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 ℃) 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 ℃) 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 ℃) was thus much lower than that for development time (28.1 ℃). Growth rate peaked at 27.9 ℃ 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 ℃ and 27.4 ℃ 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 ℃ and 29.9 ℃ and P:C ratios between 0.99:1 and 3.98:1. Our results have implications for improving the production and nutritional value of T. molitor larvae.
