Harman and norharman, metabolites of entomopathogenic fungus Conidiobolus coronatus (Entomopthorales), disorganize development of Galleria mellonella (Lepidoptera) and affect serotonin-regulating enzymes.
Naturally occurring entomopathogenic fungi such as Conidiobolus coronatus are important regulatory factors of insect populations. GC-MS analysis of fungal cell-free filtrates showed that C. coronatus synthesizes two β- carboline alkaloids harman and norharman. Significantly higher levels of both alkaloids are produced by C. coronatus in minimal postincubation medium than in rich medium. The beta-carboline alkaloids may have an effect on the nervous system of insects and their behavior. Harman and norharman were applied to Galleria mellonella larvae (a parasite of honeybees) either topically or mixed with food. Larvae received alkaloids in three concentrations 750, 1000 or 1250 ppm. The effect on the survival and further development of larvae was examined. Both harman and norharman delayed pupation and adult eclosion, and inhibit total monoamine oxidase activity. In addition, they increased the serotonin concentration and decreased the monoamine oxidase A level in the heads of the moths. It is likely that the alkaloids were metabolized by the insects, as their effect wore off 24 hours after topical application. This is the first study to show that C. coronatus produces alkaloids. Its aim was to identify the actions of β-carboline alkaloids on insect development and serotonin-regulating enzymes. Knowledge of the potential role of harman and norharman in the process of fungal infection might lead to the development of more effective and environmentally-friendly means of controlling insect pests.