The deaths of millions of monarch butterflies this month in Mexico may have been caused by a winter freeze, not pesticide-spraying by loggers, as one environmental group alleged last week. Mexico’s environmental agency said on Monday that it had tested 300 butterfly corpses and found no traces of toxic substances from pesticides, leading it to conclude that cold temperatures caused the die-off. The World Wildlife Fund, which has been helping preserve the monarchs’ winter habitat in Mexico, also said the freeze was the more likely cause of the deaths.