Inversely density-dependent egg parasitism in patchy distributions of the citrus pest Cryptophlebia leucotreta (Lepidoptera: Tortricidae) and its agricultural efficiency.
Field surveys of egg populations of Cryptophlebia leucotreta and the egg parasitoid Trichogrammatoidea cryptophlebiae native to southern Africa, were made on citrus fruit (navel sweet oranges) in 3 areas in 1983-85 and on guava in one area in 1985. As host-egg population size increased there was a tendency for more eggs to be laid on individual fruits and for more fruits to be infested. There was little evidence that total percentage parasitism on each sampling date was dependent on host population density, suggesting that egg parasitism does not play a regulatory role at the orchard level. On individual fruits, parasitism was observed in large patches of eggs more often than in small ones, but percentage parasitism within a patch was inversely related to its size. Consequently, the overall risk of parasitism was almost constant in different patch-size groups. Effective parasitism, as a measure of the proportion of infested fruit on which all eggs were parasitized, was generally lower than total percentage parasitism on each occasion. As pest population size increased, and more large patches occurred, the difference between these 2 measures became greater and the degree of fruit protection, by local extinction of the pest, decreased. It is suggested that augmentative/inundative releases of egg parasitoids, which have hitherto failed, should be aimed at local extinction of the pest at the spatial level of the individual fruit. The likelihood of this occurring could be increased by achieving very high levels of overall parasitism. Otherwise the strategy should first be directed at lowering host egg density by other means so that patches remain small; under these conditions native parasitoid populations may have greater commercial impact and may not need augmenting.