![A. gossypii on cotton leaf. Image credit: Clemson University - USDA Cooperative Extension Slide Series, , United States [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons](https://www.promegaconnections.com/wp-content/uploads/2019/01/Aphis_gossypii04-300x290.jpg)
The extensive and repetitive use of neonicotinoids has led to the development of resistance in several insect species including, the cotton aphid, A. gossypii. A. gossypii is a widely distributed pest that affects watermelons, cucumbers, pumpkin, cotton, and citrus crops, among others, making it one of the most economically important agricultural pests known. Thiamethoxam is a neonicotinoid insecticide that irreversibly binds to the nicotinic acetylcholine receptors (nAChRs) of cells in the nervous system and interferes with the transmission of nerve impulses in insects (1).
To further understand the mechanisms of resistance to thiamethoxam and other neonicotinoids, Wu et al. recently investigated (2) expression changes in the transcripts of P450 in thiamethoxam-susceptible and thiamethoxam-resistant cotton aphid strains. Nine P450 genes were significantly overexpressed in the resistant strain (especially CYP6CY14). The involvement of overexpressed P450s was examined through RNA interference (RNAi) introduced via artificial diet and dsRNA feeding.
Based on P450 sequences and predicted possible interference sites, gene fragments were amplified from cDNA and then cloned into pGEM®-T Vector. The purified plasmids served as templates for RNA synthesis using the T7 RiboMAX™ Express RNAi System. ECFP dsRNA was employed as the control. The diet was prepared in DEPC-treated water to ensure the absence of RNase activity. For the dsRNA feeding experiments, dsRNA was added to the artificial diet at a 150 ng/μL concentration. An artificial diet containing dsRNA-ECFP was employed as a control.
To analyze the efficiency of dsRNA knockdown of P450 expression, the aphids were fed an artificial diet containing dsRNA (100 ng/μL) for 48 h, and samples were then collected for RT-qPCR.
Using the known inhibitor of the CYP-450 system, piperonly butoxide, the researchers confirmed that cytochrome P450 enzymes are major enzymes involved in the detoxification and resistance to thiamethoxam in cotton aphids. Analysis of qPCR experiments indicated that transcription of several CYP450 genes was up-regulated in the thiamethoxam-resistant cotton aphids compared to the sensitive strains. RNA interference (RNAi) experiments targeting CYP6CY14 significantly increased the sensitivity of the resistant aphid to thiamethoxam.
The results of this study will be helpful in understanding the resistance mechanism to thiamethoxam so that more effective management of pests such as the cotton aphid may eventually be achieved.
Literature Cited
- Casida, J.E. and Durkin, K.A. (2013) Neuroactive insecticides: targets, selectivity, resistance, and secondary effect. Annu. Rev. Entomol. 58 99–117.
- Wu, Y. et al. (2018) Expression profile changes of cytochrome P450 genes between thiamethoxam susceptible and resistant strains of Aphis gossypii Glover Pesticide Biochemistry and Physiology 49, 1–7.
Gary Kobs
