany and Morioka, Japan. In this year, wheat companion planting significantly reduced the abundance of Myzus persicae aphids but not Brevicoryne brassicae at both locations. Either system did not significantly impact the natural enemies of aphids and cabbage harvest weight, while flea beetle populations significantly increased in plots with added sweet alyssum. Chapter 4 continues the assessments with field trials conducted at the same two locations in 2021. This chapter evaluates the effects of cereal companion planting (with wheat or barley) and flower strips (consisting of phacelia, buckwheat, and coriander) on aphids and their respective natural enemies in white cabbage. Four treatments (untreated control, insecticide control, wheat companion planting, and barley companion planting) were compared, each with and without surrounding flower strips. Results differed between locations. In Japan, both wheat and barley companion plants and flower strips increased the relative abundance of major natural enemies in relation to aphid numbers, effectively reducing aphid infestations on crop plants. In contrast, that year, no significant suppression of aphid populations was observed at the German location for wheat/barley or flower strips in that year. Chapter 5 shifts focus to an in-field “banker plant” approach in white cabbage, evaluating the potential of barley companion plants infested with the aphid species Rhopalosiphum maidis on cabbage-infesting aphids and their parasitism rates. This aphid species serves as an alternative host for the parasitic wasp Diaeretiella rapae, an important parasitoid of cabbage-infesting aphids. In the first year (2022), four treatments (1. control, 2. barley companion planting, 3. R. maidis-infested barley companion planting, and 4. barley companion plants infested with R. maidis, including mummies parasitised by D. rapae) were compared in small-plot field trials. In the second year (2023), only treatments 1 and 3 were compared. All barley treatments reduced aphid numbers, regardless ofadditional R. maidis infestation. However, treatments did not significantly influence parasitism rates, indicating that parasitism is not the primary mechanism for aphid regulation in cereal companion planting systems. In Chapter 6, I discuss the findings from the previous chapters, situating them within the context of the pest regulation potential of crop diversification practices. Possible underlying mechanisms for the pest-regulating effects of the cereal companion planting system are explored, and its advantages and disadvantages are compared to assess the system’s applicability in horticultural practice. I conclude that cereal companion planting has the potential to serve as an additional tool in Integrated Pest Management for regulating aphids and flea beetles in cabbage. While natural enemies play a critical role in pest control, particularly in regulating aphids, we could not identify them as the primary mechanisms behind the pest-regulating effects of cereal companion planting in most trials. The integration of flower plants alongside cereal companion plants can benefit natural enemy populations; however, the two tested flower plant systems require further assessment before practical recommendation: sweet alyssum due to its attractiveness to cabbage flea beetles, and phacelia-buckwheat-coriander flower strips due to inconsistent results across locations. Also, banker plant approaches by providing alternative hosts on cereal plants might benefit parasitoids, but we could not detect effects on parasitism of cabbage aphids. Ultimately, the suitability of cereal companion planting for practical application in cabbage production will depend on specific field and farm conditions. While it can supplement existing pest control strategies, potential drawbacks – such as harvest weight loss interference with other farm management practices – must be carefully considered.
Keywords: Intergrated Pest Management; Banker Plants; Brassica pests; Intercropping; Companion planting
