Spreading of emergence over several years due to prolonged diapause in some larvae was shown in the chestnut weevil. Depending on the year the larvae buried themselves in the ground, 32-56% of live adults emerged after 2 or 3 years of underground life. Variability in the duration of diapause was assumed to correspond to tactics of adaptative "coin-flipping" plasticity. This plasticity must allow the chestnut weevil to respond to the unpredictability of its habitat as measured by the irregularity of chestnut production and summer drought. Indeed, fecundity and adult longevity did not lessen after 2 years of underground life. No drastic decrease in the population size of weevils occurs after bad years; for instance when the number of chestnuts on the study tree is less than 10 000, passers-by can collect all the fruit and about 95% of larvae developing in chestnuts are destroyed. Diapause nature (simple or prolonged) may be related to moisture and gas rates in the ground from October to December. These factors acting in autumn are not known to be involved in the physiological mechanisms that control the production of chestnuts.