By Carey Gillam

July 24 (Reuters) - A class of insecticides popular with corn and soybean farmers in the U.S. Midwest but feared as a factor in the decline of U.S. honey bee colonies and other crop pollinators, has been found to be widespread through rivers and streams in Iowa, according to a government study released on Thursday.

The study, released by the U.S. Department of the Interior, U.S. Geological Survey, marks the first broad-scale investigation of multiple neonicotinoid insecticides in waterways in the Midwestern U.S., and is one of the first conducted within the entire United States, according to the government scientists.

In the report, 79 water samples from across Iowa, the top U.S. corn-producing state, were collected during the 2013 growing season. Researchers said the use of neonicotinoid insecticides has grown in recent years, and they found them to be both "mobile and persistent" with "a strong pulse of neonicotinoids associated with crop planting" in streams.

The researchers said the broad use of the neonicotinoids, "needs to be closely examined in relation to environmental impacts."

Similar studies by the USGS have found many other types of common agricultural chemicals in stream samples in Iowa, but researchers said there was a "substantially greater neonicotinoid detection frequency" observed in this study compared to historical detections of other insecticides.

Neonicotinoids, also known as neonics, are sold by agrichemical companies to boost yields of staple crops, but are also used widely on annual and perennial plants in lawns and gardens. Neonics, chemically similar to nicotine, are commonly applied to the seeds before they are planted.

As use of the neonics has grown, some scientists have linked the insecticides to large losses in honey bee colonies that are considered critical for the production of many U.S. crops. Honey bees pollinate plants that produce about a quarter of U.S. consumer foods, according to the U.S. government.

Many agrichemical companies, including Bayer, whose neonic products are top sellers around the world, say there are a mix of factors killing off the bees and that neonics are important tools for boosting crop production.

(Reporting by Carey Gillam in Kansas City; editing by Gunna Dickson)