Abstract <p>One of the most prominent features of climate change, rising temperature, disrupts the biological traits and immune systems of pest insects, threatening population stability, agricultural productivity, and the health of stored honeycombs. This study investigates the effects of constant and variable heat stress on mortality, life-history traits, and cellular immunity in the lesser wax moth <i>Achroia grisella</i> (Lepidoptera: Pyralidae). Under constant heat shock, adult emergence reached 100% at 30–32&#xa0;°C, but dropped to 36.67% at 34&#xa0;°C. The estimated LTemp₅₀ and LTemp₉₉ were 36.615&#xa0;°C and 43.016&#xa0;°C, respectively. Constant heat stress prolonged larval spinning and pupal times, shortened adult longevity, and had no significant effect on the number of eggs produced. Variable heat stress altered all life-history traits, delaying emergence, shortening longevity, and causing fluctuations in egg production, with the highest number at 34&#xa0;°C and the lowest at 40&#xa0;°C. In time-dependent immunity assays, compared to 30&#xa0;°C, heat stress at 34&#xa0;°C increased total hemocyte counts and strong encapsulation responses while suppressing weak encapsulation and mitotic activity. At 38&#xa0;°C, the combination of thermal stress and injection-induced injury proved lethal after 24&#xa0;h, highlighting the limits of immune tolerance in <i>A. grisella</i> larvae. These findings highlight the vulnerability of <i>A. grisella</i> larvae to rising and fluctuating temperatures, which significantly affect their developmental timing, reproductive capacity, and immune competence in a dose- and time-dependent manner.</p> Graphical Abstract <p></p>