The effect of two biological control agents, the weevil Neochetina eichhorniae and the mirid Eccritotarsus catarinensis on water hyacinth, Eichhornia crassipes , grown in culture with water lettuce, Pistia stratiotes

We assessed the effect of two biological control agents, the mirid Eccritotarsus catarinensis (Carvalho) and the weevil Neochetina eichhorniae (Warner), singly or in combination, on the competitive ability of their host plant, water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub., grown in a screen house, in competition with another aquatic plant (Pistia stratiotes L.). Water hyacinth plant growth characteristics measured included fresh weight, leaf and petiole lengths, number of inflorescences produced, and new shoots. Without herbivory, water hyacinth was 18 times more competitive than water lettuce (across all experimental combinations of initial plant densities), as estimated from fresh weights. Both insect species, singly or in combination, reduced water hyacinth plant growth characteristics. E. catarinensis alone was less damaging than the weevil and under normal conditions, i.e., floating water hyacinth, is not expected to increase control of water hyacinth beyond that of the weevil. When combined with the weevil, half the inoculum of weevils and half the inoculum of mirids produced the same growth reduction as the full inoculum of the weevil. Under conditions where the weevils are not effective because water hyacinths are seasonally rooted in mud, the mirid, which lives entirely on leaves, should become a useful additional biological control agent. Handling Editor: John Scott.     

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L⁻¹) and phosphorus (0.2–0.01 mg L⁻¹) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L⁻¹ N and 20 mg L⁻¹ P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.     

The Host Range ofEccritotarsus catarinensis(Heteroptera: Miridae), a Potential Agent for the Biological Control of Waterhyacinth (Eichhornia crassipes)

The host range ofEccritotarsus catarinensiswas determined using 33 plant species to assess the risk of using this insect, a native of South America, for the classical biological control of waterhyacinth (Eichhornia crassipes) in Australia and Papua New Guinea. The results, in conjunction with the results of Hillet al.(1999) who tested 67 species (mostly South African), strongly suggest thatE. catarinensisis restricted to the Pontederiaceae, a family of aquatic plants. All five species of Pontederiaceae in the Australian testing,E. crassipes, Pontederia cordata, Monochoria vaginalis, M. cyanea,andM. australasica,were suitable for insect development. Colonies persisted for at least four generations onE. crassipes, P. cordata,andM. vaginalis.Two-way choice and multiple-choice preference trials were conducted and discussed.E. catarinensisdid not exhibit a clear preference for waterhyacinth over other Pontederiaceae in these trials. Most oviposition occurred into the upper surface of the lamina during laboratory testing despite observations that the underside was preferred in the field. Although not considered suitable for release in Australia, this insect may be useful in other countries where more serious waterhyacinth problems occur and whereM. vaginalisis a serious weed, such as in Southeast Asia.     

Importation,releases, and establishment ofNeochetina spp. (Col.: Curculionidae) for the biological control of water hyacinth,Ecchhornia crassipes (Lil.: Pontederiaceae), in Benin,West Africa

Water hyacinth,Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) was first reported in Bénin in 1977 and about 10 years later became the major floating water weed in the south east, obstructing boat traffic and fisheries. Water hyacinth multiplies in permanently fresh water in the swampy upper reaches of the Sô River and in tributaries of the Ouémé River. From there it is moved by wind and water flow to the coastal lagoons. The coastal lagoons are brackish during the dry season and water hyacinth eventually dies. In 1991,Neochetina eichhorniae (Warner) (Col.: Curculionidae) of South American origin was imported from Australia via quarantine in Britain to Bénin. A small infestation of the fungusBeauveria bassiana (Bals.) Vuill. (Hyphomycetes) was eliminated from the colony before release by sterilizing eggs and rearing a fungus-free generation. Between late 1991 and mid 1993, about 23,900N. eichhorniae were released at 11 localities along the Ouémé River and in the head waters of the Sô River. Regular monitoring revealed feeding scars by adults on leaves and tunnelling by larvae in petioles at all release sites. By October 1993,N. eichhorniae had spread up to 20 km from some release sites.Neochetina bruchi Hustache was imported in 1992. A total of about 5,700 weevils has been released in six localities since mid 1992. Recoveries of offspring were made in all but one locality. Despite the negative impact of water flow, wind, penetration of salt water, and removal of infested water hyacinth by fishermen,N. eichhorniae andN. bruchi are established in Bénin in a situation typical for coastal West Africa.     

Genetic diversity of introduced populations of the water hyacinth biological control agent Eccritotarsus catarinensis (Hemiptera: Miridae)

Genetic bottlenecks can be deleterious to populations. In biological control, agent populations may be subject to severe bottlenecks during selection, importation and while in culture. The genetic variability of two collections of the water hyacinth biological control agent Eccritotarsus catarinensis Carvalho (Hemiptera: Miridae) was measured using Inter-simple Sequence Repeats (ISSR) and mtDNA cytochrome oxidase I (COI) sequences. The first collection (Brazilian) went through a bottleneck of a single gravid female, while the second collection (Peru) originated from 1000 individuals and has been maintained at a large size in culture. Two naturalised South African populations from the Brazilian collection were also sampled (Nseleni and Mbozambo). Polymorphism for ISSR was high in the Peruvian and two naturalised samples, but much less so in the Brazilian sample. The Peruvian population was shown to be highly differentiated from the Brazilian and its naturalised populations by high values of F_ST and Nei’s genetic distance, as well as in a Multidimensional Scaling (MDS) plot and an unrooted neighbour joining tree derived from Jaccard’s coefficient of similarity. In addition, sequencing of the COI region of the mitochondrial DNA revealed only two haplotypes, one Brazilian and one Peruvian, with a 5.2% sequence divergence, suggesting that recombination and not mutation is the cause of most variation in the ISSR regions. The results suggest that substantial genetic variation may be retained or recovered after a bottleneck. This may mitigate deleterious effects that are a concern for the fate of biological control agents after release.     

Leaf scarring by the weevils Neochetina eichhorniae and N. bruchi enhances infection by the fungus Cercospora piaropi on waterhyacinth, Eichhornia crassipes

Additive or synergistic effects among introduced and native insect and plant pathogen agents are necessary to achieve biological control of waterhyacinth (Eichhornia crassipes), a globally damaging aquatic weed. In field plots, plants were infested with waterhyacinth weevils (Neoechetina bruchi and N. eichhorniae) and leaves were scarred by weevil feeding. Subsequent infection by the fungal pathogen Cercospora piaropi caused necrotic lesions to form on leaves. Necrosis development was 7.5- and 10.5-fold greater in plots augmented with both weevils and C. piaropi and weevils alone, respectively, than in plots receiving only C. piaropi. Twenty-four days after weevil infestation, the percentage of laminar area covered by lesions on third-youngest and oldest live leaves was elevated 2.3–2.5-fold in plots augmented with weevils. Scar density and necrosis coverage on young leaf laminae were positively correlated, even though antipathogenic soluble peroxidases were elevated 3-fold in plots augmented with weevils alone or weevils and C. piaropi. Combined weevil and fungal augmentation decreased shoot densities and leaves per plant. In a no-choice bioassay, weevil feeding on oldest but not young leaves was reduced 44 two weeks after C. piaropi inoculation. Protein content and peroxidase activities were elevated 2–6-fold in oldest leaves three weeks after inoculation. Augmentation with both waterhyacinth weevils and C. piaropi led to the development of an additive biological control impact, mediated by one or more direct interactions between these agents, and not plant quality effects.     

Life History and Laboratory Host Range ofEccritotarsus catarinensis(Carvalho) (Heteroptera: Miridae), a New Natural Enemy Released on Water Hyacinth (Eichhornia crassipes(Mart.) Solms-Laub.) (Pontederiaceae) in South Africa

A mirid,Eccritotarsus catarinensis(Carvalho), was studied as a potentially damaging natural enemy for water hyacinth, (Eichhornia crassipes(Mart.) Solms-Laub.), in South Africa. In the laboratory, eggs were inserted into the leaf tissue parallel to the leaf surface. The four nymphal instars fed gregariously with the adults mainly on the undersurface of the leaves, causing severe chlorosis at high population levels. The duration of immature stages (egg and nymphs) was approximately 23 days, while the adults survived for approximately 50 days. Favorable biological characteristics ofE. catarinensisincluded a high rate of increase, gregarious habits, long-lived and mobile adults, and several generations per year. Laboratory host range of the mirid was determined by adult choice trials on 67 plant species in 36 families and adult no-choice trials on five species in the Pontederiaceae. Feeding was recorded on all Pontederiaceae tested and oviposition on four of the five species. However, these plant species proved to be inferior hosts forE. catarinensisin comparison to water hyacinth, suggesting thatE. catarinensiswould be an acceptable natural enemy for water hyacinth in South Africa.     

Performance and impact of the biological control agent Xubida infusella (Lepidoptera; Pyralidae) on the target weed Eichhornia crassipes (waterhyacinth) and on a non-target plant, Pontederia cordata (pickerelweed) in two nutrient regimes

Xubida infusella (Walker) (Lepidoptera: Pyralidae) is potentially a useful biological control agent targeting Eichhornia crassipes (waterhyacinth) in the USA but many regions infested with waterhyacinth are also inhabited by an alternative native host, Pontederia cordata (pickerelweed). Experiments were conducted in Australia to assess the impact of X. infusella on pickerelweed compared to waterhyacinth where both these plants were available and X. infusella had already been released. Overall X. infusella had a greater impact on pickerelweed than on waterhyacinth. More than one larva per plant was required to reduce the total shoot dry weight of waterhyacinth but only one larva per plant reduced the total shoot dry weight of pickerelweed. Insect feeding caused the number of secondary shoots (daughter plants) of pickerelweed to double whereas the number of daughter plants produced by waterhyacinth remained unchanged. We suggest this indicates a considerable impact on pickerelweed rather than effective compensation for insect damage because the shoots produced were very small. Waterhyacinth produced a constant number of daughter plants when fed on by up to three larvae per plant. Higher nitrogen status of both species of host plant increased the rate of larval development and pupal weight of X. infusella. The weight and fecundity of X. infusella reared on pickerelweed were lower than those reared on waterhyacinth but large numbers of progeny were produced on both plant species. This experiment demonstrates a considerable impact of X. infusella on pickerelweed suggesting this plant is at risk from this agent if released in the USA where pickerelweed is present. The considerable impact on waterhyacinth demonstrates the potential for this insect to contribute to waterhyacinth control in countries where risk assessment favours release.     

Abiotic and microbial decomposition of pre- and post-bloom leaves of water hyacinth (Eichhornia crassipes (mart.) Solms)

Rates of abiotic and microbial decomposition in pre- and post-bloom leaves of water hyacinth are determined under laboratory conditions. Decomposition in all types of hyacinth leaves is dominated by physical leaching in an initial phase of 4 days duration, and later by microbial processes. The largest part of physical leaching takes place within the first 4 h. Thereafter, the weight loss due to physical leaching declines exponentially. The weight loss by microbial decomposition is minimal in the initial phase but increases exponentially in the later phase. Pre-bloom leaves decompose significantly faster than post-bloom leaves, and post-bloom green leaves decompose faster than post-bloom brown leaves. The rate constants of abiotic decomposition are significantly higher in post-bloom leaves as compared with pre-bloom leaves, while microbial decomposition is significantly higher in pre-bloom leaves. After 30 days, the dry mass loss by abiotic and microbial decomposition is 15% and 55%, respectively, in pre-bloom leaves, 33% and 19% in post-bloom green leaves, and 24% and 6% in post-bloom brown leaves.     

Mycobiota associated with larval mines of Thrypticus truncatus and T. sagittatus (Diptera, Dolichopodidae) on waterhyacinth, Eichhornia crassipes, in Argentina

Thrypticus truncatus is a candidate agent for biocontrol of waterhyacinth; the larvae of this diptera mine in the petioles and feed on the phloem in the vascular bundles. The mycobiota associated with T. truncatus and T. sagittatus mines was investigated during two surveys undertaken in the spring and autumn in the Delta of the Paraná River, Argentina. Isolations were made from the mines and larval feeding points, as well as from the larvae, following dissection of the petioles, and plated onto agar. Young and upper parts of the petioles without Thrypticus mines were used as controls. Twenty eight fungal species were isolated from the mines. Pestalotiopsis guepinii, Mucor attenuatus, Phoma tropica, Achlya americana, Fusarium avenaceum, Cladosporium cladosporioides, Clonostachys rosea, Epicoccum purpurascens, Plectosporium tabacinum, Alternaria alternata, and Acremonium sp. were the most common fungi associated with mines and feeding points. Cladosporium cladosporioides, Cytospora sp., Mucor attenuatus, and Phoma tropica were associated with the larval body. The list of fungi in mines was compared with bibliographic information to determine if the species are known pathogens on waterhyacinth or other plant species. This is the first study on mycobiota associated with T. truncatus and T. sagittatus mines in waterhyacinth petioles in Argentina.     

Impact of Apion ulicis Forster on Ulex europaeus L. Seed Dispersal

The impact of the weevil Apion ulicis on the dispersal and quality of gorse (Ulex europaeus) seeds was examined in southern Chile in 1991 and 1992. Seed dispersal was assessed by counting the number of seeds captured by rows of seed traps placed at 0.7, 1.4, and 2.8 m from 12 weevil-infested and 12 weevil-free gorse plants arranged in a split-plot randomized complete block design. Seed weight and germinability were also measured. The influence of gorse pod location on host utilization by the weevil and the hypothetical subsequent change on seed dispersal were also studied. Gorse seed production and dispersal were significantly (P ≤ 0.001) decreased by A. ulicis in 1991 but not in 1992. Individual seed weight and germinability were not adversely affected by weevil infestation. Pod location did not affect host utilization by the insect, nor did it influence seed dispersal or quality.     

Use of water hyacinths as shelter,foraging place,and transport by young piranhas,Serrasalmus spilopleura

Synopsis The water hyacinth, Eichhornia crassipes, plays an important role in the early life of the piranha, Serrasalmus spilopleura in southeastern Brazil. Larvae and early juveniles are found by both day and night among the roots of this free floating waterweed, thus gaining shelter, a rich foraging place, and potential rafting dispersal. Piranha larvae up to 19 mm SL feed mainly on small aquatic arthropods, slowly searched for inside the root tangle; larger juveniles tend to leave the plants and patrol more open areas. At 24 mm SL young piranhas begin to clip out pieces from fins of other fishes and seek shelter in water hyacinths only at night. About 30% of the rafting clumps of water hyacinths may harbour one to three piranha larvae, providing dispersal during floods.     

Impact of Crown Rust (Puccinia coronata f. sp. avenae) on Competitive Interactions between Wild Oats (Avena fatua) and Stipa (Nassella pulchra)

Crown rust of oats (Puccinia coronata f. sp. avenae) was investigated as a biocontrol agent for wild oats (Avena fatua) on San Clemente Island, California. Successful restoration of the native habitats of this island will involve the reduction of wild oats and revegetation with native grasses such as stipa (Nassella pulchra). Determination of the outcome of interference between wild oats and stipa is important in the prediction of the success of the biocontrol agent. An addition series design was used to investigate these interactions, with densities of each species ranging from 0 to 2000 seeds per m². Eight replicates were established, four of which were randomly chosen and infected with the pathogen. After 75 days, plant height, dry weight, and seed production were measured. The data were fit to a hyperbola surface model by use of a nonlinear regression procedure. Results indicate that wild oat is the superior competitor at the seedling stage; however, early rust infection greatly reduces fitness of wild oats, causing an increased fitness for stipa. Simulations with a plant community model constructed from the results of the greenhouse experiment and information in the literature indicated that an equilibrium may be established between wild oat and stipa if high initial seeding rates of stipa are used in revegetation.     

Isoenzyme patterns and biology ofTrichogramma minutum as influenced by rearing temperature and host

Females ofTrichogramma minutum Riley reared at temperatures below 17°C had lower percent emergence, lower fecundity on the first day of emergence, and a reduced rate of population increase (r_i) than those reared at temperatures above 17°C. Parasitoids were more fecund and lived longer when raised onChoristoneura fumiferana (Clemens) compared toEphestia kuehniella (Zeller). The parasitoids performed better when parasitizing the same species of host on which they had been reared. Isoenzyme patterns were not affected whenT. minutum was reared at 13 or 27°C but additional bands appeared when parasitoids were reared onSitotroga cerealella (Olivier) compared toC. fumiferana. No correlation could be made between biological and biochemical changes inT. minutum. Isoenzymes have the potential to assess the quality of parasitoids during mass production.

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Einfluss von Zuchttemperatur und Wirt auf Isoenzymmuster und Biologie vonTrichogramma minutum Riley

Zusammenfassung Weibchen vonTrichogramma minutum, welche bei 17°C gezüchtet wurden, hatten eine geringere Schlüpfrate, eine geringere Fruchtbarkeit am ersten Tag nach dem Schlüpfen sowie ein reduziertes Populationswachstum (r_i) im Vergleich zu Tieren welche sich bei Temperaturen über 17°C entwickelt hatten. Wurden die Parasitoiden aufChoristoneura fumiferana gezogen, so war ihre Fruchtbarkeitsrate wie auch Lebensdauer grösser als wennEphestia kuehniella als Wirt diente. Die parasitische Fähigkeit vonT. minutum war immer auf derjenigen Wirtsart am grössten auf der die Schlupfwespen vorher gezogen wurden. Aufzuchttemperaturen von 13°C oder 27°C hatten keinen Einfluss auf den Isoenzymbändern wenn die Tiere statt aufC. fumiferana aufSitotroga cerealella gezogen wurden. Es konnte aber kein Zusammenhang zwischen den biologischen und biochemischen Veränderungen gefunden werden. Hingegen besteht die Möglichkeit während der Massenproduktion die Qualität der Parasitoiden anhand von Isoenzymprofilen zu überwachen.

     

Competitive Interactions between Hydrilla (Hydrilla verticillata) and Vallisneria (Vallisneria americana) as Influenced by Insect Herbivory

Two experiments (winter and summer) were conducted in outdoor tanks using addition-series methods to evaluate the impact of specialized feeding by two biological control agents,Hydrellia pakistanaeDeonier andBagous hydrillaeO'Brien, on competitive interactions between hydrilla [Hydrilla verticillata(L.f.) Royle] and vallisneria (Vallisneria americanaMichx). Competitive abilities of each plant species were determined using the reciprocal-yield model of mean plant weight. In the absence of the biocontrol agents, intraspecific competition from hydrilla on itself was 8.3 times stronger than interspecific competition from vallisneria.Hydrellia pakistanaeinterfered with hydrilla canopy formation by removing as much as 80% of the plant biomass in the top 30 cm of the water column. Damage byH. pakistanaealso caused a 43% reduction in hydrilla tuber production during the winter experiment. Similarly,B. hydrillaecaused up to a 48% reduction in hydrilla plant weight in the summer experiment. Neither insect species damaged vallisneria. As a result, there were significant shifts in the competitive balance between hydrilla and vallisneria due to selective insect feedings. In the presence ofH. pakistanae, hydrilla intraspecific competition was nearly equal to interspecific competition from vallisneria, indicating that hydrilla had lost its competitive edge over vallisneria.Bagous hydrillaealso produced similar, but smaller, shifts in the relative competitive abilities of hydrilla and vallisneria. These results indicate that biological control agents can disrupt the competitive balance between plant species in favor of native species, thus adding another element to the weed biological control strategies.     

Establishment and dispersal of the biological control weevil Rhinoncomimus latipes on mile-a-minute weed, Persicaria perfoliata

Mile-a-minute weed, Persicaria perfoliata (L.) H. Gross (Polygonaceae), is an annual vine from Asia that has invaded the eastern US where it can form dense monocultures and outcompete other vegetation in a variety of habitats. The host-specific Asian weevil Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae) was first released in the US in 2004 as part of a classical biological control program. The weevil was intensively monitored in three release arrays over 4 years, and field cages at each site were used to determine the number of generations produced. The weevil established at all three sites and produced three to four generations before entering a reproductive diapause in late summer. Weevils dispersed at an average rate of 1.5–2.9 m wk⁻¹ through the 50 m diameter arrays, which had fairly contiguous mile-a-minute cover. Weevils dispersing in the broader, more variable landscape located both large monocultures and small isolated patches of mile-a-minute 600–760 m from the release within 14 months. Weevil density ranged from fewer than 10 to nearly 200 weevils m⁻² mile-a-minute weed. Mile-a-minute cover decreased at the site with the highest weevil density. The production of P. perfoliata seed clusters decreased with increasing weevil populations at two sites, and seedling production declined over time at two sites by 75% and 87%. The ability of the weevil to establish, produce multiple generations per season, disperse to new patches, and likelihood of having an impact on plants in the field suggests that R. latipes has the potential to be a successful biological control agent.     

Establishment ofNeltumius arizonensis(Coleoptera: Bruchidae) on Mesquite (ProsopisSpecies: Mimosaceae) in South Africa

Species ofProsopis(Mimosaceae), or mesquites, are invasive rangeland weeds in South Africa's Western Cape and Northern Cape Provinces. Two bruchid seed-weevil species,Algarobius prosopis(Le Conte) andA. bottimeriKingsolver, were released for biological control in 1987 and 1990, respectively. Seed-feeding biocontrol agents were selected because mesquite pods are valued as livestock fodder. Livestock grazing of bruchid larvae developing in mesquite seeds, however, limits the effectiveness of these agents. Livestock grazing also exacerbates mesquite infestations because scarified seeds are dispersed widely in vertebrate dung. In response to the livestock grazing problem,Neltumius arizonensis(Schaeffer), a bruchid reputed to be capable of ovipositing on immature, tree-borne pods, was released at three sites in Western Cape Province in 1993 and 1994. Small populations ofN. arizonensishave become established at the release sites. Overall,N. arizonensiswas 18 times less abundant thanA. prosopis.In some monthsN. arizonensiseggs were heavily parasitized byUscanasp. (Trichogrammatidae), but the effect of this onN. arizonensispopulation dynamics is uncertain. Western CapeN. arizonensispopulations need more time to increase in size. The introduction of other, more injurious biocontrol agents such as the cecidomyiid bud feederAsphondylia prosopidisCockerell should be considered.     

A comparative study on the intrinsic rates of increase ofCyrtobagous singularis andC. salviniae on the water weedSalvinia molesta

The intrinsic rates of increase (r_m) ofCyrtobagous salviniae Calder & Sands from Brazil andC. singularis Hustache from Trinidad W.I., were determined in the laboratory at 23°C, 27°C and 31°C on nitrogen-rich plants of the aquatic weed,Salvinia molesta Mitchell.Variation in oviposition and immature survivorship accounted for most of the differences between species in r_m values (exponential growth of a stable-age population in a non-limiting environment). Values for r_m were higher forC. salviniae (0.210, 0.366, 0.404) than forC. singularis (0.148, 0.140, 0.064) at the three temperatures respectively. At all temperatures,C. salviniae laid seven times more eggs thanC. singularis while at 31°C oviposition was reduced for both species by 45%, and was accompanied by a reduction in egg hatch. Oviposition byC. salviniae was almost continuous (92% of weeks with some eggs laid) whereas oviposition byC. singularis was intermittent (50% of weeks) with intervals averaging 3 weeks without oviposition. Nitrogen concentration inS. molesta affected reproduction byC. singularis more thanC. salviniae, an increase of 0.1% (dry wt) increasing weekly oviposition by 7.0% and 3.6% respectively.The differences in r_m for the two weevil species are discussed in relationship to their potential as biological control agents.

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Taux intrinsèques d'accroissement deCyrtobagous singularies Hustache andC. salviniae Calder & Sands, utilsés dans la lutte biologique contreSalvinia molesta Mitchell

Résumé Les taux intrinsèques d'accroissement (r_m) deC. salviniae du Bresil et deC. singularis de Trinidad, ont été établis au laboratoire à 23°C; 27°C; 31°C, sur des plants deS. molesta riches en azote.Les différences entre les r_m des deux espèces provenaient pour l'essentiel de la ponte et de la mortalité préimaginale. Aux 3 températures, les valeurs de r_m deC. salviniae (0.210; 0.366; 0.404) étaient supérieures à celles deC. singularis (0.148; 0.140; 0.064). A toutes les températuresC. salviniae a pondu 7 fois plus d'oeufs queC. salviniae; à 31°C, la ponte a été réduite de 45% pour les 2 espèces et été accompagnée d'une diminution des taux d'éclosion. La ponte deC. salvinae était presque continue (92% des semaines avec des oeufs), tandis que celle deC. singularis était intermittent (50% des semaines avec pontes), les interruptions étant en moyenne de 2 semaines. La teneur deS. molesta en azote a affecté la reproduction deC. singularis plus que celle deC. salviniae; un accroissement de 0.1% en poids sec, augmentant les pontes hebdomadaires respectivement de 7% et de 3.6%.Les différences de valeur de r_m des 2 espèces sont examinées pour évaluer leurs potentialités comme éléments de la lutte biologique.

     

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.     

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.