Jan 21, 2003

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Research live series Part 4: A year in the trial field

Bt maize and arthropods: Species diversity under the microscope

December 2002. The maize field was harvested a long time ago. There is nothing left to be done here, but the countless insects and spiders collected in various traps over the course of the summer and preserved now have to be examined. At the end, the scientists want to be able to say whether or not Bt toxin affects the community of arthropods in the maize field.

At the moment it’s the turn of the cobs. Twenty maize cobs from each plot have been frozen here at minus twenty degrees and are now being thawed for examination. “This is another trapping method”, explains project leader Achim Gathmann, reaching into a freezer and pulling out an icy cob. “Here, between the wrapper leaves and the cob, is where we mainly find aphids and thrips.”

The cobs are weighed, the leaves counted and the animals preserved in alcohol. There are also a few frozen Europeancorn borer larvae. They had left the cobs while trying to escape the cold in the freezer.

Identify, count, compare

20 cobs from each of the 24 plots in the trial fields – that is 480 cobs to be dealt with. Then comes the catch from the various traps. Yellow pans, pit traps and trap frames have been emptied once a week for three months. That’s over 1500 individual samples. And then there are the pollinators collected using the ‘inverted umbrella’ method - a further 240 samples.

It is obviously necessary to make a selection from this vast quantity of creatures preserved in the freezer and in glass tubes. The scientists therefore started by drawing up a list of arthropod species to be identified. Experience played an important role here. The team selected animal groups that occupy an important position within the food chain, particularly those that have been thoroughly investigated and about which much is already known.

In the first step, the animals from the individual samples are roughly pre-sorted by eye and then identified properly under the microscope or binocular. There is an identification key for each animal group or family.

This work is very time-consuming and requires some experience, sometimes even special preparation. For instance, to identify the very small thrips, they must first be treated. They are placed in caustic soda, which makes them transparent and makes it easier to recognise the differentiating features.

Once the individual animals have been identified and classified, it is possible to quantify the numbers of individual species, i.e. record the population density.

In a final step, the densities of the selected arthropod species are compared for the three different maize variants.

Effect paths of Bt toxin

To make a meaningful selection it was important to consider the effect paths of Bt toxin, i.e. how animals come into contact with it within the food chain.

Many insects feed directly on the plant. These are the herbivores, which include aphids, cicadas, bugs, thrips and the cereal leaf beetle. Among the herbivores there is a special group – the flower visitors, which eat pollen. Of interest here are the hoverflies, soldier beetles and, once again, the thrips.

In biology, the direct food link between herbivore and plant is called the first trophic level. The next trophic level is that of the antagonists of the herbivores – so-called ‘predators’. These include e.g. spiders, ground beetles and various aphidivorous insects like the ladybird and hoverfly.

The food links are many and varied. The pollen-eating thrips, for instance, also suck on leaf tissue, and ‘generalists’ like the bugs are both plant-eaters and predators. They are not very choosy and are just as happy eating plant material as they are sucking on aphids and other insects.

It is also significant which plant parts are used for food. It has been found that aphids – herbivores of the first trophic level – are not actually directly exposed to the toxin because they suck juices (phloem) from the nutrient pathways in the plant. And it has been shown that the phloem does not contain Bt toxin. But because they occupy a very important position in the species structure and therefore within the food chain, it is still important and worthwhile to investigate aphid numbers.