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.     

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.

     

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.

     

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.     

Importation,colonization and establishment ofAnagyrus indicus [Hym.: Encyrtidae] onNipaecoccus viridis [Hom.: Pseudococcidae] in Jordan

Anagyrus indicus Shafeeet al. was collected on the island of Guam and released in the Jordan River Valley of Jordan in order to regulateNipaecoccus viridis (Newstead) on citrus.A. indicus was originally released in February 1984 and became colonized by April of the same year. It also dispersed by natural means up to 61 km following the original release and colonized that same year on infestedZizyphus sp. By spring of 1985 it became the dominant parasitoid attackingN. viridis. Another parasitoid,Anagyrus kamali Moursi, was collected in Jordan in June of 1985 for the first time. This latter parasitoid apparently moved into the Jordan River Valley by means of natural dispersal from a neighboring country. Infestations ofN. viridis have been greatly reduced in areas whereA. indicus became established. This paper reports the results of research only. Mention of a proprietary product does not constitute an endorsement for its use by USDA.     

Biology ofEpidinocarsis [=Apoanagyrus] lopezi [Hymenoptera: Encyrtidae] an exotic parasite of cassava mealybug,Phenacoccus manihoti [Homoptera: Pseudococcidae] in Nigeria

The biology ofEpidinocarsis (=Apoanagyrus) lopezi (DeSantis) was studied under fluctuating laboratory conditions of 24–31°C and 79–90% R.H. It is a solitary internal parasite. There are 4 tailed larval instars. The total developmental time from egg to adult ranged from 11–25 days, average 18 days. The duration of each developmental stage was: egg 2 days, 1 st instar 1 day, 2nd instar 1 day, 3rd instar 2 days, 4th instar 2 days, prepupa 4 days and pupa 6 days. Males mated several times, females mated once. Mated females produced both males and females, unmated females produced only males. Mated ovipositing females lived for 13 days while non-ovipositing mated females lived for 25 days. Parasitization decreased with increasing host density from 30% at a density of 10 hosts/plant/female to 11% at a density of 50. There was virtually no pre-oviposition period. Based on 10 females, the highest mean daily progeny production was 10. Females have an average oviposition period of 11 days and an average of 67 adult progeny. The sex-ratio was 1∶2.3 in favour of females.     

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 biology ofHyperaspis jucunda [Col.: Coccinellidae] an exotic predator of the cassava mealybugPhenacoccus manihoti [Hom.: Pseudococcidae] in southern Nigeria

The biology ofHyperaspis jucunda (Muls.) was studied at 27°C and the incubation period averaged 5.1 days. The 1st, 2nd, 3rd and 4th larval instar averaged 2.5; 2.8; 3.4 and 5.0 days respectively. Larval development was completed in about 13.8 days while it took 7.1 days for the pupae. The total developmental time averaged 26.4 days. Mean longevity was 100 and 101 days for males and females respectively. The premating period was 19–24 h while the preoviposition period averaged 6.3 days. The generation cycle (egg to egg) averaged 32.8 days. The oviposition period was about 93 days during which an average female laid 456 eggs.     

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.

     

Influence de l'hyperparasitisme sur les populations d'un hymenoptère encyrtidae,Epidinocarsis lopezi,parasitoide de la cochenille du manioc Phenacoccus manihoti introduit au Congo

Résumé Une étude au champ de la dynamique des populations de la cochenille du manioc Phenacoccus manihoti Matt.-Ferr. (Hom. Pseudococcidae) et des agents biologiques qui lui sont associés a été entreprise dans 4 parcelles de manioc Manihot esculenta Crantz (Euphorbiacée) situées dans 2 localités différents du Congo. Elle a essentiellement montré que les 7 à 16% de parasitisme développés par le parasitoïde exotique Epidinocarsis lopezi (De Santis) (Hym. Encyrtidae), 4 ans après son introduction, n'affectent pas significativement la dynamique des populations du ravageur. Les taux importants d'hyperparasitisme (67 à 100%) dont le parasitoïde primaire est l'objet limitent la reproduction de ses populations. Toutefois il est difficile de conclure quant à l'impact réel des hyperparasites sur l'efficacité de l'auxiliaire dans la lutte contre la cochenille.

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A field study of cassava mealybug Phenacoccus manihoti Mat.-Ferr. (Hom. Pseudococcidae) population dynamics, together with that of associated biological agents was made on 4 sample plots in cultivated manioc Manihot esculenta Crantz (Euphorbiacée) in two different areas of the Congo. It showed that four years after introduction, the 7 to 16% of parasitism attributable to the exotic parasitoid Epidinocarsis lopezi (De Santis) (Hym. Encyrtidae), did not significantly affect pest population dynamics. High levels of hyperparasitism (67% to 100%) to which the parasitoid is itself subject limits its population levels. It is, however, impossible to determine the real effect of hyperparasites on the efficiency of the auxiliary in the biological control of the mealybug.

     

Aleurocanthus woglumi [Hom.: Aleyrodidae] andEncarsia opulenta [Hym.: Encyrtidae]: Density-dependent relationship between adult parasite aggregation and mortality of the host

Field and laboratory choice tests in which searching adultEncarsia opulenta Silvestri were exposed to variable densities of citrus blackfly,Aleurocanthus woglumi Ashby, indicated the following: (1) a direct functional relationship between adult parasite aggregation and host density, resulting in (2) a direct density-dependent mortality ofA. woglumi within a single generation timespan. The implications of such nonrandom searching patterns byE. opulenta on stability of the host-parasite interaction on Texas citrus are discussed.     

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.

     

The influence of constant temperature on population growth rates of the cassava mealybug, Phenacoccus manihoti

Life table studies were conducted to assess the effect of constant temperature on the rate of population growth of the cassava mealybug, Phenacoccus manihoti Matile-Ferrero. Four temperatures, between 20 and 30.5°C, were tested. An inverse relationship was observed between temperature and most demographic parameters.The intrinsic rate of natural increase (r_m) increased from 0.1 at 20°C to 0.2 at 27°C and 30.5°C. The net reproductive rate varied between 426.3 at 30.5°C and 584.7 at 20°C. The mealybug population reached 50% mortality after 37.5, 21.5, 19.0 and 19.0 days respectively at 20, 23.5, 27 and 30.5°C. The results indicate that P. manihoti can persist and increase in numbers within the range between 20 and 30.5°C.

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Influence de températures constantes sur les taux de croissance de populations de la cochenille du manioc Phenococcus manihoti

Résumé Des tables de vie ont été établies pour déterminer l'effet de températures constantes sur les taux de croissance de populations de la cochenille du manioc, Phenacoccus manihoti Mat.-Ferr., et ainsi comprendre les changements au sein des populations du ravageur dans les champs et mener à bien le programme de lutte biologique organisé à l'I.I.T.A. contre cette cochenille. P. manihoti, introduit à partir de l'Amérique latine en Afrique, y menace la production du manioc (Manihot esculenta Crantz).Le taux intrinsèque d'accroissement natural (r_m) a augmenté de 0.114 à 20°C, à 0.185 à 27°C, avant de descendre à 0.182 à 30.5°C. Le taux net de reproduction (R_o) a été relativement élevé (426–584 oeufs femelles/génération). Dans nos conditions expérimentales, la mortalité a atteint 50% au bout de 37.5, 21.5, 19.0 jours respectivement à 20, 23.5, 27 et 30.5°C. La durée du cycle et le coefficient d'accroissement () étaient inversement liés à la température. Le ravageur possède la capacité de doubler sa population en 6.08 jours à 20°C alors que 3.81 jours seulement suffisent pour doubler la population à 30.5°C.Ces résultats nous ont permis de comprendre et d'expliquer l'énorme pouvoir de pullulation de la cochenille observé dans les champs pendant la saison sèche; il s'ensuit que les lâchers des entomophages produits en élevages doivent se faire très tôt au début de la saison sèche, afin de contrecarrer la grande fertilité et la capacité d'augmentation rapide des populations de P. manihoti.

     

Sex allocation in Epidinocarsis lopezi: local mate competition

Epidinocarsis lopezi is used as a biological control agent against the cassava mealybug, Phenacoccus manihoti, a serious pest of cassava in Africa. The efficiency of parasitoid mass-rearing is maximized when maximum numbers of healthy female wasps are obtained, since only female parasitoids attack the mealybugs.Highly variable sex ratios are often found in parasitic Hymenoptera. Local mate competition (LMC) is one of the evolutionary models which provide predictions about sex allocation. In this paper we show that E. lopezi does not respond to parasitoid density with a change in sex ratio. We also show that in the field, no local mating structure exists, and that mating is random. Therefore, a shift in sex ratio in response to parasitoid density as predicted by LMC theory would not be adaptive. E. lopezi also does not change its sex allocation when ovipositing in already parasitized hosts. Hence host-size distribution and differential mortality are the only factors that can influence sex ratio in mass-rearings.

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Résumé E. lopezi est utilisé dans la lutte biologique contre Phenacoccus manihoti, important ravageur du manioc en Afrique. Puisque seules les femelles du parasitoïde attaquent la cochenille, l'efficacité de l'élevage de masse de l'entomophage sera optimale quand le maximum de femelles saines sera obtenu.Les rapports des sexes des hyménoptères parasites varient très souvent. La compétition sexuelle locale (LMC) constitue l'un des modèles qui fournissent des prédictions de la distribution des sexes. Cette note montre que la proportion des sexes de E. lopezi n'est pas modifiée par la densité du parasitoïde. Par ailleurs, les accouplements s'effectuent au hasard dans la nature et il n'y a pas de structure locale d'accouplement. Par conséquent, le biais, prévu par la théorie du LMC, et introduit par la densité du parasitoïde dans la distribution des sexes, n'a pas de valeur adaptative. E. lopezi ne modifie pas non plus la distribution du sexe de ses descendants quant il pond dans de hôtes déjà parasités. Ainsi, la répartition en taille des hôtes et la mortalité différentielle sont les seuls facteurs qui influent sur la proportion des sexes dans les élevages de masse.

     

Introduction and establishment ofTrioxys pallidus [Hym.: Aphidiidae] in Oregon,U.S.A. for control of filbert aphidMyzocallis coryli [Hom.: Aphididae]

Hazelnut orchards in Spain, France, and Italy were searched for parasitoids of the filbert aphid,Myzocallis coryli (Goetze). A biotype ofTrioxys pallidus Haliday was found to parasitize the aphid throughout western Europe. Wasps were imported, quarantined, mass-reared, and released in Oregon orchards. The wasp successfully attacked and completed its development on Oregon populations of the filbert aphid. A greenhouse culture of the parasitoid was maintained continuously for over 50 generations, and approximately 30,000 adult wasps were released in western Oregon. Overwintering survival has been documented in at least 12 different locations. In 3 commercial hazelnut orchards, the parasitoids proved capable of reducing aphid population peaks by 33–48%.      

A survey of black scale,Saissetia oleae [Hom.: Coccidae] parasitoids [Hym.: Chalcidoidea] in southern California

A survey of black scaleSaissetia oleae (Olivier) parasitoids present in southern California was conducted between September 1987 and September 1989. From 308 collections of black scale-infested citrus, olive, and oleander twigs from 19 sites in southern California, 1,610 specimens were collected. Nine primary and six secondary parasitoid species were identified. Four primary species were abundant in southern California:Metaphycus bartletti Annecke & Mynhardt,M. helvolus (Compere),Scutellista caerulea (Fonscolombe) (=S. cyanea Motschulsky), andDiversinervus elegans Silvestri. The most common secondary parasitoids wereMarietta mexicana (Howard),Cheiloneurus noxius Compere, andTetrastichus minutus (Howard). In the coastal region of southern California,M. bartletti was the most abundant parasitoid, followed in order byD. elegans, S. Caerulea, andM. helvolus. In the intermediate and interior regions,M. helvolus was most abundant.D. elegans was second most abundant in the intermediate region, but was rare in the interior region.M. bartletti was second in abundance in the interior region and third in the intermediate region.      

Sex ratios in field populations of Epidinocarsis lopezi, an exotic parasitoid of the cassava mealybug, in Africa

Abstract.

• 1 In cassava fields in Africa, population sex ratios of Epidinocarsis fopezi vaned from 0.44 (males to total parasitoids) at low host densities to highly male-biased ratios of 0.70 at high host densities.
• 2 This variability is caused by the difference in allocation of sons and daughters to hosts of different sizes, through the following mechanisms: (a) small, i.e. second instar, hosts are mainly used for the production of male offspring, whereas in large, i.e. third instar, hosts a variable, female-biased sex ratio is produced; (b) E.fopezi does not selectively oviposit into large hosts but always accepts both small and large hosts for oviposition upon encountering; (c) in the field, this parasitoid is time-limited, and not egg-limited. On the basis of an optimal diet model, such general host acceptance is shown to be the best strategy.
• 3 Thus, sex ratio increases with host density for three reasons: the proportion of small hosts encountered in the field increases with increasing host density, small hosts are used for male production, and hosts are always accepted when encountered.

     

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.

     

Interaction betweenRastrococcus invadens [Hom.: Pseudococcidae] and two natural enemies

Two natural enemies of the mealybugRastrococcus invadens, the parasitoidGyranussoidea tebygi and the pathogenHirsutella cryptosclerotium were tested. Levels of parasitism by the parasitoid were reduced by the pathogen but overall mortality of the mealybug was greater both when agents were acting together.