Abstract: 【Aim】 The study aims to further explore the mechanism of pathogenicity of entomapathogenic nematodes Heterorhabditis bacteriophora strain Cangzhou infecting white grubs. 【Methods】 Transmission electron microscopy (TEM) was used to observe the morphological changes in fat body and midgut cells of the 2nd instar larvae of Hololtrichia oblita (Faldermann) and H. parallela Motschulsky, respectively. After hemocoel injection, ultrastructural changes of the white grubs were examined by TEM. 【Results】 Changes in the ultrastructure of fat body and midgut cells of the 2nd instar larvae of H. oblita and H. parallela at 24 and 48 h after injection of H. bacteriophora were observed. At 24 h after infection, fat droplets, located at fat body cells, became smaller, and their electron density became lighter in the infected larvae than those in the untreated control. Endoplasmic reticulum and mitochondria in both fat body and midgut cells became distended and mcrovilli of the midgut began to fall off in the infected larvae. At 48 h after infection, the membrane structure containing a lot of fat droplets was found broken in the infected larvae. Chromatin in fat body and midgut cell nucleus dissociated and mitochondria as well as endoplasmic reticulum were disintegrated completely in both cells of the infected larvae. Likewise, microvilli of the midgut were almost completely lost in the infected larvae. 【Conclusion】 After infection by the entomopathogenic nematode H. bacteriophora, the fat body and midgut of the 2nd instar larvae of the two white grub species, H. oblita and H. parallela, show similar pathological changes, which may be associated with the rapid death of white grubs. This study may enrich the theoretical basis for using entomopathogenic nematodes as a biological control strategy in IPM of white grubs.

Key words: Entomopathogenic nematode, Heterorhabditis bacteriophora, white grubs, Hololtrichia oblita, Holotrichia parallela, fat body, midgut