Pomacea canaliculate is a freshwater snail native to subtropical-temperate South America that has invaded several countries around the world. Temperature is probably one of the main limitations to the expansion of this and other apple snails to higher latitudes in invaded regions. Egg masses are aerial, and the duration of embryonic development varies greatly with air temperature. We compared different methods for determining the lower temperature thresholds and the cumulative degree-days (DD) required for the completion of the embryonic development of P. canaliculate. The lower temperature threshold was estimated with four methods: the least standard deviation from the mean of degree-days, the least standard deviation from the mean of days, the coefficient of variation in days and the linear regression coefficient method. The cumulative degree-days were estimated using hourly records and daily averages (calculated according to the single triangle and the single sine methods) of air temperature. The lower temperature thresholds ranged between 15.8°C and 18.3°C and the cumulative DD between 88.8°C.d and 133.8°C.d. The estimations obtained with the single triangle and the single sine methods were exactly the same. The values obtained with the method of the least standard deviation in degree-days and the corresponding cumulative DD were the poorest estimations. The estimates obtained with daily mean temperatures were close to those obtained with hourly records, indicating that recording only maximum and minimum temperatures should be sufficient. The use of degree-day models for egg development in Pomacea will serve to increase the effectiveness and efficacy of control measures targeted to egg masses through a better timing in their application, especially in localities with highly variable temperatures.
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Vol. 55 • No. 2