Soy flour versus skimmed milk powder in artificial diets for Galleria mellonella, a factitious host for Exorista larvarum

Galleria mellonella (L.) (Lepidoptera Pyralidae) is worldwide produced and utilized for different purposes. In the entomology laboratories of the University of Bologna, Italy, this insect has long been maintained in continuous colony, and used as a factitious host for Exorista larvarum (L.) (Diptera Tachinidae), a polyphagous, gregarious larval parasitoid, mainly of Lepidoptera, which lays eggs on the host body. The “standard” diet used to feed G. mellonella larvae, which contains skimmed milk powder, white and whole wheat flour, maize flour, honey, bee wax, brewer’s yeast and glycerine, has long proven to be efficient, but presents the drawback to be rather expensive. We tested the possibility to reduce diet costs by replacing skimmed milk powder (added for its protein and energy intake) with soy flour, an alternative and cheaper protein source. The development times and adult yields of G. mellonella were not negatively affected, when the “soy” diet was used to feed the larvae for one generation. Host acceptance by E. larvarum (measured as number of parasitoid eggs laid on G. mellonella) was higher for the larvae fed on the “soy” than on the “standard” diet. The use of soy flour in lieu of skimmed milk powder, however, proved inappropriate for the rearing of G. mellonella in the long run, because some quality traits, e.g. pupal weights and development times were lower in the “soy” than in the “standard” diet and further worsened through generations. Moreover, the number of eggs laid was dramatically lower for the F2 G. mellonella females obtained on the “soy” than on the “standard” diet. These results suggested that the costs of the artificial diet currently used for G. mellonella may not be lowered by replacing skimmed milk powder with soy flour on a regular basis, possibly due to nutrient imbalance.