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This great success led to further introductions of beneficial insects into the USA.
In 1905 the USDA initiated its first large-scale biological control program, sending entomologists to Europe and Japan to look for natural enemies of the gypsy moth, Lymantria dispar dispar, and brown-tail moth, Euproctis chrysorrhoea, invasive pests of trees and shrubs.
Control is greatest if the agent has temporal persistence, so that it can maintain its population even in the temporary absence of the target species, and if it is an opportunistic forager, enabling it to rapidly exploit a pest population.
Joseph Needham noted a Chinese text dating from 304 AD, Records of the Plants and Trees of the Southern Regions, by Hsi Han, which describes mandarin oranges protected by large reddish-yellow citrus ants which attack and kill insect pests of the orange trees.
To be most effective at controlling a pest, a biological control agent requires a colonizing ability which allows it to keep pace with changes to the habitat in space and time.Damage from Hypera postica, the alfalfa weevil, a serious introduced pest of forage, was substantially reduced by the introduction of natural enemies.20 years after their introduction the population of weevils in the alfalfa area treated for alfalfa weevil in the Northeastern United States remained 75 percent down.Two control agents were introduced to help control the spread of the plant, the cactus moth Cactoblastis cactorum, and the scale insect Dactylopius.Between 19, tens of millions of cactus moth eggs were distributed around Queensland with great success, and by 1932, most areas of prickly pear had been destroyed.