| Summary: | The combined effect of the temperature and relative humidity on the survival and development time of the eggs of Spanish populations of the weevil <i>Hypera postica</i>, 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 (T<sub>0</sub>) 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 <i>H. postica</i> in the Iberian Peninsula and southern Europe.
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