Aug 20, 2008

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Effects of Cry3Bb1 on the Theridion impressum spider

(2005 – 2008) Agroscope Reckenholz-Tänikon ART research station, Zurich, Switzerland

Topic

This project investigates the effects of Bt toxin (Cry3Bb1) on spiders. The model spider chosen for the experiments is Theridion impressum, a species found throughout Europe which breeds successfully in maize. It helps with natural pest control because its webs primarily catch aphids, cicadas, thrips and other pests.

Summary

Laboratory and field studies show that Theridion impressum spiders ingest Bt toxin if it is present in their prey. Lower concentrations appear to indicate that the Bt toxin is broken down and excreted. No accumulation was observed in the spiders. Laboratory experiments with prey fed on Bt maize produced no negative effects on the spider’s survival or reproduction rates.

Experiment description

Spiders and prey were collected in the trial Bt maize field of the current joint BMBF-SiFo project.

Prey spectrum: The spectrum of prey species was recorded over several years and compared with published data from Hungary.

Bt content in prey: The Bt content of both prey and spiders was measured using the ELISA method.

Feeding experiments: Female spiders were collected for feeding experiments before they produced eggs and kept individually in the laboratory. Over the course of eight weeks, prey insects were fed a relatively high Bt content to simulate a realistic worst-case scenario. The spiders were fed alternately on beetles (Diabrotica virgifera virgifera) that had been fed on Bt maize silks and lacewings (Chrysoperla carnea) that had been fed on Bt maize pollen. In another experiment, newly emerged spiders were given Bt-fed two-spotted spider mite (Tetranychus urticae) for eight weeks.

The Cry3Bb1 protein concentration was measured both in the spiders and in their prey. Bt toxin values after short-term feeding were compared with those obtained after several weeks’ exposure.

Results

Prey spectrum: The prey spectrum of T. impressum in Bt maize fields primarily comprises aphids, heteropterans and thrips, but also other arthropod groups such as beetles, flies and hymenopterans.

Bt content in prey: Among the prey, there were organisms with no detectable Bt content (e.g. aphids) and those with a relatively high Bt content (e.g. a Trigonotylus bug). The T. impressum spiders collected in the Bt maize showed a correspondingly high variation between undetectable toxin levels and Cry3Bb1 in average concentrations.

Feeding experiments: Results show no effect on the lifespan of the spiders and the number of offspring as a result of being given prey fed on Bt maize. Even with young spiders, the Bt toxin had no effect on mortality or weight gain. Interestingly, newly hatched spiderlings were able to survive and grow when fed with maize pollen only. No differences between pollen from Bt maize or from a conventional control variety were observed. The concentration of Cry3Bb1 measured in the spiders was much lower than that measured in the prey that had been fed on Bt maize and that measured in the pollen. The constant intake of Bt toxin over several weeks resulted in Cry3Bb1 concentrations in the spiders that were comparable with short-term feeding. No accumulation of the Bt protein was observed.