Nov 5, 2003

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Oil seed rape - Pollen dispersal and outcrossing

“We were surprised by the large number of wild bee species.”

When oilseed rape comes into flower, the fields are teeming with insects. Stefan Kühne and his colleagues were able to identify countless different species. Insects are responsible for transporting the rape pollen over vast distances, regardless of whether it is conventional or genetically modified oilseed rape. GMO Safety spoke to Stefan Kühne from the Federal Biological Research Centre for Agriculture and Forestry (BBA) in Kleinmachnow.

GMO Safety: In 1998 and 1999 you conducted trials of genetically modified herbicide-resistant rape on land belonging to the BBA in Dahnsdorf. You were particularly interested in pollen dispersal and outcrossing. The pollen is spread by wind and insects, regardless of whether or not the rape is genetically modified. Can you briefly explain what you found out about this?

Stefan Kühne: Oilseed rape flowers from late April to early June and we are all cheered by the sight of those magnificent yellow fields. Innumerable insects are drawn to this rich source of pollen and nectar: honey bees and many wild bee species in particular, but also hover flies, ladybirds and lacewings, which in our view are among the most important beneficial insects in the rape fields. Butterflies, especially white ones, sawflies, stinging wasps and St Mark’ s flies are also frequent visitors. Then there is the whole group of oilseed rape pests including the rape blossom beetle and various weevils. Insects can transport the pollen clinging to their hairs to other flowers even outside the field and against the wind.

Wind dispersal of pollen is quite a different matter. It is heavily influenced by wind direction, temperature and precipitation. Our investigations have shown that the pollen concentration in the air falls sharply at just a short distance from the field margin. At a distance of 10 metres from the field margin, the concentration of pollen in the air was only 4.5% of that found in the field itself. Pollen can of course be carried over considerable distances, but the probability of a pollen grain in the air pollinating a flower two kilometres away is fairly low. On the other hand, if the activity radius of the bumblebee (up to two kilometres) and honeybee (from five to a maximum of fourteen kilometres) is taken into account, it becomes clear that the spread of rape pollen to flowers outside the field is primarily carried out by insects.

GMO Safety: You looked more closely at which insects visit the rape flowers and contribute to pollen dispersal. You were particularly interested in bees. How many species did you find and how do they behave? Did you find any differences between conventional and transgenic oilseed rape plants in terms of their behaviour?

Stefan Kühne: We were particularly surprised by the large number of wild bee species. During the short time that the rape was in flower we identified a total of 94 different species. Six of these were even on the list of highly endangered bee species in Germany. Wild bees have very varied lifestyles. The 16 species of bumblebee identified form colonies like honey bees and some build their nests in holes in the ground, making use of old mouse holes.

Most wild bees are solitary, in other words they live alone, not in a colony with workers. Furthermore they do not feed their larvae directly, as bumblebees do. The gathered pollen is stored in a brood cell together with an egg, providing a food supply for the hatching larvae.

The red mason bee (Osmia bicornis) cares for its brood in this way. It nests above the ground in cracks in old walls or in dead hollow plant material. During the bee’s foraging flights its nest remains unguarded and so-called cuckoo bees take advantage of its absence to invade the foreign brood cell unnoticed and parasitise it. We were able to identify 25 different species of cuckoo bee, which often specialise in particular solitary bee species.

To find out which flowers the bees visit, we caught 129 individual bees from various different species on the trial field, marked them with a coloured dot and released them again. We found that the same bees visit both transgenic and conventional oilseed rape or even other cruciferous plants. We were unable to find any difference in flower visits to these plants.

GMO Safety: Bees not only disperse pollen, they also consume it. Have you investigated whether pollen from GM oilseed rape has a particular effect on the bees?

Stefan Kühne: We are currently examining one aspect of this problem in collaboration with Dr. Tebbe from the Federal Agricultural Research Centre in Braunschweig. For these studies we have established a honeybee colony, several bumblebee colonies and several thousand solitary mason bees on the trial field. Many bees have gathered transgenic oilseed rape pollen and fed it to their offspring. The bee’s intestine contains a large number of micro-organisms, which play an important role in food digestion. Can these micro-organisms incorporate the modified gene from the oilseed rape pollen into their own DNA? To find out, we sent prepared digestive tract samples from the bee larvae to our colleagues in Braunschweig for further investigations. We expect the final results to be available in early 2004.

Since the genetic trait of herbicide resistance has no relevance to the bees, our investigations served as a model to study the dispersal routes of the transgenes. If we know these, we can indicate possible risks which could result from the introduction of other transgenic traits. For example if an insect-resistant oilseed rape were to be grown commercially, it would be unacceptable if several flower-visiting insect species and their brood were endangered by insect toxins in the pollen and nectar. In this scenario, breeders would be required to prevent gene expression in the pollen right from the start.