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Bees and Bt maize: Interactions with pathogens?

Bees in stress test

Bees are swarming around a pot containing yellow pollen feed. The newly emerged worker bees are obviously hungry. Over the next four weeks they will have to make do with pollen from just one maize variety. They are part of a feeding experiment being conducted at the University of Würzburg. A team at the Department of Animal Ecology and Tropical Biology is conducting a series of laboratory experiments to investigate whether bees can digest pollen from genetically modified Bt maize.

Dr Stephan Härtel from the Department of Animal Ecology and Tropical Biology at the University of Würzburg is leading the research on genetically modified Bt maize.

Dr Stephan Härtel, who is leading the research into genetically modified Bt maize, by an observation hive on the University of Würzburg site.

For each feeding experiment, Theresa Hügel places 56 newly emerged worker bees in small containers.

For each feeding experiment, Theresa Hügel places 56 newly emerged worker bees in small containers.

The bees are fed various pollen diets and sugar water for four weeks.

At the end of the trial the bee intestines are examined. The entire bee intestine can be pulled out with the sting.

Half of the bees were infected with Nosema, an intestinal parasite. Now the team will investigate whether the Nosema infection has affected the bees’ digestion.

A largely undigested grain of maize pollen

The Nosema spores are also clearly visible under the microscope.

Katja Rahn is conducting a behavioural experiment with bees.

Katja Rahn is conducting a behavioural experiment with bees.

A lemon fragrance is wafted over the bees shortly before they are given sugar water. The idea is to teach them to stretch out their proboscis in response to the fragrance signal.

“The bees are all no more than 24 hours old. They eat a lot of pollen particularly at the start of the growth phase, which is important for our experiments.” Theresa Hügel uses tweezers to place a young bee carefully in a small metal cage with a glass front.

A total of 30 of these containers are each stocked with 56 insects. Six containers form a trial group that will be fed one of five different diets over the next four weeks. The insects are given either pollen from genetically modified Bt maize, pollen from the same maize variety but without the genetic modification – the isogenic variety – or a mixture of pollen from other plants. All the trial insects are also given a sugar solution. “There is another diet as well,” says Theresa Hügel pushing a little jar of pollen solution into one of the cages, “pollen from the isogenic variety that has been treated with an insecticide. And one group is given only sugar solution.”

Combined stress factors

The Bt protein formed in Bt maize can only display its insecticidal effect if insects have the corresponding receptors in their intestinal wall. Different Bt proteins target specific groups of insects. The maize being studied in this project produces Bt proteins that are effective against the European corn borer, a moth, and the Western corn rootworm, a beetle. Bees do not have the necessary receptors for these Bt proteins so, as is to be expected, Bt maize has not yet been found to have any harmful effect on healthy bees. But what happens if the insects have already been weakened by disease? If they are exposed to several potential stress factors at the same time?

It is this question that Theresa Hügel is investigating with her feeding experiment. She wants to find out whether bees infected with intestinal parasites are less able to digest Bt maize pollen than healthy bees.

A few years ago in a field experiment at Jena University, bees fed exclusively on Bt maize pollen inside flight tents were found to be more sensitive to a chance infection by the intestinal parasite Nosema than bees fed on conventional maize pollen.

Clues from the bee intestine

The bees are kept in the laboratory in a well-controlled climate chamber at 26°C. After six days, half of the trial insects are infected with the intestinal parasite Nosema.

In order to detect a potential interaction between the Bt protein and the intestinal parasite, the life expectancy of the bees in the various trial groups is documented. At the end of the trial, the bee intestines are removed and examined to see whether Bt maize pollen is digested less easily than the other pollen varieties when bees are infected with Nosema. This could have an influence on bee health.

“When you pull out the sting, you can pull out the whole intestine easily,” says Theresa Hügel, as she valiantly grabs a sting with the tweezers and pulls an intestine out in one piece. She then examines the contents of the hindgut under the microscope. Using a counting chamber, a glass slide with a depression in the centre into which a grid has been etched, she counts exactly how many pollen grains have been eaten and to what extent they have been digested. The results of Theresa Hügel’s experiments will be ready later this year.

Citrus fragrance = sugar water

Katja Rahn gently clamps a bee into a small plastic column. “The bees go into the fridge first to stop them stinging,” she explains and picks up the next insect that has been made dopey by the cold. She places ten trial insects in a semicircle on the table in front of her and starts an interesting experiment.

Katja Rahn is interested in the learning behaviour of bees. Using a standard method from behavioural biology she is investigating whether genetically modified Bt maize has an effect on the learning ability of bees. Like Theresa Hügel, she feeds the insects different pollen diets, but only for two weeks with 30 bees at a time in one cage. Then she starts the conditioning process.

“If you offer bees sugar water, they stretch out their proboscis,” says Katja Rahn. She squeezes a drop of sugar water out of a pipette and watches the bee take it with its proboscis. Shortly beforehand she wafted a lemon fragrance over the bee. The aim is to teach the insects during the experiment, which lasts several days, that the proffered sugar water is connected with the fragrance so that they will eventually respond to the fragrance signal by sticking out their proboscis.

So far it does not appear that bees fed on Bt maize pollen learn any more slowly than the others.

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