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Simple Summary: The alfalfa weevil, Hypera postica, is a destructive pest around the world and has become, in recent years, the most important pest of alfalfa in Spain. The damage from these weevils reduces the alfalfa forage yield and quality. The factors that affect the population development are poorly known in Europe. This study integrates laboratory experiments and field sampling to address population growth at different temperature and humidity conditions. In the laboratory experiments, we evaluated the combined effect of eight temperatures (8, 12, 16, 20, 24, 28, 32, and 36 °C) and three relative humidity regimes (low, medium, and high) on the survival and development time of eggs of Spanish pest populations. The lowest egg survival occurred at high temperatures and low relative humidity. The egg developmental time decreased from 8 to 32 °C, and, at low relative humidity (RH), this time was longer. The minimum threshold and thermal requirements for egg development were determined. In the field sampling, we obtained information regarding the occurrence of eggs and larvae (in winter) and the adult reproductive status (in autumn). Combining the laboratory and field results, the oviposition period was determined. The results of the study contribute to a better understanding of the annual cycle of H. postica in Spain and Europe and are useful for developing sustainable pest management strategies. The combined effect of the temperature and relative humidity on the survival and development time of the eggs of Spanish populations of the weevil Hypera postica, a key pest of alfalfa around the world, was evaluated under laboratory conditions. The experimental temperatures ranged from 8 to 36 °C, in 4 °C increments. Three relative humidity ranges were defined: high, medium, and low. Eggs of the alfalfa weevil successfully developed until larval emergence at all of the 24 conditions tested. However, the temperature and relative humidity affected the survival of the eggs. The egg developmental time decreased as the temperature increased from 8 to 32 °C, and the longest time was recorded at a low relative humidity (RH). The relationship between the development rate and temperature fit well to the lineal model for relative humidity. The minimum development threshold (T0) and the thermal requirement for egg development (K) ranged between 3 and 4 °C and 209 and 246 degree-days, respectively. According to these values and the occurrence of eggs and larvae (in winter) and adults (in autumn) in field samplings, the period of oviposition was determined. The results of the study contribute to better understanding the annual cycle and phenology of H. postica in the Iberian Peninsula and southern Europe. [ABSTRACT FROM AUTHOR]