The susceptibilities of laboratory and field-collected western corn rootworm populations (Diabrotica virgifera virgifera LeConte) to methyl-parathion and aldrin were estimated by topical application of insecticide during 2002 to determine the stability of resistance in the absence of selective pressures. Most of the laboratory-reared and field-collected populations were significantly resistant to both insecticides. Average LD 50 values of laboratory and field-collected populations were 19-and 13-fold greater than the susceptible population in methyl-parathion bioassays, respectively, and 204-and 125-fold greater in the aldrin bioassays, respectively. The presence of aldrin and methyl-parathion resistance in field-collected populations strongly suggests that both resistance traits are stable in the absence of selection pressure and that neither mechanism is associated with a strong fitness disadvantage. r

We have previously determined that cytochrome P450‐based oxidation is involved in resistance to the insecticides methyl parathion and carbaryl in geographically distinct Nebraska western corn rootworm populations. Three new family 4 cytochrome P450 (CYP4) gene fragments (CYP4AJ1, CYP4G18 and CYP4AK1) were cloned and sequenced from insecticide‐resistant and ‐susceptible western corn rootworms. Insecticide bioassays indicated the resistant population employed in this study was significantly resistant to the insecticides methyl parathion and carbaryl. CYP4AJ1 and CYP4G18 were cloned from both genomic PCR and RT‐PCR products, although only CYP4AJ1 contains an intronic region. Alignments of inferred amino acid sequences with other homologous insect CYP4 genes indicates a high degree of similarity. Northern analysis concurrently employing mixed probes representing each of the three rootworm CYP4 fragments identified increased mRNA transcript signals (i) in resistant rootworms and (ii) following induction by the P450 inducer pentamethyl benzene. These results support our previous documentation of P450‐based insecticide resistance and suggest increased CYP4 transcript abundance can serve as a molecular resistance‐associated marker.

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Key message Sustainable control of fall armyworm (FAW) requires implementation of effective integrated pest management (IPM) strategies, with host plant resistance as a key component. Significant opportunities exist for developing and deploying elite maize cultivars with native genetic resistance and/or transgenic resistance for FAW control in both Africa and Asia. Abstract The fall armyworm [Spodoptera frugiperda (J.E. Smith); FAW] has emerged as a serious pest since 2016 in Africa, and since 2018 in Asia, affecting the food security and livelihoods of millions of smallholder farmers, especially those growing maize. Sustainable control of FAW requires implementation of integrated pest management strategies, in which host plant resistance is one of the key components. Significant strides have been made in breeding elite maize lines and hybrids with native genetic resistance to FAW in Africa, based on the strong foundation of insect-resistant tropical germplasm developed at the International Maize and Wheat Improvement Center, Mexico. These efforts are further intensified to develop and deploy elite maize cultivars with native FAW tolerance/resistance and farmer-preferred traits suitable for diverse agro-ecologies in Africa and Asia. Independently, genetically modified Bt maize with resistance to FAW is already commercialized in South Africa, and in a few countries in Asia (Philippines and Vietnam), while efforts are being made to commercialize Bt maize events in additional countries in both Africa and Asia. In countries where Bt maize is commercialized, it is important to implement a robust insect resistance management strategy. Combinations of native genetic resistance and Bt maize also need to be explored as a path to more effective and sustainable host plant resistance options. We also highlight the critical gaps and priorities for host plant resistance research and development in maize, particularly in the context of sustainable FAW management in Africa and Asia.

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