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Effects of Bt maize on flower-visiting insects and predatory spiders

(2001 – 2004) Bavarian State Research Center for Agriculture; Institute for Plant Protection, Freising

Topic

Orb-weaver: Spiders come into contact with the Bt toxin through the pollen attached to their prey. They also rebuild their webs frequently to catch prey, which involves recycling, i.e. eating the old web. Pollen grains are eaten in the process.

Aculepeira ceropegia (“oakleaf spider”)

Pollen in a spider’s web

Pollen in a spider’s web

Wasp spider

Potential impact paths of Bt maize on spiders (exposure)

Insects and spiders both have important functions in the agricultural ecosystem. The main aim of the project was therefore to investigate the impact of Bt maize/Bt maize pollen on flower-visiting flying insects and predatory spiders in the herbaceous layer.

  • Many flying insects are beneficial pollinators. Even though maize is pollinated by the wind, its pollen also serves as food for various species of insect. It is possible that some insects, including beneficial species like aphid predators, are sensitive to Bt preparations or to the Bt toxin produced by the maize plant.
  • Spiders play an important role in the biological reduction of insect pests. Orb-weavers come into contact with the Bt toxin via the pollen attached to their prey. In addition, they frequently remake their webs to catch prey, recycling - i.e. eating - the old web in the process. Particles in the web like e.g. pollen grains are consumed as well.

Another project aim was to monitor the influence of Bt maize on the interactions between pollen-collecting flying insects and the spiders.

A further aim was to make a scientifically founded selection of “indicator animals” which must be included in the risk assessment of genetically modified plants and in the development of a monitoring programme.

Summary

The number of spiders in maize fields was assessed over three years. In the first year fewer spiders were counted in the Bt maize compared with the non-transgenic control, in the second year there were the same number and in the third year there were more. According to these results, Bt maize can have an impact on the spider populations, but the strength/direction of the effect appears to be determined by additional, as yet unknown (environmental) factors.

The amount of pollen in spiders’ webs fell significantly as the distance from the maize field increased. Less pollen was found overall when it rained.

Depending on the spider species, up to 40 per cent of the prey in webs at the edge of the maize field can be pollen-collecting bees. Where there was a low flower density in the field margin the proportion of bees in the prey spectrum practically disappeared.

In laboratory experiments three species of orb-weavers showed no negative effects after consuming the Bt toxin, Bt maize pollen and/or bees that forage in Bt maize.

Experiment description

Looking for spiders

Looking for spiders

Stem eclector = funnel with catch container

Orb webs erected at various distances from the field

Outdoors

Recording flower visitors and spiders. The project recorded flower-visiting flying insects and spiders in the herbaceous layer (i.e. living on the plants) in Bt maize fields at three locations in Bavaria (each of two hectares, from May to September 2001-2003).

In 2001, spiders were collected using inverted umbrellas, battery-powered aspirators, stem eclectors (= funnel with catch container) and by removing whole plants.

From 2002 onwards the spiders were collected using only battery-powered aspirators, since this proved to be the most effective collecting method.

Flying insects were collected using so-called Malaise traps. These are tent-like net lace structures. When the flying insects try to escape upwards they end up in a catch jar. The trap is several metres long and yields an extremely large number of insects.

The results were compared with those from fields of isogenic maize strains.

Pollen quantity and prey in orb webs. Orb webs constructed by orb-weavers in the laboratory in pre-assembled frames were used to investigate the range of effectiveness of the maize pollen in the field margins. The orb webs were also analysed with regard to the prey they contained.

Laboratory

Experiments with Bt maize pollen and spiders. In the laboratory Bt maize pollen was applied to the orb webs and its effect on the spiders was analysed (mortality, lifespan, weight gain and speed of development as well as various web-building parameters).

Feeding experiments with garden cross spiders and bees. To investigate the interactions between true spiders and pollen-collecting flying insects, feeding experiments (using the garden cross spider and bees) were carried out in the laboratory.

Results

Outdoors

Recording flower-visitors and spiders. In the research period, a total of 50 different species of spider were recorded (33 species in maize fields, 42 species in field margins). The outdoor spider counts in 2001 were significantly lower in Bt maize fields and adjacent field margins than in the control fields; in 2002 there was no difference and in 2003 more spiders were found in the Bt maize fields than in the control fields. It follows that the cultivation of Bt maize can have an influence on spider populations, but the strength/direction of the effect appears to be governed by other, as yet unknown (environmental) factors.

Pollencount. Vaseline-coated glass specimen slides were laid out to record pollen deposition.

Pollen quantity and prey in orb webs. The project measured the distribution of the pollen in the maize fields. Orb webs were set up for a day inside and outside the field and then analysed. The pollen count varied according to the web size, proximity to the pollen and distance from the field. Orb webs in the maize field contained the most pollen (maximum value = 7130 pollen grains in a web), while the number in the orb webs at a distance of ten metres was several times lower (maximum 1644 pollen grains). In addition, Vaseline-coated glass specimen slides were laid out to record pollen deposition. It emerged from this experiment that pollen decreases with distance from the field. Much less pollen is “on the move” on rainy days, regardless of the distance from the field.

In the maize field margins pollen-collecting bees can account for up to 40 per cent of the prey in a wasp spider’s web. Pollen-collectors made up a much smaller proportion of the prey in garden cross spiders’ webs. Where the flower density in the field margin was low, however, the proportion of bees in the prey spectrum of both spider species fell to almost zero.

Laboratory

Experiments with Bt maize pollen and spiders. In laboratory experiments the consumption of the Bt toxin, Bt maize pollen and/or bees which had foraged in Bt maize had no negative effect on the mortality rate, lifespan, weight gain, reaction to prey or web parameters of three species of orb-weaver (mangora acalypha, garden cross spider, wasp spider).