Monarda fistulosa L. hydrolate as antimicrobial agent in artificial media for the in vitro rearing of the tachinid parasitoid Exorista larvarum (L.)

Exorista larvarum (L.) (Diptera Tachinidae), a larval parasitoid of Lepidoptera, can be reared in vitro from egg to fecund adult on artificial media composed of crude components. The standard in vitro culture is performed in 24-well plastic rearing plates. Exorista larvarum eggs, removed from superparasitized larvae of Galleria mellonella (L.) (Lepidoptera Pyralidae), are individually placed in the wells, each containing a cotton ball soaked in liquid medium. The plates are then sealed until parasitoid puparium formation. Newly-formed puparia are collected from the medium and transferred into cages for adult emergence. To avoid contamination by microorganisms, the artificial medium is routinely added with 1% solution of gentamicin. Concern over occurrence of resistant microorganisms, following the prolonged use of this broad spectrum antibiotic, has prompted research on more natural antimicrobial agents. Experiments were carried out to assess whether gentamicin may be replaced with hydrolate of the wild bergamot Monarda fistulosa L. (Lamiaceae), which was selected due to its high activity against pathogenic microorganisms for humans and plants. The hydrolate was either added to the artificial medium (0.01% w/w) (first trial), or placed in an empty well of the rearing plate (0.5% w/w), to be supplied in the aerosol form due to evaporation (second trial). In both trials, a standard medium with gentamicin and an antimicrobial-free medium were maintained as positive and negative controls, respectively. In the first trial, E. larvarum development from the egg to the adult stage in the hydrolate-added medium was comparable to that observed in the standard medium. No significant difference was found between the number of eggs laid by the females obtained in either substrate. In the second trial, however, the hydrolate in the aerosol form significantly decreased E. larvarum egg hatching, puparium formation and female fecundity compared to the standard medium. In both trials, little parasitoid development occurred in the antimicrobial-free media. The results suggest that artificial media added with M. fistulosa hydrolate solution may be competitive with those containing gentamicin for the in vitro rearing of E. larvarum. The hydrolate aerosol form was shown, instead, unsuitable.