An updated classification of the order Hymenoptera is provided with the current numbers of genera and species described so far specified. The order is composed of 2 suborders, 27 superfamilies, 132 families, 8423 extant genera with an additional 685 extinct genera. Considered one of the most species-rich insects orders a total of 153088 extant species have been described, in addition to 2429 extinct species.

We measured survival, growth rate, and initial egg complement of California red scale, Aonidiella aurantii, when reared on foliage and bark of four citrus cultivars in order to detrmine the extent that variation among citrus cultivars may introduce variation in the number and quality of scale for utilization by the introduced parasitoid Aphytis melinus. Among bark substrates, lemon (Citrus limon) and grapefruit (Citrus paradisi) were most suitable for scale survival and growth. Bark of mandarin (Citrus unshiu) was nearly unsuitable for California red scale survival, and bark of sweet orange (Citrus sinensis) was intermediate in suitability. Foliage was a more uniformly suitable substrate for California red scale in that variation in survival and adult sizes was generally insignificant when scale were reared on different cultivars. Substantial differences in concentrations of free animo acids and soluble protein occurred in foliage and bark among citrus cultivars, but these differences were not correlated with differences in scale performance. The more uniform densities and sizes of scale on leaves across cultivars may have contributed to the development of A. melinus' propensity to search the leaves on the tree's exterior more frequently and regularly than the bark on the tree's interior for individual scales suitable for the reproduction of female progeny.

California red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984—1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were °100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density—dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluctuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and prediction rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size—dependent interactions between red scale and Aphytis.

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Abstract. 1. Laboratory experiments and field studies were conducted to explain the coexistence of an endoparasitoid, Encarsia perniciosi Tower, and an ectoparasitoid, Aphytis melinus DeBach, both of which were introduced into California to control the California red scale, Aonidiella aurantii (Mask.). 2. Encarsia parasitized all scale stages but it preferred first and second instar scales. This is in contrast to Aphytis melinus, in which previous studies have shown that it parasitizes second and third instar females and second instar males but prefers third instar female scales. Encarsia developed most rapidly when it parasitized an early second instar and slowest when it parasitized the mature female scale. However, on early second instar scales it was about 80% as fecund as a wasp that emerged from a mature female scale. 3. Second instar scales parasitized by Encarsia were accepted by Aphytis as readily as unparasitized scales. 4. Encarsia did not distinguish between unparasitized hosts and those previously parasitized by Aphytis. 5. Encarsia always outcompeted by Aphytis when both species parasitize the same host. 6. Encarsia prefers scale on stems whereas Aphytis prefers those on leaves and fruits. This, too, may be a result of interspecific competition with Aphytis. 7. The partitioning of the scale resource by the two species explains why they coexist in coastal southern California but it does not explain why Encarsia disappeared from citrus groves in the inland valleys coincident with the introduction of Aphytis melinus into southern California.