Macronutrients profoundly affect both lifespan and reproduction and also modulate the fundamental trade-off between these two components of fitness in many insects. Beetles represent the largest group of insects, but nutritional interventions in lifespan and reproduction have never been thoroughly explored in this taxon. Here, we used nutritional landscape methodology to determine the effects of protein and carbohydrate intake on lifespan and reproduction in the mealworm beetle, Tenebrio molitor (Coleoptera: Tenebrionidae). The time of death and last reproduction, the number of eggs laid and protein-carbohydrate intake were recorded from beetles maintained on one of 35 chemically defined foods varying in protein-to-carbohydrate ratio (P:C=0:1, 1:5, 1:2, 1:1, 2:1, 5:1 or 1:0) and in protein plus carbohydrate concentration (P+C=25.2%, 33.6%, 42%, 50.4% or 58.8%). Lifespan and reproductive traits increased with higher caloric intake, but their respective trait maxima occurred at different P:C ratios. Female reproductive traits peaked at higher P:C ratios (reproductive lifespan 1:1.06; lifetime egg production 1.31:1; egg production rate 1.75:1) than those maximizing lifespan (male lifespan 1:1.38; female lifespan 1:1.36). This divergence indicates a nutrient-mediated trade-off between lifespan and reproduction in this species. Despite this, the nutritional conflict in T. molitor appeared to be less pronounced than what has been observed in other species commonly used in insect ageing research. When given a food choice, T. molitor beetles selected a P:C ratio close to 1:1, which simultaneously supported extended lifespan and high reproductive output.

Abstract The yellow mealworm, Tenebrio molitor L. (Coleoptera: Tenebrionidae), is one of the few edible insects commercially produced around the globe as alternative protein sources for food and feed. Temperature and nutrition are the two most influential environmental determinants of growth and development in insects, but little is known about their combined effects in T. molitor . Here we examined how temperature and dietary protein:carbohydrate (P:C) balance act in concert to influence key performance traits in T. moltior larvae reared on one of the 36 treatment combinations of six temperatures (19, 22, 25, 28, 31, and 34 °C) and six protein:carbohydrate ratios (P:C = 1:5, 1:2, 1:1, 2:1, 5:1, and 1:0). Apart from females accumulating higher lipid content than males, there were no sex differences in all measured traits. Survivorship was high at low temperatures (<25 °C) and high P:C ratios (>1:1), but decreased with increasing temperature and decreasing P:C ratio. An increase in temperature accelerated development but resulted in reduced pupal mass. Thermal optimum for pupal mass (19.3 °C) was thus much lower than that for development time (28.1 °C). Growth rate peaked at 27.9 °C and P:C 1.65:1 and decreased as both temperature and P:C ratio deviated from their optimum. All four key performance traits (survivorship, development time, pupal mass, and growth rate) were expressed at high levels and, hence, optimized at temperatures between 25.7 °C and 27.4 °C and P:C ratios between 1.17:1 and 2.94:1. Food consumption rate was the highest at a high P:C ratio of 8.4:1, whereas post-ingestive food utilization was the most efficient at a moderate P:C ratio of 1:1.27. The rate and efficiency of food processing were optimized at temperatures between 24.5 °C and 29.9 °C and P:C ratios between 1:1.01 and 3.98:1. Our results have implications for improving the production and nutritional value of T. molitor larvae.