In 1959, Mao, realizing his mistake, replaced the sparrows as a target pest with beetles, but the damage had already been done. The almost total lack in China not only of sparrows (which had to be reintroduced from the USSR) but of practically all other birds led to an immeasurable increase in the insect population. The number of locusts began to increase exponentially, and immense swarms of insects making their way through the fields of China destroyed most of the crops. From 1959 to 1961, a series of ill-starred events partially related to natural disasters and partly caused by the mistaken reforms of the Great Leap Forward (the idea to exterminate the sparrows being one of the worst), led to three years of famine so harsh that it caused the deaths of an estimated 20 to 40 million people.