Citricola Scale Research Articles

A Taxonomic Study of Some Species of Hyperparasitoids Attacking the Primary Parasitoid Microterys nietneri of the Citricola Scale Insect, Coccus pseudomagnoliarum in Syria

Saleh, A.T. and A.M. Basheer. 2023. A Taxonomic Study of Some Species of Hyperparasitoids Attacking the Primary Parasitoid Microterys nietneri of the Citricola Scale Insect, Coccus pseudomagnoliarum in Syria. Arab Journal of Plant Protection, 41(3): 292-305. https://doi.org/10.22268/AJPP-41.3.292305 The citricola scale insect, Coccus pseudomagnoliarum, (Kuwana) (Hemiptera: Coccomorpha: Coccidae) is an economically important pest that occurs on citrus trees along the Syrian coast. The samples of host insects and infested plant parts were collected from April to June during 2016 and 2017 from two locations (Al-Sanobar, Dabba) in Lattakia Province and brought to the Biological Control Studies and Research Center (BCSRC), Faculty of Agriculture, Damascus University. Based on the information gathered, Microterys nietneri (Motschulsky) (Hymenoptera: Encyrtidae) may play an important role as a biological control agent of citricola scale insect, but high hyperparasitoids levels reduced M. nietneri efficiency as a citricola scale parasitoid. The objective of this study was to describe and identify naturally occurring hyperparasitoids. The following parasitoids were identified and reported for the first time in Syria: Cheiloneurus claviger (Thomson), Pachyneuron muscarum (Linnaeus), and Marietta leopardine (Motschulsky). Microscopic images were provided to illustrate the morphological and morphometric characteristics of these species. Keywords: Hyperparasitoids, Citricola scale, Microterys, Syria

Bionomy of Coccus pseudomagnoliarum (Kuwana) (Hemiptera: Coccidae), a new species in the fauna of Serbia

Citricola scale, Coccus pseudomagnoliarum (Kuwana) (Hemiptera: Coccidae) was for the first time registered in Serbia in 2015 in the area of Belgrade on Celtis occidentalis L. C. pseudomagnoliarum develops one generation annually and overwinters as the second-instar nymph on host twigs. It forms numerous colonies on infested plants, and symptoms of its feeding appear in the form of drying leaves and twigs. In addition, large amounts of honeydew that this scale secretes reduce photosynthesis and transpiration in plants, which accelerates their decay. Citricola scale attracts many entomophagous insects which are able to reduce pest population. The parasitoid wasps Coccophagus lycimnia (Walker), Coccophagus piceae Erdos, Coccophagus scutellaris (Dalman), Coccophagus shillongensis (Hayat and Singh) (Aphelinidae), Cheiloneurus claviger Thomson and Metaphycus stanleyi Compere (Encyrtidae) were reared. C. piceae and M. stanleyi are new species in the fauna of Serbia. C. pseudomagnoliarum is a new host for the species M. stanleyi. The predators Coccinella septempunctata L., Exochomus quadripustulatus (L.) (Coccinellidae) and Chrysoperla carnea (Stephens) (Chrysopidae) were found in scale colonies. The most efficient natural enemy of C. pseudomagnoliarum nymphs was C. lycimnia, reducing scale populations by 11-26%.

VIRULENCE OF ENTOMOPATHOGENIC NEMATODES TO THREE SOFT SCALE INSECT SPECIES

Virulence of commercial strains of entomopathogenic nematode (EPN) Steinernema carpocapsae (Weiser), Steinernema feltiae Filipjev and Heterorhabditis bacteriophora Poinar (Rhabditida: Steinernematidae, Heterorhabditidae), were tested on young females of Pulvinaria hydrangeae Steinweden, Partenolecanium corni (Bouché) and Coccus pseudomagnoliarum (Kuwana) (Hemiptera, Coccidae) and a native strain of H. bacteriophora was tested on citricola scale, C. pseudomagnoliarum. The laboratory tests were carried out in small plastic Petri dishes with infective juveniles (IJ) applied on filter paper covering the bottom. Results showed that all EPN tested were virulent to all three soft scale insect species. Recorded mortality of insects at the highest concentration of approximately 50 IJ per cm2 exceeded 64% in all combinations of nematode and scale species tested. Within the same nematode concentration, scales’ mortality of 90% and above was recorded for P. corni with all nematode species, P. hydrangeae with S. carpocapsae (Nemastar) and C. pseudomagnoliarum with native H. bacteriophora strain. All scale species were good hosts for all nematode species. This is the first report of EPNs to parasitize soft scale insects and one of the few among Coccoidea. The level of parasitism recorded is promising for potential use in management of scale insects.

Arthropod Infestation Levels on Mandarins in California.

Integrated pest management (IPM) guidelines for horticulture are typically established from years of experimental research and experience for a crop species. Ecoinformatics methods can help to quickly adapt these guidelines following major changes in growing practices. Citrus production in California is facing several major challenges, one of which is a shift away from sweet oranges [Citrus sinensis (L.) Osbeck Sapindales: Rutaceae] toward mandarins (including mostly cultivars of C. reticulata Blanco and C. clementina hort. ex Tanaka). In the absence of IPM guidelines for mandarins, growers are relying on pest information developed from oranges. We mined a database of management records from commercial growers and consultants to determine densities for four arthropod pests: cottony cushion scale (Icerya purchasi Maskell Hemiptera: Monophlebidae), citricola scale (Coccus pseudomagnoliarum Kuwana Hemiptera: Coccidae), European earwig (Forficula auricularia Linnaeus Dermaptera: Forficulidae), citrus red mite (Panonychus citri McGregor Acari: Tetranychidae), and a natural enemy, predatory mites in the genus Euseius (Congdon Acarina: Phytoseiidae). Densities of cottony cushion scale were approximately 10-40 times higher in the two most commonly grown mandarin species than in sweet oranges, suggesting this pest is reaching outbreak levels more often on mandarins. Densities of the other pests and predatory mites did not differ significantly across citrus species. This is a first step toward establishing IPM guidelines for mandarins for these pests; more research is needed to determine how arthropod densities relate to crop performance in mandarins.

Citricola Scale Insecticide Trial, 2014: Table 1

Citricola scale causes loss of tree vigor, yield reduction and downgrading of fruit in the packing house due to sooty mold growth on honeydew deposited by the insect. A field trial was conducted in a 22-yr-old ‘Parent’ navel orange orchard at the Lindcove Research and Extension Center, Exeter, California to compare various timings and rates of insecticides for efficacy against citricola scale. Insecticides were applied diluted in 100 or 300 gpa of water to 8 trees per treatment using a …

Citricola Scale Insecticide Trial, 2013: Table 1

Citricola scale causes loss of tree vigor, yield reduction, and downgrading of fruit in the packing house due to sooty mold growth on honeydew deposited by the insect. A field trial was conducted in an 11-yr-old “Washington” navel orange orchard at the Lindcove Research and Extension Center, Exeter, CA, to compare various timings and rates of insecticides for efficacy against citricola scale. Insecticides were applied diluted in 300 or 500 gpa water to eight trees per treatment using a 100 gal high-pressure D30 diaphragm pump sprayer with mechanical agitation. Treatments were assigned to trees on 25 …

Classical Biological Control of Invasive Legacy Crop Pests: New Technologies Offer Opportunities to Revisit Old Pest Problems in Perennial Tree Crops.

Advances in scientific disciplines that support classical biological control have provided “new tools” that could have important applications for biocontrol programs for some long-established invasive arthropod pests. We suggest that these previously unavailable tools should be used in biological control programs targeting “legacy pests”, even if they have been targets of previously unsuccessful biocontrol projects. Examples of “new tools” include molecular analyses to verify species identities and likely geographic area of origin, climate matching and ecological niche modeling, preservation of natural enemy genetic diversity in quarantine, the use of theory from invasion biology to maximize establishment likelihoods for natural enemies, and improved understanding of the interactions between natural enemy and target pest microbiomes. This review suggests that opportunities exist for revisiting old pest problems and funding research programs using “new tools” for developing biological control programs for “legacy pests” could provide permanent suppression of some seemingly intractable pest problems. As a case study, we use citricola scale, Coccus pseudomagnoliarum, an invasive legacy pest of California citrus, to demonstrate the potential of new tools to support a new classical biological control program targeting this insect.

Co-clustering Spatial Data Using a Generalized Linear Mixed Model With Application to the Integrated Pest Management

Co-clustering has been broadly applied to many domains such as bioinformatics and text mining. However, model-based spatial co-clustering has not been studied. In this paper, we develop a co-clustering method using a generalized linear mixed model for spatial data. To avoid the high computational demands associated with global optimization, we propose a heuristic optimization algorithm to search for a near optimal co-clustering. For an application pertinent to Integrated Pest Management, we combine the spatial co-clustering technique with a statistical inference method to make assessment of pest densities more accurate. We demonstrate the utility and power of our proposed pest assessment procedure through simulation studies and apply the procedure to studies of the persea mite (Oligonychus perseae), a pest of avocado trees, and the citricola scale (Coccus pseudomagnoliarum), a pest of citrus trees.

Biology and behavior of Metaphycus angustifrons Compere (Hymenoptera: Encyrtidae), a newly discovered parasitoid of soft scale insects (Hemiptera: Coccidae) in California

Metaphycus angustifrons Compere has recently been found to be the most abundant parasitoid of brown soft scale, Coccus hesperidum L., in southern California. In laboratory experiments we examined several biological parameters of this species. M. angustifrons both oviposits and host feeds in brown soft scale and is a facultatively gregarious endoparasitoid of this soft scale insect. In contrast with other Metaphycus spp., M. angustifrons is a koinobiont parasitoid, allowing its host to grow up to 40% beyond its size at parasitism. Despite its high abundance on brown soft scale in the field, in the laboratory, high rates of parasitoid egg encapsulation are observed; about half of parasitized hosts failed to issue parasitoids. Furthermore, host scales that encapsulated parasitoids eggs showed significant reduction in development. Increased scale size at oviposition influences the size of emerging females but not the size of males. Female M. angustifrons are synovigenic. They emerge from their hosts without mature eggs and begin maturing eggs after they are provided a carbohydrate source. Carbohydrates prolong the life span of both female and male M. angustifrons. The size of female wasps influences egg load but not longevity. Finally, based on laboratory observations, M. angustifrons uses citricola scale almost exclusively for host feeding and not for oviposition. These results suggest that the role of this species in citricola scale’s decline in southern California in the 1950s–1960s was negligible.

Taxon-specific multiplex-PCR for quick, easy, and accurate identification of encyrtid and aphelinid parasitoid species attacking soft scale insects in California citrus groves

Citricola scale, Coccus pseudomagnoliarum Kuwana (Hemiptera: Coccidae), is a serious pest of citrus in California’s San Joaquin Valley, but not in southern California where a complex of Metaphycus spp. Mercet (Hymenoptera: Encyrtidae) suppress it. This has created interest in using these (and other Metaphycus) species for biological control in the San Joaquin Valley. A critical step in assessing an organism’s potential for biological control is the ability to accurately identify it. For Metaphycus spp., this currently requires slide mounted adult specimens and expert taxonomic knowledge. We present a simple, quick and accurate method to identify any life stage of the ten major parasitoids of soft scales in California citrus, based on amplification of ribosomal DNA, using the polymerase chain reaction (PCR). Three multiplex-PCR protocols amplify products of taxon-specific sizes, allowing direct diagnosis of taxa accommodated by the PCR, and reducing identification time to a fraction of that of existing methods.

Chlorpyrifos Bioassay and Resistance Monitoring of San Joaquin Valley California Citricola Scale Populations

The responses to chlorpyrifos of six populations of citricola scale, Coccus pseudomagnoliarum (Kuwana) (Hemiptera: Coccidae), were tested using a leaf dip bioassay, and two- to nine-fold resistances were found. LC50 responses of nymphs ranged from 7.5 to 68.9 ppm and LC90 responses ranged from 20 to 222 ppm chlorpyrifos. A population tested monthly during August-October showed up to 3.5-fold differences in LC50 responses but no differences in LC90 responses as scale size increased. A diagnostic concentration of 178 ppm chlorpyrifos was used to test 93 populations from throughout the San Joaquin Valley California during 2006-2009 by using a leaf dip bioassay. Of the populations tested, 41% showed > 20% survival after exposure to the diagnostic concentration of chlorpyrifos, indicating resistance problems. Research is needed to relate the level of survival of the scales in the bioassay to the field efficacy of the insecticide. Tulare County citrus growers applied a higher number of organophosphate and carbamate insecticides during the 15-yr period from 1994 to 2008, and these orchards showed a higher average scale survival of chlorpyrifos and a higher number of locations with resistant scale compared with the other San Joaquin Valley counties. Chlorpyrifos resistance is a significant issue for citricola scale management because biological control is ineffective in the San Joaquin Valley and the alternative neonicotinoid and insect growth regulator (IGR) insecticides require more frequent application.

Influence of Spray Equipment and Water Volume on Coverage of Citrus and Control of Citricola Scale, <I>Coccus pseudomagnoliarum</I> (Hemiptera: Coccidae) With Mineral Oil

A trial was conducted in a commercial Citrus sinensis L. variety 'Washington' navel orange orchard to compare the coverage and efficacy against citricola scale Coccus pseudomagnoliarum (Kuwana) (Hemiptera: Coccidae) of 45.5 liters/ha of an nC24 agricultural mineral oil treatment applied by two different methods: a conventional air blast sprayer and a rotary atomizer. Three water volumes (2,340, 4,680, and 7,020 liters/ha) were applied with the air blast sprayer to determine the optimal spray volume for that equipment. A single volume (2,340 liters/ha) was applied with the rotary atomizer to compare its effectiveness with that of the air blast sprayer at this same volume. Results demonstrated that all treatments reduced citricola scale densities. Moreover, all treatments conducted with the air blast sprayer provided significantly greater coverage and significantly reduced citricola scale densities compared with the treatment made with the rotary atomizer. Larger water volume applications with the air blast sprayer did not significantly reduce citricola scale densities, although significantly better coverage was attained in the interior of the tree when spraying with 4,680 and 7,020 liters/ha. As a consequence, this study demonstrated that the increased coverage obtained by applying higher water volume with the air blast sprayer was not required for an optimal treatment in August, when the citricola scale population consisted of nymphs inhabiting the outside leaves of the tree.

Suitability of citricola scale Coccus pseudomagnoliarum (Hemiptera: Coccidae) as host of Metaphycus helvolus (Hymenoptera: Encyrtidae): Influence of host size and encapsulation

Citricola scale Coccus pseudomagnoliarum (Kuwana) (Hemiptera: Coccidae), a widespread citrus pest in California and Europe, has been recently found in Spain. A preliminary survey suggests that Metaphycus helvolus (Compere) (Hymenoptera: Encyrtidae) is its main parasitoid. This parasitoid has been also documented in other citrus areas where its efficiency as biological control agent seems to be limited. Several hypotheses have been previously proposed to explain its lack of success. In this paper we study the relationship between citricola scale and M. helvolus in both field and laboratory to elucidate some of these hypotheses. According to our data, M. helvolus may play an important role as a biological agent when the scale length ranges from 1 to 2 mm. For those scale sizes, M. helvolus develops as a solitary parasitoid and its sex allocation pattern appears to be host size dependent. Once the scale length reaches 2 mm at the end of the spring, high encapsulation levels reduce M. helvolus efficiency as a citricola scale parasitoid. During that time, as well as in summer and fall, when citricola scale is too small to be parasitized, M. helvolus may survive parasitizing alternative soft scales present in Mediterranean groves.

Role of Imidacloprid in Integrated Pest Management of California Citrus

Portions of three commercial citrus orchards were treated for 1 yr with foliar imidacloprid or for 2 yr with a systemic formulation in a replicated plot design to determine the impact of this neonicotinoid on the San Joaquin Valley California citrus integrated pest management (IPM) program. Foliar-applied imidacloprid had little effect on California red scale, Aonidiella aurantii (Maskell); cottony cushion scale, Icerya purchasi Maskell; or citrus red mite, Panonychus citri (McGregor), populations. Short-term suppression of the parasitoids Aphytis melinus DeBach and Comperiella bifasciata Howard; vedalia, Rodolia cardinalis (Mulsant); and the predacious mite Euseius tularensis (Congdon) were observed. Suppression of natural enemies allowed scales and mites to maintain higher populations in the treated areas compared with the nontreated areas. Thus, foliar imidacloprid did not exhibit control of these citrus pest species, and it disrupted biological control. Systemically applied imidacloprid suppressed California red scale and citricola scale populations 2-3 mo after treatment. Suppression of parasitoids of the California red scale also was observed. Thus, treatments of systemic imidacloprid applied in areawide management programs for invasive pests would provide a benefit of California red scale and citricola scale suppression. However, this treatment provided only single-season control of citricola scale, it was somewhat disruptive of biological control, and it did not suppress densities of either scale as low as a treatment of the organophosphate chlorpyrifos for citricola scale or the insect growth regulator pyriproxyfen for California red scale. Insecticides with longer periods of efficacy and greater IPM compatibility than imidacloprid should be used for a sustainable IPM approach in California citrus.

Role of Imidacloprid in Integrated Pest Management of California Citrus

Portions of three commercial citrus orchards were treated for 1 yr with foliar imidacloprid or for 2 yr with a systemic formulation in a replicated plot design to determine the impact of this neonicotinoid on the San Joaquin Valley California citrus integrated pest management (IPM) program. Foliar-applied imidacloprid had little effect on California red scale, Aonidiella aurantii (Maskell); cottony cushion scale, Icerya purchasi Maskell; or citrus red mite, Panonychus citri (McGregor), populations. Short-term suppression of the parasitoids Aphytis melinus DeBach and Comperiella bifasciata Howard; vedalia, Rodolia cardinalis (Mulsant); and the predacious mite Euseius tularensis (Congdon) were observed. Suppression of natural enemies allowed scales and mites to maintain higher populations in the treated areas compared with the nontreated areas. Thus, foliar imidacloprid did not exhibit control of these citrus pest species, and it disrupted biological control. Systemically applied imidacloprid suppressed California red scale and citricola scale populations 2–3 mo after treatment. Suppression of parasitoids of the California red scale also was observed. Thus, treatments of systemic imidacloprid applied in areawide management programs for invasive pests would provide a benefit of California red scale and citricola scale suppression. However, this treatment provided only single-season control of citricola scale, it was somewhat disruptive of biological control, and it did not suppress densities of either scale as low as a treatment of the organophosphate chlorpyrifos for citricola scale or the insect growth regulator pyriproxyfen for California red scale. Insecticides with longer periods of efficacy and greater IPM compatibility than imidacloprid should be used for a sustainable IPM approach in California citrus.

Survey of brown soft scale Coccus hesperidum L. parasitoids in southern California citrus

The parasitoid complex of brown soft scale, Coccus hesperidum L., a multivoltine soft scale, was determined in southern California citrus over the period February 2004–March 2006. The survey was conducted by placing brown soft scale-infested yucca leaves in the canopy of citrus trees and subsequently rearing individually isolated parasitized scales in the laboratory. A total of 14 species parasitized brown soft scale in the field, the most abundant ones belonging to the genus Metaphycus Mercet (75%). The most abundant parasitoid species was Metaphycus angustifrons Compere (38% parasitism), and this is a new record of establishment for this species in California. Coccophagus species accounted for only 11% parasitism. There were important spatio-temporal differences across the parasitoid complex survey locations. We also found that the five most abundant encyrtid parasitoid species showed preferences for scales of different sizes. Our results have implications for biological control of citricola scale, Coccus pseudomagnoliarum (Kuwana), an important pest of citrus in the San Joaquin Valley of central California. Notably, this species is nearly absent in southern California. Brown soft scale is considered to be an alternate host for parasitoids of citricola scale, a univoltine soft scale, at times when the latter species is unavailable for parasitism.

Role of Two Insect Growth Regulators in Integrated Pest Management of Citrus Scales

Portions of two commercial citrus orchards were treated for two consecutive years with buprofezin or three consecutive years with pyriproxyfen in a replicated plot design to determine the long-term impact of these insect growth regulators (IGRs) on the San Joaquin Valley California integrated pest management program. Pyriproxyfen reduced the target pest, California red scale, Aonidiella aurantii Maskell, to nondetectable levels on leaf samples approximately 4 mo after treatment. Pyriproxyfen treatments reduced the California red scale parasitoid Aphytis melinus DeBach to a greater extent than the parasitoid Comperiella bifasciata Howard collected on sticky cards. Treatments of lemons Citrus limon (L.) Burm. f. infested with scale parasitized by A. melinus showed only 33% direct mortality of the parasitoid, suggesting the population reduction observed on sticky cards was due to low host density. Three years of pyriproxyfen treatments did not maintain citricola scale, Coccus pseudomagnoliarum (Kuwana), below the treatment threshold and cottony cushion scale, Icerya purchasi Maskell, was slowly but incompletely controlled. Buprofezin reduced California red scale to very low but detectable levels approximately 5 mo after treatment. Buprofezin treatments resulted in similar levels of reduction of the two parasitoids A. melinus and C. bifasciata collected on sticky cards. Treatments of lemons infested with scale parasitized by A. melinus showed only 7% mortality of the parasitoids, suggesting the population reduction observed on sticky cards was due to low host density. Citricola scale was not present in this orchard, and cottony cushion scale was slowly and incompletely controlled by buprofezin. These field plots demonstrated that IGRs can act as organophosphate insecticide replacements for California red scale control; however, their narrower spectrum of activity and disruption of coccinellid beetles can allow other scale species to attain primary pest status.

Augmentative releases of Metaphycus sp. nr flavus against citricola scale on oranges in the San Joaquin Valley of California: are early releases better than late ones?

We conducted an augmentative release trial with the encyrtid parasitoid Metaphycus sp. nr flavus (Howard) against citricola scale, Coccus pseudomagniolarum (Kuwana), in three citrus orchards in the San Joaquin Valley of California. Three parasitoid release timings (14 October-13 November, 4 January-5 February, and 26 February-16 March; hereinafter "early," "intermediate," and "late") were compared with a no-release check. Approximately 2,400 female parasitoids were released per central release tree. Late releases were associated with reduced scale survival on the release trees but no evidence of scale suppression was found on adjacent trees. Early and intermediate releases resulted in less pronounced scale suppression than late releases. Intermediate and late, but not early, releases were associated with an increase in the numbers of recovered female M. sp. nr flavus. Early releases, however, increased the numbers of recovered male Metaphycus spp. Thus, at the time of early releases, the scales may have been too small for the production of female parasitoids. A multiple regression model relating the degree of scale mortality in one orchard to the levels of recovered female Metaphycus helvolus (Compere), M. sp. nr flavus, or M. luteolus (Timberlake) (these latter two species are morphologically indistinguishable), and Coccophagus spp., suggested that the number of Coccophagus spp. females followed by M. helvolus females, were most strongly associated with scale mortality.

Augmentative releases of a parasitoid (Metaphycus sp. nr. flavus) against citricola scale (Coccus pseudomagnoliarum) on oranges in the San Joaquin Valley of California

Abstract Augmentative release trials with Metaphycus sp. nr. flavus (Howard), an encyrtid parasitoid of citricola scale, Coccus pseudomagnoliarum (Kuwana), were conducted in two orange groves in the southern San Joaquin Valley of California. Between mid-February and early May 1998, over 4000 female parasitoids were released per experimental tree. Control trees, where no parasitoids were released, were at least eight trees (45 m) distant from release trees. Samples taken between 18 April and 20 May indicated that levels of live citricola scales on release trees were significantly reduced on twigs, greater numbers of female M . sp. nr. flavus were recovered from scales, and the fraction of scales parasitized by Metaphycus spp. was significantly higher. These data support the hypothesis that parasitism by the released wasps suppressed the scale population. By mid-May, more M. helvolus (Compere) were recovered from control than from release trees, suggesting that released M . sp. nr. flavus had depleted the scales available to M. helvolus . Additionally, in May, more male Coccophagus lycimnia (Walker) were recovered from release trees. C. lycimnia produces male offspring by heterospecifically or conspecifically hyperparasitizing parasitoids of citricola scale. The implications of these results for developing augmentative parasitoid release strategies against citricola scale in California citrus are discussed.

Seasonal and Scale Size Relationships between Citricola Scale (Homoptera: Coccidae) and Its Parasitoid Complex (Hymenoptera: Chalcidoidea) on San Joaquin Valley Citrus

Abstract The phenology of citricola scale, Coccus pseudomagnoliarum (Kuwana), and its associated parasitoid complex were studied on citrus in the San Joaquin Valley of central California over the period April 1995–March 1997. A total of 10,237 parasitoid specimens of 10 species were collected. Two of these species, Marietta mexicana (Howard) and Encyrtus lecaniorum (Mayr), each recovered from individually isolated scales, represent new parasitoid records for citricola scale. A third species, Encarsia citrinus citrinus (Craw), may represent a new parasitoid record, but this requires further confirmation because a single (male) specimen was recovered from individually isolated scales. The three most dominant parasitoid species, Coccophagus lycimnia (Walker), Metaphycus helvolus (Compere), and Metaphycus luteolus (Timberlake), accounted for the majority (>97%) of the specimens recovered. In contrast to the situation on citrus in southern California, where citricola scale is under effective biological control and is very rarely seen, citricola scale on citrus in the San Joaquin Valley is reemerging as a major pest, especially in groves employing integrated pest management with minimal use of broad-spectrum insecticides. Possible reasons uncovered in this study for the lack of effective biological control of citricola scale in the San Joaquin Valley include: (i) reduced presence of Metaphycus spp. because of hyperparasitism by the heteronomous hyperparasitoid C. lycimnia; (ii) absence of alternate hosts for those species of Metaphycus present; and (iii) absence of hosts of suitable size for Metaphycus at critical times of the year. Recommendations for improving the level of biological control in the San Joaquin Valley are discussed.