Bacillus thuringiensis serovar morrisoni biovar tenebrionis impact and persistence in Tenebrio molitor larvae

The yellow mealworm (Tenebrio molitor L.; Coleoptera: Tenebrionidae) has been proposed during the last decade as a suitable insect species for feed and food production. The inclusion of by- and side-products of cereal and vegetable production in the feed of yellow mealworm increases the probability of exposure to Bacillus thuringiensis sv. morrisoni biovar tenebrionis (Btt), an entomopathogen used in the biological control of coleopteran pest species. The lack of studies on effects of Btt spores and toxin on T. molitor in mass-rearing systems led us to explore its effect on survival and growth of larvae and its persistence in larvae and in frass. Larvae of different body weight (3, 5, 10 and 40 mg) were exposed by free feeding for 72 hours to a range of Btt spore - crystal concentrations spiked to wheat bran (WB). Susceptibility to the entomopathogen was concentration-dependent and linked to the larval body weight, larvae of higher weight being less susceptible. Growth and feed conversion ratio of survivors showed significant impact for low-weight larvae while less or no effect was recorded for larvae having higher weight. Btt was still recovered from larvae and frass 14 days after feeding with non-contaminated WB, indicating a certain level of persistence in the tested conditions, on alive 40-mg larvae, implying a potential risk in T. molitor mass rearing.