Along the Coastal Bend of Texas, the rice stink bug, Oebalus pugnax (F.), is a major pest of grain sorghum and rice that is primarily managed by insecticide applications. Reports of rice stink bug resistance to pyrethroids in Texas first surfaced in 2015 and continued to spread. To determine the status of pyrethroid resistance, rice stink bug populations across Texas and Louisiana were evaluated from 2021 to 2023. Mortality was assessed through glass vial exposures to eight concentrations (0, 0.03, 0.1, 0.3, 1, 3, 10, and 30 mu g/vial) of a pyrethroid, lambda-cyhalothrin. The concentration of lambda-cyhalothrin required to kill 50% (LC50) of each population was estimated by probit analysis. Furthermore, the efficacy of insecticides, including lambda-cyhalothrin, dimethoate, and dinotefuran, were evaluated in field experiments conducted in 2021. Our results indicated that 14 of the 21 rice stink bug populations sampled were resistant to lambda-cyhalothrin, with LC50 values ranging from 42 to 1,600 times higher than a susceptible population. In the field trial, lambda-cyhalothrin did not control rice stink bugs. Dinotefuran provided excellent control of nymphs, but dimethoate provided greater control of adult rice stink bugs. To our knowledge, this is the first study to thoroughly evaluate the extent or geographic range of pyrethroid resistance in Texas for rice stink bugs.

The wide occurrence of resistance to Cry1A and Cry2A insecticidal toxins from Bacillus thuringiensis (Bt) in the corn earworm/bollworm Helicoverpa zea (Boddie) leaves the Vip3A toxin produced during the vegetative stage of Bt as the only fully active toxin expressed in transgenic crops to control H. zea in the U.S.A. During 2021, the first unexpected survival of H. zea and injury (UXI) on a maize hybrid expressing Cry1A.105, Cry2Ab2, and Vip3Aa in Louisiana, U.S.A. were observed in two sentinel plots used for resistance monitoring. A follow-up intensive investigation was conducted with two H. zea populations established from larvae collected from the two UXI plots. The main goal of this study was to reveal if the unexpected damage was due to resistance development in the insect to the Bt toxins expressed in the maize hybrid. Diet-overlay bioassays showed that the two populations were highly resistant to Cry1A.105, moderately resistant to Cry2Ab2, but still highly susceptible to Vip3Aa when compared to a reference susceptible strain. In 10 d assays with detached ears, the larvae of the two UXI populations exhibited survival on ears expressing only Cry toxins but presented near 100% mortality on maize hybrids containing both cry and vip3A transgenes. Multiple field trials over three years demonstrated that natural H. zea populations in Louisiana were highly resistant to maize expressing only Cry toxins but remained susceptible to all tested hybrids containing cry and vip3A genes. Altogether, the results of this study suggest that the observed UXIs in Louisiana were associated with a resistance to Cry toxins but were not due to a resistance to Vip3A. The possible causes of the UXIs are discussed. The results generated and procedures adopted in this study help in determining thresholds for defining UXIs, assessing resistance risks, and documenting field resistance.

In the U.S., Helicoverpa zea (Boddie) is a major pest targeted by both transgenic maize and cotton expressing Bacillus thuringiensis (Bt) proteins. Resistance of insect to Bt maize and cotton containing cry1A and cry2A genes has widely occurred in the U.S. In this study, two trials were performed to investigate larval survival and development of a Cry1A.105/Cry2Ab2 dual-protein resistant (VT2P-RR), a susceptible, and an F1 heterozygous (VT2P-RS) populations of H. zea on ears of nine Bt and three non-Bt maize hybrids. The Bt maize hybrids evaluated represent five common pyramided traits expressing two or three of the Cry1A.105, Cry1Ab, Cry1F, Cry2Ab2, and Vip3Aa20 proteins. In the laboratory, neonates of the three H. zea populations were inoculated on silks of ears collected from maize at R1-R2 plant stages; and larval survivorship was checked 10 d after neonate release. All three insect populations survived normally on non-Bt maize ears. Varied numbers of VT2P-RR and VT2P-RS survived on ears of Cry1A.105/Cry2Ab2 maize, while all larvae of the three populations died or could not develop on ears of Vip3Aa20-expressing maize. The results demonstrated that the dual-protein resistant H. zea was not cross-resistant to Vip3Aa20-expressing maize, and thus traits with vip3Aa20 gene should be effective to manage Cry1A.105/Cry2Ab2-resistant H. zea. The resistance in VT2P-RR was determined to be incomplete on Cry1A.105/Cry2Ab2 maize. The effective dominance levels varied greatly, from recessive to incompletely dominant, depending on maize hybrids and trials, suggesting that proper selection of maize hybrids could be important for mitigating the Cry1A.105/Cry2Ab2 resistance. The data generated should aid in modeling multiple-protein Bt resistance in H. zea.

A seed blend refuge has been implemented in the U.S. Corn Belt for Bt maize resistance management. The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a target pest of Bt maize in the Americas. The larvae of this pest are mobile, which may affect the efficacy of seed blend refuges. In this study, field and greenhouse trials were conducted to determine the performance of Bt-susceptible (aabb) and -heterozygous dual-gene-resistant (AaBb) genotypes of S. frugiperda in seed blends of non-Bt and pyramided Bt maize. Three field trials evaluated larval survival, larval growth, and plant injury with aabb in seed blends of Bt maize expressing Cry1A.105/Cry2Ab2/Vip3A with 0-30% non-Bt seeds. Greenhouse tests investigated the performance of aabb and AaBb in seed blends of Cry1A.105/Cry2Ab2 with 0-30% non-Bt seeds. In pure non-Bt maize plots, after 9-13 d of neonates being released on the plants, 0.39 and 0.65 larvae/plant survived with leaf injury ratings of 4.7 and 5.9 (Davis's 1-9 scale) in the field and greenhouse, respectively. In contrast, live larvae and plant injury were virtually not observed on Bt plants across all planting patterns. Larval occurrence and plant injury by aabb on non-Bt plants were similar between seed blends and pure non-Bt plantings, suggesting that the blended refuges could provide an equivalent susceptible population as structured refuge under the test conditions. In the greenhouse, the two insect genotypes in seed blends performed similarly, indicating that the seed blends did not provide more favorable conditions for AaBb over aabb. The information generated from this study should be useful in managing S. frugiperda and evaluating if send blends could be suitable refuge options for Bt resistance management in the regions where the insect is a primary target pest.

The Vip3A, a vegetative Bacillus thuringiensis (Bt) gene, has been introduced into many Cry maize and cotton varieties that can manage the recently occurred Cry1/Cry2 resistance in Noctuidae pests including Helicoverpa zea (Boddie) in the USA. A seed blend refuge has been used for providing susceptible insect populations for Bt maize resistance management. Four field trials were deployed in this study to investigate the effects of Bt protein contamination due to gene flow in seed blend refuges of Viptera maize containing Cry1Ab and Vip3Aa20 genes with 0–30% refuges on the survival, development, and reproduction of the parental and F1 generations of H. zea. Viptera maize is highly effective against H. zea and likely expresses a ‘high dose’ for the insect. Compared to the survival on the structured refuge, seed blends reduced approximately 70% of the pupal and adult productions from refuge ears, and the reductions were not related to the refuge percentage. Pupae from the seed blend refuge weighed 22.1% less than those from the structured refuge, but the body mass reduction did not significantly affect the reproduction per female. All field populations were highly resistant to Cry1Ab, but susceptible to Vip3Aa20. Fitness of the F1 generations from the structured and seed blend refuges in diet rearing was generally similar. Data generated from the study can be used in simulation modeling to evaluate the feasibility of seed blends for resistance management.