The headline reads "'Natural' nitrogen-fixing bacteria protect soybeans from aphids", but naturalized is a more accurate and useful word to use.

"Soybeans are from Asia and so there were originally no nitrogen-fixing bacteria that would colonize soybeans in U.S. soils," said Consuelo De Moraes, associate professor of entomology. "The rhizobia had to be transferred here."

The soybean aphid is also not native to North America. This pest only began to infest soybean fields about 10 years ago but are now fully established pests requiring pesticide applications to avoid the loss of as much as 40 percent of the crop. . .

"Our results demonstrate that plant–rhizobia interactions influence plant resistance to insect herbivores and that some rhizobia strains confer greater resistance to their mutualist partners than do others," . . .

"The bacteria that were used initially to inoculate the first crops of soybeans are growing wild in the soil now," said Mark C. Mescher. "They are now considered "naturally occurring" and are different from the inoculants purchased with the soybean seeds."

They become natural because they change through generations of contact with other rhizobia. While they may not provide as much nitrogen to the plant as commercial types, the trade off between optimal growth and heavy insect damage may still be worthwhile.

"In most cases, the inoculant companies provide rhizobia for inoculation that gives plants the maximum yield," said Jennifer M. Dean, postdoctoral fellow in entomology. "Their rhizobia are highly competitive against naturally occurring nitrogen-fixing bacteria. The inoculant companies treat the natural rhizobia almost as a pest."

Because of this, soybeans almost uniformly incorporate the specially developed rhizobia rather than the natural ones. However, the researchers found that the plants associated with the naturally occurring rhizobia had lower aphid densities than either the artificially fertilized plants or the plants inoculated with commercial rhizobia. They also found the same level of nitrogen in both soybean plants inoculated with natural rhizobia and those inoculated with commercial varieties. . .

The researchers do not yet know what the natural nitrogen-fixing bacteria do to repel aphids.

To repeat, they are not "natural" so much as naturalized imports from Asia, and given that other rhizobia strains have been imported from around the world the best that can be said is that they are mongrels . . . just like most of us Americans.

The classic aphid defense is to raise brix (a measurement of the dissolved sugar-to-water mass ratio of a liquid) which makes plant fluids more viscous and so harder for aphids to suck out of them. I'd check the soy for brix. If that is the reason for better aphid defense then I'd check the general fertility of the fields in question for nutrient balance. It may be that the highly productive rhizobia strains would be better utilized if fertility was balanced to match the higher nitrogen availability.