Fungus shows potential against Phthorimaea operculella

Researchers at Yunnan Agricultural University have identified an isolate of Metarhizium robertsii with potential for biological control of the potato tuber moth, Phthorimaea operculellaThe fungus, named ML-2, caused 94,5% corrected mortality in larvae and 83,07% in pupae on the seventh day after inoculation, at a concentration of 1 × 10⁸ conidia/mL. In eggs, mortality reached 20,28%. The results came from laboratory tests and indicate greater susceptibility of larvae and pupae to the isolate.

The potato moth, Phthorimaea operculellaThis pest attacks crops such as potatoes and tobacco. The larvae bore into roots, stems, and leaves. During storage, they form galleries in the tubers. This damage reduces quality and commercial value. The article also highlights the difficulty of managing the pest in the field and in storage, associated with the excessive use of insecticides, monoculture, and the reproductive capacity of the pest.

The isolated Metarhizium robertsii ML-2 came from larvae of Phthorimaea operculella Naturally infected fungi were collected from a potato field in Malong County, Yunnan Province, China. The team purified the fungus and identified it through morphological characteristics and molecular analysis of the ITS and EF1-α regions. Phylogenetic analysis grouped the isolate with... Metarhizium robertsii ARSEF2575, with 98% bootstrap support.

Bioassays

The bioassays used immersion of eggs, larvae, and pupae of Phthorimaea operculella in conidia suspensions of Metarhizium robertsii ML-2. Concentrations ranged from 1 × 10³ to 1 × 10⁸ conidia/mL. Researchers monitored mortality for seven days. The viability of the suspensions needed to exceed 90% for use in the assays. Each replicate used 25 individuals, with three replicates per treatment.

The response varied according to the stage of development of Phthorimaea operculellaThree-day-old larvae showed the greatest sensitivity. One-day-old pupae also responded to the fungus. Eggs showed low mortality compared to other stages. According to the authors, protective characteristics of the egg may reduce infection. Larvae and pupae have intersegmental membranes and spiracles. These structures favor the adhesion of conidia and increase the opportunities for penetration. Metarhizium robertsii.

The effect of the fungus also increased with time and concentration. From the fifth day after inoculation, all treatments with conidial suspension differed from the control in larvae and pupae. On the seventh day, larval mortality ranged from 12,01% to 94,50%, depending on the concentration. In pupae, it ranged from 21,67% to 83,07%.

Analysis of results

Analysis using the time-concentration-mortality model confirmed this relationship. For larvae of Phthorimaea operculellaThe LT50 values ​​at 1 × 10⁶, 1 × 10⁷, and 1 × 10⁸ conidia/mL were 4,98, 2,89, and 2,34 days, respectively. For pupae, the corresponding values ​​were 5,91, 3,70, and 2,72 days. At the highest concentration, the LT90 for larvae reached 4,35 days. For pupae, the LT90 could not be estimated because the final mortality rate was below 90%.

The authors also evaluated growth and conidia production of Metarhizium robertsii ML-2 in two culture media: PDA and SDAY. Colony growth did not differ between the media. Sporulation, however, changed. The PDA medium produced approximately three times more spores compared to the SDAY medium on day 15. The difference may involve carbon and nitrogen sources and the carbon:nitrogen ratio of the medium.

The images from the study reinforce the symptoms of infection by Metarhizium robertsii em Phthorimaea operculellaIn larvae, the body changed from white to yellowish-brown and hardened. Then white hyphae appeared, followed by green conidia. In pupae, there was a change in color, growth of white mycelium, and formation of green conidia on the surface. By the fifth day, dead larvae and pupae showed a covering of conidia.

More information at doi.org/10.3390/insects17050474