Spécificité des hyphomycètes entomopathogènes pour les larves de lepidoptèresNoctuidae

Résumé L'étude de la sensibilité des 4 espèces de noctuelles,Mamestra brassicae L.,Spodoptera littoralis Boisduval,Spodoptera frugiperda Smith etScotia ipsilon Hübner, aux hyphomycètes a été conduite avec 10 souches dePaecilomyces fumoro-roseus (Wize) Brown & Smith, 6 deNomuraea rileyi (Farlow) Samson, 1 deBeauveria bassiana (Bals.) Vuill. et 1 deMetarhizium anisopliae (Metsch.) Sor. Les chenilles nouvelles-nées agées de moins de 16 h ont été exposées pendant 24 h sur des rondelles de feuilles contaminées par aspersion des inoculums, puis élevées individuellement à 25±1°C. A la dose de 3.10⁵ conidiospores/cm², l'activité pathogène des germes sur les chenilles varie dans de larges proportions suivant l'espèce-h?te et suivant l'isolat considérés. L'espèce la plus sensible à la plupart des souches estM. brassicae et la plus résistanteS. littoralis. Les germes les plus performants présentent des temps létaux 50% compris entre 2 et 3,5 j (à la dose de 3.10⁵ spores/cm²) et des doses létales 50% de 10³ à 10⁴ conidiospores/cm² (mortalités cumulées le 6e j après traitement); ce sont les isolats:P. fumoso-roseus n^o 13, 32, 39, 40 et 45 pourM. brassicae, N. rileyi n^o 5 pourS. littoralis; P. fumoso-roseus n^o 32 pourS. frugiperda etM. anisopliae n^o 139 pourS. ipsilon. La virulence élevée du pathotypeN. rileyi n^o 5 pourS. littoralis et son agressivité à l'égard des 3 autres noctuelles, confirment l'intérêt de cet hyphomycète en dehors de son aire d'origine (Sud-Est des Etats-Unis). Par ailleurs, les aptitudes des souches européennes deP. fumoso-roseus montrent que cette espèce présente des potentialités pour la lutte biologique contre divers noctuides.

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Summary Bioassays were carried out on 1st instar larvae to determine the relative susceptibility of 4 noctuid species,Mamestra brassicae L.,Spodoptera littoralis Boisduval,Spodoptera exigua Smith andScotia ipsilon Hübner to 10Paecilomyces fumoso-roseus, 6Nomuraea rileyi, 1Metarhizium anisopliae and 1Beauveria bassiana isolates. Neonate larvae, 2 to 16 h old, were exposed for 24 h to leaf pieces treated with conidial suspensions of each fungal strain and then transferred to untreated foliage at 25±1°C. When tested at 3×10⁵ conidia/cm², pathogenic activity of hyphomycetes towards noctuids showed high variability depending on both host-species and fungal isolate.M. brassicae was more susceptible to tested isolates than the other 3 noctuids. In contrast,S. littoralis was the most resistant species. The most active pathotypes had LT 50 values ranging from 2 to 3,5 days at the concentration of 3×10⁵ conidia/cm² and LD 50 values ranging from 10³ to 10⁴ conidia/cm² (mortality recorded at 6 days post exposure). According to these data, there were 5P. fumoso-roseus isolates (nos 13, 32, 39, 40 and 45) highly active againstM. brassicae, oneN. rileyi isolate (n^o 5) againstS. littoralis, oneP. fumoso-roseus isolate (n^o 32) againstS. frugiperda and oneM. anisopliae isolate (n^o 139) againstS. ipsilon. Bioassays establishing the high virulence of the FloridaN. rileyi isolate (n^o 5) to the Egyptian cottonworm and its pathogenic activity against the other tested noctuids, confirmed the interest for this fungal entomopathogen which has been demonstrated on noctuid populations in the southeastern United States. Moreover results of the host range assays of European isolates ofP. fumoso-roseus showed that this species appears to be an other potentially valuable microbial control agent against noctuid pests.

     

Isolation and characterization of plasma membrane-associated cortical granules from sea urchin eggs

Cortical granules, which are specialized secretory organelles found in ova of many organisms, have been isolated from the eggs of the sea urchins Arbacia punctulata and Strongylocentrtus pupuratus by a simple, rapid procedure. Electron micropscope examination of cortical granules prepared by this procedure reveals that they are tightly attached to large segments of the plasma membrane and its associated vitelline layer. Further evidence that he cortical granules were associated with these cell surface layers was obtained by (125)I-labeling techniques. The cortical granule preparations were found to be rich in proteoesterase, which was purified 32-fold over that detected in a crude homogenate. Similarly, the specific radioactivity of a (125)I-labeled, surface glycoprotein was increased 40-fold. These facts, coupled with electron microscope observations, indicate the isolation procedure yields a preparation in which both the cortical granules and the plasma membrane-vitelline layer are purified to the same extent. Gel electrophoresis of the membrane-associated cortical granule preparation reveals the presence of at least eight polypeptides. The major polypeptide, which is a glycotprotein of apparent mol wt of 100,000, contains most of the radioactivity introduced by (125)I-labeling of the intact eggs. Lysis of the cortical granules is observed under hypotonic conditions, or under isotonic conditions if Ca(2+) ion is present. When lysis is under isotonic conditions is induced by addition of Ca(2+) ion, the electron-dense contents of the granules remain insoluble. In contrast, hypotonic lysis results in release of the contents of the granule in a soluble form. However, in both cases the (125)I-labeled glycoprotein remains insoluble, presumably because it is a component of either the plasma membrane or the vitelline layer. All these findings indicate that, using this purified preparation, it should be possible to carry out in vitro studies to better define some of the initial, surface-related events observed in vivo upon fertilization.     

Bioecology of some key cashew insect pests and diseases in diverse habitats and landscapes in Tanzania

Cashew (Anacardium occidentale) is an economically important cash crop for many rural households in Tanzania. However, its production is constrained by some insect pests and diseases. As a prerequisite for the development of a more sustainable integrated insect pest and disease management strategy for cashew, information on the biology and ecology of the key insect pests and diseases in a changing environment, and on influencing biotic and abiotic factors, is needed. Surveys were conducted in the major cashew nut‐producing areas of Tanzania for two seasons: August to December, 2009, and August to December, 2010. Data on number of infested and infected shoots by key insect pests and diseases, natural enemies and associated farmer practices, namely synthetic pesticide use and intercropping systems, were collected from different subzones within agroecological zones. Our data showed that abundance and diversity of key cashew insect pests and diseases were influenced by agroecological zones and subzones. Intercropping was more commonly practised in the northern than in the southern zone. Agrochemicals were most frequently used in the southern agroecological zone and affected the occurrence of natural enemies, notably the weaver ant that was more abundant in the northern zone. Furthermore, our findings revealed that Helopeltis sp. and the powdery mildew remained the major constraints to cashew nut production in Tanzania.     

A Device for Infecting Adult Tsetse Flies,Glossinaspp., with an Entomopathogenic Fungus in the Field

Various chamber designs for infecting natural populations ofGlossina pallidipes, G. longipennis,andG. fuscipes fuscipeswith the entomopathogenic fungusMetarhizium anisopliaewere tested in the field. All three species of tsetse flies entered the chambers and became infected with the fungus. Mortality attributed to infection byM. anisopliaeranged from 0 to 76% forG. pallidipes/G. longipennisand from 0 to 80% forG. fuscipes.One design proved to be more efficient than the others in permitting the passage of flies and contaminating them with fungal conidia. Dry conidia ofM. anisopliaein the infection chamber retained their infectivity for more than 21 days in the field.     

Efficacy of Metarhizium anisopliae in controlling the two‐spotted spider mite Tetranychus urticae on common bean in screenhouse and field experiments

The efficacy of aqueous and emulsifiable formulations of the fungus Metarhizium anisopliae isolate ICIPE78 was evaluated on the population density of Tetranychus urticae infesting common bean plants under screenhouse and field conditions. Synthetic acaricide abamectin was included as a check. Bean plants were artificially infested with T. urticae and allowed to multiply. Three treatments were applied in the screenhouse and 1 treatment in field trials. Mite density was recorded 2 d before spraying and weekly postspraying. The number of pods per plant, number of seeds per pod, and the dry weight of seeds per plant were recorded only in the screenhouse trials. In both screenhouse and field trials, fungal formulations applied at the concentration of 10⁸ conidia/mL and the acaricide reduced the population density of mites as compared to the controls. There were significant differences in T. urticae population densities between the treatments at the various post‐spraying sampling dates. In the screenhouse, the mite densities were near zero from 3‐week postspraying in the treated leaves. At 4‐week postspraying, there were no more leaves in the untreated control (T1) and in the control water + Silwet‐L77 (T2). Fungal formulations were as effective as abamectin in reducing mite densities in both screenhouse and field experiments. There were significant differences in the production parameters during the 2 screenhouse trials, with fungal and abamectin treatments generally having the highest yield. Results of this study underline the potential of the M. anisopliae isolate ICIPE78 as an alternative to acaricides for T. urticae management.     

Effect of temperature on virulence of Beauveria bassiana and Metarhizium anisopliae isolates to Tetranychus evansi

The virulence of three isolates of Beauveria bassiana (Bals.) Vuill. and 23 isolates of Metarhizium anisopliae (Metschnik.) Sorok. (Ascomycota: Hypocreales) against the tomato spider mite, Tetranychus evansi Baker and Pritchard (Acari: Tetranychidae), was assessed in the laboratory. The effect of temperature on germination, radial growth and virulence of selected isolates (two isolates of B. bassiana and nine of M. anisopliae) on T. evansi was also investigated in the laboratory. All the fungal isolates tested were pathogenic to the adult females of T. evansi, and there were significant differences in mortality between fungal isolates. The lethal time to 50% mortality (LT(50)) values ranged from 4.2 to 8.1 days and the LT(90) values from 5.6 to 15.1 days. Temperature had significant effects on germination, radial growth and virulence of the various isolates. The best fungal germination was observed at 25 and 30 degrees C, while for the fungal radial growth it was 30 degrees C. All the isolates germinated and grew at all temperatures, but germination and radial growth varied with isolate and temperature. The selected isolates were all virulent to T. evansi, but virulence varied also with isolate and temperature.     

Mortality in Three African Tephritid Fruit Fly Puparia and Adults Caused by the Entomopathogenic Fungi, Metarhizium anisopliae and Beauveria bassiana

The pathogenicity of 13 isolates of Metarhizium anisopliae and two isolates of Beauveria bassiana to Ceratitis capitata and Ceratitis var. rosa fasciventris exposed as late third instar larvae in sand was evaluated in the laboratory. All isolates caused a significant reduction in adult emergence and a corresponding large mortality on puparia of both species. All isolates also induced large deferred mortality in emerging adults following treatment as late third instar larvae. On C. capitata , seven isolates ( M. anisopliae ICIPE 18, 20, 32, 60 and 69 and B. bassiana ICIPE 44 and 82) caused significantly higher mortality on puparia than other isolates. With the exception of ICIPE 32, the other four isolates of M. anisopliae above were the most pathogenic against C. r. fasciventris . Dose-response study carried out with these isolates of M. anisopliae on the two species of flies above plus another species, Ceratitis cosyra showed that the dose-mortality regression lines of ICIPE 18 and 20 were steeper with lower LC 50 values when compared with ICIPE 60 and 69 on the three species. When these two isolates were evaluated with regard to their pathogenicity to different pupal age, adult emergence was found to increase with increasing pupal age with a corresponding decrease in mortality in puparia and emerging adults in the three species of fruit flies. M. anisopliae ICIPE 18 and 20 were equally pathogenic to all pupal ages tested in C. capitata and C. cosyra but ICIPE 18 was more pathogenic to older puparia of C. r. fasciventris than ICIPE 20. Our results suggest that soil inoculation with M. anisopliae under mango trees might form an important component of integrated pest management strategies in areas where these three species of fruit fly coexist.     

Active aggregation among sexes in bean flower thrips (Megalurothrips sjostedti) on cowpea (Vigna unguiculata)

Male sexual aggregations are a common territorial, mating‐related or resource‐based, behaviour observed in diverse organisms, including insects such as thrips. The influence of factors such as plant substrate, time of day, and geographic location on aggregation of thrips is uncertain, therefore we monitored the dispersion of male and female bean flower thrips (BFT), Megalurothrips sjostedti (Trybom) (Thysanoptera: Thripidae), on cowpea, Vigna unguiculata (L.) Walp. (Fabaceae), over three cowpea growth stages and across three cowpea‐growing areas of Kenya. Our results indicated that for all the crop growth stages, the density of BFTs varied over the time of day, with higher densities at 10:00, 13:00, and 16:00 hours than at 07:00 hours. Thrips densities did not differ among blocks at the budding stage, but they did at peak flowering and podding stages. Dispersion indices suggested that both male and female BFTs were aggregated. Active male aggregation occurred only on green plant parts and it varied across blocks, crop stages, and locations. Similarly, active female aggregation was observed in peak flowering and podding stages. Such active aggregation indicates a semiochemical or behaviour‐mediated aggregation. Identification of such a semiochemical may offer new opportunities for refining monitoring and management strategies for BFT on cowpea, the most important grain legume in sub‐Saharan Africa.     

Effects of temperature and photoperiod on development and oviposition of the legume flower thrips, Megalurothrips sjostedti

Chrysomya albiceps, the larvae of which are facultative predators of larvae of other dipteran species, has been introduced to the Americas over recent years along with other Old World species of blowflies, including Chrysomya megacephala, Chrysomya putoria and Chrysomya rufifacies. An apparent correlate of this biological invasion has been a sudden decline in the population numbers of Cochliomyia macellaria, a native species of the Americas. In this study, we investigated predation rates on third instar larvae of C. macellaria, C. putoria and C. megacephala by third instar larvae of C. albiceps in no-choice, two-choice and three-choice situations. Most attacks by C. albiceps larvae occurred within the first hour of observation and the highest predation rate occurred on C. macellaria larvae, suggesting that C. albiceps was more dangerous to C. macellaria than to C. megacephala and C. putoria under these experimental conditions. The rates of larvae killed as a result of the predation, as well as its implications to population dynamics of introduced and native species are discussed.     

Size-spectra as indicators of the effects of fishing on coral reef fish assemblages

The data requirements and resources needed to develop multispecies indicators of fishing impacts are often lacking and this is particularly true for coral reef fisheries. Size-spectra, relationships between abundance and body-size class, regardless of taxonomy, can be calculated from simple sizeabundance data. Both the slope and the mid-point height of the relationship can be compared at different fishing intensities. Here, we develop size-spectra for reef fish assemblages using body size- abundance data collected by underwater visual census in each of ten fishing grounds across a known gradient of fishing intensity in the Kadavu Island group, Fiji. Slopes of the size-spectra became steeper (F_9,69=3.20, p<0.01) and the height declined (F_9,69=15.78, p<0.001) with increasing fishing intensity. Regressions of numbers of individuals per size class across grounds were negative for all size classes, although the slope was almost zero for the smallest size class. Response to exploitation of each size class category was greatest for larger fish. Steepening of the slope with increasing fishing intensity largely resulted from reductions in the relative abundance of large fish and not from the ecological release of small fish following depletion of their predators. The slope and height of the size-spectrum appear to be good indicators of fishing effects on reef fish assemblages.