Discussion
We asked how interactions between two hypothesized faba bean stressors – herbivory and lack of insect pollination – affect yield components (Fig. S1B). While lack of insect pollination is a clear stressor of faba bean, as it related negatively to several yield components, effects of herbivory by B. rufimanus were less straightforward.Bruchus rufimanus effects changed depending on the scale investigated: individual plant or plant-stand scale, when averaging individual plant scale responses at the cage scale. While no interactions between lack of insect pollination and herbivory were observed at the plant-stand scale (Fig. S1B), at the plant scale interactions were antagonistic, with increasing B. rufimanusdamage increasing yield components but only in the absence of pollinators. No negative effects of B. rufimanus on yield components were found at the plant-stand scale. Differences between the plant-stand and plant scale analyses in the effect on B. rufimanus are likely due to high variation in the level of herbivory damage among individual plants within cages (Fig. S3).
Despite high variation between faba bean cultivars in their dependency on pollinators (Bishop et al., 2020; Bishop & Nakagawa, 2021), lack of insect pollination generally leads to lower yield in faba bean crops (Cunningham & Le Feuvre, 2013; Free & Williams, 1976; Raderschall et al., 2021; Riedel & Wort, 1960; Suso et al., 1996; Suso & del Río, 2015). This study confirms that lack of pollination decreases several faba bean yield components and is in line with a recent cage experiment using the same cultivar (Tiffany), where bean weight per plant and beans per pod but not individual bean weight benefitted from insect pollination (Raderschall et al., 2021). In the absence of pollinators, bean weight and number of beans per plant decreased by 15% and 17%, respectively. Costs of lacking insect pollination were evident despite high frequencies of B. terrestris robbing, a common behaviour in this crop (Marzinzig et al., 2018), supporting the idea that robbing may benefit yield by increasing selfing in faba bean (Kendall & Smith, 1975). Counterintuitively, visitation rate and proportion of legitimate flower visits decreased with number of open flowers (after sugar-water addition), while proportion of nectar robbing increased (Fig. S6). This is likely due to nectar deprivation at the start of the study, which meant that B. terrestris shifted from robbing nectar to legitimate visit later in the season when there were less flowers open. The low frequency of legitimate flower visits could, explain why the decrease in bean weight per plant due to lack of pollination (15%) was modest when compared to results from a recent meta-analysis of faba bean pollinator dependency (37% on average, Bishop & Nakagawa, 2021) or an experiment using the same cultivar (Tiffany) and bumblebee hives (61%, Raderschall et al., 2021). In addition, a recent field study using the cultivar Tiffany, found no dependence on insect pollination (Lundin & Raderschall, 2021), indicating high variation in pollination benefit within this cultivar, likely mediated by environmental conditions. Finally, despite observed yield benefits with B. terrestris in this experiment, field studies have found that less frequent and more specialised pollinator species with long tongues, such as B. hortorum , improve pollination and cross-fertilisation of faba bean compared to B. terrestris (Marzinzig et al., 2018). Further studies investigating pollination and herbivory interaction in faba bean should include a greater diversity of pollinator species varying in their behaviour.
The effect of the pest B. rufimanus on crop yield was inconclusive. While there was a higher proportion of damaged beans in the presence of B. rufimanus, which meant that the stand was non-marketable for human consumption due to aesthetic damage to the beans (> 3% damage threshold, Bachmann et al., 2020; Roubinet, 2016) (Fig. S3b), this did not translate to a lower total or individual bean weight, number of beans or pods per plant. This is in contrast to bean aphid herbivory, which reduced all yield components (Raderschall et al., 2021). However, we found a marginal interaction (p=0.06) between pollination and B. rufimanus herbivory on individual bean weight, whereby individual bean weight was heavier with pollination, but only when B. rufimanus herbivory was absent. While we found no evidence that bumblebees legitimately visited flowers less frequently, proportion of legitimate flower visits decreased and EFN visits increased in the presence of B. rufimanus . Insect pollination has been shown to increase individual bean weight (Suso & del Río, 2015), therefore, reduced proportion of legitimate flower visits due to B. rufimanus direct interference with pollinators or indirect damage caused by feeding on pollen, could lead to lower individual bean weight. There is, to our knowledge, no literature investigating damage to flowers or pollen by B. rufimanus adults. This interaction was, however, not apparent on other yield components, such as number of beans per pod and per plant. This might be because the number of beans is primarily determined by ovule fertilisation (pollination), while individual bean weight might also be affected by plant resource allocation after fertilisation, which could have been adversely impacted by the presence of B. rufimanus (Patrick & Stoddard, 2010). Adverse herbivory effects of leaf chewers and sap feeders on bean weight have been found in Fabacea (Cuny et al., 2018; Raderschall et al., 2021). Therefore, resource allocation to developing beans might have been compromised by B. rufimanus and lead to lower individual bean weight when pollinators were present.
We detected a high variation in B. rufimanus damage between plants within cages (Fig. S3). When analysing the effect of B. rufimanus, at the individual plant scale we detected interactions and, generally, positive relationships between B. rufimanus herbivory damage and yield components, but only in the absence of pollination byB. terrestris . This indicates that the level of damage caused byB. rufimanus affects plant resource allocation, and that this response depends on the presence of pollinators. Two non-exclusive mechanisms, plant overcompensation and pollination by adult B. rufimanus , explaining these results are discussed below.
A biological process, which potentially underlies the differential response to herbivory depending on pollination treatment, is the capacity of faba bean plants to overcompensate for damage or stress (López-Bellido et al., 2005). Faba bean might overcompensate for early and high pod damage by B. rufimanus by increasing pod production or bean weight during their growth. This is because the growth of faba beans is indeterminate, and while pods are developing at the lower nodes, flower production continues at the upper ones. This leads to competition for resources within the plant between the first set of pods, the roots, bean growth, as well as with the developing pods further up the stem (Jaquiery & Keller, 1978; Smith, 1982). Roots were on average 5% shorter in the presence of B. rufimanus and root length correlated negatively with herbivory damage. This indicates alterations in resource allocation from roots to other plant parts in the presence of B. rufimanus (Heinze, 2020). Further studies quantifying faba bean root nodules in interaction with herbivory and lack of pollination would shed some light on plant resource acquisition and allocation in relation to biotic stress. Plants might have overcompensated for herbivory damage in the absence of pollinators, as several yield components in highly damaged plants were higher compared to undamaged plants (Fig. 2). Overcompensation in terms of seed-set in response to herbivory is common in other crops such asBrassicaceae (Gagic et al., 2016; Rusman et al., 2018). Increase in yield components with increasing B. rufimanus damage were only visible in the absence of pollinators, when plants were stressed by both a lack of pollination and high herbivory damage. Plant resource allocation is complex, especially so in plants with indeterminate growth. We found that in the absence of pollinators, faba bean plants had a higher above-ground biomass, likely to compensate for insufficient fertilisation (Raderschall et al., 2021). Higher plant biomass might enhance photosynthetic capacity and result in higher ability to overcompensate to B. rufimanus damage in the absence of pollination. However, very little is known about how plants optimize their resource allocation under multiple stressors.
Another non-exclusive biotic process that can explain the observed increases in yield with increasing herbivory in the absence of bumblebee pollination is that of pollination by B. rufimanus . Indeed, florivorous herbivores potentially act as pollinators if they transfer pollen between (cross-pollination) or within (self-pollination) plants. For example, damage by the bud-clipping weevil Anthonomus signatus lead to an increase in self-pollination in strawberries (Penet & Collin, 2009). Faba bean plants that had greater level of herbivory damage might have had flowers that were visited more often by B. rufimanus , leading to positive associations between damage and pollination by B. rufimanus and increased yield components in highly damaged plants. The positive effects of B. rufimanus on yield were only visible in the absence of bumblebee pollination, an indication that when more efficient pollinators are present, B. rufimanus does not benefit faba bean pollination. While the net effect of bumblebees on yield components was positive, in the presence of both bumblebees and B. rufimanus , some plants likely received high numbers of visits and sustained more flower damage than what they could compensate for (Sáez et al., 2014), leading to lower bean weight per plant in the pollination treatment under high levels of herbivory damage. The fact that B. terrestris mainly robbed nectar instead of conducting legitimate visits could have influenced the results. Indeed we hypothesize that more legitimate visits would lead to higher rates of cross-pollination and higher yields in the presence of pollinators. In iris, a short-tongued bumblebee shifted to more nectar robbing and longer flower handling time during legitimate flower visits than long-tongued species because of increased competition with a florivorous sawfly (Ye et al., 2017). We did not find an effect ofB. rufimanus on nectar robbing, but there was a negative effect on proportion of legitimate flower visits. . While nectar robbing is a common foraging behaviour in the field (Marzinzig et al., 2018), further studies also with long-tongued bumblebee species present, which mainly conduct legitimate visits, are necessary to understand interactions between florivory and pollination in faba bean.
Differences in B. rufimanus effects at different scales (plant-stand versus individual plant level) are likely due to differences between plants in the amount of florivory, oviposition and pollination visits they received. At the plant-stand scale, taking into account variation in B. rufimanus herbivory damage, pollinators have a positive effect while B. rufimanus only have a tendency to interact with pollinators and negatively affect individual bean weight. On the other hand, at the individual plant scale, herbivory damage byB. rufimanus strongly and positively correlates with yield components but only in the absence of pollinators. Effects of B. rufimanus on yield components were only visible when high levels of pest damage per plant (>50%, see Fig. 2a,b) were included in the analysis – damage levels, which are not occurring at plant-stand-level means in the cages, which are below 50% (Fig. S3b). In the field, B. rufimanus damage is, to our knowledge, generally lower than 50% and so interactions with pollinators might be less relevant for commercial production. In addition, the scenario of pollinators being absent, which is where B. rufimanus benefitted yield components, is not, yet, realistic in the field. Therefore, under field conditions, it is unlikely that B. rufimanus directly affects faba bean crop yield. However, the negative and indirect effects of B. rufimanus on individual bean weight and proportion of legitimate visits by pollinators call for further studies of these interactions in the field.
In summary, we confirm an insect pollination benefit on several faba bean yield components despite low rates of legitimate pollination, whereas plant responses to B. rufimanus herbivory differed on the individual versus plant-stand scale. Interestingly, positive effects ofB. rufimanus herbivory were found on faba bean yield components, but only in the absence of pollinators and under high levels of herbivory damage. Another interesting result is that of a tendency for higher individual bean weight due to pollination, but only in the absence of B. rufimanus . Further studies at the plant scale level to understand how the plant allocates its resources under varying levels of pollination and herbivory would be required to clarify the mechanisms driving these interactions. In addition, to disentangle the effects of pollen limitation from other factors, such as flower damage due to high visitation rates, it will be important to investigate effects of herbivory in hand pollinated plants. This is the first experimental evidence of interactive effects of B. terrestris and B. rufimanus on faba bean plants. While our results strengthen the case for management of pollinators to maximize pollination benefits in faba bean, there is no evidence for direct yield losses, in terms of total bean weight and numbers, at infestation level of B. rufimanusthat typically occur in the field despite high bean damage levels. Bean damage by the larvae will decrease faba bean saleability and germination and thus requires control. However, findings of a negative and indirect association between B. rufimanus and individual bean weight and proportion of legitimate visits by pollinators call for an improved understanding of these interactions in the field.