By Alex Kirby BBC News Online environment correspondent

US scientists say monarch butterflies suffer no significant harm from pollen from genetically-modified (GM) maize plants.

A two-year study led by the US Agricultural Research Service (ARS) has concluded that the risk of harm is negligible.

A group of federal, university and biotechnology industry scientists was involved in the study.

The ARS says it shows definitively that the pollen poses no "immediate significant risk".

The research was published in the Proceedings of the National Academy of Sciences (PNAS).

The debate over a possible risk to the butterflies from GM pollen on maize plants (known also as corn) has been simmering for several years.

In 1999 researchers from Cornell University reported that maize genetically engineered with the bacterium Bacillus thuringiensis (Bt) to resist the corn-borer pest killed monarch caterpillars in the laboratory.

Problem strain

In 2000 researchers from Iowa State University said monarch caterpillars were seven times likelier to die from eating milkweed near GM maize than those eating milkweed leaves with no GM pollen on them.

Milkweed, the monarch's staple (USFS)

The ARS scientists sought to establish how much Bt pollen was needed to cause toxic effects in the caterpillars, and how likely they were to encounter that level in natural conditions.

It assessed the feeding behaviour of caterpillars on milkweed leaves dusted with pollen from six Bt maize types, with no-pollen and no-Bt pollen controls.

The lead ARS scientist was Richard Hellmich, an entomologist from the Corn Insects and Crop Genetics Research Unit, Ames, Iowa.

He said: "We looked at larval weight and survival and found it took large amounts of pollen to get any statistically significant effect."

Below 1,000 pollen grains per square centimetre, the caterpillars' weight and survival rate were unaffected. Above that level the caterpillars were smaller, but they survived as long as the controls.

The exception was one of the six types of maize, BT176, which caused some harm at levels of ten grains per square cm.

Results corroborated

ARS says this was the earliest Bt maize developed and is likely to be phased out entirely by 2003.

To find how likely the caterpillars were to be exposed to significant amounts of pollen, one team measured pollen deposition around seven fields in Iowa.

Pesticides harm insects too

Dr Hellmich said: "We found that, on average, less than 30% of the pollen that corn produces ends up on milkweed leaves, even when conditions are perfect, and most of that gets deposited on milkweed within the cornfield."

Similar studies in Maryland, Nebraska and Ontario in Canada confirmed this pattern: they showed average pollen density on milkweed leaves within cornfields was about 170 grains per square cm, and seldom went above 600 grains.

Chemical decline

Dr Hellmich said this meant caterpillars in the fields would encounter the lowest observable effect dose less than 1% of the time.

He said maize pollen was too heavy to blow far, and rain easily washed it off milkweed leaves.

He added: "You need to compare the potential for risk to monarchs from Bt corn with the alternative, which is chemical insecticide use."

The US Environmental Protection Agency says use of insecticides recommended against European corn borers has fallen by about one-third since Bt maize was introduced to the marketplace.