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Genetically modified Bt maize: New test with bee larvae

Scientists at the University of Würzburg have for the first time investigated how well bee larvae cope with Bt maize pollen under controlled laboratory conditions. The results of their experiments have now been published: Pollen from Bt maize MON810 and from a Bt maize variety that produces three different Bt proteins was not found to have any harmful effect on the sensitive larval stages of bees. The larval test is part of a biological safety research project funded by the German Federal Ministry of Education and Research (BMBF).

Honey bee gather maize pollen

A bee collecting maize pollen

Bee larva with maize pollen diet

Bee larva in food solution with pollen

The Bt curve shows the 100% survival rate of the larvae that were given Bt pollen (both Bt maize treatments combined, 40 larvae). The C curve shows the survival rate of the larvae that were given conventional maize (three conventional maize varieties, 59 larvae). Compared with the other treatments, the larvae fed on Heliconia pollen (H; 10 larvae) had a significantly lower survival rate.

Until now, the effect of Bt maize on bees has largely been investigated using adult insects, despite the fact that Bt proteins attack the larval stages of the pests they target. Where impacts on bee larvae have been investigated, it has been only at colony level in field trials, and not under controlled laboratory conditions.

Now Harmen Hendriksma and his colleagues at the University of Würzburg have successfully carried out the first larval test of this kind. Since raising bees in the laboratory or transferring newly emerged bee larvae to the laboratory is very difficult, they came up with a system in which the queen lays her eggs in artificial honeycombs in the beehive, which are then completely removed about 90 hours after the eggs have been laid without disturbing the new larvae, so that they can be transferred to the climate chamber. Here the larvae develop at a constant temperature of 35°C unaffected by environmental factors or the brood care behaviour in the beehive.

Flower pollen is the only source of protein for honeybees. If GM plants are grown near a beehive, the bees will collect pollen from them and carry it back to the hive. A worker bee consumes around four grams of pollen per day. Hardly any of it is found in the brood food that the nurse bees produce with their glands and feed to the bee larvae, but they do add pollen to the brood food. Swiss scientists have found 1720 to 2310 maize pollen grains, or 1.5 to 2 milligrams, in the gut of bee larvae (the colonies were kept in flight tents where the only pollen available was maize pollen).

In the in vitro larval test developed by the Würzburg scientists, each bee larva was given a realistic dose of two milligrams of maize pollen, which was added to its food. Each group of 20 larvae was given maize pollen from one of five maize varieties tested in the experiment, including two types of Bt maize: Bt maize MON810, which produces a Bt protein that targets the European corn borer, and MON89034 x MON88017, which produces three different Bt proteins – two that target the European corn borer and one that targets the Western corn rootworm. Some groups of bees were given pollen from one of three conventional maize varieties as a control. As a positive control to test the sensitivity of the test, pollen from Heliconia rostrata, an ornamental plant that is known to be poisonous to bees, was used, and one trial group was given food containing no pollen.

The test started with three-day-old larvae, which were then given a diet of brood food and aqueous solution for 120 hours until they stopped eating and growing shortly before the pupation stage. In order to simulate natural pollen consumption, fresh pollen was added directly to the food solution once only on the sixth day when the insects were in the third larval stage. From this development stage onwards the brood food in the beehive also contains pollen. The larvae were observed throughout the duration of the trial and the mortality rate was recorded. In order to obtain information about any potential chronic, non-fatal effects, the weight of the larvae before pupation was also recorded.

All larvae that received Bt maize pollen survived until pupation. Even the combined Bt proteins, which are found in much greater concentrations in pollen than the Bt protein produced by MON810 maize, evidently pose no risk to honeybees. The survival rate of bee larvae fed on pollen from the conventional maize varieties was slightly lower. Significantly higher mortality rates were observed in the bee larvae fed on Heliconia pollen, as was to be expected. The weight of the larvae shortly before pupation was not affected by the different types of maize pollen.

The Würzburg scientists believe the test they developed can be used as a standard method for assessing the risk of GM plants and crop protection products.

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