The microclimate within western cotton (Gossypium spp.) seems favorable for the western tarnished plant bug (Lygus hesperus Knight) (Hemiptera: Miridae), provided adequate soil moisture is available. Diminishing water supplies and increasing costs in the West will likely change irrigation practices and induce at least periodically unfavorable conditions for L. hesperus. Knowledge of L. hesperus thermal ecology has been limited to constant temperatures for which relevance to variable temperature environments is unknown. Eggs of L. hesperus were reared under low (mean = 15°C), medium (mean = 22°C), or high (mean = 29°C) constant (±<0.5°C) or diurnally fluctuating (±8°C) temperatures. Developmental time and survival were similar between constant and variable regimes at the medium temperature. In contrast, variable low temperatures hastened egg development and increased survival, whereas variable high temperatures delayed development and reduced survival compared with constant regimes. Within the studied temperature range, the relationship between temperature and egg developmental rate was linear for constant temperatures, but a quadratic term was needed to describe this relationship under variable temperatures. Under medium variable temperatures, egg hatch was disproportionately high during the warmest period of the day (1300–1900 h) compared with the constant regime. Differences between regimes were less pronounced at high temperatures, except for the conspicuous absence of hatch between 1300 and 1900 h in the variable regime when temperatures were always >32°C. These results indicate the limited utility of constant temperature data for understanding L. hesperus thermal ecology and provide baseline information to better plan and interpret applied studies of L. hesperus thermal ecology.