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Summary Genetically engineered (GE) rice lines expressing Lepidoptera‐active insecticidal cry genes from the bacterium Bacillus thuringiensis (Bt) have been developed in China. Field surveys indicated that Bt rice harbours fewer rice planthoppers than non‐Bt rice although planthoppers are not sensitive to the produced Bt Cry proteins. The mechanisms underlying this phenomenon remain unknown. Here, we show that the low numbers of planthoppers on Bt rice are associated with reduced caterpillar damage. In laboratory and field‐cage experiments, the rice planthopper Nilapavata lugens had no feeding preference for undamaged Bt or non‐Bt plants but exhibited a strong preference for caterpillar‐damaged plants whether Bt or non‐Bt. Under open‐field conditions, rice planthoppers were more abundant on caterpillar‐damaged non‐Bt rice than on neighbouring healthy Bt rice. GC–MS analyses showed that caterpillar damage induced the release of rice plant volatiles known to be attractive to planthoppers, and metabolome analyses revealed increased amino acid contents and reduced sterol contents known to benefit planthopper development. That Lepidoptera‐resistant Bt rice is less attractive to this important nontarget pest in the field is therefore a first example of ecological resistance of Bt plants to nontarget pests. Our findings suggest that non‐Bt rice refuges established for delaying the development of Bt resistance may also act as a trap crop for N. lugens and possibly other planthoppers.

Corn, Zea mays L. (Poales: Poaceae), growers in the US Cotton Belt are required to plant 20% of total corn acres to non-Bt hybrids for resistance management (non-Bt refuge). Most growers do not meet this requirement, in part, because they perceive non-Bt hybrids to yield less than Bt hybrids. We planted multiple non-Bt and Bt hybrids from a single company in small-plot replicated trials at a single location from 2019 to 2023, as well as in small-plot replicated trials at multiple locations during 2022 and 2023. In the single location, we measured kernel injury from corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae), and we recorded yield at all locations. In the single location trial, yields only separated among hybrids in 3 out of 5 years. In the multiple location trial, yields were variable between both years. We found that Bt hybrids tended to yield higher than non-Bt hybrids overall, but this was influenced by the inclusion of non-Bt hybrids that had a lower overall genetic yield potential in the environments we tested them in. In both tests, when hybrids were analyzed during each year, both Bt and non-Bt hybrids were among the statistically highest yielders. Our study demonstrates the importance of comparing multiple Bt and non-Bt hybrids to draw yield comparisons. This highlights the need for corn seed company breeders to put effort into improving yield for non-Bt hybrids. Hopefully this effort will translate into increased planting of non-Bt refuge corn for growers in the US Cotton Belt.

Economists often treat pesticide resistance as a common-pool resource problem. While pecuniary economic incentives are the standard prescription for open-access market failures arising from such resources, non-pecuniary behavioral approaches (e.g., \"nudges\") are also effective in some cases. Yet non-pecuniary instruments have not previously been evaluated for managing pesticide resistance. I empirically evaluate the performance of such an intervention to manage pest resistance to genetically engineered Bacillus thuringiensis (Bt) corn. The U.S. Environmental Protection Agency permits sale of Bt seed conditional on seed producers compelling customers to plant mandated levels of non-Bt refuge to delay the evolution of Bt resistance. Because of compliance challenges, the Bt seed producer Monsanto piloted a social marketing program to promote refuge in 17 North Carolina counties in 2013-2014. Using 2013-2016 sales data, I use difference-in-differences, fractional regression, discrete changes-in-changes, and matched differences econometric models to identify the average treatment effect of the program on refuge planting. Results suggest that if it had covered all corn growers in North Carolina, the intervention would have led the average grower to plant between 2.6% (preferred estimate) and 5.8% more refuge in 2014 compared to the counterfactual. The program increased by at least 12% the average probability of planting any refuge in 2014. I find little evidence that effects of the program persisted in subsequent years after cessation, nor that the program increased compliance with mandated refuge thresholds. Informed by behavioral economics research on other environmental and resource policies, I discuss the implications of these findings for pesticide resistance management.

Summary China is the world's second‐largest maize producer and consumer. In recent years, the invasive fall armyworm Spodoptera frugiperda (J.E. Smith) has adversely affected maize productivity and compromised food security. To mitigate pest‐inflicted food shortages, China's Government issued biosafety certificates for two genetically modified (GM) Bt maize hybrids, Bt‐Cry1Ab DBN9936 and Bt‐Cry1Ab/Cry2Aj Ruifeng 125, in 2019. Here, we quantitatively assess the impact of both Bt maize hybrids on pest feeding damage, crop yield and food safety throughout China's maize belt. Without a need to resort to synthetic insecticides, Bt maize could mitigate lepidopteran pest pressure by 61.9–97.3%, avoid yield loss by 16.4–21.3% (range −11.9–99.2%) and lower mycotoxin contamination by 85.5–95.5% as compared to the prevailing non‐Bt hybrids. Yield loss avoidance varied considerably between experimental sites and years, as mediated by on‐site infestation pressure and pest identity. For either seed mixtures or block refuge arrangements, pest pressure was kept below established thresholds at 90% Bt maize coverage in Yunnan (where S. frugiperda was the dominant species) and 70% Bt maize coverage in other sites dominated by Helicoverpa armigera (Hübner) and Ostrinia furnacalis (Guenée). Drawing on experiences from other crop/pest systems, Bt maize in se can provide area‐wide pest management and thus, contribute to a progressive phase‐down of chemical pesticide use. Hence, when consciously paired with agroecological and biodiversity‐based measures, GM insecticidal crops can ensure food and nutrition security, contribute to the sustainable intensification of China's agriculture and reduce food systems' environmental footprint.

Crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are planted on millions of hectares annually, reducing the use of conventional insecticides and suppressing pests. However, the evolution of resistance could cut short these benefits. A primary pest targeted by Bt maize in the United States is the western corn rootworm Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae). We report that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury. In all cases, fields experiencing severe rootworm feeding contained Cry3Bb1 maize. Interviews with farmers indicated that Cry3Bb1 maize had been grown in those fields for at least three consecutive years. There was a significant positive correlation between the number of years Cry3Bb1 maize had been grown in a field and the survival of rootworm populations on Cry3Bb1 maize in bioassays. However, there was no significant correlation among populations for survival on Cry34/35Ab1 maize and Cry3Bb1 maize, suggesting a lack of cross resistance between these Bt toxins. This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary.

In late 2018, the highly destructive and polyphagous fall armyworm was first detected in China. It is now a major economic threat to corn production. In this article, the main control strategies that are available are reviewed and prospects to manage this pest with Bacillus thuringiensis (Bt) corn in China are discussed.

Bt cotton has played a great role in bollworm management since its approval for commercial cultivation in India. To ensure the longevity of Bt technology, structured refuge was advocated wherein, the non-Bt seeds were provided as separate pack along with Bt seeds. Poor compliance of refuge planting is considered as a major factor for resistance development in Pink bollworm against Bt toxins and therefore, an alternate concept of refuge-in-bag (RIB) was recommended for Bt cotton. This strategy ensures automatic compliance of refuge, since a prescribed range (5–10%) of non-Bt seeds are blended with Bt seeds within each seed packet sold in the market. Since, Bt and non-Bt seeds are mixed and indistinguishable, assessment of Bt trait purity and ascertaining the proportion of refuge within each seed packet becomes critical for the success of RIB. A multi-phased study was conducted to determine the minimum seed sample size to be drawn and tested for RIB compliance in seed lots. Initially, the probable estimate to detect 5% non-Bt in blended seed lot was calculated theoretically which was later analyzed through visual observation of differentially colored seeds on various sample sizes. The minimum seed sample size thus arrived was validated through ELISA testing on a custom-built seed lot in which prescribed level of non-Bt seeds (5%) were blended with the Bt seeds. This was later revalidated using commercial Bt seed packets available in the open market. Based on the results, we propose a minimum sample size of 180 seeds which shall be tested individually for presence/absence of transgenes to determine proportion of non-Bt seeds in the Bt cotton seed packets. The need for strict care and critical cognizance by the seed producer organizations during the seed blending and packaging is specifically emphasized so as to ensure homogeneity of seed lot and prevent its possible misrepresentations. The proposed sample size may be used for ascertaining RIB compliance in the commercially available Bt cotton seed lots.

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is among the most serious pests of maize in the United States. Since 2003, transgenic maize that produces insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) has been used to manage western corn rootworm by killing rootworm larvae, which feed on maize roots. In 2009, the first cases of field-evolved resistance to Bt maize were documented. These cases occurred in Iowa and involved maize that produced Bt toxin Cry3Bb1. Since then, resistance has expanded to include other geographies and additional Bt toxins, with some rootworm populations displaying resistance to all commercially available Bt traits. Factors that contributed to field-evolved resistance likely included non-recessive inheritance of resistance, minimal fitness costs of resistance and limited adult dispersal. Additionally, because maize is the primary agricultural crop on which rootworm larvae can survive, continuous maize cultivation, in particular continuous cultivation of Bt maize, appears to be another key factor facilitating resistance evolution. More diversified management of rootworm larvae, including rotating fields out of maize production and using soil-applied insecticide with non-Bt maize, in addition to planting refuges of non-Bt maize, should help to delay the evolution of resistance to current and future transgenic traits.

The invasion of fall armyworm Spodoptera frugiperda poses a significant threat to the maize production of smallholder farmers in Asia and Africa. Bt maize is an effective measure for controlling this pest, but resistance management strategies tailored to the smallholder farming systems in the old world remain poorly understood. Surveys conducted from 2021 to 2022 in key infestation regions of Yunnan and Guangxi, China, revealed that an average administrative village includes 633 households, each cultivating 0.22 ha of maize per season, with 95.68% of fields smaller than 0.33 ha. Laboratory and field studies indicated that the high dispersal ability of fall armyworm larvae facilitated frequent larval movement between Bt and non-Bt maize within seed mixtures and structured refuges in smallholder farming systems. Resistance evolution models showed that establishing structured refuges covering 10–20% of households at the village level significantly slowed resistance development. This study proposes a village-based structured refuge strategy, proportionally allocated according to household distribution. The strategy is simple and feasible for smallholder farming systems in developing countries, offering a novel approach for managing resistance to Bt maize in fall armyworms.