The Summer Nectar Dearth And How To Spot It...

Many places experience a summer nectar dearth; a period when there is a shortage of nectar producing flowers for honey bees to forage on. The timing, length and severity of a summer nectar dearth will vary from place to place and season to season. A common dearth occurs in the transition from spring to summer. During spring, large volumes of nectar and pollen are produced by a variety of trees and plants bursting back into life after winter. In some rural areas oilseed rape (canola) will also provide a glut of nectar. However once this is all finished, there is often a ‘gap’ in available forage until summer plants are in full bloom. In the UK this phenomenon is enshrined in beekeeping folklore as the ‘June Gap’.

A summer nectar dearth is potentially devastating for a colony. It hits when the population is high and the bees are active. Stores can be wiped out very quickly. If the beekeeper has already harvested honey, then the problem could be worse.

Spotting a summer nectar dearth is therefore very important. However this time of shortage can be missed by new beekeepers as it is counter intuitive, after all it is summer! Everything is lush and green, some plants will be flowering, and the bees are flying. However even experienced beekeepers that know the timing and sequence of flowerings in their area can be caught out. A summer dearth is greatly affected by the prevailing weather, but other issues are having a major impact. Climate change is not only affecting weather patterns but also plant diversity; increased invasions by alien species are occurring, as conditions become more suitable for exotic species whilst native species become less well suited to their environment. Also changes in local land use and agricultural practices will have an impact.

By remotely monitoring hive weight beekeepers can accurately track nectar flows and dearth on a daily basis and respond accordingly. The graphs below show hive weights of colonies in the UK and Italy, from which it is easy to spot the nectar dearth.

At the UK hive, the ‘June Gap’ started on 14 May and a nectar flow returned about 10 June! This hive lost 17kg, about 650g/day. In Italy the hive weight started to decline on 21 May and fell by 8kg until a nectar flow returned on 11 June, losing an average of 400g/day. The rate of weight decline will depend on the size of the colony and the severity of the nectar dearth – whether there is some forage available, or a complete absence of nectar. However in both cases the data also shows us that the weight loss during the dearth is less than the weight gain in the previous nectar flow (after allowing for the addition and removal of supers, which can be identified from the graph). Thus beekeepers can track the real status of the nectar dearth on a daily basis, assess the store levels available and identify if and when supplementary feeding is needed. This data when collected from large numbers of hives over a wide area will also have great value for scientists investigating issues such as climate change, biodiversity, bee health and nutrition. Bees are true sentinels of the environment.

Behaviour of bees during a summer nectar dearth

Honey bees behave differently when nectar shortages occur. One of the most common problems is robbing. Strong colonies will rob weaker colonies of their nectar stores. Predators such as wasps and hornets may also be drawn to the scene of the crime. Once robbing begins, a colony can be quickly stripped of its food supply.The graph below shows the hive weight of a colony which has a sharp fall in weight 2 days in a row – the total weight loss is 11kg! This size and suddenness of the weight drop is very different to the steady weight decline exhibited when bees are consuming their stores.

Adding flight noise to the graph (pink line, graph below) clearly shows the significant rise in activity at the hive coinciding with the weight loss; this confirms that the colony was subjected to robbing. Another potential consequence of robbing is the transfer of Varroa mites back to the raiding colony. The robbers may return with mites as well as stolen honey. By following real colony status on a daily basis the beekeeper can spot behaviours such as robbing and take prompt action, in this case reducing the hive entrance.

Another interesting behaviour during times of nectar shortage is that honey bees, which are generalist pollinators, will turn to what seems a less suitable source of nutrition. They will visit Anemophilous (wind pollinated) plants. These plants have little or no nectar and an abundance of nutritiously inferior pollen. So why do bees bother with the second best? Research in this area has been scant but there has been plenty of anecdotal evidence of the phenomenon.

Most beekeepers know that hazel and willow are welcome early sources of pollen and yet both of these plants have evolved to be pollinated by wind. The catkins bearing an abundance of small, non-sticky pollen grains are a perfect example of a wind pollination adaptation. Similarly, wind pollinated olive and grape plants in the Mediterranean are frequently visited by bees in early June. This raises a number of interesting questions, such as do the these instances have a significance from an adaptive point of view? For the honey bee the significance of pollen collection and for the plant the significance of pollen dispersal? This is a question for evolutionary biologists but what is interesting to note is that some plants are wind and insect pollinated (ambophilous plants). For example sweet chestnut is an important nectar and pollen source for honey bees, yet its flowers bear physionomical features of a wind pollinated plant. Ash is another example of an ambophilous plant. Are these relationships incidental or do they uncover new understanding of the evolution of plant – pollinator interactions?

If you have any observations about bee behaviour during a summer nectar dearth please get in touch.

Images courtsey of Sandra Evans. Graphs copy right AgriSound Ltd.