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Effect of Bt pollen on butterflies

(2001 – 2003)

Topic

The genetically modified Bt maize lines Bt176 and Mon810 are effective against the European corn borer. The larvae of this small butterfly eat through the plants’ stems.

Female European corn borer

The Bt toxin (Cry1AB) formed in the plants could also harm other species of butterfly. Whether the species are at risk will depend on how sensitive the caterpillars are and to what extent they ingest the toxin when feeding.

The project investigated the potential risk to non-target butterflies posed by Bt maize pollen (Bt176 and Mon810):
* Does Bt pollen lead to increased mortality of these butterfly species? * Does it affect the development of the caterpillars? (=sublethal effects) * Can caterpillars differentiate between Bt pollen and non-transgenic maize pollen? (= choice experiments)

In addition, butterfly mapping was carried out to help identify butterfly species that may be at risk as the result of Bt maize cultivation.

Summary

It is not possible to make a general statement about the endangering potential of Bt176. Both mortality and sublethal effects on the butterfly species under investigation varied widely from species to species.

The cultivation of Mon810 is seen as less problematic than Bt176 maize in terms of butterfly protection. Mon810 pollen has a very low Bt toxin content.

Pollen can be ingested with fodder plants in the vicinity of transgenic maize fields.

A high probability of exposure to maize pollen must be assumed for the larvae of 26 diurnal and 53 nocturnal butterfly species identified by the mapping project in the research area.

The following recommendations are made:

  • Authorization of lines with low Bt toxin concentrations in the pollen
  • Establishment of a cordon sown with conventional maize
  • Minimum distances to nature reserves
  • Use of diamond-back moth as a test organism in future studies with new Bt maize varieties because it is the most sensitive to Bt toxin and can be reared easily

The transferability of the laboratory experiments is not yet clear, since there were problems with the field trials. There is therefore a need for further research here.

Experiment description

Seven native butterfly species were selected for the experiments:

Peacock butterfly

Large white

Turnip moth caterpillar eating a piece of leaf to which maize pollen has been applied.

They include both potentially endangered species and species that are controlled, like the cabbage white.

European corn borer (Ostrinia nubilalis)
target organism of the Bt maize lines under investigation (Bt176 and Mon810)

Peacock butterfly (Inachis io)
potentially endangered

Large white (Pieris brassicae), cabbage white (Pieris rapae) and diamond-back moth (Plutella xylostella)
all three are controlled with pesticides, including Bt sprays, on cabbages.

Small tortoiseshell (Aglais urticae)
potentially endangered

Turnip moth (Agrotis segetum)
controlled in other crops

Feeding experiments

Feeding experiments were carried out with Bt176 and Mon810 pollen to investigate mortality. Pollen suspensions were applied to caterpillar fodder plants and caterpillars were placed on leaf pieces from the plants.

Pollen from the Mon810 line was fed only to caterpillars of the diamond-back moth, the most sensitive species to Cry1Ab toxin. Feeding experiments were also conducted with pieces of stamen. The stamens have a much higher BT toxin content than the Mon810 pollen.

Research into sublethal effects with Bt176 pollen

Small quantities of Bt176 pollen (5 pollen grains each) were fed to diamond-back moth (L4) and peacock (L1 and L2) caterpillars once/several times. The development of the caterpillars was observed.

Such small concentrations are still found even at some distance from maize fields. For instance, up to 34 pollen grains per cm² were collected at a distance of 32 metres in some cases.

Choice experiments

The caterpillar were offered a choice of cabbage leaves with and without Bt pollen.

Butterfly species mapping

Butterflies and moths were mapped outdoors with the aim of identifying potentially endangered species. Species were defined as potentially endangered if the caterpillars are exposed to the pollen and their host plants are found in the vicinity of maize fields.

Results

The different species exhibited very different levels of sensitivity to the toxin. Within the same species, younger caterpillars were much more sensitive than older caterpillars. Among the older caterpillars, damage occurred only at very high pollen quantities which are unlikely to be found outside maize fields under natural conditions.

LD50 values after feeding experiments with pollen from a transgenic Bt176 maize variety.

Species (larval stage) LD50 value
Diamond-back moth (L4) 8 pollen grains
European corn borer (L2) 32 pollen grains
Small tortoiseshell (neonat) 32 pollen grains
Peacock (neonat) 37 pollen grains
Cabbage white (L2) 39 pollen grains
Peacock (L2) 61 pollen grains
Peacock (L4) > 80 pollen grains
Large white (L2) 139 pollen grains
Turnip moth (L1 und L2) > 500 pollen grains

Number of pollen grains on the feed plant at which 50% of test animals died. Calculating the LD50 value is a standard procedure for quantifying the toxicity of a substance. (LD=lethal dose)

The LD50 values measured with Bt176 pollen (see table) relate to the pollen quantity offered and not to the quantity eaten, since the caterpillars often ate only part of the offered leaf pieces and pollen. LD50 values for the quantity of pollen actually eaten would certainly be lower.

With pollen from Bt maize line Mon810 no damage was found on the most sensitive species, the diamond-back moth, even with 80 pollen grains per caterpillar. Whether the ingestion of Mon810 maize pollen can cause sublethal effects was not investigated in this context. The pieces of stamen with their high Bt toxin content did, however, lead to significantly increased mortality rates in all cases.

Sublethal effects with Bt176 pollen

The importance of the sublethal effects appears to be greater than the direct toxic impacts.

Small quantities of Bt176 pollen led to delayed development, delayed pupation and delayed emergence of diamond-back moths and peacock butterflies. Shortly before pupation, however, the reduced average weight was back up. Younger caterpillars were better able to make up the development deficit than older caterpillars.

Turnip moths (L2 and L3) that were fed more than 500 transgenic pollen grains exhibited only slightly reduced weight gain compared with the untreated control animals.

Storing the pollen for three weeks did not lessen the effects observed.

No avoidance reaction was observed in the choice experiments, either for food intake or for egg-laying. In other words, the caterpillars ingest pollen if the fodder plants are coated in pollen.

Butterfly mapping (butterflies and moths)

A high probability of exposure to maize pollen must be assumed for the larvae of 26 diurnal and 53 nocturnal butterfly species identified in the research area in the course of the mapping project. This means that when cultivating Bt maize lines with high toxin expression in the pollen there is a potential risk to these butterfly species.