Fungicides and strawberry pollination-Effects on floral scent, pollen attributes and bumblebee behavior

Abstract

Fungicides are used in agriculture to protect crops from various fungal diseases. However, they may modulate the plants metabolism. Moreover, fungicides can accumulate in the environment and may cause toxic effects on non-target organisms such as nectar microbes and pollinators. Nectar microbes contribute to the volatile profile of flowers and can influence pollinators behaviour. Thus, fungicide treatment could potentially affect the pollination. In this study, we investigated the influence of fungicide treatment on floral attributes as well as the behavioural impact on bumblebees. In separate experiments, we used one or both strawberry cultivars (Fragaria × ananassa var. Darselect and Malwina), which were either kept untreated (control) or treated with either Cuprozin® progress or SWITCH® fungicide. We analysed various flower traits including volatiles, pollen weight, pollen protein, and the attraction of bumblebees towards the flowers in the greenhouse. Additionally, we analysed the viability of pollen and pollen live-to-dead ratio, as well as the composition of nectar fungi in the field. A treatment with Cuprozin® progress led to a lower emission of floral volatiles and a slightly lower pollen protein content. This had no impact on the visit latency of bumblebees but on the overall visit frequency of these flowers. The treatment with the fungicide SWITCH® resulted in a higher emission of floral volatiles as well as a delayed first visit by bumblebees. Furthermore, flowers of control plants were visited more often than those treated with the two fungicides. Plant-pollinator interactions are highly complex, with many contributing factors. Fungicides can have an impact on the pollen quality and pollinator attraction, potentially leading to an altered pollen dispersal by pollinators and a change in fruit quality.

Fig 1. Flower volatiles of fungicide treated strawberry plants grown in the greenhouse (2020).

Plants (Fragaria × ananassa var. Malwina) were untreated [control (CTR)] or fungicide-treated [Cuprozin^® progress (CU), SWITCH^® (FR)]. (A) The volatile composition (averaged over replicates within groups) and (B) non-metric multidimensional scaling (NMDS; with Kulczinsnky distance matrix) of the volatile composition with scores (coloured symbols; samples within each group are surrounded by convex hulls and the corresponding medians of the groups shown as crosses) and loadings (blue compound names). Results of the ADONIS are shown in the graph (B); n = 7–8 replicates per fungicide treatment.

Fig 2. Amplicon sequence variants (ASV) of nectar fungi of fungicide treated plants placed in the field (2021).

Displayed are the richness (A), Shannon diversity (B) and the composition (C) of nectar fungi of flowers from control (CTR; green) or fungicide-treated [Cuprozin^® progress (CU; yellow), SWITCH^® (FR; red)] plants (Fragaria × ananassa var. Malwina). Data points of the richness (A) and Shannon diversity (B) is presented as strip plot while the composition (C) is presented in a stacked bar plot which represents the mean distribution in ASV over the fungicide treatment groups. Presented are the fungi genera with an abundance of more than 5% per sample; n = 2–3 replicates per fungicide treatment.

Fig 3. Pollen traits of fungicide treated strawberry plants placed in the field (2020).

Total number of pollen grains (A) and live-to-dead ratio of pollen grains (B) of pollen from control (CTR; green) or fungicide-treated [Cuprozin^® progress (CU; yellow), SWITCH^® (FR; red)] plants (Fragaria × ananassa var. Malwina). Data is presented as box-whisker plots with interquartile ranges (IQR; boxes) including medians (horizontal lines) and whiskers (extending to the most extreme data points with a maximum of 1.5 times the IQR), while black dots indicate the means; individual values are given as circles. Significant values (P < 0.05) of the generalised linear models are highlighted in bold and the Tukey post hoc test is indicated by different lowercase letters; n = 10–12 replicates per fungicide treatment.

Fig 4. Behavioural responses of Bombus terrestris workers towards treated flowers (2020).

Plants [Fragaria × ananassa var. Darselect (DS), Malwina (MW)], grown and offered in a greenhouse were untreated [control (CTR)] or fungicide-treated [Cuprozin^® progress (CU), SWITCH^® (FR)]. Displayed are the latency until the first flower visit (A), duration of that first visit (B), frequency of plant visits (C) and total duration of plant visits (D). Data is presented as box-whisker plots with interquartile ranges (IQR; boxes) including medians (horizontal lines) and whiskers (extending to the most extreme data points with a maximum of 1.5 times the IQR), while black dots indicate the means; individual values are given as circles. Significant differences are shown in Table 2; A, B: n = 4–9, C, D: n = 40–56 replicates per fungicide treatment.