Heteronomous Hyperparasitoids Research Articles

Can heteronomous hyperparasitoids recognize host abundance and adjust offspring ratio?

Can heteronomous hyperparasitoids recognize host abundance and adjust offspring ratio?

Benefits of remating of a hyperparasitoid acting as a biocontrol agent

Encarsia sophia is the dominant parasitoid of invasive whitefly pest such as Bemisia tabaci. This heteronomous hyperparasitoid species lays fertilised diploid eggs in whitefly nymphs, which serve as primary hosts. Larvae develop as female progeny by consuming whitefly hosts, providing direct biocontrol benefits. However, male progeny originate from unfertilised eggs laid on secondary hosts (primary parasitoids within whitefly nymphs) and develop by consuming primary parasitoid larvae rather than whitefly hosts. Therefore, it is crucial for indoor rearing and field release of the hyperparasitoid to determine whether a female with single-mating experience can parasitise primary hosts and produce female progeny for a whole lifetime and, if not, whether it is remating events and prolong the period of female production, thereby increasing the amount of parasitism on whiteflies. Our fecundity experiments of females with single-mating experience, equally provided with primary and secondary hosts, showed that single mating was insufficient for E. sophia females to lay fertilised eggs in whitefly nymphs for their whole lifetime. In multiple mating experiments, 76.7 % of male adults completed their second mating 2 h after the first, whereas female adults accepted the second mating only if primary hosts were available thereafter. Remating was beneficial for female adults as it prolonged the ovipositional period of fertilised eggs (from 13.3 ± 0.6 days for single to 17.1 ± 0.7 days for twice mating) and increased parasitism on B. tabaci primary host (from 53.1 ± 3.8 for single to 76.4 ± 5.9 for twice mating respectively). The mating experience of males increased their copulation success, whereas that of females decreased it. In conclusion, recommendations are provided for large-scale indoor rearing and field release of E. sophia. Firstly, sufficient time for providing males can be shortened to within 2 h, and a single male can engage in 2–3 mating sessions. Following the 15-day peak oviposition period, the addition of newly emerged males is advised. Secondly, for field release of adult parasitoids, it is recommended to concentrate on releasing females after completing mating indoors. For the release of pupae cards, ensure a balanced ratio of males and females on each card to facilitate mating upon emergence. Additionally, release extra males and females 2 weeks after the initial batch of releases.

Host-feeding capacity and parasitism performance of heteronomous hyperparasitoid Encarsia lutea Masi (Hym.: Aphelinidae) on different host plant species

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is one of the most economically significant and widely distributed whitefly species. Encarsia lutea Masi (Hymenoptera: Aphelinidae) is a heteronomous hyperparasitoid and is reported to be one of the key factors in the management of B. tabaci populations in Mediterranean countries. This study aimed to evaluate the interactions among E. lutea, B. tabaci, and the host plant, obtaining comprehensive data to enhance its application in biological control programmes. Therefore, laboratory studies were carried out to determine the host-feeding capacity and parasitism of E. lutea at six B. tabaci nymph densities (10, 20, 40, 60, 80, and 100) on three host plant species: bean, cotton, and eggplant. The number of nymphs killed by the parasitoid through feeding and parasitism increased with increasing nymph densities for all host plants. The lowest (4.06) and the highest (27.5) mean numbers of parasitised nymphs were observed at host densities of 10 and 100 nymphs, respectively. The feeding behaviour of E. lutea was affected by the host plant, and the mean number of nymphs killed by feeding was found to be statistically significant and higher in cotton (6.7) than in bean (4.6) and eggplant (4.3), when pooled over all nymph densities. In conclusion, although the results showed that E. lutea can act as a density-dependent parasitoid, and its performance varies depending on the host plant, further studies on its hyperparasitism behaviour should be conducted before evaluating it as a potential candidate for the biological control agent of B. tabaci.

High nymphal host density and mortality negatively impact parasitoid complex during an insect herbivore outbreak.

Insect herbivore outbreaks frequently occur and this may be due to factors that restrict top-down control by parasitoids, for example, host-parasitoid asynchrony, hyperparasitization, resource limitation and climate. Few studies have examined host-parasitoid density relationships during an insect herbivore outbreak in a natural ecosystem with diverse parasitoids. We studied parasitization patterns of Cardiaspina psyllids during an outbreak in a Eucalyptus woodland. First, we established the trophic roles of the parasitoids through a species-specific multiplex PCR approach on mummies from which parasitoids emerged. Then, we assessed host-parasitoid density relationships across three spatial scales (leaf, tree and site) over one year. We detected four endoparasitoid species of the family Encyrtidae (Hymenoptera); two primary parasitoid and one heteronomous hyperparasitoid Psyllaephagus species (the latter with female development as a primary parasitoid and male development as a hyperparasitoid), and the hyperparasitoid Coccidoctonus psyllae. Parasitoid development was host-synchronized, although synchrony between sites appeared constrained during winter (due to temperature differences). Parasitization was predominantly driven by one primary parasitoid species and was mostly inversely host-density dependent across the spatial scales. Hyperparasitization by C. psyllae was psyllid-density dependent at the site scale, however, this only impacted the rarer primary parasitoid. High larval parasitoid mortality due to density-dependent nymphal psyllid mortality (a consequence of resource limitation) compounded by a summer heat wave was incorporated in the assessment and resulted in density independence of host-parasitoid relationships. As such, high larval parasitoid mortality during insect herbivore outbreaks may contribute to the absence of host density-dependent parasitization during outbreak events.

Unravelling mummies: cryptic diversity, host specificity, trophic and coevolutionary interactions in psyllid \u2013 parasitoid food webs

BackgroundParasitoids are hyperdiverse and can contain morphologically and functionally cryptic species, making them challenging to study. Parasitoid speciation can arise from specialisation on niches or diverging hosts. However, which process dominates is unclear because cospeciation across multiple parasitoid and host species has rarely been tested. Host specificity and trophic interactions of the parasitoids of psyllids (Hemiptera) remain mostly unknown, but these factors are fundamentally important for understanding of species diversity, and have important applied implications for biological control.ResultsWe sampled diverse parasitoid communities from eight Eucalyptus-feeding psyllid species in the genera Cardiaspina and Spondyliaspis, and characterised their phylogenetic and trophic relationships using a novel approach that forensically linked emerging parasitoids with the presence of their DNA in post-emergence insect mummies. We also tested whether parasitoids have cospeciated with their psyllid hosts. The parasitoid communities included three Psyllaephagus morphospecies (two primary and, unexpectedly, one heteronomous hyperparasitoid that uses different host species for male and female development), and the hyperparasitoid, Coccidoctonus psyllae. However, the number of genetically delimited Psyllaephagus species was three times higher than the number of recognisable morphospecies, while the hyperparasitoid formed a single generalist species. In spite of this, cophylogenetic analysis revealed unprecedented codivergence of this hyperparasitoid with its primary parasitoid host, suggesting that this single hyperparasitoid species is possibly diverging into host-specific species. Overall, parasitoid and hyperparasitoid diversification was characterised by functional conservation of morphospecies, high host specificity and some host switching between sympatric psyllid hosts.ConclusionsWe conclude that host specialisation, host codivergence and host switching are important factors driving the species diversity of endoparasitoid communities of specialist host herbivores. Specialisation in parasitoids can also result in heteronomous life histories that may be more common than appreciated. A host generalist strategy may be rare in endoparasitoids of specialist herbivores despite the high conservation of morphology and trophic roles, and endoparasitoid species richness is likely to be much higher than previously estimated. This also implies that the success of biological control requires detailed investigation to enable accurate identification of parasitoid-host interactions before candidate parasitoid species are selected as biological control agents for target pests.

Confirmation of the existence of alloparasitoids in nature – host relationships of an Australian Coccophagus species that parasitizes mealy bugs

AbstractHeteronomous hyperparasitoids are parasitic wasps with sex‐related host relationships that are unique to a group of genera in the chalcidoid family Aphelinidae. Females are primary parasitoids of various sedentary bugs (mainly, scale insects, mealy bugs, and whiteflies). Males, in contrast, are hyperparasitic, and they frequently develop at the expense of female conspecifics. Alloparasitoids constitute a special category of heteronomous hyperparasitoids, for their males never develop through female conspecifics. The existence of alloparasitic host relationships and the utility of the category ‘alloparasitoid’ have both been questioned. Here, we present results that confirm the existence of the alloparasitic way of life among heteronomous aphelinids. We investigated an undescribed species of Coccophagus (Hymenoptera: Aphelinidae), an Australian parasitoid that attacks the introduced lantana mealy bug, Phenacoccus parvus Morrison (Homoptera: Pseudococcidae), in Queensland. A year‐long field survey regularly returned large numbers of female Coccophagus spec. near gurneyi individuals from P. parvus (total n = 4212), but only few males (n = 11). Males emerged from samples only when the encyrtid parasitoid Anagyrus diversicornis (Howard) (Hymenoptera: Encyrtidae) was present in samples in relatively high numbers. Laboratory oviposition tests confirmed that A. diversicornis is a male host and showed that males do not develop at the expense of conspecific females. Other studies show that males are attracted in numbers to virgin females held in cages above mealy bug‐infested Lantana montevidensis (Spreng.) Briq. (Verbenaceae) in the field, demonstrating that they are common in the population as a whole. This confirms that the males need hosts other than conspecific females and that their usual hosts are present outside of the lantana/P. parvus system. The implications of these results for developing a realistic classification of heteronomous host relationships are discussed.

Host selection by the heteronomous hyperparasitoid Encarsia pergandiella: multiple‐choice tests using Bemisia argentifolii as primary host

AbstractThe ovipositional patterns of the heteronomous hyperparasitoid Encarsia pergandiella Howard (Hymenoptera: Aphelinidae) in the presence of its primary host Bemisia argentifolii Bellows & Perring (Hemiptera: Aleyrodidae), and in the presence or absence of conspecific and heterospecific secondary hosts (Encarsia formosa Gahan and Eretmocerus mundus Mercet; Hymenoptera: Aphelinidae) were examined to assess host species preferences. Host preferences by heteronomous hyperparasitoids may affect the relative abundance of co‐occurring parasitoid species and may influence host population suppression by the parasitoid community. Four combinations of hosts were tested: (1) B. argentifolii, E. mundus, and E. formosa, (2) B. argentifolii, E. formosa, and E. pergandiella, (3) B. argentifolii, E. mundus, and E. pergandiella, and, (4) B. argentifolii, E. mundus, E. formosa, and E. pergandiella. Arrays of hosts (24) were constructed in Petri dishes using leaf disks, each bearing one host. Thirty arrays of each host combination were exposed to single females for 6 h. All hosts were dissected to determine number of eggs per host. Encarsia pergandiella parasitized E. formosa hosts as frequently as E. mundus hosts. However, E. pergandiella parasitized either of these heterospecific hosts more frequently than conspecific hosts in treatments including two secondary host species. When a third parasitoid species was included in host arrays, E. pergandiella parasitized conspecific hosts as frequently as heterospecific hosts. Developmental stage of the hosts did not significantly influence host species selection by E. pergandiella. Our results indicate that host selection and oviposition by heteronomous hyperparasitoids like E. pergandiella, vary with the composition of hosts available for parasitization, and suggest a preference for heterospecific over conspecific secondary hosts.

Autoparasitism, Interference, and Parasitoid-Pest Population Dynamics

Autoparasitoids (“heteronomous hyperparasitoids”) are parasitoids that lay female eggs on homopteran hosts and male eggs on juvenile parasitoids of either the same species or another species. Males develop as hyperparasitoids and eventually kill the juvenile parasitoid. We present a series of stage-structured models that investigate the effects of autoparasitism on population dynamics. Autoparasitism causes density-dependent mortality on juvenile parasitoids and therefore has a stabilizing effect. This also leads to an increase in host population abundance. In most cases an autoparasitoid leads to higher host equilibrium densities than a comparable primary parasitoid (except when the primary parasitoid is arrhenotokous (sexual) and the autoparasitoid has a low preference for attacking parasitized hosts or can attack the parasitized host for only a small portion of its development). When male autoparasitoids are followed explicitly in the models, mate limitation reduces the stabilizing effect of autoparasitism and leads to a further increase in host abundance. Coexistence of an autoparasitoid with a nonprimary parasitoid or second autoparasitoid is possible when the level of conspecific autoparasitism is greater than the level of heterospecific autoparasitism. When an autoparasitoid coexists with a primary parasitoid, the resulting host density is always greater than that with only the primary parasitoid. Therefore, autoparasitoids have the potential to disrupt control achieved by primary parasitoids. When two autoparasitoids coexist, the resulting host density is always lower than that attained by either autoparasitoid alone. The effects of autoparasitism are compared with those of other forms of interference competition.

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.

Geographical distribution and species composition of parasitoids (Hymenoptera: Chalcidoidea) of Trialeurodes vaporariorum and Bemisia tabaci-complex (Homoptera: Aleyrodidae) in Japan.

To determine the geographical distribution and composition of parasitoid species from Trialeurodes vaporariorum and the Bemisia tabaci-complex (B. tabaci and B. argentifolii), parasitized whiteflies were collected at different sites in Japan and parasitoids were reared for adult emergence. Amitus sp., Encarsia citri, E. formosa, E. japonica, E. lutea, E. transvena, Encarsia sp., Eretmocerus sp. and Syrphophagus sp. were reared from Trialeurodes vaporariorum sampled at 125 sites from 1977 to 1998. Amitus sp., E. azimi, E. citri, E. formosa, E. japonica, E. lutea, E. transvena and Eretmocerus sp. were reared from a B. tabaci-complex sampled at 36 sites from 1990 to 1998. The indigenous species E. japonica and E. transvena are widely distributed in Japan and are predominant parasitoids of T. vaporariorum and the B. tabaci-complex. The introduced parasitoid E. formosa has recently been found at many sites near greenhouses in which it has been used commercially by seasonal inoculative release to control T. vaporariorum and B. argentifolii. This is because some E. formosa leave the greenhouses to parasitize whiteflies in the field. Single species composition per site was dominant for T. vaporariorum and not numerous for the B. tabaci-complex. At the sites containing two or three parasitoid species, E. formosa was attacked frequently by E. japonica and E. transvena which are facultative heteronomous hyperparasitoids. In addition, male Syrphophagus sp. was also a hyperparasitoid of E. formosa.

Invasion and Displacement of Experimental Populations of a Conventional Parasitoid by a Heteronomous Hyperparasitoid

Aphelinid parasitoids have an outstanding record of success in programmes of classical biocontrol against whiteflies and scale insects. Heteronomous hyperparasitoids are aphelinids in which the sexes develop on or in different hosts. The female always develops as a primary endoparasitoid of Homoptera. The male develops as a secondary parasitoid hyperparasitoid of his own or another species of homopteran endoparasitoid. Caged experiments were performed with the cabbage whitefly, Aleyrodes proletella, to examine the invasion of a population of a conventional parasitoid, Encarsia inaron both sexes primary endoparasitoids by a heteronomous hyperparasitoid, E. tricolor. In all cages the heteronomous hyperparasitoid successfully invaded an established population of the conventional parasitoid and the conventional species population declined to very low levels within 8 weeks of the introduction of the heteronomous hyperparasitoid. The patterns of invasion were different in each cage. In two cages, high levels of...

A test of kin recognition in a heteronomous hyperparasitoid

Introduction equilibrium sex ratio depended on the probability that males developed at the expense of their sisters. When this probability was high, fue sex ratio was strongly female biased, whereas if low, fue sex ratio moved in favour of males. However, altemative explanations for adaptive sex ratios in heteronomous hyperparasitoids have since been proposed (Godfray & Waage, 1990). The theory of Godfray & Waage, which focuses on fue behaviour of female parasitoids facing egg or time constraints on their reproduction, has received strong support from subsequent empirical s~dies (Hunter & Godfray, 1995). In this study I address fue question as to whether or not a female heteronomous hyperparasitoid shows discriminative behaviour in fue production of males when ofreced her own progeny or that of an unrelated

A re-examination of host relations in the Aphelinidae (Hymenoptera: Chalcidoidea)

Host relations among the Aphelinidae (Hymenoptera: Chalcidoidea) are highly intriguing from an evolutionary view. Females are usually primary endoparasitoids of whiteflies or scale insects, whereas the development of the male is different and has been used for classification. In heteronomous hyperparasitoids (adelphoparasitoids or autoparasitoids) the male develops as a hyperparasitoid of conspecific females or other endoparasitoid species. We review the consequences of this mode of development which is unique because decisions concerning host selection are inextricably linked with progeny sex ratio. Autoparasitoid field sex ratios can fluctuate dramatically concurrent with changes in the relative availability of male and female hosts. A recent adaptive explanation for these sex ratios involves understanding the reproductive constraints acting on heteronomous parasitoids. Host relations in these parasitoids can show a degree of plasticity. We argue that in many instances autoparasitism may be facultative in nature and should not be used for classification. Heterotrophic parasitism, wherein the male develops as a primary parasitoid of lepidopterous eggs, has been poorly understood in the past due to uncertainties in reports of the biology and taxonomy of heterotrophic parasitoids. The host relations of this group are clarified.

A re-examination of host relations in the Aphelinidae (Hymenoptera: Chalcidoidea)

Host relations among the Aphelinidae (Hymenoptera: Chalcidoidea) are highly intriguing from an evolutionary view. Females are usually primary endoparasitoids of whiteflies or scale insects, whereas the development of the male is different and has been used for classification. In heteronomous hyperparasitoids (adelphoparasitoids or autoparasitoids) the male develops as a hyperparasitoid of conspecific females or other endoparasitoid species. We review the consequences of this mode of development which is unique because decisions concerning host selection are inextricably linked with progeny sex ratio. Autoparasitoid field sex ratios can fluctuate dramatically concurrent with changes in the relative availability of male and female hosts. A recent adaptive explanation for these sex ratios involves understanding the reproductive constraints acting on heteronomous parasitoids. Host relations in these parasitoids can show a degree of plasticity. We argue that in many instances autoparasitism may be facultative in nature and should not be used for classification. Heterotrophic parasitism, wherein the male develops as a primary parasitoid of lepidopterous eggs, has been poorly understood in the past due to uncertainties in reports of the biology and taxonomy of heterotrophic parasitoids. The host relations of this group are clarified.

FACTORES CLAVE EN LA BIOLOGIA DE HIPERPARASITOIDES HETERONOMOS (HYM.: APHELINIDAE): AGENTES PARA EL CONTROL BIOLOGICO DE MOSQUITA BLANCA Y ESCAMAS

Las mosquitas blancas y las escamas son consideradas plagas de gran importancia económica en la actualidad. El uso de parasitoides afelínidos (Hym.; Aphelinidae) ha tenido un éxito sin igual en programas de control biológico clásico contra dichas plagas. Los afelínidos son un grupo muy interesante debido a que su sistema reproductivo es peculiar. Por ejemplo, en algunas especies, sus relaciones con el huésped varían de acuerdo al sexo. Las hembras de este grupo siempre se desarrollan como ondoparasitoides solitarios primarios de homópteros (e.g. ninfas de la mosquita blanca) mientras que el desarrollo del macho es diferente. Esta característica ha sido utilizada para su clasificación. Lo más intrigante de estos parasitoides son los "hiperparasitoides heterónomos", en cuales los machos se desarrollan como hiperparasitoides solitarios de hembras de su propia especie o especies afines. Modificaciones recientes han simplificado la clasificación de estos parasitoides y han llamado la atención al hecho de que el fenómeno de hiperparasitismo de conespeciticos u otras especies es probablamente más flexible de lo que se cree. En este artículo, se revisan los aspectos de la biología de los hiperparasitoides heterónomos con relevancia al control biológico, tal como la gama de huéspedes, desarrollo, longevidad del adulto, fecundidad, comportamiento oviposicional y preferencias en la explotación de huéspedes. En los hiperparasitoides heterónomos la selección del tipo de huésped y la relación de los sexos están entrelazados de modo inexplicable. Una hipótesis reciente ha considerado a los factores que limitan la reproducción del parasitoide para explicar las fluctuaciones en la relación de los sexos en el campo. La teoría predice que la relación de los sexos para hembras individuales se verá influenciada por la tasa de descubrimiento de huéspedes y la disponibilidad de huevecillos. Datos recientes apoyan tales predicciones. Otros factores que podrían influir en la relación de los sexos incluyen el apareamiento dirigido (no fortuito) e infección por microorganismos. Observaciones sobre la habilidad de hiperparasitoides heterónomos de descriminar entre huéspedes conespecíficos y no conespecíflcos para la producción de machos indican que ésta tiene consecuencias importantes para el uso de estos parasitoides en programas de control biológico. Los hiperparasitoides heterónomos parecen ser muy competitivos en sistemas con parasitoides convencionales. Se requiere de evaluaciones cuidadosas sobre el impacto probable de introducciones de hiperparasitoides heterónomos antes de su liberación en sistemas que ya contienen parasitoides convencionales. La notable biología de los hiperparasitoides heterónomos puede explicar éxitos pasados pero debe servir a su vez como un elemento que incite a la prudencia en el presente y el futuro.

Heteronomous hyperparasitoids, sex ratios and adaptations: a reply

Heteronomous hyperparasitoids, sex ratios and adaptations: a reply

Heteronomous hyperparasitoids, sex ratios and adaptations

Heteronomous hyperparasitoids, sex ratios and adaptations

Sex ratios of heteronomous hyperparasitoids: adaptive or non‐adaptive?

Sex ratios of heteronomous hyperparasitoids: adaptive or non‐adaptive?

Host selection and sex ratio in a heteronomous hyperparasitoid

Abstract. Encarsia tricolor Förster is a heteronomous hyperparasitoid: females develop as primary endoparasitoids of Homoptera, whereas males develop hyperparasitically in primary endoparasitoids, including conspecific females. When offered pupae of Encarsia inaron (Walker) or conspecific pupae, E.tricolor showed a distinct preference to exploit E.inaron for male production. Parasitoids were given the opportunity to gain different types of ovipositional experience on male and female hosts and were then offered patches containing mixtures of male and female hosts in various ratios. The mean sex ratio which they laid was not dependent on the relative availability of each type of host, whereas prior ovipositional experience had a significant effect. Adaptive reasons for the observed oviposition strategies are suggested.

Transition from Bisexual to Unisexual Cultures in Encarsia perniciosi (Hymenoptera: Aphelinidae): New Data and a Reinterpretation

Three forms of Encarsia perniciosi Tower have been recognized based on their mode of reproduction and host choice: (1) a thelytokous form parasitizing California red scale, Aonidiella aurantii (Maskell), (2) a thelytokous, and (3) an arrhenotokous form parasitizing San Jose scale, Quadraspidiotus perniciosus (Comstock). The arrhenotokous E. perniciosi is a heteronomous hyperparasitoid (males develop as secondary parasitoids, either on their own species, usually a female, or that of some other species). Several studies have suggested that the arrhenotokous form will become thelytokous in one generation when cultured under constant temperatures in the laboratory. We tested this possibility by starting four arrhenotokous E. perniciosi cultures from different geographic localities and exposing them to constant temperatures. All four cultures remained arrhenotokous after 2, 3, 11, and 19 generations, respectively, under constant temperatures. A diagnostic allozyme variant of the enzyme system, PGM, allowed us to assign our cultures (four arrhenotokous, four thelytokous on San Jose scale, and one thelytokous on California red scale) unambiguously to form. Crossing tests indicated that thelytokous females did not mate with arrhenotokous males. We concluded that it is unlikely that the thelytokous forms were recently derived from the arrhenotokous forms in our sample. Therefore, the reports in the literature about cultures of E. perniciosi changing their mode of reproduction are more likely explained by the competitive displacement of the arrhenotokous by the thelytokous form in mixed cultures under laboratory conditions, or, alternatively, by inadequate attention to the fact that females of the arrhenotokous form emerge from 6 to 13 d before the males.