Effects of Bt maize cultivation on various arthropods found in maize fields

(2001 - 2004) RWTH Aachen University, Institute of Environmental Research (Biology V) Chair of Ecology, Ecotoxicology, Ecochemistry

Topic

The aim of the project was to evaluate the effects of genetically modified Bt maize on the species composition and numbers of selected groups of arthropods (e.g. butterflies, beetles, flies, spider mites) compared with conventional maize cultivation.

The following questions were investigated:

Are certain groups of arthropods (e.g. aphidivorous insects) as common in Bt maize as in isogenic maize not treated with insecticide?
What effect does insecticide treatment in conventional maize crops have on the arthropod life community compared with the cultivation of Bt maize?
Are the numbers and spectrum of pollinators different in Bt maize compared with isogenic maize?
Are the numbers of butterflies living on associate plants near Bt fields reduced as a result of contact with Bt pollen?

Summary

The arthropod life community in the soil and herbaceous layer was surveyed over the course of the three trial years. Over 500 000 insects were caught, sorted and identified.

The analyses provide no indication that Bt maize has any effect on the frequency and species diversity of non-target organisms.

However, a clear effect from sprayed insecticide was established for some insect groups.

No significant effects of Bt maize on caterpillars was found at any time.
As part of the development of a suitable monitoring programme, recommendations for suitable monitoring organisms in maize were made based on the results. These could provide early indications of possible harmful consequences of Bt maize cultivation. Suitable organisms for monitoring include soil-dwelling ground beetles and spiders, thrips, various pollinators and caterpillars on weeds. With the exception of the yellow pan (lack of specificity), all methods used proved practicable for monitoring purposes.

Experiment description

The comparative studies took place on a trial field near Bonn on plots planted with a Bt maize variety or with its isogenic conventional variety. Another variant involved the conventional variety being sprayed with insecticide. The trial was repeated eight times for each trial variant on a total of 24 plots (each about a quarter of a hectare).

In this field trial (on a total of about six hectares) the populations of various organisms were recorded and investigated more closely. The field trials were conducted over three cultivation periods and ended in 2003.

Pollinators are collected using the “inverted umbrella” method

Yellow pan

Pit trap

Trap frame

Herbivores. Species of arthropods that eat or suck maize plants were caught in various traps and quantified. In addition, cob samples were taken and whole maize plants (leaves, stalk, cob and panicle) were regularly examined.

Most of the recorded arthropods are herbivores which suck juices from the plant’s nutrient pathways (aphids), those that suck cell sap (thrips, cicadas) and predators specialising in these herbivores (heteropterans, lacewing, ladybird and syrphid larvae).

Extended monitoring of cicadas. Since cicadas suck cell sap and are therefore exposed to the Bt toxin, and were also found in large numbers in 2002, extended cicada monitoring was carried out in 2003. Three different methods were used (yellow pan, net, sticky boards) to determine the numbers and diversity of cicadas.

Pollinators. A particular emphasis was placed on pollen-eating hoverflies, thrips and soldier beetles.

Flower-visiting thrips were collected from male flowers using standard netting methods. Other pollinators were collected during the flowering period using colour dishes.

Predators on the soil surface. Pit traps and trap frames were used to catch ground beetles and spiders. These two groups are important predators of the soil fauna, but also eat insects like aphids which fall from the maize plants.

Butterfly larvae on associate flora. Further studies looked at the possible impact of Bt maize pollen (Mon810) on caterpillars in the adjacent weed area. Narrow weed strips in the trial plots were planted with yellow mustard and white goose-foot as forage for the caterpillars. The natural caterpillar population was recorded before, during and after dusting with Bt maize pollen. The animals were identified and counted on-site and returned to the same area.

Results

The arthropod life community of the soil and herbaceous layer was surveyed over the three trial years. Over 500 000 insects were caught, sorted and identified. The analyses give no indication that Bt maize has any effect on the frequency and species diversity of non-target organisms.

However, a clear effect from the sprayed insecticide was established for some insect groups. For instance, aphids treated with insecticide showed stronger population growth compared with the other variants. By contrast, cicadas and thrips and some predatory arthropods declined in numbers as a result of the insecticide treatment.

Comparison between Bt maize and isogenic control

The comparison of the Bt and isogenic variants revealed effects on various species in certain years, but not consistently for any one species. For instance, opposite effects were found with thrips and spiders in consecutive years – i.e. reduced numbers one year and increased numbers the following year.

The effects were observed with all methods used.

Herbivores. Aphids represented the largest fraction of the herbivores, but there were also relatively large numbers of thrips and cicadas. Spiders and heteropterans were the most common of the predators. Other predators were found only rarely.

Extended monitoring of cicadas. In 2003 it was possible to demonstrate using an ELISA test that cicadas from the Bt plots had ingested the Cry 1Ab toxin along with their food. Zyginidia scutellaris represents over 94 per cent of all cicadas. Only small numbers of the other species were found.

Pollinators. In all three years, thrips and aphids were the most numerous of the herbivores in the maize flowers. In 2001 the number of aphids was much lower than in subsequent years.

Predators on the soil surface. A total of 155 251 ground beetles and 17 364 spiders were trapped and identified. 64 species of ground beetle and 57 species of spider were recorded. The number of species remained relatively constant over the years. The common black ground beetle Pterostichus melanarius was the most common species. The dominant spider species all belong to the family of dwarf and sheetweb weavers (Linyphiidae). Much higher numbers were found in 2003 than in the previous years. No consistent direct or indirect toxic effect on predators on the soil surface was found for all years.

Caterpillars on associate flora. The assessments in the part of the experiment dealing with the "effect of Bt maize pollen on butterflies living in associate flora" are almost complete. No differences between the Bt variant and the isogenic control were found in any of the three trial years. By contrast, a clear insecticide effect was observed in all years, i.e. a reduction in numbers of caterpillars on their fodder plants in the plots treated with insecticide.

Under the selected field conditions, no negative effects of Bt maize pollen (MON810) were found on caterpillars. Eleven species were identified on yellow mustard and goose-foot during the three trial years. Apart from the diamond-back moth (Plutella xylostella) and the cabbage white (Pieris rapae), only very small numbers of each species were found, making further analysis of the data impossible. The maize flowering period did not overlap with the incidence of various species of butterflies every year, and the incidence of the various species also fluctuated considerably from year to year. No effect of Bt maize on caterpillars was demonstrated at any time.