Insects are masters of innovation and so, perhaps, are their gut microbiota. Researchers examining Western corn rootworms have discovered the insects’ gut microbes are helping it adapt to crop rotation, a common pest control method.
Corn and soy are the dominant crops in the Midwest corn belt. After a season of growing corn, farmers typically plant soy in the same field. The rotation should wipe out corn rootworms (Diabrotica virgifera virgifera), a devastating pest responsible for perhaps $1 billion dollars in annual loss (Dow AgroSciences). The rootworms, technically a beetle, must feed on corn during their larval stage to develop properly.
Typically, corn rootworms feed in corn fields during summer and lay eggs in the fall. The next generation then emerges as larvae the following spring. To survive and develop properly they must feed on corn roots. If they emerge to find soy plants they die. Since the rootworms reproduce just once per year, crop rotation has been effective for decades.
Yet in the mid-1990s farmers began seeing rootworm damage even in rotated fields and today a rotation-resistant type of corn roomworm is widespread. These insects were able to survive for a longer time on soy plants, generally toxic to rootworms. By laying eggs in soy fields, their young emerged the following year to find new corn plants, ready for the munching.
In 2012 Manfredo Seufferheld and Joseph Spencer, both of the University of Illinois, and colleagues found the resistant beetles were better able to tolerate the soy plant’s defensive chemicals due to digestive enzymes in their gut. But the difference in expression of genes coding for these enzymes did not fully explain the resistant type’s survival rate.
To follow up on the work, Chia-Ching Chu a graduate student in the Crop Science department, and his colleagues analyzed the microbiota of resistant and non-resistant beetles. After dissecting more than 4000 insects (not a task for the easily daunted) they found significant differences in both the kinds of microbes and the relative abundance between the two types of beetles. The specialized microbe community, they found, was aiding the resistant rootworms physiological adaptation to living on soybeans. Once they gave the resistant rootworms a dose of antibiotics they had no more success feeding on soy plants than their wildtype kin. They report their results in PNAS Early Edition.
“Perhaps one of the most surprising findings is that in reality, it is not just a bacteria taxa but rather, the structure (relative abundance) of the microbial community that is important,” says Seufferheld. “The community behaves as a ‘multicellular organ’ that responds accordingly when the host is exposed to soy-toxins. The community structure is like a blueprint of a house–it will determine the shape and functionality of the house.”
They found the community structures of non-resistant populations were largely homogenous, but the communities of rotation resistant populations varied. The degree to which they varied correlated with insect motility, gut proteolytic activity, survival and landscape diversity.
“Beyond studying the WCR as an important agricultural pest,” Seufferheld says, “the Western corn rootworm model has far-reaching implications.”
It raises general questions, he says, about how microbial community structures and function and interact with a host as that host evolves. Beyond addressing basic questions of evolutionary biology, Seufferheld hopes a deeper understanding of the microbe-host relationship will lead to more sustainable management.