Banker Plant Bonuses? The Benefits and Risks of Including Brassicas in Field Margins to Promote Conservation Biocontrol of Specialist Pests in Oilseed Rape

Abstract

European agri-environment schemes include the use of flower-rich field margins to promote on-farm biodiversity, but species mixtures rarely include Brassicaceae. As pests of oilseed rape (OSR; Brassica napus) and their parasitoids are mostly brassica specialists, including brassica 'banker plants' in the mixtures would help support these important biocontrol agents and improve pest control throughout the crop rotation. We assessed the potential of six brassicaceous plants (replicated plots grown in the field) to enhance populations of parasitoids of OSR pests whilst minimising proliferation of their pest hosts. Fodder radish (Raphanus sativus) facilitated high production of parasitoids of the pollen beetle pest (Brassicogethes aeneus) but may proliferate Ceutorhynchus weevil pests due to low parasitism. Turnip rape (B. rapa) and the B. rapa hybrid 'Tyfon' showed potential to perform a trap cropping function for pests, but their early flowering phenology resulted in B. aeneus larvae escaping parasitisation, potentially assisting proliferation of this pest. Forage rape B. napus exhibited similarly high B. aeneus parasitoid production characteristics to R. sativus but did not potentiate problems with other pests, indicating that it would be a favourable banker plant option. Careful selection of plants in field margin mixtures is therefore needed to maximise their benefits and ideally the whole crop pest-beneficial complex needs to be studied, as focus on a single major pest risks unintended consequences with other pest problems.

Keywords: Brassica napus; Brassicogethes aeneus; Ceutorhynchus; Dasineura; Meligethes; agri-environment schemes; ecosystem services; rapeseed; regenerative agriculture.

Figure 1

Key brassica plant growth stages relative to suction sample dates (dotted lines), averaged over four plots of each Brassicaceae treatment ^†; horizontal bars represent the range of dates over which the key growth stages were attained. Filled green circles: ≥25% of plants at GS 51 (green bud); filled yellow triangles: ≥25% of plants at GS 60 (onset of flowering); crosses (x): flowering finished for ≥95% of plants. ^† Brassicaceae treatments: Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR; Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (hybrid of B. rapa Rapifer group × B. rapa Pekinensis group).

Figure 2

Mean numbers (log scale—left axis, and back-transformed -right axis) of pollen beetle (Brassicogethes aeneus) (a) adults, (b) larvae and (c) parasitoids in suction samples taken on five different dates from plots (n = 4) of different Brassicaceae: Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR, Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (a hybrid of B. rapa Rapifer group × B. rapa Pekinensis group). Confidence intervals (CIs; vertical bars) are valid for within-date comparisons; for between-date comparisons, CIs should be based on SEM values of (a) 0.1711, (b) 0.1219 and (c) 0.0827.

Figure 3

Mean numbers (log scale—left axis and back-transformed –right axis) of cabbage seed weevil Ceutorhynchus obstrictus (a) adults and (b) parasitoids in suction samples taken monthly from plots (n = 4) of different Brassicaceae: Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR, Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (a hybrid of B. rapa Rapifer group × B. rapa Pekinensis group). CIs (vertical bars) are valid for within-date comparisons; for between-date comparisons, CIs should be based on SEM values of (a) 0.1022 and (b) 0.0999.

Figure 4

Mean (log scale—left axis and back-transformed -right axis) numbers of Brassica pod midge (Dasineura brassicae) (a) adults and (b) parasitoids in suction samples taken monthly from plots (n = 4) of different Brassicaceae: Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR, Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (a hybrid of B. rapa Rapifer group × B. rapa Pekinensis group). CIs (vertical bars) are valid for within-date comparisons; for between-date comparisons, CIs should be based on SEM values of (a) 0.1169 and (b) 0.0974.

Figure 5

Mean numbers (log scale—left axis and back-transformed right axis) of cabbage stem weevil (Ceutorhynchus pallidactylus) (a) adults and (b) parasitoids in suction samples taken monthly from plots (n = 4) of different Brassicaceae: Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR; Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (a hybrid of B. rapa Rapifer group × B. rapa Pekinensis group). CIs (vertical bars) are valid for within-date comparisons; for between-date comparisons, CIs should be based on SEM values of (a) 0.0806 and (b) 0.0723.

Figure 6

Proportion (back-transformed logistic regression predictions, ±CIs) of (a) pods infested by cabbage seed weevil (Ceutorhynchus obstrictus) larvae; (b) seed weevil-infested pods with parasitised larvae; (c) pods infested by brassica pod midge (Daseneura brassicae) larvae. Pods sampled from plots of different Brassicaceae: Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR; Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (a hybrid of B. rapa Rapifer group × B. rapa Pekinensis group).

Figure 7

Mean number (log scale, ± SEM) of parasitoids of (a) Pollen beetle (Brassicogethes aeneus), (b) Cabbage stem weevil (Ceutorhynchus pallidactylus), and (c) Brassica pod midge (Dasineura brassicae) captured emerging from the site of different Brassicaceae plots (n = 4): Fodder radish (Raphanus sativus) cv Apoll; Oilseed rape (OSR; Brassica napus) cv Castille; Forage rape (Brassica napus) cv Emerald and cv Hobson; Turnip rape (Brassica rapa) cv Jupiter and Tyfon (a hybrid of B. rapa Rapifer group × B. rapa Pekinensis group). Back-transformed values are shown above each bar.