Hopes that tomato growers will once again be able to use non-native bumblebees for crop pollination remain unrealised despite evidence collected by the British Tomato Growers Association that the current ban on these subspecies – introduced by Defra in 2015 – might have been unnecessary.
AHDB-funded work has looked at various characteristics of the native bumblebee, Bombus terrestris audax, which growers were mandated to use instead, and of tomato flower development and pollen production to understand why fruit set on many nurseries has suffered under the ban.
Dave Chandler of Warwick Crop Centre said studies on the genetic structure of B. terrestris populations point to the non-native subspecies B.t. terrestris already being established naturally at low levels in England, before growers had started using it to pollinate tomato crops.
The Tomato Conference in September heard that poor fruit set appeared more common in periods of hot weather. Results from practical studies suggest most B.t. audax colonies go into decline soon after being introduced to tomato crops and are less active in the glasshouse environment.
Taking its cue from research 20 years ago, which found fruit set is most affected by high-temperature periods between 13 and seven days before flowers have opened, the most recent work looked at the effect of high temperatures alone on tomato pollen production, in glasshouse experiments at Warwick Crop Centre overseen by Ken Cockshull.
Speaking at the conference, Dr Cockshull, who has spent a lifetime researching the crop physiology of tomato, said three varieties were grown at a standard constant temperature regime of 20°C or subjected to an elevated air temperature of 32°C during the day for seven days.
In all varieties, flowers produced much less pollen when high temperatures coincided with that seven-day period of sensitivity, which ends a week before flowers open. As little as two days of high temperature during the sensitive period was enough to have an impact. Production in flowers sampled 14 days after the high temperature period ended were unaffected.
Numbers of seed from flowers exposed to the higher temperature during their period of sensitivity were also reduced. “There were days when there was no seed in the fruit at all,” he said. “There is a clear impact on the number of fruit being set, especially on Piccolo.”
While the 32°C imposed on the flowers was extreme, Dr Chandler said temperatures lower than that, but higher than average, were likely to affect fruit set to the extent of having an economic effect on yield.
Read more news and features from the protected crop industry in our monthly publication The Commercial Greenhouse Grower.