By Richard Levine
Agricultural pests, such as insects and weeds, can be incredibly adept at developing resistance to control methods. When you mention the word “resistance,” most people probably think of pests becoming resistant to certain chemicals — weeds becoming herbicide resistant, or insects becoming resistant to insecticides, for example.
However, there are many other types of resistance. Dr. Alison Van Eenennaam, the 2014 recipient of the Borlaug CAST Communication Award, recently explained that weeds can become resistant to mechanical control methods — such as mowing or tilling.
“Anyone that’s ever mowed down dandelions in their lawn knows that those little guys kind of evolve a way to avoid getting mowed by kind of growing out laterally and trying to really hurry up and flower before you come back and mow next time. They’re evolving an approach around that control strategy,” she said. “Or you might use tilling, and then some plants grow deep roots to avoid getting tilled.”
Another example involves behavioral resistance. Researchers at North Carolina State University published a paper in Science in 2013 showing that German cockroaches (Blattella germanica) had evolved to avoid insecticidal baits that contained glucose. According to a press release from that year, these cockroaches “evolved to become glucose-averse. These roaches aren’t attracted by glucose, so they tend not to consume insecticide baits.” The following video shows a glucose-adverse population (right) avoiding jelly containing glucose, alongside a non-resistant population of cockroaches that are chowing down on it (left):
Now a new study published in Ecology and Evolution shows how insects can develop resistance to an old agricultural control method — crop rotation.
Western corn rootworms (Diabrotica virgifera) thrive in cornfields, but die when farmers switch to soybeans or other crops — at least that used to be the case. By 1995 some growers started seeing rootworm damage even in rotated fields. Today rotation-resistant rootworms are widespread in the Midwest cornbelt, where corn and soybeans dominate the landscape.
So how does the western corn rootworm persist in fields that alternate between corn and soybeans? The answer, according to the authors, has to do with enzyme production in the rootworm gut.
Their focus was prompted by observations made by Jorge Zavala, who knew from previous research that levels of protein-degrading enzymes in the insect gut, called proteinases, rise and fall in response to chemical defenses in soybean leaves. He saw that rotation-resistant rootworms survived longer on soybeans and inflicted more damage on soybean leaves than their non-resistant peers. He also detected differences in levels of proteinases in rotation-resistant and non-resistant (wild-type) rootworms.
The new study tested these results in a broad sample of western corn rootworms from Illinois, Iowa, Nebraska and Missouri.
“We indeed found that the rotation-resistant rootworms could eat more foliage than the (non-resistant) wild type (rootworms),” said University of Illinois crop sciences professor Manfredo Seufferheld, who led the new study. “They are also able to survive a little longer on the soybean than the wild-type rootworms.”
When insects feed on their leaves, soybeans ramp up production of proteinase inhibitors to combat the insects’ ability to digest proteins in their leaves. The researchers hypothesized that the rotation-resistant rootworms had evolved the ability to compete a little longer in this chemical warfare with the soybeans.
Tests confirmed that rotation-resistant rootworms had higher levels of a special class of proteinases than wild-type rootworms to begin with, and that they increased production of one of these proteinases, Cathepsin-L, in response to soybean defenses. The wild-type rootworms increased levels of another proteinase, Cathepsin-B, when feeding on soybeans, the researchers found. But this enzyme appears to be ineffective against the plant’s defenses.
This difference allows the rotation-resistant beetles to survive on soybeans for two or three days — just long enough, the researchers said, for some of them to lay their eggs in bean fields. In spring, when the same fields are planted in corn, the rootworm larvae emerge to feed on corn roots.
Illinois insect behaviorist and co-author Joseph Spencer of the Illinois Natural History Survey said that before this study, researchers studying rotation resistance were looking at the insects and insect behavior in isolation, thus missing their interaction with plants as a potential clue to the problem.
“You have to include the soybean in the equation,” Spencer said. “It is not a passive player. The beetle has changed its behavior but what facilitates this change in behavior is this change in expression of these digestive proteinases. That allows them to stay in the soybeans longer. We had ignored this aspect of the biology.”
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Richard Levine is Communications Program Manager at the Entomological Society of America and editor of the Entomology Today Blog.