For those of you who love planting, are you often troubled by the pest crisis? Moreover, each season, there are different pests; often, a pesticide is only effective on a pest. Along with the use more times, the pest resistance has increased, and the effect is gradually not as good as expected.

So now, many people are promoting biological control methods, such as using natural enemies of pests or things that pests are afraid of to prevent pests. This will avoid the use of pesticides that cause harm to the environment and avoid the problem of pesticide residues that cause a physical burden. Nevertheless, this control’s effect is limited, so this method can only be described as an auxiliary.

Because of the recent global pandemic of Exotic Infectious Pneumonia (COVID-19), I now have an idea that there might be something that can make pests sick and die, so I have been looking through much literature lately. I found a fungal pathogen called Metarhizium anisopliae.

What is Metarhizium anisopliae

Metarhizium anisopliae is a widespread, soil-borne fungal pathogen of insects, ticks, and mites that has excellent potential as an environmentally friendly alternative to traditional chemical pesticides for controlling pests of “socio-economic importance.” It can infect more than 200 species of pests in total and can be used as a biopesticide to harm crop pests by infecting each other and causing death in the population. Moreover, it does not infect humans and other vertebrates and is relatively safe for use as an insecticide and acaricide in agriculture.

The infection route of Metarhizium anisopliae

The infection route of Metarhizium anisopliae is similar to other entomopathogenic fungi in that they require conidia to attach to the surface of the host. After contact with the insect’s surface, the spores can germinate and grow mycelium, making enzymes break down the insect’s epidermis and forming attachments to penetrate the insect’s body wall and enter its body for further growth. When the mycelium grows to the hemolymph, it will form free-growth hyphal bodies, which can separate from the mycelium and replicate and grow freely in the hemolymph like yeast. Because it has particular proteins covering the surface, it can obscure the Β-1,3-glucan and other pathogen-associated molecular patterns (PAMP) of the cell wall so that it cannot be recognized by the insect’s immune cells and is not attacked. The mycelium of Metarhizium anisopliae can grow inside the insect tissues and outside, eventually leading to the insect’s death. The epidermis of the killed insect will usually be red. If the environmental humidity is high enough, the mycelium will grow from the insect carcass and produce conidia that will appear green and may turn grey-green to black as the spore pile increases.

Host range of Metarhizium anisopliae

Its host range covers more than 200 species of insects in 7 orders. The seven orders are Orthoptera, Isoptera, Hemiptera, Diptera, Lepidoptera, Coleoptera, and Hymenoptera. Lepidoptera and Coleoptera are the majority of the infected insects found in Taiwan, such as the Taiwan yellow poison moth, the striped butterfly, the diamondback moth, the Spodoptera litura, the corn borer, the weevil, the scarab, the kowtow beetle, the golden flower worm and the larvae of the unicorn.

Restrictions on the use of Metarhizium anisopliae

The operation of Metarhizium anisopliae is the same as most of the agents, starting with an average of one insect per crop and spraying the field evenly twice, about seven days apart. “The restriction of use is the environmental humidity.” It requires a relative humidity of 90% to maintain high infectivity because high humidity allows the fungus to germinate and infect the pest quickly. Therefore, it is best to spray in the evening or at night to take advantage of the high humidity at night to enhance the effect of the strains. As for vegetable cultivation, we can consider humidifying with the spraying system. Nevertheless, many orchards are difficult to humidify because of the low relative humidity, so the effect of Metarhizium anisopliae control may be low. In addition, the crops cultivated in the greenhouses can control the humidity of the environment, and the effect of using Metarhizium anisopliae for pest control is also very high.

Metarhizium anisopliae has five characteristics that chemical pesticides do not have.

Metarhizium anisopliae not only has the effect of pest control but also has the advantages that the chemical control method does not have, which are as follows.

1.Economical

Research and development of fungal products are far less expensive than chemical insecticides and can be done in large quantities using simple natural culture machines.

2.With security

Metarhizium anisopliae has a high degree of safety for humans and animals, is a beneficial natural enemy, and will not pollute Wyoming after all; no residual pesticide toxicity and pesticide harvesting period concerns, crops can be harvested at any time, and not afraid of the problem of just spraying has not yet evaporated, and can not be harvested or accidentally eaten.

3.With compatibility

It can be used with other pest control methods to reduce the frequency and dose of pesticides, such as pheromones, sticky insect versions, or other biological control methods; it can achieve twice the effect with half the effort.

4.Spreadability

Metarhizium anisopliae can survive in the natural environment and cause pests epidemics, actively balancing the density of pests and spreading the infection in the colony if only one of the colonies is infected.

5.No drug resistance problems.

Because Metarhizium anisopliae is a living organism, there is no concern about resistance even after a significant amount of time; at least so far, there are no reports of resistance to Metarhizium anisopliae.

Reasons why Metarhizium anisopliae is not commonly used

1. In the field, it is affected by the adverse environment of nature (mainly the destruction of ultraviolet light and unsuitable temperature and humidity conditions), which makes the application effect unstable, resulting in the field application effect is not as expected.

2. Metarhizium anisopliae products are not easy to preserve and have a Short shelf life. Therefore, it affects the marketing and promotion of commercial products. Although the disadvantages mentioned above exist, the insecticidal ability and safety cannot be ignored.

Therefore, with the gradual change in agricultural ecology and the increase of organic farmers, if combined with the improvement of the resistance of Metarhizium anisopliae, it will be beneficial to the promotion and application of Metarhizium anisopliae in pest control.

Improvement methods

Scholars around the world not only keep screening local strains with high insecticidal ability but also for strains to improve the above-mentioned poor environment, among which the African region for up to three years of screening, and finally, the tremendous strain of Metarhizium anisopliae that can withstand high-temperature growth conditions was selected by nature. In addition, the strains that can withstand high temperatures are screened by mutation, and although they have not reached 36 degrees, they have initially obtained the goal of surviving high temperatures. In the future, we will add UV-absorbing substances to improve the problem of UV damage and use emulsified oil formulations to improve the shortcomings of Metarhizium anisopliae due to low humidity conditions not easy to germinate and infest the pest.

In addition, In addition to the improvement of strains and formulations, the timing of the application of biological control agents can be used to avoid the damage of the adversities as mentioned above. for example: if applied at dusk or after rain, it can avoid the damage caused by high sunlight during the day and provide a favorable environment with high humidity. As the Metarhizium anisopliae can complete the invasion within a few hours when it touches the surface of the pest, it can still achieve good results as long as the application is made correctly.