Experimental evaluation of the efficiency of Epidinocarsis lopezi, a parasitoid introduced into Africa against the cassava mealybug Phenacoccus manihoti

The capability of Epidinocarsis lopezi (De Santis) (Hymenoptera: Encyrtidae) to control the cassava mealybug (CM) Phenacoccus manihoti Mat.-Ferr. (Homoptera: Pseudococcidae) was investigated in Nigeria using physical and chemical exclusion experiments. In two sleeve cage experiments CM populations, about 2 months after artificial infestation, were 7.0 and 2.3 x lower on artificially infested cassava tips covered with open cages than on tips in closed cages which excluded most parasitoids. On similarly infested but uncovered tips, CM populations were 24.3 and 37.5 x lower, and parasitisation rates were higher. In an artificially infested field which was treated weekly with carbaryl, parasitisation rates were below 10% and CM populations exceeded 200 per tip. In the chemically untreated plot, parasitisation rates were up to 25% and CM densities were mostly below 10 per tip. This study demonstrates the efficiency of E. lopezi in controlling its host under the experimental conditions.

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Résumé La capacité d'E. lopezi de contrôler la cochenille farineuse du manioc a été évaluée au Nigéria en excluant le parasitoïde de son hôte par des moyens physiques et chimiques. Dans deux expériences utilisants des manchons les populations de la cochenille deux mois après l'infestation artificielle étaient 7.0 et 2.3 fois plus basses sur les branches couvertes d'un manchon ouvert que sur les branches couvertes par un manchon fermé, qui excluait la plupart des parasitoïdes. Sur les apex sans manchons, également infestés artificiellement, les populations de la cochenille étaient 24.3 et 37.5 fois plus bas tandis que le degré de parasitisme était plus élevé. Dans un champ infesté artificiellement et partiellement traité chaque semaine avec du carbaryl, le pourcentage de parasitisme restait au-dessous de 10%, et les populations de la cochenille dépassaient 200 par apex. Dans la partie non-traitée, le parasitisme atteignait 25% et la densité de la cochenille restait pour la plupart du temps au-dessous de 10 cochenilles par apex. Ces expériences démontrent la capacité d'E. lopezi de maintenir son hôte à un bas niveau dans des conditions expérimentales.

     

Interspecific interference between Apoanagyrus lopezi and A. diversicornis, parasitoids of the cassava mealybug Phenacoccus manihoti

The parasitoids Apoanagyrus lopezi De Santis and A. diversicornis (Howard) (Hymenoptera: Encyrtidae) have been introduced into Africa for the biological control of the cassava mealybug Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae). We have studied competition between these species to investigate if they can coexist. Here we report on the influence of the simultaneous presence of non-conspecific adult females on searching efficiency on patches. Wasps of either species foraged on discs of cassava leaf with mealybugs, while at the same time different numbers of non-conspecifics were also depleting the patch. Patch area per parasitoid and number of hosts available to each parasitoid were equal in all treatments.In both species, the presence of other foragers clearly affected several aspects of the parasitoids' behaviour. Patch residence time increased with the number of non-conspecifics in A. diversicornis. In both parasitoid species, the proportion of hosts left unparasitized after the patch visit decreased with increasing numbers of females on the patch. The proportions of super- and multiparasitism did not change with the number of females. Both species produced more offspring during a patch visit in the presence of more non-conspecifics. These behavioural changes did not, however, lead to a change in the offspring production rate on patches. A. diversicornis produced offspring at a rate three times that of A. lopezi when one A. lopezi and one A. diversicornis foraged simultaneously. This is the first report of an aspect of interspecific competition where A. diversicornis has an advantage over A. lopezi. Interference between adult females thus promotes coexistence of the two species on P. manihoti.     

Release and establishment in Nigeria of Epidinocarsis lopezi, a parasitoid of the cassava mealybug, Phenacoccus manihoti

The encyrtid wasp Epidinocarsis (= Apoanagyrus) lopezi (De Santis) was imported from Paraguay into Nigeria for the biological control of the cassava mealybug, Phenacoccus manihoti Matile-Ferrero. It was mass-reared and released at four localities in Nigeria. The parasitoid is now established and it is dispersing throughout cassava growing areas of Nigeria.

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Libération et installation au Nigéria d'Epidinocarsis lopezi, parasitoïde de la cochenille du manioc Phenococcus manihoti

Résumé Epidinocarsis lopezi (Apoanagyrus) lopezi a été introduit du Paraguay au Nigéria pour lutter contre la cochenille du manioc, Phenacoccus manihoti. Il a été lâché dans quatre champs de manioc pour étudier son acclimatation et son installation au Nigéria. Trois ans après les lâchers, les résultats ont permis de conclure que E. lopezi s'est établi avec succès et se disperse dans la plupart des zones de culture du manioc au Nigeria; il a aussi survécu à trois saisons pluvieuses pendant lesquelles les populations de P. manihoti ont été très faibles. Quatorze mois après les premiers lâchers, cet encyrtide a été obtenu à environ 150 km du lieu de libération.

     

The role of kairomones in prey finding by Diomus sp. and Exochomus sp., two coccinellid predators of the cassava mealybug, Phenacoccus manihoti

We studied searching behaviour of Diomus sp., a coccinellid predator introduced into Africa as a natural enemy of the cassava mealybug, when searching on cassava leaves, and compared its behaviour with the searching behaviour of Exochomus sp., an African predator of mealybugs (MB's). Female adults of Diomus and Exochomus spent more time searching on cassava leaves previously infested with cassava MB than on clean cassava leaves, in response to substances produced by MB's (wax and/or honeydew) still present on these leaves as kairomones after removal of the MB's. Both species were also arrested by wax and/or honeydew of the citrus mealybug, Planococcus citri. When offered a choice between kairomones of both MB species only experienced Diomus (reared on cassava MB) showed a clear preference for kairomone of cassava MB. Separate influences of wax and honeydew were tested. Wax from cassava MB was an arrestment stimulus for both coccinellid species. Honeydew produced by cassava MB arrested Exochomus and inexperienced Diomus.

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Zusammenfassung Das Suchverhalten auf Cassaveblätter des coccinelliden Raübers Diomus, eines aus Süd Amerika importierten natürlichen Feindes der Cassaveschmierlaus wurde studiert und verglichen mit dem Suchverhalten eines afrikanischen Räubers von Schmierläusen, Exochomus sp.. Weiblichen Adulten von Diomus un Exochomus verwendeten mehr Zeit auf Cassaveblätter wenn diese Blätter vorher mit Cassaveschmierläuse infiziert waren als auf uninfizierte Blätter, und gebrauchten Substanzen (Wachs und/oder Honigtau) die von Schmierläuse produziert wurden und nach der Entfernung der Schmierläuse auf die Blätter zugeblieben waren als Kairomonen.Beide Arten wurden auch von Wachs und/oder Honigtau der Citrusschmierlaus, Planococcus citri, auf den Blätter arretiert. Wenn die Tiere wahlen könnten zwischen Kairomone der beiden Schmierlausarten wurden die Kairomonen der Cassaveschmierlaus nur von erfahrenen Diomus-Individuen, die auf Cassaveschmierlaüse aufgezogen wurden, bevorzugt.Die Einflüsse von Wachs und Honigtau wurden separat geprüft. Wachs von Exuvien der Cassaveschmierlaus ist ein Arretierungsstimulus für beide Räuberarten. Von Cassaveschmierläusen produzierten Honigtau arretierte Exochomus-und unerfahrene Diomus-Individuen.

     

Biological control of the cassava mealybug Phenacoccus manihoti (Hom., Pseudococcidae) by Epidinocarsis lopezi (Hym., Encyrtidae) in Malawi

From 1985 to 1989 five large scale surveys were made to document the spread of the cassava mealybug (CM) Phenacoccus manihoti Matile‐Ferrero (Hom., Pseudococcidae) and the releases and successful establishment of its exotic parasitoid Epidinocarsis lopezi (De Santis) (Hym., Encyrtidae) through most cassava‐growing areas of Malawi. In a multiple regression analysis involving 29 meteorological, agronomic and plant variables from 476 fields, the duration of E. lopezi's presence was the major factor influencing CM population densities. In the first year the CM was recorded in a particular place, 25% of all tips had more than 100 CM. Wherever E. lopezi had been present for two years or more, CM populations were reduced on average seven times and tips infested with more than 100 CM became rare (1%). In parallel, damage on cassava tips stabilized at a low level. Similarly, along Lake Malawi, 84.9% of all tips had more than 10 CM in 1986; this value stabilized at 3.3%‐4.0% in 1988–1989. Indigenous coccinellids were often abundant in the first year of the CM infestation, but their populations collapsed later, while the frequency of fields with E. lopezi increased. In 1987, farmers abandoned 28.4% of all fields where E. lopezi was just introduced as compared with 2.7% where the parasitoid had been present for two years or more. Overall, satisfactory control was achieved by E. lopezi in all but a few fields concentrated on extremely poor soils characterized by sand dune vegetation. The socioeconomic implications of this ongoing and apparently successful biological control program are discussed.     

Temperature-dependent growth rates for the cassava mealybug, Phenacoccus herreni, and two of its encyrtid parasitoids, Epidinocarsis diversicornis and Acerophagus coccois in Colombia

Developmental rates for Phenacoccus herreni Cox & Williams were determined at 18°, 20°, 22°, 25°, 30° and 35°C for the egg, all juvenile male and all juvenile female stadia. Longevity was determined for adult females and adult males. Developmental rates for the P. herreni parasitoid Epidinocarsis diversicornis (Howard) were determined at 18°, 20°, 25° and 30°C for the oviposition-to-mummy-formation period and the mummy-formation-to-adult-eclosion period. Developmental rates were determined for the P. herreni parasitoid Acerophagus coccois Smith for the same two life stages at 20°, 25° and 30°C. Least-squares-derived polynomial equations or logistic equations were fitted to each data set (except for A. coccois) so that rates could be interpolated for temperatures between observed points for use in an analysis of the impact of these parasitoids on population dynamics of P. herreni. Results of this analysis are presented separately.

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Résumé Les vitesses de développement des oeufs et de tous les stades larvaires mâles et femelles de P. herreni Cox & Williams ont été déterminées à 18°, 20°, 22°, 25°, 30° et 35°C. La longévité des adultes mâles et femelles a été déterminée. Les vitesses de développement de la ponte à la formation du cocon et de celleci à l'émergence de E. diversicornis Howard, encyrtide parasite de P. herreni ont été déterminées à 18°, 20°, 25° et 30°C. Il en a été de même pour un second encyrtide parasite A. coccois Smith à 20°, 25° et 30°C. Les équations polynomiales des derniers carrés dérivés et les équations logistiques ont été ajustées pour chaque lot de données (à l'exception de A. coccois) de façon à ce qu'elles aient pu être interpolées pour analyser l'impact de ces parasites sur la dynamique de population de P. herreni. Les résultats de cette analyse sont présentés à part.

     

Comparative life table statistics of Apoanagyrus lopezi reared on the cassava mealybug Phenacoccus manihoti fed on four host plants

The demographic statistics of Apoanagyrus lopezi De Santis (Hymenoptera: Encyrtidae) were studied in the laboratory on its host, the cassava mealybug Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae), reared on four host plants characterized by different levels of antibiotic resistance to the mealybug: two cassava varieties, Incoza and Zanaga (Manihot esculenta Crantz: Euphorbiaceae), the faux-caoutchouc (hybrid of M. esculenta × M. glaziovii Muel. Arg.) and Talinum (Talinum triangulare Jack:Portulacaceae).Total and daily mean fecundities of female parasitoids were strongly influenced by the host plant but there was no link with antibiotic resistance. The mean duration of the oviposition period was also significantly modified by the host plant and was again not correlated to the level of antibiotic resistance. Net reproduction rates of female parasitoids were nearly 2 times higher on hosts fed on Zanaga and Talinum than on Incoza and Faux-caoutchouc, and were not related to the total fecundities. The generation time was significantly longer on Faux-caoutchouc than on the other three plants. The intrinsic rate of increase varied significantly with host plants and was higher on Talinum and Zanaga than on Incoza and Faux-caoutchouc. With the varieties of cassava, Incoza and Zanaga, it was observed that antibiosis had a significant negative effect on the survival of A. lopezi, which would influence the effectiveness of the parasitoid.     

Influence of secondary compounds in the phloem sap of cassava on expression of antibiosis towards the mealybugPhenacoccus manihoti

Identification and assay of cyanogenic and phenolic compounds in phloem sap of cassava (Manihot esculenta Crantz, Euphorbiaceae) and in honeydew of the cassava mealybugPhenacoccus manihoti Matt. Ferr. (Homoptera, Pseudococcidae) were realised. Cyanogenic glucosides and three flavonoid glycosides (rutin, kaempferol glycoside-1 and kaempferol glycoside-2) were found to be translocated in cassava phloem sap and consumed by the mealygug. Differences in profiles of secondary compounds of phloem sap and honeydew samples, characterised mainly by the appearance of free cyanide and of a free flavonoid, suggest the metabolic processing of at least some of the ingested compounds. The relationship between foliar concentrations of these different compounds and expression of the antibiotic resistance of cassava towardsP. manihoti was also investigated in 7 varieties of cassava and in the ‘faux-caoutchouc’ hybrid. Infestation by mealybug was followed by a clear increase in levels of both rutin and kaempferol glycoside-2, while no modification in cyanide contents was noted. The best rank correlation between antibiotic resistance (measured by the intrinsic rate of increase r _c ) and secondary compounds analyzed is observed with rutin contents of infested plants (ρ=−0.73; p=0.05). The possible implication of this compound in the biochemical mechanisms accompanying cassava defence reaction to mealybug attack is discussed.

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Résumé Ce travail présente l'identification et le dosage des substances secondaires (composés cyanés et phénoliques) présentes dans la sève phloémienne du manioc (Manihot esculenta Crantz, Euphabiaceae) et dans le miellat de la cochenille farineusePhenacoccus manihoti Matt. Ferr. (Homoptera, Pseudococcidae). Des glycosides cyanogéniques et trois flavono?des glycosylés majeurs (rutine, kaempferol glycoside-1 et kaempferol glycoside-2) sont transportés par la sève phloémienne du manioc et consommés par la cochenille. Le profil sensiblement différent des composés secondaires du meillat de la cochenille, en particulier l'apparition d'un flavonoide libre et de cyanure, suggère une modification de certains composés lors du transit intestinal. La relation entre les teneurs de ces différentes substances dans le liquide foliaire de 7 variétés de manioc et du ‘faux-caoutchouc’ (hybride deM. esculenta et deM. glaziovii) et l'expression de leur résistance antibiotique vis-à-vis deP. manihoti a également été étudiée. L'infestation par la cochenille se traduit par une importante augmentation des teneurs en rutine et en kaempferol glycoside-2, alors qu'aucune modification des teneurs en cyanure libre n'est enregistrée. La meilleure corrélation entre la résistance par antibiose, exprimée par la capacité intrinsèque d'accroissement r _c , et les teneurs en substances secondaires analysées est observée avec les teneurs en rutine des plants infestés (ρ=−0.73; p=0.05). L'association éventuelle de ce glycoside aux mécanismes biochimiques de défense du manioc à la cochenille farineuse est discutée.

     

Sustained biological control of the cassava mealybugPhenacoccus manihoti [Hom.: Pseudococcidae] byEpidinocarsis lopezi [Hym.: Encyrtidae] in Nigeria

Following the successful introduction ofEpidinocarsis lopezi (De Santis) for biological control of the cassava mealybug (CM)Phenacoccus manihoti Mat.-Ferr. in southwestern Nigeria in 1981 and 1982, 11 groups of cassava fields were sampled every 2 weeks up to 1988 for impact assessment. After 1984, CM populations remained mostly below 10 per tip despite the presence of native hyperparasitoids, demonstrating the long-term success of biological control byE. lopezi in the region. Indigenous polyphagous coccinellids were found only during peak host densities, whereas the specificE. lopezi was common throughout the year. During some periods, percentage parasitism indicated delayed density dependence. Since 89% of all sampled cassava tips had no CM at all and the parasitisme is very mobile, parasitization rates were also calculated for individual infested tips (N=4,878). Parasitism increased slightly with host density on tips having between 1 and 10 CM of the 3^rd and 4^th instars, indicating positive density dependence. Such tips comprised 64% of all infested tips. At higher host densities, parasitism rates fell rapidly. The results are discussed in view of different theories on population regulation by biological control agents.      

Measuring host finding capacity and arrestment of natural enemies of the cassava mealybug,Phenacoccus manihoti, in the field

In unifested fields, 80 cassava tips were artificially infested with 0, 1, 2, 4, 8, 16, 32, or 64 third instars, and 20 or 100 eggs of cassava mealybug,Phenacoccus manihoti Matile-Ferrero (Hom., Pseudococcidae). Another 80 uninfested tips served as a control. Tips were arranged in a circle of 28 m diameter, in the centre of which the following exotic natural enemies ofP. manihoti were released:Apoanagyrus (Epidinocarsis) lopezi De Santis andA. diversicornis (Howard) (Hym., Encyrtidae),Hyperaspis notata (Mulsant) andDiomus hennesseyi Fürsch (Col., Coccinellidae), and others. This experiment was repeated six times. During the 4–14 days following release, all experimental tips were inspected at two-hour intervals during each day and the presence of exotic as well as indigenous natural enemies, likeExochomus troberti Mulsant (Col., Coccinellidae), ants and spiders was noted. The experiment was repeated six times measured the aggregative response by the natural enemies to different host densities, achieved through host attractance and arrestment. All exotic natural enemies, except the males ofApoanagyrus spp., were fast attracted to the host colonies. As compared to the control tips, they concentrated on the infested tips about 50-fold for the twoApoanagyrus spp. and 10 to 20-fold for the exotic coccinellids. By contrast, non-coevolved indigenous coccinellids, as well as generalist predators like ants and spiders were attracted to the infested tips only 2 to 5-fold.A. lopezi responded best to different host densities, followed byA. diversicornis and the coccinellids, followed by ants and spiders. None of the parasitoids or predators was particularly attracted to egg masses. These results correspond closely to the known efficiencies of these natural enemies,A. lopezi standing out among all candidates. The results of such aggregation studies are compared with those of life-table studies.     

不同光周期条件下棉花粉蚧的生长发育和种群增长能力

【目的】 研究阐明不同光周期条件下棉花粉蚧Phenacoccus solenopsis Tinsley实验种群的发育历期、繁殖和数量动态规律,为制定该虫防治对策、研究治理措施等提供科学依据。【方法】 以扶桑为寄主,在温度27±1℃、相对湿度70%±5%、光照强度4 000 lx及不同光周期（8L∶16D, 10L∶14D, 12L∶12D, 14L∶10D和16L∶8D）条件下,观察了棉花粉蚧实验种群的生长发育特性和繁殖能力,组建了实验种群生命表。【结果】 不同光周期处理下棉花粉蚧1龄若虫和3龄若虫的发育历期及雌成虫产仔量均发生了变化,但不同龄期若虫存活率之间差异不大。随着光照时间增加,棉花粉蚧1龄若虫、3龄若虫发育速率加快,8 h光照时发育历期分别为5.43 d和6.35 d,而16 h光照时发育历期分别缩短至4.16 d和4.75 d;整个若虫发育历期也由8 h光照时的15.54 d缩短至16 h光照时的12.74 d,世代历期由27.75 d缩短至23.45 d。长光照时雌虫更早进入产仔期,也有利于雌虫繁殖后代,16 h光照时进入产仔期是18日龄,8 和10 h光照时均推迟至22日龄;16 h光照时单雌产仔量为417.06头,明显高于12 ,10 和8 h光照时的353.59,347.61和336.00头。长光照条件下实验种群趋势指数(I)明显较大,增长潜能更大,16,10 和8 h光照时种群趋势指数分别为 175.37,133.94和141.99。该虫生命表参数表明,随着光照时间加长净增值率（R₀）呈逐渐增大趋势,8, 10,12,14和16 h光照时分别为88.57,86.85,100.77,124.16和126.86;内禀增长率、周限增长率表现类似的规律,8～16 h光照时内禀增长率（r_m）为0.1616～0.2066,周限增长率（λ）为1.1754～1.2294;而种群倍增时间（t）则随着光照时加长而缩短,8～16 h光照时为4.29～3.36 d。【结论】 长光照有利于棉花粉蚧生长发育和繁殖,世代历期缩短,种群增长潜能变强。     

The role of host- and host-plant odours in the attraction of a parasitoid, Epidinocarsis lopezi, to the habitat of its host, the cassava mealybug, Phenacoccus manihoti

In the field, the encyrtid wasp, Epidinocarsis lopezi (DeSantis) can find its host, the cassava mealybug (Phenacoccus manihoti Matile-Ferrero) on cassave plants when the host is scarce. As a step towards understanding the parasitoid's effectiveness we investigated the cues with which it locates its host-habitat. Using a fourarmed olfactometer, we determined the attactivity of various host-and host-plant odours to female E. lopezi which had been previously exposed to infested plants. Attractivity or preference of an odour was determined from the proportion of 50 choices made between the odour and a blank, or between two odours. Female E. lopezi were attracted by mealybug-infested cassava leaves (CML), but not by the odour of cassava mealybugs (CMB) alone or by uninfested plants (ULU). An artificial combination of CMB and ULU was also not attractive. CML was preferred over ULU and CMB + ULU. These results suggest that the attractive element arises from the cassava plant itself in response to CMB infestation. We therefore tested the attractivity of uninfested leaves from partly infested plants, and found that these were indeed attractive. E. lopezi probably uses the odour which is emitted from infested cassava plants to guide it to its host in the field.

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Zusammenfassung Epidinocarsis lopezi ist imstande, seine Wirte auf den Cassavapflanzen zu finden, auch wenn die Wirte nur in geringer Dichte vorkommen. Die Reaktion der Schlupfwespe auf Gerüche der Wirte und der Wirtspflanzen wurde untersucht. Weibchen der Schlupfwespe, die vorher mit Schmierläusen auf Cassavapflanzen zusammengebracht worden waren, wurden individuell in einem vierarmigen Olfaktometer verschiedenen Kombinationen dieser Gerüche ausgesetzt. Die relative Attraktivität jedes Geruchs wurde bestimmt anhand von 50 Entscheiden zwischen dem Geruch und einer Kontrolle oder zwischen zwei Gerüchen. E. lopezi Weibchen wurden von schmierlausinfizierten Blättern angezogen, aber nicht von Cassavaschmierläusen (CMB), oder von uninfizierten Cassavapflanzen (ULU). Die künstliche Mischung von CMN und ULU wurde auch nicht anlockend. CML war relativ mehr attraktiv als ULU und als die Mischung. Die Vermutung liegt vor dass E. lopezi von einem Pflanzenduft angezogen wird, der von mit Schmierläusen infizierten Cassavapflanzen ausgeschieden wird. Die Parasitoide wurden auch von uninfizierte Blättern teilweise infizierter Pflanzen angezogen. Wahrscheinlich wird im Freiland der Geruch infizierter Cassavapflanzen durch E. lopezi zur Auffindung weiterer wirtstragenden Cassavapflanzen gebraucht.

     

Field placement of mealybug-infested potted cassava plants for the study of parasitism of Phenacoccus herreni

During 4-day field exposure intervals, Phenacoccus herreni Cox & Williams, third-instar mealybugs on artificially infested potted cassava plants were reduced 73% from initial densities of 235.7–1100.2/infested plant to 63.7–314.2/plant due to dispersal, rain, and predation. Frequency of parasitoid attack did not differ between high and low mealybug densities. The same species of parasitoids, in approximately the same proportions, were reared from mealybugs on artificially infested potted plants as from field mealybugs (Acerophagus coccois) Smith, 88.1% and Epidinocarsis diversicornis (Howard), 11.3%. Parasitoid attack rates on artificially exposed mealybugs, when combined with a simulation model of parasitism, accurately predicted observed levels of stage-specific parasitism in the field population.

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Résumé Pendant des expositions de 4 jours dans la nature de plants en pots de M. esculenta contaminés artificiellement par des P. herreni du troisième stade, les densités de cochenilles ont été réduites de 73%: de 235.7–1100.2/par plant contaminé à 63.7–314.2/par plant, par suite des évasions, de la pluie et de la prédation. Les attaques de parasitoïdes ne changent pas suivant les densités de cochenilles. Les mêmes espèces de parasitoïdes, ont été élevées dans à peu près les mêmes proportions (Acerophagus coccois, 88.1%, et Epidinocaris diversicornis, 11.3%) à partir de cochenilles sur pots artificiellement contaminés ou de manioc de la nature. Les taux d'attaques par les parasitoïdes sur les cochenilles artificiellement exposées, associés à un modèle de simulation du parasitisme, ont permis de prévoir avec précision les niveaux de parasitisme observés pour chaque stade dans la population de la nature.

     

Host preferences of two encyrtid parasitoids for the Columbian Phenacoccus spp. of cassava mealybugs

In a choice test among six life stages of Phenacoccus herreni Cox & Williams, Epidinocarsis diversicornis (Howard) used its antennae to examine adult and 3rd stadium females more than other stages and preferentially attempted to oviposit in these plus 2nd stadium females. Success of ovipositor insertion was unaffected by host stage. The outcome of these behaviors was preferential oviposition by E. diversicornis in the large female host stages. Acerophagus coccois Smith also preferentially examined larger female mealybugs (second and third stadium nymphs and adults) more than other stages and successfully inserted its ovipositor in these stages more often than in second stadium male nymphs and male cocoons, resulting in a similar preference in this species for larger female host stages. When given a choice between adult female hosts of two species, P. herreni and Phenacoccus gossypii Townsend & Cockerell, E. diversicornis exhibited a clear preference for P. herreni; whereas A. coccois preferred P. gossypii.

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Résumé Epidinocarsis diversicornis (Howard), ayant la possibilité de choisir entre six stades différents de Phenacoccus herreni Cox & Williams, examine avec ses antennes plus particulièrement les adultes et les larves femelles du 3ème stade, et essaie de pondre de préférence dans ces stades et les larves femelles de second stade. L'insertion de la tarière s'effectue aussi bien quel que soit le stade de l'hôte. Il résulte de ces différents aspects du comportement que E. diversicornis pond de préférence dans les femelles des stades les plus avancés. Acerophagus coccois Smith préfère aussi examiner les cochenilles femelles les plus grosses (second et 3ème stade larvaires et adulte), et introduit sa tarière avec succès dans ces stades plus souvent que dans les larves mâles de second stade ou les cocons mâles; il en résulte aussi pour cette espèce une préférence pour les femelles des stades les plus gros.Quand on leur a donné le choix entre des femelles des deux espèces de cochenilles (P. herreni et Phenacoccus gossypii Towsend & Cockerell), E. diversicornis manifestait une nette préférence pour P. herreni, tandis que A. coccois préférait P. gossypii.

     

Étude de l'évolution d'une mycose aNeozygites fumosa [Zygomycetes,Entomophthorales] dans une population de la cochenille du manioc,Phenacoccus manihoti [Hom.: Pseudococcidae]

Résumé L'action régulatrice d'un champignon du groupe des Entomophthorales,Neozygites fumosa (Speare) Remaudière et Keller, est mise en évidence pour la lère fois dans une population de la cochenille du maniocPhenacoccus manihoti Matile-Ferrero (Homoptères: Pseudococcidae) en République Populaire du Congo en 1982. Le pathogène est de loin l'ennemi naturel qui intervient le plus significativement dans la régulation des effectifs. L'extension de la maladie dans la population h?te appara?t conditionnée par l'apparition simultanée d'une humidité relative supérieure ou égale à 90% et d'une température journalière minimale supérieure à 20°C. Elle semble également liée à la densité du ravageur. On notera enfin que les formes adultes sont systématiquement plus infectées que les formes larvaires.

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Summary The regulatory action of the entomophthoraceous fungusNeozygites fumosa (Speare) Remaudière & Keller (Zygomycetes) is shown for the ist time in a population of the Cassava mealybug,Phenacoccus manihoti (Hom.: Pseudococcidae), in the People's Republic of the Congo in 1982. This pathogen is by far the natural enemy which intervenes the most significantly in the regulation of this insect. The development of the epizootic appears to be influenced by a relative humidity of at least 90% along with a minimal daily temperature greater than 20°C. The spread of the disease also seems to be affected by the host density. Throughout the entire duration of this study, the adult forms were more infected than the larval forms.

     

The use of embryo culture for the recovery of plants from cassava (Manihot esculenta Crantz) seeds

Cassava fertility and seed viability are frequently low, which can be a disadvantage in a breeding programme. An embryo culture method is described whereby embryonic axes are excised from mature seeds and placed on a culture medium containing 1.23 M indolebutyric acid (IBA) at 30°C under continuous light. The number of plants recovered by embryo culture was much greater than the number recovered from conventional seed germination procedures.     

L'entomophthoraleNeozygites fumosa pathogène de la Cochenille du manioc,Phenacoccus manihoti [Hom.: Pseudococcidae], en République populaire du Congo

Résumé Uniquement connue du sud des Etats-Unis d'Amérique, l'EntomophthoraleNeozygites fumosa (Speare) Remaudière & Keller a été retrouvée en République populaire du Congo, où elle attaque la Cochenille du manioc,Phenacoccus manihoti Matile-Ferrero [Hom.: Coccoidea Pseudococcidae]. De légères variations d'ordre biométrique sont mises en évidence au sein de l'espèce fongique. A ce jour, parmi les Entomophthorales mentionnées dans la littérature comme pathogènes de Cochenilles, seules deux espèces peuvent être retenues avec certitude:Neozygites fumosa etConidiobolus pseudococci Speare (Tyrrell & MacLeod), toutes deux pathogènes dePseudococcidae.

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Summary The entomophthoraceous fungus,Neozygites fumosa (Speare) Remaudière and Keller, which was only known from the south of the U.S.A., was found in the People's Republic of the Congo, on the Cassava mealybug,Phenacoccus manihoti Matile-Ferrero [Homoptera: Coccoidea, Pseudococcidae]. Slight biometrical variations appear within the fungal species. At the present time, among the Entomophthorales cited in the literature as parasitizing Coccoidea, two species only can be retained with certainty:Neozygites fumosa andConidiobolus pseudococci Speare (Tyrrell and MacLeod), both attackingPseudococcidae.