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Investigating the influence of Bt maize on aphids and their specialized antagonists

(2001 – 2004) University of Göttingen, Institute of Plant Pathology and Plant Protection, Agricultural Entomology section

Topic

The purpose of the research was to investigate possible effects of Bt maize plants on the interaction between aphids and their specialized antagonists (parasitoids and hyperparasitoids) using frequently occurring cereal aphids as a model.

Aphids on maize cobs

Aphids on maize cob

Parasitic wasp emerging from an “aphid mummy”

The population development of cereal aphids is regulated by an entire complex of antagonists. In addition to ladybirds, syrphid flies and other predatory insects, these include, in particular, parasitic wasps of the genera Aphidius and Praon. The females of the species Aphidius ervi can parasitize more than 200 aphids, laying one egg inside each. After one to two days a larva emerges which feeds off the insides of the aphid. It evolves through several stages before it pupates after approximately ten days in what is now the completely hollowed-out aphid shell, resulting in a so-called “aphid mummy”. After five more days the fully formed parasitic wasp emerges from the mummified aphid.

These antagonists, which are also known as primary parasitoids, are themselves attacked by parasitic wasps. The secondary or hyperparasidoids parasitize the larva of the primary parasitoid developing in the aphid.

Summary

The cereal aphid infestation of seven different maize varieties was recorded in a field trial. The transgenic maize varieties studied do not appear to have any impact on the population development of aphids. The transgenic maize strains did not appear to have any impact on the parasitization activity of the parasitoids.

The composition of the parasitic wasp populations (parasitoids und hyperparasitoids) is not recognizably affected.

Experiment description

Outdoors

Seven different maize varieties were included in the investigations: Two Bt strains (Mon810 and Bt176) and their respective isogenic strains as well as three conventional varieties.

Recording aphid and antagonist density. Individual plants within the different varieties were marked and inspected regularly at weekly intervals. During the inspections, the numbers of aphids their natural antagonists were carefully counted on all levels of leaf growth.

Exclusion tents: The effect of various maize types on aphids and their antagonists is being investigated using the example of the parasitic wasp in insect-proof tents measuring eight by two metres in field conditions.

Inclusion tent where aphids and parasitic wasps were released.

Exclusion tent from the inside

Recording the species spectrum of parasitoids. Both living and already mummified aphids were collected from the various maize varieties and the parasitoids and hyperparasitoids were encouraged to emerge. This enabled the species spectrum of the parasitoids to be determined for each of the different maize varieties.

Direct impact of maize varieties on aphid and antagonist parasitic wasp. The direct impact of the maize varieties on aphids and their antagonists was investigated in inclusion tents (insect-tight gauze tent, ground area 4 x 2 metres, housing approx. 90 maize plants) in outdoor conditions using the parasitic wasp Aphelinus abdominalis and the aphid Metopolophium dirhodum. Aphids and parasitic wasps were released in the tents and the development of the aphids and aphid mummies was tracked over several weeks.

Laboratory

Investigating the development stages of aphids. The breeding rate, reproduction and life span of female aphids and the duration of larva development were established using maize plants (Bt maize and isogenic varieties) previously raised in the greenhouse. These findings were compared with the results of the field trials.

Results

Outdoor

Recording aphid and antagonist density. Between 2001 and 2003 the cereal aphid infestation of seven different maize varieties was recorded as part of a large-scale field trial. The number of aphids was generally found to be lower on Bt176 and the related isogenic variety than on the Mon810 pair and the conventional reference varieties in all years. However, no difference was found in the population development of crop aphids on Bt maize varieties and their corresponding isogenic varieties. A smaller field trial carried out in 2003 at a location in Göttingen using the same methods of investigation led to very similar results. The investigated transgenic maize varieties have no recognizable impact on the population development of the aphids.

Tent trials. The transgenic maize strains did not appear to have any impact on the parasitization activity of the parasitoids in the tent trial.

No effects on the composition of the parasitic wasp populations (parasitoids and hyperparasitoids) are discernible.

Laboratory

Investigating the development stages of aphids. Similar trends to the field trials were found here. No differences can be discerned between the transgenic and isogenic varieties.

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