A pervasive agricultural insecticide that has been linked to the decline of honeybees is now a near-constant presence in the small and great rivers that flow through Midwestern farm country, according to the first major review of its kind.

Scientists at the U.S. Geological Survey tracked the toxins called neonicotinoids in six states and nine Midwestern rivers, including the portion of the Mississippi that drains southern Minnesota, and found that they were universally present throughout the growing season in every watershed tested.

The results, published this week, raise significant questions about possible threats to the insects that form the base of the food chain in aquatic ecosystems, and they follow another study last month that found sharp declines in birds wherever the insecticides were widely used in Holland.

"If you get enough rain to transport it over land or into tile drains, then it gets into streams quite quickly at higher concentrations," said Kathryn Kuivila, a scientist at the USGS Oregon Water Science Center in Portland, Ore., and a lead author of the study.

The concentrations found by the study are lower than those the U.S. Environmental Protection Agency (EPA) considers fatal to aquatic insects, she said. But other scientists have found that the EPA's estimates for toxicity may be too high.

"Even more importantly, these organisms are not exposed to just one neonicotinoid," Kuivila said. "And there are other pesticides, other stressors."

Neonicotinoids, a synthetic nicotine, are neurotoxins whose use has exploded since they were first introduced in the mid-1990s. They are now the most widely used insecticide in the world, having quickly replaced older classes of chemicals that were far more toxic to humans and mammals.

The manufacturers, Bayer CropScience and Syngenta, say that neonicotinoids provide significant increases in yield for farmers and that there is no evidence that they are harmful to the environment. But recent studies have found that they may play a major role in the decline of honeybees, other pollinating insects and wildlife.

The compounds are most often used as a seed treatment for corn, soybeans and other cash crops, and — because they are water-soluble — they become part of the plant's vascular system as it grows. But only a tiny portion of the toxin is absorbed into the plant, while the rest remains in the soil, where it can leach into ground and surface water.

The USGS study, however, is the first to measure how widely the toxins have spread through surface waters. The researchers took monthly measurements at eight sites from spring through fall in 2013. They looked at small watersheds such as the Little Sioux, which drains a tiny portion of southern Minnesota at the Iowa border, and the huge watersheds of the Missouri and Mississippi rivers. At a ninth site, a tiny watershed in Iowa surrounded by agricultural fields, they took more frequent measurements to track how the pollutant levels changed during the season and with rain.

They found one or more of three different kinds of neonicotinoids in each of the 79 samples. The highest concentrations were found in smaller watersheds where farming was the dominant use of the landscape; lower concentrations were found in the big rivers that drained areas with more diverse uses.

"We are finding them throughout the season," Kuivila said. "They tend to be more water-soluble than older insecticides."

What is not clear, however, is what impact they have in aquatic ecosystems, she said. Levels considered toxic by the EPA are many times higher than those found in the USGS samples. But Kuivila said that other studies have found that toxicity can be much lower for some species, and others have found that the number of tiny worms and other soil insects drops precipitously at very low concentrations.