Joachim MÖSER, Stefan VIDAL
Georg-August-University, Göttingen

The main objective of this study is the relation of Diabrotica virgifera virgifera (Western Corn Rootworm, WCR) to its main host plant corn (Zea mays) and alternative hosts. The following questions were addressed:
1. Do adult WCR feed on other plants than corn?
2. If so, is there any preference for specific plants outside the cornfield or is the choice correlated with the flowering plants occurring in the field?
3. Does the diet of adult WCR change over time due to changing corn phenology or do they move to fields with younger corn, that still provide their supposedly necessary corn diet (pollen and silk)?
4. If it changes, do adult WCR switch to the pollen of other plants or rather to different parts of the corn plant (leaves, kernels) after the period of flowering?

To answer these questions adult beetles were collected and the gut content was analyzed. The sampling was done in two categories of cornfields: ones with low abundance of weeds and ones with a high abundance of weeds inside. This was done to investigate if WCR uses the weeds inside the corn fields or if it leaves the field to forage on pollen of weeds outside.
Six replicas of each field-category were sampled, resulting in a total of 12 fields. Each field was sampled weekly, collecting 20 beetles from corn per sampling date. Further adult beetles were collected separately from the weeds inside the field using sweep netting.

Besides that, the species diversity and number of individuals of flowering weeds were recorded weekly for each field along a 20m transect between the cornrows. Also the corn phenology in each field was recorded weekly.

Pollen samples were taken from all plants occurring as weeds inside the cornfields to allow later identification in the pollen analyses.

No-choice feeding tests were done with different corn tissues separately (leaves, tassel, silk, and kernel) to be able to analyze the utilization of distinct plant parts by WCR.

The gut content analyses comprised two parts: 1. Half of the samples were analyzed using the untreated gut contents focusing on changes in the use of different corn tissues (comparing the relation of leaves, silk, pollen and kernel tissue). 2. The pollen content of the guts was analyzed, using acetolyzation technique treating the whole insect which chemicals leaving only the pollen at the end.

Preliminary results correlating the diet composition with the changes in corn phenology and diversity and abundance of weeds inside the cornfields will be discussed in connection with their impact on possible control mechanisms.