Volume41,Nt~mber 20 • April, 1973 Bioecological and Population Studies of the Cottony-cushion Scale, Icerya purchasi Mask., and Its Natural Enemies, Rodolia cardinalis Mul. and Cryptochaetum iceryae Will., in Southern California Jose R. Quezada and Paul DeBach HILGA" "1(20) 6n~88 (1973) Competitive interactions between the two natural enemies of leery" purehllSi-Rodolia eardinalis and Cryptoehaetum ieeryae­ were studied in detail in order to throw more light on the question of the introduction of multiple natural enemies as a biological control policy. The host, a major citrus pest accidentally intro­ duced in 1868 from Australia, has been under control since 1888­ 1889, when its highly specific natural enemies (also from Aus­ tralia) were imported for that purpose. Three different climatic regions of southern California were the study sites: the desert, the coasr, and the interior. Life table studies of the scale indicated that it is maintained under control at very low population levels in the desert by vedalia and on the coast by Cryptoehaetum, with competition causing displacement in both areas. In the interior, the two enemies seasonally share their prey in different propor­ tions and are fairly even in their competitive abilities-vedalia usually taking more prey during the summer and fall, the By taking more during the winter and early spring. In the interior, three generations of the host occurred in a year, while those of vedalia and Cryptoehaetum averaged 12 and 8, respectively. Some host plants restricted vedalia development, and certain species of ants interfered with Cryptoehaetum. Competition results in micro­ habitats also varied, depending on prevailing environmental con­ ditions and the time at which an enemy finds a scale colony. The "coe istence" of vedalia and Cryptochaetum in the interior is thought to be only apparent and maintained by the dispersal of Cryptoehaetum from their coastal area of dominance. Coexistence experiments in the interior showed displacement of the By by vedalia to occur after nine generations. Competition between the two natural enemies did not increase host survival. These extensive studies provided strong support for the policy of importation of multiple natural enemies. THE AUTI-IORS: Jose R. Quezada, formerly at Departamento de Biologia, Uoiversi­ dad de EI Salvador, is Associate Research Entomologist, Division of Biological Control, Riverside. Paul DeBach is Professor of Biological Control and Entomologist, Division of Biological Control, Riverside. Jose R. Quezada andPaul DeBach Bioecological and Population Studies of the Cottony-cushion Scale, Icerya purchasi Mask., and Its Natural Enemies, Rodolia cardinalis Mul. and Cryptochaetum iceryae Will., in Volume41,Nt~mber Southern California':20 • April, 19732 INTRODUCTION CONTROL OF THE COTTONY-CUSHION (1889) emphasized the potential of BioecologicalSCALE, Icerya purchasiandMaskell,Populationby its CryptochaetumStudiesandofexpressedthe his hope natural enemy, the vedalia beetle, Ro­ for the success of the parasite. A few Cottony-cushiondolia cardinalis (Mulsant),Scale,in Califor­Iceryaother purchasiworkers haveMask.,suggested that the nia was the first outstanding and now fly may be equally effective, or even andclassicalItsexampleNaturalof biologicalEnemies,control Rodoliasuperior, to cardinalisthe vedalia beetle in the (fig. 1). Icerya purchasi, accidentally control of the cottony-cushion scale in introducedMul. andin 1868Cryptochaetumfrom Australia certainiceryaeareasWill.,of the scale's in distribution (Doutt, 1964), became so abundant in (Smith and Compere, 1916; Essig, 1958; the absence of its Southernnatural checks Californiathat Thorpe, 1930; Clausen, 1956; Bartlett in a few years it threatened the whole and Lagace, 1960). citrus industry of California. The in­ Icerya purchasi has a wide range of troduction of the predatory vedalia distribution throughout the state and beetle from Australia in 1888-1889 can be found in the mild coastal areas, brought aboutJosethe rapidR. Quezadacontrol ofandthe Paulthe interiorDeBachregions, and in the hot, dry, pest in southern California. At about desert areas. It is found not only on cit­ the same time, or perhaps even a little rus but on a large number of alternate earlier (Smith and Compere, 1916), a host plants as well, some of which pro­ parasitic fly, Cryptockaetum iceryae vide good protection to and are impor­ (Williston), was also introduced and tant in the survival of the pest. colonized in San Mateo and Los Angeles The cottony-cushion scale is usually counties. The value of the fly, however, maintained at low population levels by was over-shadowed by the outstanding either or both of its natural enemies on success of vedalia on commercial citrus all of the host plants and in all places in the south. Thus, nearly all the credit where it occurs in California. Climate, for the control of the scale has been however, directly or indirectly influ­ given to the beetle, although Riley ences the distribution and abundance 1 Submitted for publication June 6, 1972. I The senior author's studies were jointly supported by Universidad de EI Salvador and the Organization of American States (OAS), with partial support by NSF Grants GB-7444, GB­ 17829, GB-6776, GB-14489, and GB-30843X. [ 631 ] 632 Q1.lczada and DrBach : Bioecoloqicai and Population Studies: Cottonu-cushion. Scale Fig. 1. Adult and larvae of vedalia attacking Icerya purchasi. of both enemies of the scale. Crupio­ feed or reproduce on any other host in chaetum is dominant in coastal regions, California. From this point of view, the while vedalia is dominant in the desert two species can be said to be ecological areas. In the interior, both species over­ homologues as defined by DeBach and lap and share their prey in different Sundby (1963). The ecological niches proportions throughout the year. Com­ of both natural enemies are not identi­ petitive advantage between the two is cal in the strictest sense. Vedalia preys rather evenly balanced in this region, on all stages of the scale, while Crypto­ and in spite of a variable seasonal dom­ chaetum parasitizes the host from the inance of either species over the other, second stage onward. However, as these both are common throughout the year. authors suggest: "If one essential com­ Occasional upsets of the scale's popula­ ponent of the niches of two species is tion balance may occur in all districts identical (such as identical food), then on individual plants or in small areas. the two species are ecological homo­ These can be ascribed to the adverse logues to this extent." They are con­ effect of various factors on natural ene­ sidered true ecological homologues for mies, such as ants and insecticides. all practical purposes in this study. Both vedalia and Oryptochaetum Our examination of the competitive have been found to be highly specific in interactions between Rodolia cardinalis relation to the host scale and will not and Oryptochaetum has thrown some HILGARDIA • Vol. 41, No. 20 • April, 1973 633 light on a controversial basic ecological vedalia beetle and the parasitic fly, as question. Turnbull and Chant (1961) well as their relative and absolute effi­ have questioned the value of multiple ciency in population regulation of the introductions of natural enemies and scale. suggested that two or more biotic agents Three different climatic regions of attacking a single host species may in­ southern California were selected for terfere with each other to the advantage the studies: the desert (Palm Springs), of host survival. According to this hy­ the interior (Riverside), and the coast pothesis' Icerya purchasi populations in (Rancho Santa Fe). It was necessary to California, for example, may be en­ find out if the general average densities hanced rather than suppressed as a re­ attained by the scale in the three differ­ sult of competition between its two ent regions were different, and if the natural enemies. control exerted by its natural enemies The studies reported here were under­ was hindered in any way as a result of taken to explore this hypothesis and the their competition, in order to evaluate mechanisms of competition between the the assumptions of Turnbull and Chant. MATERIALS AND METHODS All laboratory tests were conducted temperature of 80 ± 2° and 65 ± 5 per under insectary conditions where the cent relative humidity. By allowing the temperature was maintained at 80 ± 2° crawlers to settle at different times, dif­ F, and the relative humidity at 55 ± 5 ferent scale stage.s were available as per cent, unless otherwise stated. Un­ needed. Plants infested were princi­ disturbed citrus orchards-that is, or­ pally Citrus spp., Pittosporum tobira, chards that had not been sprayed for Cocculus laurifolius, Acer oblongum, several years-were secured in each of Spartium junceum, Cassia artemisoides, the three regions of southern California and Nandina domestica. Of all these to make field observations under natural hosts, P. tobira proved to be the ideal conditions. For the interior, the Bio­ one for rearing purposes, supporting logical Control Grove at the University colonies of scales for longer times than of California, Riverside, was used. An other plants without deterioration. undisturbed Valencia orange orchard Rodolia cardinalis. The vedalia beetle was secured in Rancho Santa Fe near was reared by placing one or two pairs the coast. In the desert (Palm Springs) , in a 2 x 2 x 2 ft sleeve cage whose bot­ only a small, untreated, backyard or­ tom fit a pot containing a citrus or Pit­ tosporum tobira plant infested with chard could be obtained. scales. The beetles were also reared, Rearing methods when closer observations were needed, in one-pint carton containers covered Icerya purchasi. In order to have a with a petri dish bottom. Scale-infested good supply of seale colonies available leaves from the same plants were reg­ for different aspects of the work, a rou­ ularly provided for the beetles and their tine rearing method was used. Repro­ growing larvae. ducing scales were obtained in the field Cryptochaetum iceryae. The fly para­ and their crawlers allowed to settle on site, as pointed out by Thorpe (1930), different host plants as was convenient.