As I mentioned in a soybean article, corn earworms are early this year, populations are high, and moths are flying. A lot of growers, curious about how their corn might have pollinated after this period of intense drought, have found corn earworm in their Bt corn. What gives?
We have documented corn earworm resistance to all hybrids with Cry toxins since 2016- eight years now. During that year, we observed unexpected injury to Bt corn and recommended growers spray Bt cotton. Common brand names with corn earworm resistance are Optimum Intrasect and VT Double PRO. This table lists all the brand names and and indicates which ones could have resistant earworm populations. Corn hybrids such as Leptra and Trecepta, which express Vip toxins, should have minimal earworm infestations, as these toxins remain effective.
In general, I am not concerned about earworms impacting yield. Earworms must eat 40-60 kernels on the tip before yield loss is experienced. The plant compensates by filling out kernels that are heavier on the base of the cob. That being said, I think if corn is drought stressed and earworm populations are heavy, the ability for the plant to compensate is less. I have no evidence for this, just observations. However, in my opinion the drought stress is more of an issue for yield than corn earworm. To put it another way, the corn has bigger problems.
We have many many lines of evidence that timely planted non-Bt refuge hybrids can yield as well as Bt hybrids. Recently, we put them in the OVTs and you can read about their performance. Be sure to click on links in that article for a deeper dive. We also completed a large on-farm project with a graduate student. We planted and harvested large plot strips of Bt and non-Bt hybrids using grower equipment across the state (around 60 sites over 2 years). We found expected levels of corn earworm feeding and Bt and non-Bt hybrid pairs (similar genetic background) yielded equally. We will post about this more during the fall.
In addition, past publications from small plots have shown the same result. These studies were from Alabama, South Carolina, North Carolina and the Midsouth. Timely planted Bt and non-Bt hybrids from similar genetic backgrounds yield the same.
Why am I emphasizing timely planted? These results will be published soon, but Bt hybrids do have a slight yield advantage, on average, compared to non-Bt hybrids planted late (after May 20). Bt hybrids actually control corn earworms less as planting date increases. However, injury from both corn earworm and fall armyworm increases as planting date increases, in both Bt and non-Bt hybrids. Don't plant corn late!
Some growers have commented that they want to spray for corn earworm in field corn. However, this approach is ineffective. As mentioned previously, corn earworm rarely causes yield loss. So even if growers could reduce their numbers (which is unlikely), it would not be a cost-effective measure. Additionally, foliar insecticides are ineffective against corn earworm in field corn. Sweet corn growers, who spray daily or every other day, still struggle to manage these pests. In studies conducted in South Carolina, North Carolina and the Midsouth, spraying directly on the silks twice a week did not effectively control corn earworm populations.
Growers should plant the best yielding corn hybrids for their farm, whether they are Bt or non-Bt. Check out the NC OVT data for 2022 and 2023 for non-Bt hybrid yield information. Note that refuge in a bag is not a permitted way to plant refuge. Refuge in a bag actually accelerates resistance. We recently demonstrated that selecting for resistance can occur within a single generation when corn earworm feed on refuge-in-a-bag corn.
North Carolina growers that plant Bt corn are required plant at least 20% of their corn acres to non-Bt hybrids. In addition, growers can save $13.7 and $27.9/A buying non-Bt hybrid seed compared to Bt hybrid seed. Planting non-Bt corn will help slow resistance to the trait that is currently working for earworm. I recently covered the reasons to plant non-Bt refuge corn here.
