Aug 2, 2005

Research Projects

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Impacts of Bt maize on sciarid fly larvae as decomposers

(2001 – 2004) Federal Biological Research Centre for Agriculture and Forestry (BBA), Institute for Plant Protection in Field Crops and Grassland; Braunschweig

Topic

The project deals with the possible impacts of plant parts of genetically modified maize plants which produce the Bacillus thuringiensis (Bt) toxin (Cry1AB). In the maize plants the toxin acts as protection against grazing by the European corn borer. The project investigated to what extent other insects, particularly soil-dwelling decomposers (saprophagous dipterans), are also damaged by the toxin.

This project focused particularly on sciarid flies, whose larvae play an important role in the material life cycle of the field ecosystem as soil-dwelling “consumers” of dead plant material.

Key questions are:

• Are sciarid fly larvae affected by the intake of maize plant parts containing the Bt toxin?
• Could there be a reaction of the soil life community, leading to a change in soil fertility through a disturbance of the material life cycles?

Summary

The most common groups of two-winged insects (dipterans) were gall-midges, sciarid flies and humpbacked flies.

For three of the parameters investigated outdoors a trend reversal was observed over the course of the three research years. The Bt variant initially had the greatest species diversity, the highest hatching rate and the highest decomposition performance. However, the preference for the Bt variant did not continue in subsequent years.

In the laboratory, feeding the sciarid fly species Lycoriella castanescens on maize litter from Bt-Mon810 Novelis and from its isogenic parent variety, which had been treated with insecticide, resulted in a longer time to pupation than when maize litter from the other maize variants was used.

The delayed development effect was also demonstrated in tests in which sciarid fly larvae fed on Mon810 Bt maize litter were fed to the larvae of predatory species of beetle.

Bt176 did not produce this effect, despite the fact that in this variety the toxin content was thirty times higher than for Mon810.

Experiment description

The experiments took place outdoors, in the greenhouse and in the laboratory.

Outdoors

The outdoor experiments ran in parallel on individual parcels of the trial field which were sown with a Bt maize variety or with the corresponding isogenic variety (with and without insecticide treatment). Another conventional maize variety was also planted on the same field and treated with an insecticide against the European corn borer.

Species composition. In order to record the various species of decomposers outdoors, the creatures were caught in the relevant trial plots with photoeclectors, which were moved to a different spot within the plot every four weeks (apart from in winter). In this type of trap, hatched insects are attracted by the light and end up in a transparent container filled with trapping fluid. In this way, the species composition and abundance can be analysed at regular intervals. Changes to the life community can therefore be well documented even over a longer period of time.

Grazing behaviour and decomposition activity. To enable a better analysis of the grazing behaviour and decomposition activity of the soil-dwellers, sections of the soil were examined using so-called “bait strips” and mini containers. The bait strips contained a defined bait mass; the mini containers were filled with maize substrate from the different variants planted. Both containers were then buried for a certain length of time in the trial field. The quantities of bait mass and maize substrate eaten can be used to compare decomposition performance on plots with Bt maize and plots with conventional maize.

Laboratory

Grazing experiments with sciarid fly larvae. Another part of the project consisted of laboratory experiments to investigate the direct effect of the Bt toxin on the sciarid fly larvae. In these experiments, home-bred larvae were fed over a long period of time on plant material containing the toxin. The toxin content of the larvae was then monitored using a biochemical procedure (ELISA) in collaboration with our project partner at the Federal Agricultural Research Centre (FAL). This procedure can establish the presence of the Bt toxin in the sciarid fly larvae and their droppings, as well as in other predator insects that have eaten the sciarid fly larvae.

In addition, in collaboration with our project partner at the Dienstleistungszentrum Ländlicher Raum (DLR) Rhineland Palatinate, the Bt toxin was monitored during the different stages of decomposition of the plant parts using biochemical methods.

In order to test different reactions of the sciarid fly larvae to plant material containing Bt, plant material without Bt and plant material treated with insecticide, feeding experiments were carried out: plant material of the different varieties was fed to home-bred sciarid fly larvae. During these laboratory experiments, the development of the larvae was observed and important factors like the pupation rate, hatch rate and death rate of the sciarid fly larvae were recorded. These factors can be used to describe the reactions of the sciarid fly larvae to the different types of plant material.

Results

Outdoors

Species composition. In the 2001 vegetation period 25 per cent more species of two-winged fly (dipterans) were recorded in the trial plots with Bt maize Novelis (Mon810) than in the isogenic maize without insecticide treatment. These differences were attributable solely to the decomposers among the dipterans. As early as the fallow phase following the first harvest (November to April), however, the highest species numbers were no longer found in the Mon810 Bt maize plots, and in the 2002 vegetation period, ten per cent more species of saprophagous dipterans were found in the plots with conventional maize.

Abundance. The most common groups of two-winged insects (dipterans) were the gall-midges with 35 per cent of the total, the sciarid flies with 29 per cent and the humpbacked flies with 18 per cent. The outdoor catches in the trial plots using photoeclectors produced higher proportions of the investigated insect groups in the Bt maize in the 2001 vegetation period, (May – October), but it was not possible to support this result with statistics.

In this first cropping period there were also hardly any Bt maize residues on the field. Only after the harvest in October, once a large amount of Bt plant residue had been incorporated in the soil for decomposing for the first time, were very many (significantly) more hatchings registered in the Mon810 Bt plots than in the plots with the other varieties. This preference for the Bt variant was not continued in the following vegetation period (2002). Instead, no differences in the abundance of flies or midges were noted between the varieties.

Grazing behaviour and decomposition activity. It was not possible to start evaluating the decomposition performance with Mon810 Bt and non-Bt maize litter in mini containers in the outdoor plots until the second research year, since before then there was no Bt maize litter. Each year’s evaluation was carried out with the litter from the previous year.

The preference for the Mon810 Bt maize litter in the first year, which was deduced from the species numbers and hatching densities, is confirmed by the experiment with the mini containers: in 2002 the decomposition rate was approx. five percent higher in the Mon810 Bt maize plots throughout the exposure period than in plots with isogenic maize. Whole-plant analyses using the VDLUFA method provided indications that Mon810 Bt maize decomposes more easily than other maize varieties, judging by the proportion of components that are difficult to decompose, such as fibre content, hemicellulose, cellulose and lignin.

Despite this, in 2003 (the third research year) an average of seven per cent less litter decomposition was recorded in the Mon810 Bt maize plots compared with the plots with conventional maize litter. It is worth noting that the Bt toxin content of the Mon810 Bt maize litter weighed out into the mini containers in 2003 was about 2.5 times higher than in the previous year. It is not yet possible to reach a final conclusion about whether the differences in species composition and decomposition behaviour can be explained by these differences.

Laboratory

Grazing experiments with sciarid fly larvae. It was apparent that different sciarid fly species can react in different ways. The sensitivity of the investigated sciarid fly species Bradysia difformis was lower during the feeding experiments with Bt maize pollen than that of the sciarid fly species Lycoriella castanescens. Whereas Bradysia difformis rarely occurs in fields, Lycoriella castanescens is usually the dominant species of sciarid fly here, accounting for 20-30 per cent of individuals, as was the case on our maize plots. The two species had very different reactions to the same conditions. This raises the question of suitable test organisms for any monitoring programme.

During the feeding experiments in the laboratory, feeding Lycoriella castanescens with maize litter from Bt-Mon810 Novelis and from its isogenic parent variety that had been treated with insecticide resulted in a longer time to pupation compared with feeding with maize litter from the other maize variants like the BT176 variety (Valmont), the isogenic control cultivar Prelude and the conventional variety Eurostar.

The delayed development effect was also demonstrated in experiments in which sciarid fly larvae that had been fed on Mon810 Bt maize litter were fed to the larvae of predatory beetle species: both the rove beetle Atheta coriaria and the ground beetle Poecillus cupreus took more days to pupation when their prey had been fed Mon810 maize.

It is not yet possible to judge what factors lead to the evidently lower nutritional quality of the Mon810 Bt maize litter for Lycoriella castanescens and its predators.

In feeding experiments with maize pollen the effects noted in connection with the feeding of Lycoriella castanescens were found for only one of the two Bt maize varieties in the test. Valmont (Bt176) did not produce the effect, despite the fact that, at 2962 ng/g of pollen, the toxin content for this variety was thirty times higher than for Novelis (Mon810) at 97 ng/g. It can be deduced from this that there is evidently no relationship between the observed effect and the absolute toxin content for different Bt maize varieties.