In the middle ages, there was a rule of thumb that indicated that for every three grains harvested, one was for eating, the second was for sowing and the third was for pests. As the yield today is bigger than it was then, seeds for sowing only takes up a tiny share of the harvest. In the meanwhile, pests still take up much of the harvest. However, the greatest loss happens on the fields. On a world basis, 35 % of all crops are destroyed by pests before harvest. There is also a sizeable loss in stored goods today. The loss is due to pests, fungus and respiration which are difficult to separate as one often follow the other. In many development countries storage loss can exceed 50 %. In the USA the storage loss is 9 %. In Denmark the granary weevil causes a loss of stored grain at about 2 %, but usually this loss is noted as regular stock loss. Among other quantitatively important storage pests are rats, mice and birds.
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