Stage-Dependent Toxicity of 1,8-Cineole and Diatomaceous Earth, Alone and Combined, Against Tenebrio molitor (Coleoptera: Tenebrionidae), and Observations on F1 Larvae

Growing interest in environmentally compatible stored-product pest control has highlighted diatomaceous earth (DE) and 1,8-cineole as promising agents, both alone and in combination. Their different modes of action, together with the limitations associated with higher-dose single applications, support evaluating their combined use at lower doses. This study was conducted to compare the effects of DE and 1,8-cineole, applied alone and in combination, on the larval, pupal, and adult stages of Tenebrio molitor. Five different concentrations were tested for each substance (DE at 25, 50, 100, 250, and 500 ppm, and 1,8-cineole at 2.5, 5, 10, 15, and 20 ppm), and four DE + 1,8-cineole combinations were evaluated within the same experimental system. Mortality was monitored over time, LC50 values were calculated by probit analysis, and larval output observed after adult treatments was also evaluated. The findings indicated that the biological response was associated with developmental stage. The lowest LC50 for DE was recorded in larvae at 86.11 ppm on day 3, whereas for 1,8-cineole the lowest LC50 was recorded in adults at 94.83 ppm on day 3. Combined treatments generally tended to produce faster and stronger mortality; in particular, the DE250 + CIN20 treatment reached 100% mortality in larvae and adults and 93.33% mortality in pupae by day 7. In addition, larval output decreased in the single-treatment groups, the proportion of dead larvae among the observed larvae increased to 96–100%, and no larval output was detected in the combination groups. Combinations of DE and 1,8-cineole tended to produce more pronounced mortality responses than the single treatments, particularly in the larval and adult stages. The present findings indicate that combining DE with 1,8-cineole may provide a promising stage-specific strategy for improving the control of T. molitor under laboratory conditions.