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.