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Safety research project: Environmental effects of Bt gene

(2000 – 2004) Bavarian State Research Center for Agriculture, Freising

Topic

The environmental effects of growing Bt maize were examined in a joint project in which the following points were considered:

  • Does cultivation of Bt maize affect soil micro-organisms?
  • Is there any effect on soil fauna (earthworms, springtails and nematodes)?
  • Is there any effect on selected non-target organisms?

Observations were made of non-target organisms in maize (aphids, cicadas, parasitic wasps, lacewings, hoverfly larvae, ladybirds, predatory bugs and spiders) and also of butterflies on field margins.

Summary

No differences were found between Bt maize and conventional maize for most of the parameters investigated.

Bt maize (Bt176) pollen feeding tests were carried out in the laboratory on swallowtail butterfly caterpillars (Papilio machaon). The negative effects demonstrated in these tests cannot be ruled out with certainty for field margins. Risks could be reduced by planting a cordon of conventional maize or by using Bt maize cultivars with lower toxin content in the pollen (MON810, Bt11).

However, long-term studies and a comprehensive risk analysis are recommended before any large-scale plantings of Bt maize because of isolated findings of Bt effects on individual beneficial organisms and on butterfly larvae.

Experiment description

The studies took place on five trial farms in Bavaria, where Bt maize lines (Bt176 and Mon810) were compared with conventional, isogenic cultivars grown under worst-case conditions (continuous monoculture with a high proportion of plant remains after harvesting). On half of each plot, insecticide was applied to control the European corn borer. Normal crop rotation was carried out on control plots forming part of a long-term soil observation programme.

Soil microbiology

Since Bt toxins can enter the soil via plant remains after harvesting and via root secretions, soil micro-organisms were observed for any effects. This involved examining microbial biomass and its enzyme activity.

Soil fauna (earthworms, springtails and nematodes)

  • Earthworms and springtails: In the autumn of 2001 and 2002, a four-centimetre-deep soil sample was taken from plots growing Bt maize, the conventional variant, and an insecticide-sprayed reference crop. The samples were tested to determine the species, density of colonisation and biomass.
  • Nematodes: Selected groups of nematodes were recorded annually before and after planting in soil samples with and without Bt maize.

In order to confirm the bioavailability of Bt toxins in soil clinging to roots, a biotest was carried out with the nematode Caenorhabditis elegans.

Wasp spider (Argiope)

Swallowtail (Papilio)

Photos: Claudia Ludy

Non-target organisms in maize fields

The aim was to examine the possible effects of Bt maize cultivation on non-target organisms. This was done by recording and comparing colonisation densities of aphids, cicadas, thrips, parasitic wasps, lacewings, hoverfly larvae, ladybirds, predatory bugs and spiders in the different maize variants.

Butterflies on field margins

In order to identify potentially endangered species of butterfly, day-flying lepidoptera were mapped on maize field margins.

After establishing the abundance of each species, a random-sample estimate was calculated for future monitoring.

Feeding tests were carried out in the laboratory with L1 swallowtail butterfly caterpillars (Papilio machaon), to test the acute toxicity of Bt176 pollen. In addition, the effect of pollen dispersal on the caterpillars was studied during field trials carried out at the margins of Bt maize fields.

Results

Soil microbiology

Within the microbial parameters studied (biomass and enzyme activity), differences were found only between locations. No differences were observed between Bt maize and conventional maize. Microbial activity increased, as expected, between spring and autumn. No effects were observed through use of insecticide.

Soil fauna (earthworms, springtails and nematodes)

  • Earthworms and springtails: 14 species were identified among the 21,000 springtails examined. More than 18,000 earthworms were examined, and ten species identified. There were no significant differences between Bt and non-Bt maize. There were, however, significant differences between locations as a result of soil differences. Because of the possibility of long-term effects, it is recommended that trials be repeated after a few years and after further continuous maize cultivation at the same locations, and that soil characteristics should be recorded at the same time.
  • Nematodes: In addition to nematodes that parasitise plants and that can reproduce readily on maize, there are also so-called saprophagous nematodes that feed on dead matter and have a function in metabolic activity in the soil. An increase was noted between initial and final infestation. No differences were found in the samples of soil clinging to roots of Bt and conventional maize, whether with or without insecticide.

In laboratory tests with Caenorhabiditis elegans, a slight reduction in body size, number of eggs and rates of reproduction was noted in the maize line Mon810.

This test – already standardised for ecotoxicological analysis – is to be validated in a subsequent project for Bt monitoring with other Cry toxins.

Non-target organisms in maize fields:

Most non-target organisms showed no negative effects as a result of Bt maize cultivation. It was noticed that there were shifts in occurrence of some groups in Bt maize fields. However, these shifts did not always occur consistently during the trial period. In order to find an explanation for the decrease in abundance of predatory bugs in Bt maize fields and identify possible long-term effects, it is recommended that a monitoring programme should be carried out over a period of several years, with accompanying laboratory experiments.

Application of pyrethroid insecticide had a clear negative effect on various species.

Butterflies on field margins:

36 different species of day-flying lepidoptera were found on maize field margins.

In order to establish the effects of Bt maize on butterflies, a monitoring programme is required using random samples varying in size according to the species of butterfly.

In the laboratory, Bt maize pollen was found to have negative effects on swallowtail butterfly caterpillars (Papilio machaon). Tests used Bt176 maize pollen, which has a high toxin content. The amount of pollen used in the laboratory was based on the amount found in the open on the leaves of food plants such as wild carrot. Depending on the age of the larvae, the LD50 value – the quantity needed for 50 percent of the larvae to die – was between 13 and 36 pollen grains.

The maize flowers and pollen dispersal period overlap with the development of the larvae.

Feeding on BT176 maize pollen had no effect on garden tiger moth (Arctia caja) L2 and L3 larvae.

By contrast, in the field trials no evidence was found of Bt maize pollen having a negative effect on mortality rates of swallowtail butterfly caterpillars. However, the larvae’s natural mortality rate was already so high that a possible Bt effect might have been masked and have gone undetected. Potentially sub-lethal effects were not investigated.