From the OU Kosher website (link):
OU Public Advisory regarding raisins(emphasis added)
The OU has long maintained that raisins packed and stored under normal industry conditions do not pose a halachic infestation concern and may be consumed without further checking on the part of the consumer.
Recent public reporting of widespread infestation in packaged raisins has led the OU to reexamine its prior held position vis-à-vis raisins. Following careful investigation, extensive testing and consultation with our Halachic authorities, the OU upholds its original position that raisins, when stored under normal conditions (cool, dry and clean environments) do not require checking for the presence of worms or insects. (עיין בט"ז יו"ד פד, יב)
Many food items, when packed or stored in warm, moist or unclean environments should be avoided until fully checked for the presence of insects, particularly when exhibiting telltale signs of infestation (webbing, insect feeding damage).