Bt maize compatible with bees
Bees are very important as pollinators of many wild and cultivated plants. They fly from flower to flower collecting nectar and pollen and paying no attention to field boundaries. If genetically modified Bt maize is grown, bees will come into contact with the Bt toxin via the pollen. Are they harmed as a result? Swiss scientists have been investigating whether Bt maize is harmful to honey-bees in a number of experiments.
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Pollen is rich in proteins and bees collect it to feed to their larvae and young bees. Maize is not particularly attractive to bees because the male flowers at the top of the plant do not produce nectar, but maize does play a role as a provider of pollen. The Bt toxin produced by insect-resistant Bt maize plants is also found in the pollen. This means that bees come into contact with it. In order to be able to judge the potential effect of the Bt toxin on bees, the amount formed in the pollen is of interest and the specific toxicity, but above all the extent to which bees are exposed to the toxin. How much pollen do the larvae and adult bees actually consume? A Swiss study looked at this question and investigated how many grains of pollen are consumed and digested by individual honey-bee larvae over the course of their development. Non-transgenic maize was sown in three tunnel tents, each measuring 8x14 metres. When the maize flowered, two honey-bee colonies, each consisting of around 1000 bees and a queen bee, were introduced into each tent. The bees were supplied with a protein-free candy made from saccharose and honey and were given water. There were no pollen reserves in the honeycombs, so the larvae could be fed only on maize pollen. Since the larvae do not empty their intestine at all during the growth phase, it was possible to analyse the total amount of pollen consumed. |
Between 26 and 35 larvae from five colonies that hatched successfully, were weighed and dissected and the pollen grains in their intestines were counted. Between 1720 and 2310 pollen grains were found in the intestine of each honey-bee larva, around 75% of them fully digested. Based on the fresh weight of maize pollen, this represents 1.5 to 2 milligrams of pollen.
This quantity was then compared with the total amount of protein that a larva needs during its development. The authors calculated that the protein from the pollen grains accounts for just around 2.5% of their total protein requirement. By far the largest proportion of the larvae’s protein comes from the brood food. The authors draw the conclusion that honey-bee larvae’s exposure to the Bt toxin has been overestimated. The study is designed to provide a basis for more realistic toxin quantities in feeding experiments.
Does Bt toxin have an impact on the brood food gland?
In another experiment researchers investigated the effects of the Bt toxin on the development of the brood food gland. The workers feed the larvae with protein-rich brood food that they produce in their brood food glands. This means that the glands are also very important for the offspring’s development.
In this experiment, small honey-bee colonies with a brood and queen bee and around 250 workers were held in climate chambers. The bee colonies were fed maize pollen and sugar solution in five different variants. Maize pollen from the transgenic maize variety Mon810 and, as a control, pollen from the non-transgenic, isogenic line , was offered in combination with sugar solution without any additive. Since Mon810 contains only a small amount of Bt toxin in its pollen, the bees were also given pure Bt toxin (0.0014%) in sugar solution in combination with non-transgenic maize pollen. And finally, there were two treatments with a soya bean trypsin inhibitor (SBTI) in different concentrations in sugar solution (0.1 and 1%). SBTI is known to have a negative impact on bees. Trypsin inhibitors like SBTI inhibit certain enzymes in insects’ digestive tracts, thereby lowering digestion.
For each of the treatments, and each of the two repetitions in each case, an additional 50 freshly hatched workers were added to each bee colony.
Neither the Bt pollen nor the Bt toxin were found to have any effect on the brood food gland or the survival of the bees after ten days – after around ten days the brood food glands are fully developed. By contrast, the treatment with SBTI reduced the average weight and average diameter of the brood food glands. With the high concentration, the amount of sugar solution consumed was lower, which inhibited brood rearing.
The researchers also attempted to detect Bt toxin in the brood food glands using molecular methods. The Bt toxin was not detected in the bees fed with Bt pollen; small quantities were found in the bees fed with Bt sugar solution – about one thousandth of the concentration fed to the bees.
The micro-organism community – an indicator for bees’ health?
In a third experiment, the authors investigated the effect of the Bt toxin Cry1Ab on the micro-organism community in the bee intestine. The bacterial community in the bees’ intestine could be an important indicator for the health of the bees.
In the laboratory the bees were fed sugar solution and pollen from the transgenic maize Mon810 or from an isogenic line. In addition, the Bt toxin – and, in another variant, the soya trypsin inhibitor (SBTI) – was added directly to the sugar solution in two concentrations. For comparative purposes, free-flying bees from two locations in Switzerland were investigated.
In order to be able to cover the whole of the bacterial community, a method was used that studies the community as a whole at DNA level and which therefore also includes non-culturable bacteria. The DNA from the bee intestines was isolated. Fragments of specific bacterial genes (16S rRNA genes) were amplified using PCR . The analysis of the fragments provides indications of the diversity and composition of the bacterial community. Differences can be identified through differences in the number of fragments.
The composition of the bacterial communities in the bee intestine proved very similar in all the bees under investigation. There were hardly any differences between the free bees and the laboratory bees. It is known from other studies that the bacterial communities in bee intestines demonstrate a relatively low diversity. Neither Bt pollen nor high concentrations of the Bt protein affected the microbial communities in the bee intestine to any significant extent.
Neither Bt pollen nor high concentrations of the Bt protein affected the microbial communities in the bee intestine to any significant extent.
Only the high concentration of SBTI significantly reduced the number of fragments in the intestine. At this concentration the mortality rate among the bees also increased.
The authors conclude that the bacterial communities are not a suitable indicator for the impact of Bt toxin on the health of bees.
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