More than 80% of field corn grown in the U.S. has been genetically engineered to express one or more insecticidal toxins from Bacillus thuringiensis, called Bt toxins. Bt corn was first commercialized in the U.S. in 1996, expressing a single Bt toxin to control the European corn borer. Newer Bt corn hybrids express two or more Bt toxins, which has increased the number of target pests to include the corn earworm and the fall armyworm. Benefits of planting Bt corn can include protecting yield from insect damage, reducing the need to use insecticide, and improved grain quality. However, such benefits can be impacted by the development of resistance in target insects.
Funded by a grant from the South Carolina Corn Board, Drs. Francis Reay-Jones (Clemson Pee Dee REC, Florence, SC) and Michael Plumblee (Clemon Edisto REC, Blackville, SC) conducted field trials in 2025 to evaluate the efficacy of Bt traits and the impact Bt toxins have on the survival of corn earworm, with implications on resistance development. Trials were planted in Florence on 14 April and on 25 June, and in Blackville on 17 April. The following Bt products were used, in addition to non-Bt hybrids from the same hybrid families:
| Bt product | Bt toxins expressed |
| Pioneer Optimum Intrasect | Cry1F + Cry1Ab |
| Pioneer Optimum Leptra | Cry1F + Cry1Ab + Vip3A |
| Genuity VT Double Pro | Cry1A.105 + Cry2Ab2 |
| Trecepta | Cry1A.105 + Cry2Ab2 + Vip3A |
At the whorl stage, corn can sometimes be infested with fall armyworm, as was the case in May 2025 in the trial in Blackville (Figure 1). While an average of 4% of plants were infested in a non-Bt corn hybrid, no larvae were found in any of the Bt hybrids tested. In Florence, fall armyworm infestations were found only in the late planted trial, with a range of 14-28% plants infested in the non-Bt hybrids. Again, no Bt plants were infested, underlining the efficacy of Bt corn in South Carolina with this insect.
Dr. Reay-Jones has been conducting field efficacy trials with a range of Bt corn traits since 2009. Since then, some traits have become less effective over time because corn earworm has developed resistance. We saw in particular a shift in 2015 when resistance to some of the Cry toxins expressed in Bt corn became more widespread not just in South Carolina, but across the southeastern U.S. In 2025, only hybrids expressing Vip3A provided excellent levels of control. As an example of the data collected in the early planted trial in Florence, Bt traits without Vip3A provided either no suppression or limited suppression of kernel injury (Figure 2).
Survival of corn earworm was determined by placing corn ears with large larvae in plastic boxes with soil, which were then sifted to collect cocoons or pupae. A total of 282 and 717 pupae were collected in 2025 in early and late planted corn, respectively, in Florence. Pupal weights were generally lower in Bt than in non-Bt corn. No pupa was collected from corn expressing Vip3A. Pupae from Optimium Intrasect had similar weights to non-Bt corn, suggesting limited impacts on the insect. Pupae from VT Double Pro were significantly lighter than on non-Bt corn, suggesting sublethal effects that may play a role in insect resistance management.
Yields did not vary among Bt and non-Bt traits in both early and late trials in Florence. In Blackville, yield of P1366YHR (144.4 bu/ac) was significantly greater than P1366R (122.9 bu/ac), with an intermediate yield for P1366VYHR (130.6 bu/ac). While insect pressure was light in these trials, the value of planting Bt corn for growers in the southeast is due to its ability to suppress damage from a range of potential target pests, which are generally minor pests or are less common than for instance corn earworm. Although corn earworm is generally not a significant economic pest of field corn, the same species is a serious pest in cotton, where it is known as bollworm. Because Bt cotton expresses the same or similar toxins as Bt corn, selection for resistance in Bt corn is likely driving resistance issues to Bt cotton. While Vip3A remains effective, reports of unexpected injury in recent years in the mid-south underline the need for continued resistance monitoring. Vip3A toxins are also expressed in new Bt cotton varieties, and planting of non-Bt refuge in corn is crucial, since only a single Bt toxin is currently available with very good activity for this key pest. Continued monitoring of efficacy of Bt corn in field trials is important to detect potential changes in susceptibility to Bt toxins among key pests, in order to provide timely recommendations to growers in South Carolina.
