Abstract: To explain the infection ability of Beauveria bassiana to cotton bollworm Helicoverpa armigera (Hübner), pathogenicity of B. bassiana HFW-05 strain to 2nd instar larvae of H. armigera was measured using feeding method and cuticle infection, and the mode of infection of HFW-05 strain to H. armigera was observed through histopathological section and scanning electron microscopy observation. The results showed that HFW-05 strain could infect H. armigera successfully through the digestive tract (feeding method) on 2nd instar larvae of cotton bollworm, with the corrected mortality 75.8% at 6 d after infection. But it did not infect the larvae through cuticle infection (dipping method), and the average body weight and food consumption of the test insects were similar to the control, with the corrected mortality only 17.3% at 6 d after infection. The results indicated that HFW05 strain could infect cotton bollworm mainly through the digestive tract under 26±1℃ condition and the pathogen occurred in intestinal tissue, fat bodies and the Malpighian tubules. After invading the host, it caused the intestinal tissue microvilli to shed seriously and to dissolve gradually with only membrane of the bottom left. The Malpighian tubules became atrophic and deformed, and their edges became swelled and the diameter became enlarged. Fat bodies were disintegrated and loose. The epidermal cells of the hosts had been damaged by hyphae. Cuticle infection by dipping method was observed through histopathological section, hyphae were not found in host body and the intestinal tissues were normal and integral at 6 d after treatment. The scanning electron microscope results showed that conidia failed to penetrate the host cuticle by cuticle infection, but affixed to the cuticle surface of host, and grew to break as blastospores under the right condition. The results suggest that digestive tract infection could compensate environmentally unfavorable effect to B. bassiana infection, and this may have important significance for B. bassiana application in bio-control.

Key words: Beauveria bassiana, Helicoverpa armigera, infection process, histopathology, scanning electron microscope