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Code which you have used, it works only for "old" indexes, but not for the Sitecore 7 indexes. I am sure you must be using old one.

You should try to disable/enable indexing as follows

Sitecore.ContentSearch.Maintenance.IndexCustodian.PauseIndexing();
// Add code to create item
IndexCustodian.ResumeIndexing();

Also add check whether indexing is currently paused or not with the use of the following:

IndexCustodian.IsIndexingPaused(ISearchIndex index )