莱氏绿僵菌对斜纹夜蛾的致病力及生理效应

Virulence and physiological effects of Metarhizium rileyi against Spodoptera litura

摘要:

【目的】测定莱氏绿僵菌Metarhizium rileyi Nr5772菌株对斜纹夜蛾Spodoptera litura幼虫及蛹的致病能力，研究莱氏绿僵菌侵染后在寄主体内的发育及对寄主的生理效应，探讨莱氏绿僵菌的致病机制。【方法】采用浸渍法测定莱氏绿僵菌孢子对斜纹夜蛾3?6龄幼虫及蛹的致死中浓度(LC50)和致死中时(LT50)。采用微量注射法接种莱氏绿僵菌虫菌体，在不同时间后采集斜纹夜蛾幼虫血淋巴，在显微镜下检查虫菌体的数量、形态及寄主血细胞数量，并用酶标仪测定寄主血淋巴酚氧化酶(Phenoloxidase，PO)的活性。【结果】M. rileyi孢子对3龄斜纹夜蛾幼虫毒力最强，10 d后LC50=3.12×106个孢子/mL，龄期越大，致病力越低；孢子浓度为5×109个/mL时，对3龄幼虫的致死速度最快，LT50=4.55 d，致死速度随龄期的增大和浓度的降低逐渐减缓；M. rileyi孢子对蛹的致病力远低于对幼虫的致病力。注射接种虫菌体后，64 h内，虫菌体数量在寄主血腔中以幂函数的形式增长，寄主的血细胞数量没有明显的变化；在侵染初期(接种后44 h内)，血淋巴PO活性正常；在侵染后期，虫菌体数量不再增加(55?64 h后)，逐渐转化为菌丝体，并快速杀死寄主，PO活性受到抑制。【结论】莱氏绿僵菌Nr5772菌株对斜纹夜蛾幼虫有较强的致病力，应在害虫低龄期应用；莱氏绿僵菌在侵染初期对寄主血细胞和血淋巴PO无影响，后期则完全抑制PO活性。

Abstract:

[Objective] This study aimed to determine the virulence of Nr5772 isolate of Metarhizium rileyi against larvae and pupae of Spodoptera litura, observe the development of M. rileyi in vivo and the physiological effect on host and illustrate the pathogenic mechanism of M. rileyi to insect pest. [Methods] Dipping method was adopted to assay the LC50 and LT50 of M. rileyi conidia against 3?6 instar larvae and pupae of S. litura. After the hyphal bodies of M. rileyi was injected into the hemocoel, the hemolymph was sampled at different time intervals to record the morphism, meanwhile, the number of hyphal bodies and hemocytes was counted and the phenoloxidase (PO) activity of host was assayed. [Results] As the lethal effect decreased along with the increase of instar, M. rileyi has the strongest lethal effect on 3rd instar larvae (LC50=3.12×106 spores/mL). The lethal velocity decreased with the increase of the instar and decline of spore concentration. When dipped in 5×109 spores/mL solutions, the 3rd instar larvae died rapidly (LT50=4.55 d). Within the following 64 h after inoculation, the hyphal body reproduction in vivo fit the power function model, whereas the hemocyte number in larvae was remain stable. At the primary period of infection (44 h post inoculation), PO activity in inoculated larvae was the same as control, however, it was inhibited strongly in the later stage (55?64 h post inoculation) when yeast-like hyphal bodies transformed into mycelia and killed the host. Apart from larvae, M. rileyi had no significant lethal effect on pupae. [Conclusion] Nr5772 isolate of M. rileyi has stronger lethal effect on earlier instar S. litura larvae, thus, to achieve better control efficiency, it is recommended to apply this fungus on lower instar larvae. M. rileyi has no obvious harmful effect on hemocytes and PO activity of host at the primary stage of the invasion, but it will strongly inhibit the PO activity at later stage.