Effects of the Entomopathogenic Fungus Metarhizium anisopliae on the Mortality and Immune Response of Locusta migratoria

Abstract

Entomopathogenic fungi are the key regulators of insect populations and some of them are important biological agents used in integrated pest management strategies. Compared with their ability to become resistant to insecticides, insect pests do not easily become resistant to the infection by entomopathogenic fungi. In this study, we evaluated the mortality and immune response of the serious crop pest Locusta migratoria manilensis after exposure to a new entomopathogenic fungus strain, Metarhizium anisopliae CQMa421. M. anisopliae CQMa421 could effectively infect and kill the L. migratoria adults and nymphs. The locust LT₅₀ under 1 × 10⁸ conidia/mL concentration of M. anisopliae was much lower than that under conidial concentration 1 × 10⁵ conidia/mL (i.e., 6.0 vs 11.2 and 5.0 vs 13.8 for adults and nymphs, respectively). The LC₅₀ (log₁₀) of M. anisopliae against locust adults and nymphs after 10 days was 5.2 and 5.6, respectively. Although the number of hemocytes in L. migratoria after exposure to M. anisopliae did not differ with that in the controls, the enzymatic activity of superoxide dismutase (SOD) and prophenoloxidase (ProPO) did differ between the two treatments. The activities of both SOD and ProPO under the M. anisopliae treatment were lower than that in the controls, except for the ProPO activity at 72 h and the SOD activity at 96 h. Further, the expression of the L. migratoria immune-related genes defensin, spaetzle, and attacin differed after exposure to M. anisopliae for 24 h to 96 h. Taken together, this study indicated that infection with M. anisopliae CQMa421 could cause the death of L. migratoria by interacting with the immune responses of the host, demonstrating that this fungal strain of M. anisopliae can be an efficient biocontrol agent against L. migratoria.

Keywords: Locusta migratoria; Metarhizium anisopliae; immune response; mortality; pest control.

Figure 1

Survival of L. migratoria after exposure to M. anisopliae CQMa421. (A) the corpses of L. migratoria insects infected with M. anisopliae CQMa421 or not; (B) LC₅₀ of L. migratoria adults and nymphs; (C) Survival rate of L. migratoria nymphs treated with M. anisopliae CQMa421; (D) LT₅₀ of L. migratoria nymphs after being challenged with M. anisopliae CQMa421. (E) Survival rate of L. migratoria adults treated with M. anisopliae CQMa421; (F) LT₅₀ of L. migratoria adults after being challenged with M. anisopliae CQMa421. The different letters indicate significant differences, and the bars represent the means ± SEs.

Figure 2

Concentration of hemocytes after challenge with M. anisopliae CQMa421 from 24 to 96 h. The bars represent the means ± SEs.

Figure 3

ProPO and SOD activities of L. migratoria after challenge with M. anisopliae CQMa421. (A) Enzymatic activity of L. migratoria after treatment with M. anisopliae CQMa421 for 24 h; (B) Enzymatic activity of L. migratoria after treatment with M. anisopliae CQMa421 for 48 h; (C) Enzymatic activity of L. migratoria after treatment with M. anisopliae CQMa421 for 72 h; (D) Enzymatic activity of L. migratoria after treatment with M. anisopliae CQMa421 for 96 h. The different letters indicate significant differences, and the bars represent the means ± SEs.

Figure 4

Relative expression of genes after challenge with M. anisopliae CQMa421. (A) The relative expression of defensin, spaetzle and attacin 24-h post infection; (B) the relative expression of defensin, spaetzle, and attacin 48 h post infection; (C) the relative expression of defensin, spaetzle and attacin 72 h post infection; (D) the relative expression of defensin, spaetzle and attacin 96 h post infection. The different letters indicate significant differences, and the bars represent the means ± SEs.