The pollination puzzle

New findings about the biology and behaviour of the bumblebee subspecies that tomato growers now count on are helping researchers understand why its performance could be better, writes Claire Shaddick

Few would have predicted that reform of the licensing arrangements for bumblebees, which has seen tomato growers having to rely on the native subspecies, Bombus terrestris audax,for pollination, would result in some having to return to manual methods to ensure adequate fruit set.

Painstaking monitoring of B.t. audaxactivity is looking to pin down why pollination is so much poorer than with the European subspecies growers were able to use until 2015. The AHDB-funded six-month project was commissioned following a survey of growers that suggested small-fruited varieties, which now account for more than three-quarters of UK tomato production, have been particularly affected by the switch.

Warwick University researcher Dave Chandler believes it is the two changes – adoption of the native bee subspecies and the industry move to smaller-fruiting varieties – coinciding that is the root cause. So the work is also investigating flower development and pollen release in the widely grown variety Piccolo, to see if there is a lack of synchronicity between bumblebee behaviour and peak flow of pollen.

Entomologist Rob Jacobson said bumblebee numbers should normally continue to increase following introduction to the crop and up to the point when the colony naturally crashes, giving growers six to eight weeks of good pollination. But monitoring so far in commercial crops has found that while bee colonies were in good condition on arrival at the nurseries, more than half started to decline within a fortnight and a third within two to four weeks.

“This trend overrides supplier, tomato cultivar or location,” he said. “The only common factors seem to be the B.t. audaxbumblebee and tomato, which leads us to believe that B.t. audaxdoesn’t really do very well on tomato.”

In order to understand the interaction between bee, flower and environment, and answer questions such as whether high temperatures reduce bee foraging and pollen flow and how pollen availability affects the bee’s lifespan, researchers have had to gather basic information on the physiology of ‘new’ small-fruiting varieties, and on the bee’s activity.

For instance, they have followed the development of individual flowers to measure the length of time when pollen is available, and collected and counted pollen grains to see if flow varies with the time of day. “We have discovered that Piccolo produces less pollen than ‘classic’ varieties, which produce copious pollen,” said Dr Chandler.

They have also installed on a commercial nursery and in an experimental greenhouse at Warwick Crop Centre a prototype of the remote hive-monitoring system Arnia, which was originally developed for honeybees but which may be a useful tool in future for growers to track bumblebee activity during the season. “The sensors give real-time data on the weight of the hive; the outside temperature and temperature of the brood; hive humidity; and in-hive acoustics, which detect when bees are being active in the morning, ” said Dr Chandler.

Data generated by the prototype is being validated by manual monitoring of the bumblebees’ flight movements in and out of a hive in a greenhouse and in an orchard. These records already point to a significant difference in activity between the two sites, with the bumblebees in the greenhouse much less active than those in the orchard. “If this is typical in a greenhouse environment, something quite serious is happening,” he said.

As for why the bees are behaving differently, Dr Chandler says it’s too early to say. The higher brood temperature that has been recorded may be one explanation but he believes it may also have something to do with the quality of the pollen which is hindering bee development, and which perhaps could be overcome with additional nutrition for the hive.

“It raises all sorts of questions, such as could we breed tomato varieties with bees in mind?”, he added.