In this post Associate Editor Romina Rader discusses a recent paper she handled from Fabrice Requier and colleagues ‘The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands

When we think about pollinators within intensive agricultural systems, mass flowering crops (MFCs) seemingly act as both heroes and villains.  On the upside, many pollinators congregate at local mass flowering crops during the flowering season and feed on the abundant pollen and nectar available (Westphal et al. 2003; Rundlof et al. 2014). In addition to increased pollinator abundances, MFCs can also drive patterns in pollinator reproduction like the number of brood cells (Holzschuh et al. 2013).

On the downside though, other studies have found that a high percentage cover of these crops at the landscape scale is thought to result in lower pollinator abundances relative to semi-natural habitats (Holzschuh et al. 2016), and although known to facilitate bumblebee colony growth, have no effect on the likelihood of colonies producing sexual offspring (Westphal et al. 2008).  A high proportion of MFCs in a given landscape has also been associated with declines in the reproduction of insect-pollinated plants in some landscapes (Holzschuh et al. 2011), but not in others (Diekotter et al. 2010).  Patterns also seem to differ greatly among pollinator species and functional guilds.  With increasing amounts of oilseed rape in a landscape, Diekotter et al. 2010 found that long-tongued bumblebees visiting long-tubed plants decrease but nectar robbers increase.

Oilseed rape crops. Photo © Brian Robert Marshall, licensed for reuse.

The pros and cons of MFCs to pollinators seems to be mediated by the timing of the MFC resource boom (Jauker et al. 2012; Riedinger et al. 2014) whereby early flowering can benefit reproduction later in the season.  Yet, few studies have actually unravelled the mechanisms underlying pollinator impacts as a result of the boom-bust cycles of floral resource availability.

Apis melifera / Cirsium arvense
Apis mellifera on Cirsium arvense. Photo © Thierry Tamic, INRA.

The recent study by Requier et al. has made headway towards addressing this knowledge gap by investigating how the seasonal timing of MFC availability impacts the European honeybees’ capacity to harvest pollen and in turn, the impacts upon honeybee colony dynamics in central-western France.

Between the mass flowering period of two ubiquitous crops (rapeseed and sunflower), there is a two-month period when the honeybee’s diet is generally restricted to few floral resources in semi-natural habitats.  This May-June spring period also coincides with peak brood production.  So what do honeybees do in the absence of MFCs?

Bee colony monitoring. Photo © Thierry Tamic, INRA.

Requier et al. found that this lack of availability of floral resources resulted in a decline in pollen harvest.  This in turn resulted in lower brood production, which led to a smaller adult population size later in the season, lower honey reserves and higher Varroa mite loads.

What I found most interesting was that the severity of the response depended on the individual colony level resource acquisition and allocation strategy.  As the amount of honey reserve stored by bees in the brood chamber is limited by the space dedicated to brood, colonies may allocate more space to honey reserves in the hive brood chamber by reducing the brood size.  This means colonies can exhibit two strategies – they can maximize their reserves by preferentially allocating workers to honey storage instead of brood production; or they can do the opposite, i.e. maximise the brood at the expense of honey storage.

Pelotes de pollens en plan large
Pollen monitoring. Photo © Thierry Tamic, INRA.

Understanding how current beekeeping practices promote or limit these strategies is important as management can influence the survival of the colony and its storage resources.  For example, this study found that harvesting honey in the spring resulted in less honey during the July sunflower blooming period and halved the colonies survival probability over winter.  While bees are often fed with sugar syrup in May and June to combat this response, this may not compensate for a deficiency in brood production.  Avoiding the harvest of honey in early spring may improve honeybee survival and increase honey production in sunflower crops that mass-flower in July.

So, we now know a little bit more about the importance of timing when it comes to the availability of MFC resources.  Ensuring floral resources are available year-round in intensive agriculture systems is indeed a challenge that needs to be overcome.