At the risk of stating the obvious, insects pose a major threat to agriculture. Between crop losses and management inputs, insect pests cost U.S. farmers billions of dollars every year. And naturally, they continue to thwart efforts to control them, developing resistance to sprays, seed treatments and even plant-incorporated protectants (PIPs), as in the case of corn rootworm (CRW), which by 2016 had in many cases shrugged off the Bt traits that were enlisted for CRW control starting in 2003.
Taking a simplified historical view, this adaptation is very rapid, especially when you consider that the primary previous method of control for CRW — crop rotation — delayed resistance for considerably longer. First introduced in 1910, simply rotating fields annually between corn and soybeans was recommended for several decades.
In a 2002 American Entomologist article, Eli Levine says “a corn and soybean rotation disrupts the rootworm life cycle because eggs are not normally laid outside of cornfields and larvae cannot survive on soybean roots. Crop rotation proved so effective that farmers using it could basically forget about the possibility of corn rootworms injuring the roots of the grower’s first-year corn crop.”
However, in the 1980s it became clear that the insects had developed the ability to lay their eggs in soybean and alfalfa fields, and infestations multiplied.
The introduction of Bt corn reduced the impacts of CRW considerably, but with that technology now facing challenges, scientists have turned to a new biotech tool — RNA interference (RNAi). In fact, in 2017 Bayer’s SmartStax PRO, a GM corn seed featuring an RNAi-based mode of action, became the first RNAi insect control product to be approved by the EPA and just recently achieved approval for import and food/feed use from China, opening the door to a 2022 commercial release.
Using RNAi technology is promising because it can be deployed in a very targeted way. According to Graham Head, head of global resistance management for Bayer, “RNA interference allows one to target particular aspects of an insect’s biology and use that to control them. By using this technology, we’re able to look at the genes that are most important to the corn rootworm life cycle, pick one that is critical to the overall metabolism of the insect — in this case DvSnf7 — and we can completely knock it out.”
One of the keys to this process is the “interference” aspect, which as the name suggests, means that after being ingested and absorbed, it interferes with the insect’s ability to create a specific protein. This “leads to the insect ceasing to feed within a day or two, causing mortality shortly after because the insect isn’t able to feed,” he says.
SmartStax PRO doesn’t solely rely on this RNAi technology and in fact contains other Bt traits similar to previous SmartStax products. Like any insect control agent, Head emphasizes that it should be used in conjunction with other management practices, such as crop rotation, incorporating refuges and field scouting.
The timing of the SmartStax PRO release may prove to be fortunate for Bayer. In September 2020, the EPA announced they are considering phasing out many of the Bt traits that rootworms have developed resistance to. While it’s not a done deal, this introduction of a new mode of action could be very beneficial for corn growers who are concerned about the re-emergence of CRW as a widespread threat.
Today, CRW is not nearly the devastating pest it once was, but that may largely be due to the efficacy of those Bt traits, which have only become less effective in the past 5 years. No-tillers generally have less of an issue with CRW than other farmers because thick surface residue can discourage egg laying, according to a paper by William Quarles in The IPM Practitioner. Quarles also says that growing cover crops is another practice that can reduce CRW problems, as they tend to encourage greater numbers of beneficial insects that can keep pest levels in check.
Looking into the future, Head says he believes RNAi has got good potential for additional product development but it does have it’s limitations.
“For reasons the scientific community doesn’t fully understand yet, when RNAi technologies are used, they have very clear efficacy in beetle pests, but, for example, in moths, they don’t seem to work,” he says.
Bayer did field testing with SmartStax PRO during the 2019 and 2020 growing seasons, testing it against a non-CRW trait and many commercial trait packages that contain the leading traits for CRW protection that are on the U.S. market today, including their own.
They targeted fields with moderate-to-high corn rootworm CRW pressure, including fields that previously had greater than expected damage (GTED), were on multiple years of corn-on-corn and/or other factors that caused CRW pressure to be moderate to high over 2019-2020.
Results showed that SmartStax PRO statistically outperformed all other products in the trial. While other traits with CRW protection did provide better performance than the check, they also had significantly more damage than SmartStax PRO technology.
The early results for this new RNAi mode of action look promising. Whether it remains effective for the long run, only time will tell, but one key to prolonging the efficacy of this or any technology is to use it in combination with other tools. This approach, known as “integrated pest management,” suggests that by employing a variety of pest-control technologies, farmers can counter the evolution of resistance to any one particular method.
“Fundamentally, we view the diversity of tools as being absolutely critical for grower success and for management of a pest like rootworm. It’s important that farmer have access to the broadest availability of tools,” says Head.