Combining the entomopathogenic nematodes (EPNs), Heterorhabditis beicherriana LF strain and Bacillus thuringiensis (Bt) HBF-18 strain is a practical strategy to manage the larvae of Holotrichia parallela Motschulsky. However, the intrinsic resistance mechanism between H. parallela larvae and the EPN-Bt combination is unknown. Herein, antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] and detoxifying enzymes [carboxylesterase (CarE), glutathione S-transferase (GST), and acetylcholinesterase (AChE)] of H. parallela larvae showed an activation-inhibition trend throughout the EPN-Bt exposure time course. Eight potentially key antioxidant and detoxifying enzyme genes in response to EPN-Bt infection were identified from the midgut of H. parallela larvae through RNA-seq. Spatiotemporal analysis showed they were ubiquitously expressed in all development stages and tissues. After silencing CAT, CarE, and GST1, the enzyme activities were significantly decreased by 30.29%, 68.80%, and 34.63%, respectively. Meanwhile, the mortality of grubs was increased by 18.40%, 46.30%, and 42.59% after exposure to EPN-Bt for 1 day. The expression level change trends of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), cap ‘n’ collar isoform-C (CncC), kelch-like ECH-associated protein 1 (Keap1), and CarE were consistent when exposed to EPN-Bt. Furtherly, RNAi-mediated PI3K silencing showed a similar downregulated trend between PI3K/Akt/CncC and CarE. Furthermore, silencing PI3K rendered grubs more susceptible to EPN-Bt and accelerated symbiotic bacteria multiplication in grubs. Overall, these results suggest that PI3K/Akt/CncC pathway mediates the expression of CarE and participates in the resistance of H. parallela larvae to EPN-Bt infection.

Background Combining the entomopathogenic nematode (EPN), Heterorhabditis beicherriana LF strain, and Bacillus thuringiensis (Bt) HBF‐18 strain is a practical strategy to manage the larvae of Holotrichia parallela Motschulsky (white grubs). However, the mechanisms underlying the larval defense response to this combined biocontrol strategy are unknown. Results The activities of some antioxidant enzymes (SOD, POD, CAT) and some detoxifying enzymes (AChE, P‐450, CarE, GST) in grubs showed an activation–inhibition trend throughout the EPN‐Bt exposure time course. Eight potentially key antioxidant and detoxifying enzyme genes in response to EPN‐Bt infection were identified from the midgut of grubs through RNA sequencing. After silencing CAT, CarE18, and GSTs1, the enzyme activities were significantly decreased by 30.29%, 68.80%, and 34.63%, respectively. Meanwhile, the mortality of grubs was increased by 18.40%, 46.30%, and 42.59% after exposure to EPN‐Bt for 1 day. Interestingly, the PI3K/Akt signaling pathway was significantly enriched in KEGG enrichment analysis, and the expression levels of phosphatidylinositol 3‐kinase (PI3K), protein kinase B (Akt), cap ‘n’ collar isoform‐C (CncC), kelch‐like ECH‐associated protein 1 (Keap1), and CarE18 were all up‐regulated when exposed to EPN‐Bt for 1 day. Furthermore, RNAi‐mediated PI3K silencing showed a similar down‐regulated trend between PI3K/Akt/CncC and CarE18. Moreover, silencing PI3K rendered grubs more susceptible to EPN‐Bt and accelerated symbiotic bacteria multiplication in grubs. Conclusion These results suggest that the PI3K/Akt/CncC pathway mediates the expression of CarE18 and participates in the defense response of H. parallela larvae against EPN‐Bt infection. Our data provide valuable insights into the design of appropriate management strategies for this well‐known agricultural pest. © 2022 Society of Chemical Industry.

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