Bt corn doesn’t harm earthworms

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URBANA, Ill. – Creatures in the soil, such as earthworms and mites, have a positive effect on soil quality and, in the long run, farmers may want to manage their soils to protect and nurture these creatures.

“We know soil invertebrates are beneficial,” said Ed Zaborski, a soil invertebrate ecologist with the Illinois Natural History Survey.

“They have a positive impact on the soil system and, ideally, we would like to manage farmland to produce the best possible benefits from them.”

The creatures are the focus of a study at the University of Illinois and state Natural History Survey.

Test sites. Zaborski’s study looks at earthworms and mites as possible indicators of soil condition and how they respond to insect control strategies.

He makes up bags of crop residues and buries them in field plots consisting of three different corn hybrids, each with a Bt line and a non-Bt line. Half of the plots are treated with insecticides.

Every few weeks, some of the bags are collected and the invertebrates extracted. Through the process, he can discover the numbers and species of invertebrates that are decomposing the residue.

“We can determine the impact of insecticides, the growing crop and the quality of the residues. After analyzing the residues, we’ll know the rate of decomposition for several points in time.

“So, we can look at the effect of insect control strategies on the soil process and decomposition.”

No negatives. Zaborski said there doesn’t seem to be any negative effect to earthworms caused by the Bt endotoxin.

“For the lines of corn used in our study, it looks like the impact of genetically engineering corn to produce the Bt toxin was no greater than the genetic differences between various corn hybrids.”

He has also discovered that certain soil mites might respond to a particular residue based partially on their reproductive strategy. Invertebrates that reproduce quickly and in large numbers when food is plentiful are known as ‘r’ strategists. In one corn variety, mites with these characteristics were more abundant on residue containing the Bt gene than on the non-Bt variety.

“One reason could be that, if the plant could not break down the Bt toxin, some residues would are slightly higher in nitrogen. That means there’s a greater amount of protein available there to support a rush of microbial activity, and mites would benefit from feeding on these microbes.”

Unique test. Zaborski’s approach is a unique way to study the effect of Bt crops on the soil.

“Often, researchers test a very small set of animals. I’m looking at a couple hundred different species. It’s a whole, interacting, functioning community. And I’m measuring the community, instead of individual species, which offers a much better chance of finding any risk associated with the technology,” he said.

Initial results also show that, in crops treated with insecticides, juvenile nightcrawlers were more abundant than in crops not sprayed with insecticides.

“We’re not sure why this happened,” he said. “One hypothesis is that the insecticides killed spiders and beetles that would be predators of earthworms. So the spraying improves the reproductive success of earthworms. Unfortunately, those predators can also help control crop pest.”

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