Polen theft by honeybees hits Sikkim's cardamom yield

 
By Maureen Nandini Mitra
Last Updated: Saturday 04 July 2015

Honeybees deprive flowers of p (Credit: P A SINU)sikkim's most important cash crop is ailing. India's biggest producer of large cardamom, Sikkim, has recently witnessed a sharp fall in crop yields, forcing many farmers to abandon large cardamom plantations.

The decline has been largely attributed to a spate of viral and fungal infestations over the past three decades. But new research says pollen theft by a local breed of honeybees, which farmers maintain to facilitate pollination, could also be a major cause.

What happens? Large cardamom is a key player in India's spice trade (see box Spice woes). As the fruits and seeds are the economic products of the plant, effective pollination is crucial, and to a large extent determines the yield.Two scientists from Bangalore's Ashoka Trust of Research in Ecology and the Environment (atree) have found that the honeybees (Apis cerana F) are actually leaving 70 per cent of the cardamom flowers un-pollinated; at the same time depriving the flowers of enough pollen for pollination by their only other visitor--the humble bumblebee. Their findings were published the August 25 issue of the Current Science.

K R Shivanna, the biologist who led the two-year study in Sikkim, says that a native bumblebee species (Bombus haemorrhoidalis Smith) is the only effective pollinator of the cardamom flower. "Hardly any studies have been done on the pollination biology of large cardamom. There's just one old report that says honeybees are the pollinators based on the number of times bees were observed visiting the flowers," he says.

The biologists note that the flower of the large cardamom is better adapted for pollination by the bumblebee. The flower's nectar is protected in a 3 cm long nectar tube which is formed by the fusing of its petals with the labellum--the large central petal or lip of the flower. The nectar is thus accessible only to insects with a long proboscis--elongated sucking mouthpart.
Pollen theft Though pollen grains on the anther (male, pollen-carrying part) are exposed and accessible to any visiting insect, the flower's cup-shaped stigma (female, pollen-trapping part of the flower), like its nectar tube, isn't designed for easy access. To effect pollination, the pollen has to be successfully transferred from the anther to the stigma.

The honeybee, with its short proboscis, buzzes in and out of same flower several times, trying in vain to reach the nectar. In the process, it depletes the flower of its pollen but is usually unable to penetrate it deep enough to effect pollination. Meanwhile, the bigger bumblebee, with its longer proboscis, penetrates the flower, sucks out the nectar and flits on to the next bloom, transferring pollen effectively.

So, the honeybees are "pollen robbers", says Shivanna. He adds that the decreasing population of the local bumblebee, possibly due to its habitat degradation, is yet another factor behind the low crop yields. "India lacks the technology to rear bumblebees, though the technology exists in the west," he says.

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