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.

     

Egg allocation of the autoparasitoid encarsia tricolor at different relative densities of the primary host (Trialeurodes vaporariorum) and two secondary hosts (Encarsia formosa and E. tricolor)

Parasitism, offspring sex ratio and superparasitism of the facultative autoparasitoid Encarsia tricolor Foërster (Hymenoptera: Aphelinidae) when given access to arenae with different proportions of the primary host (Trialeurodes vaporariorum (Westwood)) and two species of secondary hosts (E. tricolor and Encarsia formosa Gahan) were studied.Parasitism and offspring sex ratio were not affected by female age in the range 3–10 days old. When the secondary hosts were young E. tricolor pupae, eggs were mostly laid on primary hosts, so the offspring sex ratio was more female-biased than expected, and secondary hosts were not superparasitized at all. When the secondary hosts were fully grown E. formosa larvae, superparasitism was small and offspring sex ratio was more male-biased than expected. E. tricolor females were able to discriminate between hosts previously parasitized by themselves and non-parasitized hosts.     

Impact of mating status on egg-laying and superparasitism behaviour in a parasitoid wasp

Most parasitoid female wasps can distinguish between unparasitized and parasitized hosts and use this information to optimize their progeny and sex allocation. In this study, we explored the impact of mating on oviposition behaviour (parasitism and self‐ and conspecific superparasitism) on both unparasitized and already parasitized hosts in the solitary parasitoid wasp Eupelmus vuilleti (Crw.) (Hymenoptera: Eupelmidae). Virgin and mated females had the same oviposition behaviour and laid eggs preferentially on unparasitized hosts. The sex ratio (as the proportion of females) of eggs laid by mated females in parasitism and conspecific superparasitism was 0.67 ± 0.04 and 0.57 ± 0.09, respectively. Likewise, females laid more eggs in conspecific superparasitism than self‐superparasitism under our experimental conditions. These experiments demonstrate that E. vuilleti females can (i) discriminate between unparasitized and parasitized hosts and adapt the number of eggs they lay accordingly, and (ii) probably discriminate self from conspecific superparasitized hosts. Finally, mating does not appear to influence the host discrimination capacity, the ovarian function, or the oviposition behaviour.     

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.

     

Sex Allocation Decision Under Superparasitism by the Parasitoid Wasp Eupelmus vuilleti

Superparasitism is a widespread phenomenon in parasitoids and may be advantageous in some circumstances. In this study, offspring sex ratio was analysed in three superparasitism situations: when the second egg was laid by a random Eupelmus vuilleti (Hymenoptera: Eupelmidae) female from a group, when an isolated female was allowed to lay two eggs on the same host (self-superparasitism) or laid one egg on a host already parasitized by a conspecific (conspecific superparasitism). Females produced a different offspring sex ratio according to the different superparasitism situations tested. These sex ratios are in line with the local mate competition theory. The results further suggest that females can discriminate between hosts parasitized by a conspecific or by themselves and adapt the sex of the eggs they lay accordingly.     

Presence of a conspecific increases superparasitism but not infanticide under self‐ and conspecific superparasitism in a semisolitary parasitoid,Echthrodelphax fairchildii (Hymenoptera: Dryinidae)

Female parasitoids are expected to avoid superparasitism (ovipositing in and/or on parasitized hosts) when unparasitized hosts are available. However, when the supply of unparasitized hosts is restricted, they are expected to self‐ as well as conspecifically superparasitize. One of the cues of a reduced availability of unparasitized hosts is the presence of a conspecific. Moreover, if the focal species can perform infanticide, after encountering a conspecific female, the females are expected to kill eggs existing in and/or on hosts when superparasitizing, because the eggs are more likely to be laid by others. In this study we investigated whether females of an infanticidal semisolitary parasitoid, Echthrodelphax fairchildii, increase their frequencies of superparasitism and infanticide after encountering a conspecific female. Echthrodelphax fairchildii females are capable of discriminating between self‐ and conspecific superparasitism until up to 0.75 h after the first egg was laid (self‐superparasitism frequency < conspecific superparasitism frequency). As expected, the female parasitoids were more likely to perform self‐ and conspecific superparasitism after they had encountered a conspecific. In particular, the self‐superparasitism frequency increased highly within a short period after the first oviposition, so that no difference between the self‐ and conspecific superparasitism frequencies was found. In contrast, the infanticidal‐probing frequency remained extremely low, irrespective of whether or not the female parasitoids had encountered a conspecific. Moreover, when superparasitizing, females usually laid female eggs. Possible causes for the low frequency of infanticidal probing and the female‐biased sex ratio are discussed.     

Strange limits of stability in host-parasitoid systems

The classical Nicholson-Bailey model for a two species host-parasitoid system with discrete generations assumes random distributions of both hosts and parasitoids, randomly searching parasitoids, and random encounters between the individuals of the two species. Although unstable, this model induced many investigations into more complex host-parasitoid systems. Local linearized stability analysis shows that equilibria of host parasitoid systems within the framework of a generalized Nicholson-Bailey model are generally unstable. Stability is only possible if host fertility does not exceede ⁴=54.5982 and if superparasitism is unsuccessful. This special situation has already been discovered by Hassell et al. (1983) in their study of the effects of variable sex ratios on host parasitoid dynamics. We discuss global behaviour of the Hassell-Waage-May model using KAM-theory and illustrate its sensitivity to small perturbations, which can give rise to radically different patterns of the population dynamics of interacting hosts and parasitoids.     

Interspecific host discrimination and competition in Apoanagyrus (Epidinocarsis) lopezi and A.(E.)diversicornis, parasitoids of the cassava mealybug Phenacoccus manihoti

Abstract.

• 1 Choice experiments on interspecific host discrimination in A.lopezi and A. diversicornis were carried out on discs of cassava leaf containing four hosts (P.manihoti) that had been parasitized by the other species and four unparasitized hosts.
• 2 A.lopezi accepted both host types equally for oviposition, whereas A.diversicornis accepted fewer hosts that had been parasitized by A.lopezi than unparasitized ones. A.diversicornis is therefore capable of interspecific host discrimination, but such a capability was not demonstrated for A.lopezi.
• 3 Survival probability in singly parasitized hosts was 0.85 for both parasitoid species. When the time interval between ovipositions was 2 h or less, survival in multiparasitized hosts was 0.68 for A.lopezi and 0.17 for A.diversicornis, irrespective of priority. Increasing A.lopezi priority to 24±2h did not increase A.lopezi survival. A.diversicornis survival, however, increased to 0.43 when A.diversicornis was given 24 ± 2 h priority. A.diversicornis eggs took 19 h longer than A.lopezi eggs to hatch. This could explain the difference in competitive abilities in multiparasitized hosts.
• 4 The observed difference in host selection behaviour between A.lopezi and A.diversicornis is in accordance with the different benefits of multiparasitism: A.lopezi gains more than A.diversicornis because of its superior within-host competitive abilities.
• 5 Neither species avoided multiparasitism completely. The low survival probability of A.diversicornis in multiparasitized hosts may partly explain its failure to establish when introduced into Africa as part of a biological control programme of P.manihoti.

     

Self and conspecific superparasitism by the solitary parasitoid Antrocephalus pandens

Abstract.

• 1 The fitness consequences of superparasitism for a solitary parasitoid depend on whether the host was first parasitized by itself (‘self-superparasitism’) or a different individual (‘conspecific superparasitism’). Self superparasitism is usually expected to be avoided.
• 2 A.pandens females showed no difference in their probability of superparasitism between self-parasitized and conspecifically-parasitized hosts. The probability of superparasitism decreased as time from the laying of the first egg in a host increased, from about 0.29–0.46 at a time interval of 1 h to 0.10–0.14 at 72 h.
• 3 The egg distribution of wasps foraging alone on a patch showed significant avoidance of superparasitism, but that of wasps foraging in the presence of conspecifics was not significantly different from a random distribution. This suggests that wasps switch from avoidance of superparasitism when alone to acceptance of all hosts when in a group.
• 4 When wasps foraged in a group, the hosts had many more ovipositor puncture marks than when wasps foraged singly. This suggests that either hosts were attacked several times per encounter, or that the wasps' encounter rate with hosts was much higher when in a group. If the latter is true, it is possible that, although the egg distribution suggested a higher rate of superparasitism when wasps foraged in a group, the ratio of acceptances to contacts of parasitized hosts may in fact have been lower.

     

Using superparasitism by a stem borer parasitoid to infer a host refuge

Abstract. 1. Macrocentrus cingulum Reinhard (Hymenoptera: Braconidae), a parasitoid of the European corn borer, Ostrinia nubilalis, and the Asian corn borer, O. furnacalis (Lepidoptera: Pyralidae), has high fecundity but has been reported to parasitize a low proportion of host larvae. This was corroborated in field collections: in Hebei (China) and Delaware (U.S.A.), M. cingulum parasitized only 15 and 25%, respectively, of hosts collected. 2. Because M. cingulum females cannot oviposit through plant tissue, they must parasitize hosts either before they have bored into stalks or while they are near entrance holes, so that at any one time, many Ostrinia larvae may be unavailable to M. cingulum. This refuge, together with fluctuations in abundance of foraging M. cingulum females, may explain why M. cingulum parasitizes relatively few Ostrinia larvae. 3. To test this hypothesis, levels of superparasitism were measured in the field. Low parasitism resulting from a refuge for host larvae should cause high rates of superparasitism in hosts outside the refuge. 4. Because M. cingulum is polyembryonic, the number of parasitoids per host does not indicate the level of superparasitism. Random amplified polymorphic DNA markers were used to determine the number of different genotypes emerging from each host. The resulting frequency distributions were fitted to those expected under random oviposition to estimate the proportion of Ostrinia larvae unavailable to M. cingulum. 5. In the samples from Hebei and Delaware, the level of superparasitism was much higher than expected by chance if all hosts were available. Fitting the frequencies of genotypes per host to a Poisson distribution, the authors estimated that 74–82% and 69–74% of host larvae were unavailable to M. cingulum in these collections, respectively. This means that M. cingulum parasitized 60–84% and 82–95% of available hosts in these collections, respectively. These levels of parasitism contrast strongly with the 15–25% found when all hosts were assumed available for oviposition. 6. Genetic distances of M. cingulum within and between hosts did not differ, allowing rejection of the hypothesis that high levels of superparasitism resulted from a female laying several eggs in the same host. 7. The hypothesis that M. cingulum parasitizes few Ostrinia larvae because many larvae are in a refuge explains these data and previously published information better than other hypotheses that have been suggested.     

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.

     

Host location and discrimination mediated through olfactory stimuli in two species of Encyrtidae

The role of olfactory stimuli in host detection and evaluation was studied in two encyrtid Hymenoptera. The first, Epidinocarsis lopezi De Santis, is a monophagous parasitoid of the cassava mealybug Phenacoccus manihoti Matile-Ferrero, itself feeding exclusively on cassava, Manihot esculenta Crantz. The second, Leptomastix dactylopii Howard, is a monophagous parasitoid of the Citrus mealybug, Planococcus citri Risso, but this latter is highly polyphagous. The behaviour of females of both parasitoids (attaction and locomotion) was compared in a tubular olfactometer for the odours of their respective hosts on cassava and poinsettia. Tests were made using: 1) healthy host-plant alone; 2) host-plant infested with unparasitized mealybugs; 3) unparasitized mealybugs only; 4) host-plant infested with parasitized mealybugs and 5) parasitized mealybugs only. Only E. lopezi was attracted by the odour of the host-plant alone, but both species were attracted by the odour of an infested host-plant and that of unparasitized mealybugs. The odour of parasitized mealybugs, alone or on host-plant, induced an undirected activity. The attraction of E. lopezi to the odour of the host-plant alone could be linked to the monophagous diet of its host, whereas the attraction of the two species of parasitoids to the odours of infested host-plants and unparasitized mealybugs could be due to the fact that both parasitoids are specialists. The behavioural response of both species to the odour of parasitized mealybugs revealed a new aspect in host discrimination: the identification of parasitized hosts could be partly mediated through olfactory stimuli, and not only through gustatory stimuli.     

SEX RATIO VARIATION IN A PARASITIC WASP I. REACTION NORMS

To understand genetic and phenotypic constraints on the sex ratio in a parasitic wasp that attacks fly pupae, I carried out a laboratory study of sex ratio variability in five strains of Muscidifurax raptor (Hymenoptera: Pteromalidae). I manipulated the environment through combinations of temperature and day length, and the numbers of females that attack a group of hosts. The change of phenotype in each strain over the range of environmental conditions describes the norm of each reaction for that strain, and measures how a strain responds to environmental variation to create phenotypic variability. Sex ratio in parasitic wasps is a complex trait that has several components—the numbers of eggs laid by an ovipositing wasp and the fraction of eggs that are fertilized (female). Further, sex ratio may be influenced by a female's reaction to other females exploiting the same hosts (superparasitism). I found no strain-environment interactions in either sex ratio or fecundity when I varied environmental conditions. Although strains differed in sex ratio and fecundity, all strains produced a more female-biased sex ratio and had higher fecundity when temperature and day length increased. Sex ratio and fecundity were phenotypically correlated, and strains with greater fecundity also produced a more female-biased sex ratio. All strains facultatively shifted sex ratio toward a higher fraction of males with increasing female density, despite apparent differences in superparasitism among strains. Males and females survived equally during development, so that mortality differences among strains and across environments could not account for sex ratio variability. This study indicates that sex ratio variability among strains is constrained by the correlation between sex ratio and fecundity, and that strains display similar facultative shifts in sex ratio as female density increases because sex ratio shifts are insensitive to differing levels of superparasitism.     

Superparasitism in mass reared Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae), a parasitoid of fruit flies (Diptera: Tephritidae)

Superparasitism frequency and its effects on the quality of mass-reared Diachasmimorpha longicaudata (Ashmead) parasitoids were investigated under laboratory conditions. Percentage of adult emergence, sex ratio, survival, fecundity and flight ability of adult parasitoids that emerged from Anastrepha ludens (Loew) pupae with different levels of superparasitism were determined. A high prevalence of superparasitism was observed. The number of scars per pupa, produced by insertion of the parasitoid ovipositor, ranged from 1 to 30, with an average (±SD) of 8.3 ± 6.2. Adult parasitoid emergence decreased as the level of superparasitism increased. However, the fraction of females rose with increasing superparasitism and the flight ability was lower in adults emerging from pupae with only one scar, compared with adults emerging from superparasitized hosts. Female longevity and fecundity were not affected by superparasitism. Our results support the hypothesis that superparasitism in D. longicaudata might be adaptive, since adults emerging from hosts with moderate levels of superparasitism showed the highest percentage of emergence and there were no significant differences in the other quality control parameters tested. Our findings are relevant to the mass rearing process, where the ratio of hosts to parasitoids can be optimized as well as the distribution of eggs deposited in host larvae. This contributes to efficient mass rearing methods for augmentative biological control programs.     

Self-/conspecific discrimination and superparasitism strategy in the ovicidal parasitoid Echthrodelphax fairchildii （Hymenoptera： Dryinidae）

Superparasitism in solitary parasitoids results in fatal competition between the immature parasitoids, and consequently only one individual can emerge. In the semisoli- tary ovicidal parasitoid Echthrodelphaxfairchildii （Hymenoptera： Dryinidae）, 2 adults can emerge under superparasitism with a short interval （〈24 h） between the first and second ovipositions. We determined the female parasitoid＇s behavioral responses under self- and conspecific superparasitism bouts with first-to-second oviposition intervals of 〈2 h. The self- and conspecific superparasitizing frequencies increased up to an oviposition interval of 0.75 h, with the former remaining lower than the latter, particularly for oviposition intervals of _〈0.25 h, suggesting the existence of self-/conspecific discrimination. The superparasitizing frequency plateaued for oviposition intervals of _〉0.75 h, with no dif- ference between self- and conspecific superparasitism. The ovicidal-probing frequency did not differ under self- and conspecific superparasitism, and was usually 〈20%. The females exhibited no preference for the oviposition side （i.e., ovipositing on the side with or without the first progeny） and almost always laid female eggs for any oviposition in- terval under self- and conspecific superparasitism. The sex ratio was not affected by the type of superparasitism, oviposition sides, or the occurrence of ovicidal probing. These observed results about the oviposition side, ovicidal probing, and sex ratios differed from the predictions obtained assuming that the females behave optimally. Possible reasons for the discrepancies are discussed： likely candidates include the high cost of selecting oviposition sides and ovicidal probing, and, for the sex ratio, the low frequency of encountering suitable hosts before superparasitism bouts.     

Sex allocation and effects of superparasitism on secondary sex ratios in the gregarious parasitoid, Trichogramma chilonis (Hymenoptera: Trichogrammatidae)

The role of sex-controlling behaviour at oviposition in generating primary sex ratios, and the effect of larval competition on secondary sex ratios, were studied in the gregarious endoparasitoid, Trichogramma chilonis. The production of a fertilized (female) egg is indicated by the incorporation of a pause in abdominal movements during oviposition, while the absence of it indicates the production of an unfertilized (male) egg. During each ovipositional bout, the first male egg is deposited at the second oviposition, and thereafter at intervals of about eight eggs. This simple pattern enables the wasps to adjust their progeny sex ratios under local male competition to a wide range of host size. Inexperienced wasps do not distinguish between parasitized and healthy hosts. Immature mortality is not significantly different between the sexes when a host is attacked by a single wasp, while females suffer higher immature mortality than males when superparasitism occurs.     

Sex ratio regulation in the waspEupelmus vuilleti (crwf.), an ectoparasitoid on bean weevil larvae (Hymenoptera: Pteromalidae)

In Africa, the seeds and/or pods ofVigna unguiculata andVigna radiata (Papilionacea) are attacked in fields and storage structures by bruchid beetlesCallosobruchus maculatus andBruchidius atrolineatus, on which parasitoid communities can develop. One of these parasitoids is the solitary ectoparasitoidEupelmus vuilleti (Eupelmidae). The storage conditions ofV. unguiculata andV. radiata favor the overlapping of all host stages during several months. These conditions suggest that female parasitoids would vary the sex ratio of their offspring according to the different sizes or developmental stages of hosts. The sex ratio ofE. vuilleti was strongly related to the developmental stage of the hostC. maculatus. Under our experimental conditions, where superparasitism is rare, the proportion of daughters varied between 5 and 25% on the third larval stage but reached 70 to 90% on the pupae. The increase in the proportion of daughters was also observed in the absence of superparasitism. In this case, there was an absolute coincidence between the sex ratio of eggs laid and that of emerged adults. Manipulation of the sex of the egg by the females seems to take place at the time of the egg's fertilization. The relation between host weight and egg sex showed that the male eggs are preferentially laid on lighter host larvae and the female eggs on heavier ones.     

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.     

Host influences on sex ratio, longevity, and egg load of two Metaphycus species parasitic on soft scales: implications for insectary rearing

Metaphycus flavus (Howard) and M. stanleyi Compere (Hymenoptera: Encyrtidae) are currently being screened for use as augmentative biological control agents of citrus-infesting soft scales (Homoptera: Coccidae). Two factors were investigated, host quality-dependent sex allocation and local mate competition, which likely influence these parasitoid's sex allocation strategies and are therefore of interest for their mass-rearing. The results of these studies suggested that, under the mass-rearing protocol that is envisioned for these parasitoids, offspring sex ratios in both M. flavus and M. stanleyi are dominated by host quality (= size) influences, but not by interactions with other females. These results indicated that host size strongly influences offspring sex ratios and brood sizes; larger hosts led to more female offspring and larger broods. In contrast, increasing the number of parental females did not lead to fewer female offspring as expected under local mate competition. Additionally, within-brood sex ratios did not vary with brood size; this result is inconsistent with expected sex ratios due to local mate competition. Other results also indicated that host quality was a dominant influence on M. flavus' and M. stanleyi's sex ratios. Larger hosts led to a larger size in the emerging wasps, and larger wasps had greater egg loads and lived longer than smaller wasps. However, wasp longevity, and the influence of wasp size on longevity were mediated by a wasp's diet. Metaphycus flavus females lived the longest when they had access to hosts, honey, and water, followed by honey and water, and shortest when they had access to water alone; M. stanleyi females lived longest with honey and water, followed by hosts, honey, and water, and shortest with water alone. Greater wasp size led to greater longevity in females only when they had access to food (honey, or hosts and honey). Finally, other results suggested that both M. flavus and M. stanleyi are facultatively gregarious. Wasp size did not decrease with brood size as expected under superparasitism. Overall, the results of these studies suggested that holding newly emerged females of both M. flavus and M. stanleyi for several days in the presence of an appropriate food source before field release could enhance a female's performance as an augmentative biological control agent. It increases their initial life expectancy following release, and maximizes the females' egg load (both Metaphycus species) and resources for replacing oviposited eggs (M. flavus only).