Comparison of Foraging Behavior, Interspecific Host Discrimination, and Competition of Encarsia formosa and Amitus fuscipennis

The foraging behavior of Amitus fuscipennis MacGown & Nebeker and Encarsia formosa Gahan was studied on tomato leaflets with 20 Trialeurodes vaporariorum (Westwood) larvae in the first or third stage. Ten of the whitefly larvae were previously parasitized and contained a conspecific or a heterospecific parasitoid egg or larva. The host type (host stage and/or previous parasitization) did not influence the foraging behavior of either parasitoid species. The residence time on these tomato leaflets was about 0.9 h for A. fuscipennis and 1.9 h for E. formosa. Amitus fuscipennis hardly stood still and fed little, while E. formosa showed extensive standing still and feeding. As a result, the time walking while drumming was similar for both parasitoid species. The numbers of host encounters and ovipositions per leaflet were similar for both parasitoid species. However, the residence time of A. fuscipennis was half as long as that of E. formosa so the rate of encounters and ovipositions was higher for A. fuscipennis. Amitus fuscipennis is more efficient in finding and parasitizing hosts under these conditions. The walking activity and host acceptance of the synovigenic E. formosa diminished with the number of ovipositions, but not those of the proovigenic A. fuscipennis. Encarsia formosa is egg limited, while A. fuscipennis is time limited because of its short life span and high egg load. Both parasitoid species discriminated well between unparasitized larvae and self-parasitized larvae, but discriminated poorly those larvae parasitized by a conspecific and did not discriminate larvae parasitized by a heterospecific. Self-superparasitism, conspecific superparasitism, and multiparasitism were observed for both parasitoid species. Superparasitism always resulted in the emergence of one parasitoid and multiparasitism resulted in a higher emergence of one parasitoid of the species that had parasitized first. The data suggest that A. fuscipennis is a good candidate for use in biological control of high-density spots of T. vaporariorum when we consider its high encounter and oviposition rate.     

Survival of the parasitoidEncarsia formosa after treatment of parasitized greenhouse whitefly larvae with fungal spores ofAschersonia aleyrodis

The interaction between the entomopathogenic fungusAschersonia aleyrodis and the parasitoidEncarsia formosa on greenhouse whitefly as a host organism was studied, in particular, the survival of the parasitoid after treatment of parasitized hosts with fungal spores. The mean number of parasitized black pupae per parasitoid produced at 25°C was significantly reduced after spore treatment in the first three days following parasitization. Spore treatment four, seven or ten days after parasitization resulted in a mean number of parasitized pupae not significantly different from the number of black pupae in the control. The rather sudden change from low to high survival of parasitized hosts when treated with spores four days after parasitization in spite of high numbers of infected unparasitized larvae, coincided with the hatching of the parasitoid larva from the egg inside the host. Possible reasons for this decrease in susceptibility to infection after parasitoid egg hatch, such as induced changes in host cuticle or haemolymph, are discussed. Parasitoids emerged from treated hosts did not show differences in reproduction compared with parasitoids emerging from untreated hosts. Both natural enemeies of whitefly are compatible to a great extent.     

Influence of intrapatch experiences and temperature on the time allocation of the whitefly parasitoidEncarsia formosa (Hymenoptera: Aphelinidae)

The effect of experiences, such as contact with honeydew, rejections of hosts, and ovipositions in hosts, and of temperature on the time allocation of individualEncarsia formosa female parasitoids on tomato leaflets have been studied. Behavioral records were analyzed by means of the proportional hazards model. Analyses were carried out at two levels: (1) the tendency of leaving and (2) the tendency of changing from one leaf side to another. The patch-leaving behavior ofE. formosa can be described by a stochastic threshold mechanism, which is characterized by a certain tendency (probability per time) to leave. The median time from being placed on the leaflet or, if it occurred, from the latest encounter with a host until leaving was 18.6 min. The median time for changing from one leaf side to the other was initially 11.6 min and dropped to 5.7 min after both leaf sides had been visited. The effect of temperature, ranging from 20 to 30°C, was negligible. The presence of honeydew as well as the first oviposition in an unparasitized host decreased the tendency to leave, thus increasing the giving up time (GUT) since the latest encounter with a host. Encounters with parasitized hosts did not affect the GUT since latest encounter; as a result, the total residence time increased. After the first oviposition in an unparasitized host the tendency of changing from the lower leaf side on which hosts were present to the upper side was decreased. The presence of honeydew did not affect the tendency of changing leaf sides.     

Comparison of Searching Behaviour of Two Aphelinid Parasitoids of the Greenhouse Whitefly, Trialeurodes vaporariorum under Summer vs. Winter Conditions in a Temperate Climate

Searching behaviour of two aphelinid parasitoids, Encarsia formosa Gahan and Eretmocerus eremicus Rose and Zolnerowich, was compared in a controlled environment under simulated summer [high light intensity (83 ± 1 W/m²), and 24 ± 1°C] and winter [low light intensity (11 ± 0.5 W/m²), and 20 ± 1°C] greenhouse conditions on tomato leaflets, with and without a single 3rd instar whitefly host, Trialeurodes vaporariorum (Westwood), within a 4-cm tomato leaflet arena. Residence time of both parasitoid species was longer on infested leaflets vs. clean leaflets, and longer under winter than summer conditions. When parasitoids encountered a host on infested leaflets, residence time increased. In all cases, residence time of E. formosa was longer that of E. eremicus. Proportion of time spent searching (i.e. antennating leaf surface while walking or standing still) was longer on clean vs. infested leaflets for both E. formosa and E. eremicus. Walking speed by E. eremicus on clean leaflets was faster than E. formosa under both summer and winter conditions. Host handling time and proportion of host acceptance did not vary among parasitoids. These findings suggest that E. eremicus could be more efficient in host finding on tomato leaflets than E. formosa over all seasons, especially in the winter when natural light is limiting and where daylight temperatures are ≥20°C.     

Temperature dependent functional response ofEncarsia formosa parasitizing the Poinsettia-strain of the cotton whitefly,bemisia tabaci, on Poinsettia

The control efficiency and performance ofEncarsia formosa Gahan (Hymenoptera: Aphelinidae) as influenced by the density of its host, the Poinsettia-strain ofBemisia tabaci Gennadius (Homoptera: Aleyrodidae), were investigated by laboratory experiments on Poinsettia (Euphorbia pulcherrrima Willd.).E. formosa showed a Type II functional response to fourth instar larvae ofB. tabaci, the response plateau increasing with temperature. A response model for randomly searching parasitoids incorporating temperature-dependent handling time and temperature-independent search rate was in accordance with the results, and gave an estimated search rate of 0.033 leaf·hour⁻¹ and handling times of 1.54, 2.86 and 20.1 h at 28°C, 22°C and 16°C, respectively. From the latter, the maximum number of hosts that can be parasitized at the three temperatures are 10.4, 5.6 and 0.8 larvae per day (provided the light period is 16 h). The number of hosts with ovipositor punctures was higher than the number of parasitized hosts, especially at 22°C and 28°C, implying thatE. formosa refrains from laying eggs in some of the hosts examined with the ovipositor. About 31% of the punctured larvae did not contain any eggs. Superparasitism occured during the experiment presumably originating from young, inexperienced parasitoids. Individual larvae were occasionally punctured several times, also by non-superparasitizingE. formosa. The resulting distribution of ovipositior holes was random, indicating thatE. formosa on the basis of antennal testing is unable to determine if a larva has previously been examined with the ovipositor. Almost fifty percent of the punctures were not followed by egg-depositions. Besides parasitizationE. formosa used hosts as food source. The number of hostfed larvae was independent of density, but varied with temperature being highest at 28°C (0.12 hostfed larvae per parasitoid per day).     

Foraging Behavior of the Parasitoid Encarsia formosa on Gerbera jamesonii Leaves

Insight into the foraging behavior of the parasitoid Encarsia formosa Gahan for whitefly hosts was gained by continuous observation of individual parasitoids on leaves of the ornamental plant Gerbera jamesonii, until females left the leaf. Comparison of the parasitoid behavior on three cultivars gave similar results. Mean searching time on uninfested G. jamesonii leaves of three cultivars was 1 h 30 min and the mean percentage of walking activity of the total observation time on those cultivars was 61%. Both parameters were not influenced by different leaf structures of Gerbera cultivars. Encounters with hosts arrested the parasitoids on the leaves. The walking activity and the percentage of host encounters that resulted in an oviposition decreased with decreasing egg load of the parasitoid. In comparison with tomato, where biological control of whiteflies is successful, only minor differences in the foraging behavior occur, except for the residence time of females, which was about three to four times longer on G. jamesonii leaves, but these leaves are about seven times larger than tomato leaves. The facts that (1) the foraging behavior of E. formosa on G. jamesonii is independant of the cultivar and (2) the foraging behavior is, in many aspects, similar to that on tomato suggest that biological control of whitefly on this ornamental plant is a potential option.     

Analysis of Foraging Behaviour of the Whitefly Parasitoid Encarsia formosa on a plant: A Simulation Study

The foraging behaviour of Encarsia formosa was analyzed using a stochastic simulation model of the parasitoid's behaviour. Parasitoids were allowed to search during a day on a tomato plant infested with immatures of the greenhouse whitefly, Trialeurodes vaporariorum. The model simulates searching, host selection, host handling and patch leaving behaviour, and the physiological state of the parasitoid. The outputs of the model are the number of visited leaflets and the number of hosts encountered, parasitized or killed by host feeding. The simulation results agreed well with observations of parasitoids foraging on tomato plants. The number of encounters and ovipositions on the plant increased with host density according to a type II functional response. At a clustered host distribution over leaflets and low host densities, the most important parameters affecting the number of ovipositions were the leaf area, the parasitoid's walking speed and walking activity, the probability of oviposition after encountering a host, the initial egg load and the ratio of search times on both leaf sides. At high densities, the maximum egg load and the giving-up time on a leaflet since latest host encounter were the most essential parameters.     

Whitefly control in cut gerbera: is it possible to control Trialeurodes vaporariorum with Encarsia formosa?

We investigated the impact of inundative releases of the parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphelinidae), for control of greenhouse whitefly, Trialeurodes vaporariorum (Westwood), on cut gerbera (Gerbera jamesonii L.) under controlled greenhouse conditions. Experimental units consisted of ten plants covered and separated from other units by gauze tents. We assessed three release rates of the aphelinid parasitoid: a 7-week experiment with a standard release rate (10 m⁻²/14 days), and a subsequent 3-month trial with high (100 m⁻²/week) and very high (1,000 m⁻²/week) release rates. Experimental units without release of parasitoids served as control treatment. Gerbera plants were infested initially with 50–100 juvenile and 50–70 adult whiteflies in the first experiment, and in the second experiment with less than 50 juveniles per plant and 50–70 adults. Whitefly and parasitoid population density were assessed in weekly intervals using infestation and activity categories. Results show that parasitized whiteflies were present in all treatments within 2 weeks after initial release. Unfortunately, it was not possible to control whiteflies with standard release rates of E. formosa. Although parasitism rates slightly increased, the effect on whitefly populations was negligible. Large amounts of honeydew and growth of sooty mold fungi caused the termination of the first experiment. In a second experiment, E. formosa was tested at 10–100 times higher release densities. In contrast to the first experiment, whitefly densities increased steadily during the first 8 weeks, but remained constant until the end of the experiment in both treatments. Parasitism by E. formosa reached its maximum after 8 weeks. We discuss possible reasons for the low efficiency of E. formosa as a whitefly antagonist in greenhouse production of gerbera.     

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.     

Evidence for a marking pheromone in host discrimination byCephalonomia Stephanoderis (Hym.: Bethylidae)

The bethylidCephalonomia stephanoderis Betrem is an ectoparasitoid that prefers to oviposit on the prepupae and pupae of the coffe berry borerHypothenemus hampei (Ferrari) (Coleoptera: Scolytidae). It has the ability to distinguish unparasitized from parasitized hosts and rarely lays more than one egg per host. The mechanism of this host discrimination byC. stephanoderis was investigated under laboratory conditions. For this, parasitoid eggs that had been deposited on host pupae were removed and pupae were then offered (individually and collectively) to individual female wasps. A total of 92% of individually offered hosts and 93% of collectively offered hosts were not parasitized. It is concluded thatC. stephanoderis recognizes a marking pheromone deposited into or onto the host, preceding, during, or after oviposition which enables female parasitoids to avoid self and conspecific superparasitism.     

Egg-laying experience and acceptance of parasitized hosts by the parasitoid,Leptopilina heterotoma (Hymenoptera: Eucoilidae)

The influence of egg-laying experience on the response of females of the eucoilid parasitoid,Leptopilina heterotoma, to parasitized and unparasitizedDrosophila melanogaster host larvae was examined under more controlled conditions than those used in past studies. In laboratory assays, we precisely manipulated both the number of eggs laid by females and the kind of larvae (parasitized versus unparasitized) in which the eggs were laid. We found that the tendency to avoid laying eggs in parasitized hosts depended markedly on whether or not eggs had been laid previously, but depended little on whether those eggs had been laid in parasitized or unparasitized hosts. The observed effect of general egg-laying experience on avoidance of parasitized hosts may reflect responses to either changes in the wasp's internal state (perhaps, changes in egg load) or changes in the wasp's neural representation of the external environment (such as those presumed to occur during learning). In light of these results, we offer a tentative reinterpretation of several earlier studies.     

Competition between Gyranusoidea tebygi and Anagyrus mangicola, parasitoids of the mango mealybug, Rastrococcus invadens: interspecific host discrimination and larval competition

The competition between Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes (both Hymenoptera: Encyrtidae), exotic parasitoids of the mango mealybug, Rastrococcus invadens Williams (Homoptera: Pseudococcidae) was studied in the laboratory. No significant differences were found in the way each parasitoid species examined, attacked, stung, and oviposited into hosts, unparasitized, or previously parasitized by the other species. This suggests that neither species discriminates against each other. The total number of parasitoids of either species emerging did not significantly differ between competition experiments. When A. mangicola was the first parasitoid to attack a host, it had no significant advantage over G. tebygi. However, when A. mangicola followed G. tebygi by either 4 or 24 h, it clearly won. Overall A. mangicola won the competition in 70.9% of all cases. The level of the competition, either at the egg or larval stage, and factors responsible for the elimination of older larvae by younger ones could not be assessed in these experiments. The coexistence of the two parasitoids as complementary for the biological control of the mango mealybug is discussed.     

Host selection and survival of the parasitoidEncarsia formosa on greenhouse whitefly,Trialeurodes vaporariorum, in the presence of hosts infected with the fungusAschersonia aleyrodis

Successful control of greenhouse whitefly may be achieved by complementary activity of the parasitoidEncarsia formosa and the fungusAschersonia aleyrodis. One way to obtain an additive mortality effect of both entomopathogen and parasitoid would be achieved by the selection of healthy hosts by the parasitoid and rejection of fungus-infected hosts. Third and fourth instar larvae ofTrialeurodes vaporariorum which had been treated with a spore suspension ofA. aleyrodis 0, 4, 7, 10 or 14 days beforehand, were presented to female parasitoids. The parasitoids adopted the oviposition posture on untreated hosts as well as on treated hosts, irrespective of the different stages of infection in the hosts. However, significantly more hosts were parasitized byE. formosa in the control treatment than in the fungal treatment. The parasitoids offered treated hosts, showed rejection behaviour after probing on hosts showing detectable signs of infection (containing hyphal bodies or mycelium in the haemolymph). For instance, when hosts were offered seven days after spore treatment, the parasitoids showed an oviposition posture on a total of 83 (95.4%) out of 87 infected larvae, but laid only 4 eggs (4.6%). In contrast, on 48 (94.1%) out of 51 noninfected (or showing no detectable signs of infection) hosts an oviposition posture was adopted and 40 eggs (78.4%) were found after dissection. When infected hosts were encountered the oviposition posture lasted less than 1′40″ while rejection of non-infected hosts occurred after more than 1′40″. Other experiments were carried out offering treated hosts for 24 h to the parasitoids. The hosts were dissected afterwards. Again, significantly more eggs were laid in the non-infected hosts. When hosts were parasitized shortly after fungal spore treatment they were colonized by the fungus and the parasitoids did not develop. Transmission of the entomopathogen after probing infected hosts was observed to a limited extent. In conclusion,A. aleyrodis andE. formosa can be used together in a glasshouse situation. The parasitoid will be most effective when introduced more than seven days after application ofA. aleyrodis, because from that time onwards it is able to detect and reject fungus-infected hosts.     

Analysis of foraging behavior of the whitefly parasitoidEncarsia formosa (Hymenoptera: Aphelinidae) in an experimental arena: A simulation study

Foraging ofEncarsia formosa was analyzed using a stochastic simulation model of the parasitoid's behavior. Parasitoids were allowed to search during a fixed time in an experimental arena with immatures of the greenhouse whitefly,Trialeurodes vaporariorum. The model simulates host searching, selection, and handling behavior and the physiological state (egg load) of the parasitoid. The simulated number of hosts encountered, parasitized, or killed by host feeding agreed well with observations on leaf disks. The hypothesis of random host encounter seems to be correct. The number of ovipositions on the leaf at low host densities was strongly affected by the parasitoid's walking speed and walking activity, the probability of oviposition after encountering a host, and the initial egg load. At high densities, the initial and maximum egg load were most important. A strong temperature effect was found at 18°C or below. The number of encounters, ovipositions, and host feedings increased with host density to a maximum of 25, 6.5, and 1.5, respectively, during 2 h at 25°C. The shape of the curves resembled a Holling Type II, which may be the result of the experimental procedure, where a parasitoid was confined to a patch during a fixed time.     

The parasitoidOoencyrtus nezarae (Hymenoptera: Encyrtidae) prefers hosts parasitized by conspecifics over unparasitized hosts

Summary Two laboratory experiments were conducted to examine the ovipositional preferences of the egg parasitoidOoencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) for parasitized and unparasitizedMegacopta punctatissimum Montandon (Hemiptera: Plataspidae). Females that had never oviposited or that had not oviposite for 3 days preferred recently parasitized hosts more than unparasitized hosts. The preference for recently parasitized hosts appeared to be mediated by the punctures in already parasitized hosts made by the ovipositor of the first female. Survival of the parasitoid progeny was lower in recently parasitized hosts than in unparasitized hosts. However, handling time of parasitized hosts was extremely short relative to that of unparasitized hosts, because the superparasitizing female could use the punctures made by the previous females. It is concluded that the females preferred the parasitized hosts over unparasitized hosts because the benefit of saving time and energy for drilling was more than the cost of progeny survival.     

Nondevelopment ofCotesia kariyai(Hymenoptera: Braconidae) in Entomopoxvirus-Infected Larvae ofPseudaletia separata(Lepidoptera: Noctuidae)

Interaction between an entomopoxvirus (PsEPV) and a gregarious braconid endoparasitoid,Cotesia kariyai,inPseudaletia separatalarvae showed that infection of larvae with PsEPV was deleterious to the development and survival ofC. kariyai.The survival and development ofC. kariyaiin PsEPV-infectedP. separatalarvae depended on the length of time between parasitization and viral infection. No parasitoid larvae emerged from PsEPV-infected hosts when host larvae were exposed simultaneously to parasitization and PsEPV inoculation whereas more than 80% of the hosts produced parasitoids when PsEPV was administered 5 days postparasitization.C. kariyailarvae in PsEPV-infected hosts showed a retarded development, shrank, and died about 8 days after viral exposure. Virion-free plasma from PsEPV-infectedP. separatalarvae was toxic to the parasitoid larvae even up to a dilution level of 32 when it was injected intrahemocoelically into the host larvae. Development of parasitoids in hosts that were simultaneously parasitized and injected with the virion-free plasm never progressed beyond the egg stage. The parasitizedP. separatalarvae injected with the virion-free plasma did not pupate and died within 30 days after injection.     

Egg Size, Intrinsic Competition, and Lethal Interference in the Parasitoids Encarsia pergandiella and Encarsia formosa

Recent population dynamic theory predicts that disruption of biological control may occur when one parasitoid species' superiority in intrinsic competition is associated with a lower ability to find and exploit hosts (i.e., ability in extrinsic competition). One might expect such a trade-off, for instance, if parasitoids with larger (and fewer) eggs are more likely to prevail in intrinsic competition than species with smaller (and more numerous) eggs. We tested the idea that relative egg size could be used to predict the outcome of intrinsic competition in two closely related endoparasitoids, Encarsia pergandiella Howard and Encarsia formosa Gahan. Contrary to expectation, the parasitoid species with smaller eggs, E. pergandiella, prevailed in intrinsic competition, regardless of the order that hosts were exposed to the two species. In a literature survey, we found four studies of competing pairs of endoparasitoid species for which: (a) egg size estimates were available and (b) one species was consistently superior in intrinsic competition. In three of the four studies, the small-egged species prevailed in intrinsic competition, as we also found. Although E. formosa lost in intrinsic competition, this species negatively affected E. pergandiella's progeny production by host feeding on and killing hosts containing E. pergandiella eggs. E. formosa females also host fed on conspecific-parasitized hosts. As a mechanism of both intra- and interspecific interference competition, host feeding on parasitized hosts contradicts assumptions about the nature of interference competition in existing population dynamics models.     

Interspecific competition between the braconid endoparasitoids Glyptapanteles porthetriae and Glyptapanteles liparidis in Lymantria dispar larvae

The effect of interspecific competition between the solitary endoparasitoid Glyptapanteles porthetriae Muesebeck (Hymenoptera: Braconidae) and the gregarious Glyptapanteles liparidis Bouché (Hymenoptera: Braconidae), was investigated in larvae of Lymantria dispar L. (Lepidoptera: Lymantriidae). Host larvae were parasitized by both wasp species simultaneously in premolt to the 2^nd or the 3^rd host instar or in an additional approach with a 4-day delay in parasitization by the second wasp species. Host acceptance experiments revealed that both wasp species do not discriminate between unparasitized host larvae and larvae parasitized previously by the same or the other species. In more than 90% female wasps parasitized the larva they encountered first. During the period of endoparasitic development, larvae of the competing parasitoid species never attacked the egg stage of the other species. When host larvae were parasitized simultaneously by both wasp species, the rate of successful development of both species depended on the age of the host larva at the time of its parasitization; G. liparidis emerged successfully from 44% of host larvae parasitized during the premolt to 2^nd instar, G. porthetriae from 28%, and in 20% of the hosts both parasitoid species were able to develop in one gypsy moth larva. However, when host larvae were parasitized simultaneously during premolt to the 3^rd instar, G. liparidis was successful in 90% of the hosts, compared to 8% from which only G. porthetriae emerged. In the experiments with delayed oviposition, generally the species that oviposited first succeeded in completing its larval development. Larvae of the species ovipositing with four days delay were frequently attacked and killed by larvae of the first parasitizing species or suffered reduced growth. As the secondary parasitoid species, G. porthetriae-larvae were never able to complete their development, whereas G. liparidis developed successfully in at least 12,5% of the multiparasitized host larvae. Thus, multiparasitism of gypsy moth larvae by both Glyptapanteles species corresponds to the contest type; however, G. porthetriae is only able to develop successfully as the primary parasitoid of young host larvae.     

Differential cannibalism and population dynamics in a host-parasitoid system

The effects of host cannibalism on a host-parasitoid system were explored through experiment and modelling. In individual encounters between parasitized and unparasitized Plodia interpunctella larvae, parasitized larvae were more likely to be cannibalized. Inclusion of this differential cannibalism into a simple Lotka-Volterra-type model of host-parasitoid population dynamics generates alternative stable states-including stable coexistence and extinction of the parasitoid — which depend on starting conditions. Possible mechanisms for differential cannibalism, and its implications for studies of host-parasitoid populations and biological control programmes are discussed.     

Influence of Hairiness ofGerbera jamesoniiLeaves on the Searching Efficiency of the ParasitoidEncarsia formosa

To characterize the relationship between the leaf surface ofGerbera jamesoniiHooker cultivars and the searching behavior of the parasitoidEncarsia formosaGahan on a leaf, the trichome density and shape were described, and the walking behavior was quantified. Leaf hair density varied from 80 to more than 1000 trichomes/cm²and the hair shape varied from single erect trichomes to tapestries of entangled trichomes above the leaf surface. The walking activity, speed, and pattern of the parasitoid were tested on leaves with different hair density and shape. In spite of the large differences in leaf surface structure, the walking activity was similar and around 75% on most cultivars. The walking speed was between 0.2 and 0.3 mm/s on all cultivars and was not significantly different from the speed on tomato. On hairless sweet pepper leaves the walking speed is much higher (0.73 mm/s), so the hairs onGerberaleaves do hamper parasitoid females and result in a strong reduction of the walking speed, but the variation in hair density and shape onGerberadoes not lead to differences in walking activity, pattern, and speed. It appears that the hairs ofGerberaare less of an obstacle forE. formosathan the stiff, large hairs occurring in a high density on cucumber, where the walking speed of the parasitoid is less than 0.2 mm/s. A rise of temperature of 5°C gave a significantly higher walking speed (0.39 mm/s) ofE. formosafemales on all cultivars tested. The relative straightness of the walking track was high and the same on all cultivars. Consequences of the results of the searching behavior ofE. formosaonG. jamesoniicultivars are discussed with respect to other host plants. As parasitoid walking speeds are the same onGerberaand tomato, and whitefly reproduction is also similar on these two host plants, we conclude that biological control of whiteflies onGerberais a realistic option. This conclusion is confirmed by the strong increase of commercial biological control onGerberawhich occurred during the past 5 years.