Abstract: 【Aim】 Albizia kalkora is a low-fitness host plant for Hyphantria cunea larvae. The purpose of this study is to explore the effect of H. cunea larval feeding on the induced defense of A. kalkora and the effects of induced defense of A. kalkora on the fitness of the 2nd batch of H. cunea larvae. 【Methods】 On the 7th and 21st days after feeding by the 4th instar larvae of H. cunea at low-density (LD) (30 individuals/plant) and high-density (HD) (50 individuals/plant), the contents of nutrients (amino acids, soluble sugar and total proteins) and secondary substances (tannin, total phenols, total flavones, lignin and total alkaloids) in the damaged and healthy leaves of A. kalkora were measured. The expression levels of the key genes PGT1, CHS7 and ANR in flavonoid biosynthesis pathway, and AOC2, OPCL1 and LOX2S in α-linolenic acid pathway in the damaged and healthy leaves of A. kalkora were measured by RT-qPCR. On the 3rd and 7th days after feeding the 3rd instar larvae of the 2nd batch of H. cunea with the damaged leaves of A. kalkora, the mortality, body weight, body length and head shell width of larvae were measured, and the expression levels of GADD45 and MYC related to growth and development, HK1 and OGDH related to energy metabolism, digestive enzyme genes TRY7, AMY2 and LIP10, and detoxification enzyme genes GST18, CARE14 and CYPAE178 in the 2nd batch of H. cunea larvae were measured by RT-qPCR. 【Results】 The contents of soluble sugar and total proteins in the leaves of A. kalkora in various treatment groups decreased significantly at different time points after feeding the 4th instar larvae of H. cunea with A. kalkora leaves as compared with those in the non-feeding control (CK) group. On the 7th day after feeding, the contents of total phenols, total flavones and lignin in the damaged leaves of A. kalkora in LD and HD feeding groups were significantly increased and the tannin content in the damaged leaves of A. kalkora in the LD feeding group was significantly increased, as compared with those in the CK group. The contents of total phenols and total flavonoids in the healthy leaves of A. kalkora in the HD feeding group were significantly increased as compared with those in the CK group. On the 21st day after feeding the 4th instar larvae of H. cunea with A. kalkora leaves, the contents of tannin and total flavonoids in the damaged A. kalkora leaves in LD and HD feeding groups were significantly increased, the content of total alkaloids in the healthy A. kalkora leaves in the LD feeding group was significantly increased, and the contents of tannin, total phenols and total alkaloids in healthy A. kalkora leaves in the HD feeding group were significantly increased, as compared with those in the CK group. On the 7th day after feeding by the 4th instar larvae of H. cunea, the expression levels of ANR and CHS7 in both the damaged and healthy A. kalkora leaves were significantly up-regulated as compared with those in the CK group. On the 21st day after feeding by the 4th instar larvae of H. cunea, the expression levels of PGT1 and CHS7 in both the damaged and healthy A. kalkora leaves were significantly up-regulated as compared with those in the CK group. On the 7th and 21st days after feeding by the 4th instar larvae of H. cunea, the expression levels of OPCL1 and LOX2S in both the damaged and healthy A. kalkora leaves were significantly up-regulated as compared with those in the CK group. On the 21st day after feeding by the 4th instar larvae of H. cunea, the expression levels of AOC2 in both the damaged and healthy A. kalkora leaves was significantly up-regulated as compared with those in the CK group. On the 3rd and 7th days after feeding on the A. kalkora leaves in which the induced defense had been activated in LD and HD feeding groups, the body weight, body length and head shell width of the 2nd batch of H. cunea larvae decreased significantly, and the larval mortality increased to a certain extent as compared with those in the CK group. The expression levels of GADD45 in the 2nd batch of H. cunea larvae in the LD feeding group on the 3rd and 7th days after feeding were significantly up-regulated and those of MYC in the 2nd batch of H. cunea larvae in LD and HD groups on the 7th day after feeding were significantly up-regulated, as compared with those in the CK group. On the 3rd and 7th days after feeding, the expression levels of TRY7, AMY2, LIP10, HK1, OGDH, GST18, CARE14 and CYPAE178 in the 2nd batch of H. cunea larvae in LD and HD feeding groups were significantly down-regulated as compared with those in the CK group. 【Conclusion】 H. cunea larval feeding activates the induced defense of A. ralkora, and the 2nd batch of H. cunea larvae can not form an effective counter-defense strategy to the induced defense of A. kalkora, which may be an important reason why A. kalkora is a low-fitness host plant for H. cunea larvae.

Key words: Hyphantria cunea; feeding stress, host plant, induced defense, defensive escape, fitness