Bioaccumulation of cadmium in Tenebrio molitor, Zophobas morio and Hermetia illucens

Insects have been proposed as a high quality, efficient and sustainable alternative protein source via feed or as a direct food source for humans. Mealworms and black soldier fly are already mass-produced as feed. One of the potential risks associated with their consumption is the chemical contamination (i.e. heavy metals). This study focuses on the potential bioaccumulation of cadmium from the feeding media in the mealworm species Yellow mealworm (Tenebrio molitor) and Giant mealworm (Zophobas morio) and in the Black soldier fly (Hermetia illucens). A colony of the three species was set up and maintained in a climate chamber. Larvae were used to define the chemical risk by assessing their storage capacity, when fed on artificially contaminated diet with cadmium at a concentration of 1.109 mg/kg. The control substrate had an initial cadmium concentration of 0.030 mg/kg for H. illucens and 0.114 for the mealworms. For each trial (n=3), 300 larvae were used for H. illucens, while 50 for T. molitor and Z. morio. The bioaccumulation factor (BAF) was estimated for mature larvae in mealworm species. In BSF specimens the accumulation was tested at each larval instar, pupal and adult stage. Each sample was processed by inductively coupled plasma mass spectrometry. For BSF data showed a cadmium concentration, in larvae and in pupae, always greater than 2 mg/kg, with a decrease in the last stages of development. The concentration of cadmium in the residual substrate tends to remain almost constant up to IV larval instar and then decrease during the prepupal and pupal stage. The BAF showed a substantial increase in the prepupal and pupal stage. Development times in contaminated diet were higher than the control group. In mealworm larvae, bioaccumulation was less relevant, with a value of 0.79 and 0.44 mg/kg respectively. These values are below the values of the initial diet, both for the control larvae and for those fed on the artificially contaminated diet. No differences were noticed on the survival of the two mealworms between the contaminated and the control diet, while final larval weight of Z. morio was impacted from the presence of cadmium. The study reveals a high ability of H. illucens to accumulate cadmium, while in mealworms larvae, most likely the longer development allowed their elimination. As the accumulation of heavy metals varies according to the type of metal, the species of insects and the stage of development, for food safety it is important to evaluate the potential risks in a case by case approach.