Cumulative impact of a clover cover crop on the persistence and efficacy of Beauveria bassiana in suppressing the pecan weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan. Endemic levels of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin can occur in pecan orchards and contribute to natural control of C. caryae. Commercial formulations of the fungus can also be applied for suppression of C. caryae. We hypothesized that a clover cover crop enhances B. bassiana efficacy and persistence (e.g., by protecting the fungus against abiotic environmental stresses). The hypothesis was tested by conducting field trials in a pecan orchard in Byron, GA, in 2009 and 2010. The study included four treatments arranged in a factorial with two levels of fungus (endemic fungus only, and application of a commercial B. bassiana product), and two levels of clover (white clover, Trifolium repens L., and no clover). Fungal persistence was measured by determining the number of CFUs per gram of soil over time (during 42 d postapplication of B. bassiana in 2009 and 29 d in 2010). Efficacy was measured by capturing naturally emerging C. caryae and subsequently determining mortality and mycosis (over 24 d in 2009 and 17 d in 2010). In 2009, greater prevalence of B. bassiana conidia was detected in plots receiving fungal applications compared with no fungus applications, and no clear effect of clover was observed in plots receiving B. bassiana applications in either year. In 2010, B. bassiana prevalence in the endemic fungus plus clover treatment was higher than fungus without clover, and was similar to plots receiving additional B. bassiana applications. Given that we observed enhanced persistence of endemic B. bassiana in 2010 but not 2009, the impact of clover appears to be a cumulative effect. Mortality of C. caryae (averaged over the sampling periods) ranged between 68-74% in plots receiving B. bassiana applications and 51-56% in plots with endemic fungus only. C. caryae mortality and mycosis data also provided evidence that endemic B. bassiana efficacy was enhanced by clover relative to plots without clover (with no clear clover effect on plots receiving fungus applications). Thus, we conclude that natural control of C. caryae can increase when clover is grown in pecan orchards with endemic populations of B. bassiana.     

Effects of combining microbial and chemical insecticides on mortality of the Pecan Weevil (Coleoptera: Curculionidae)

The pecan weevil, Curculio caryae (Horn), is a key pest of pecan [Carya illinoinensis (Wangenh.) K. Koch]. Current control recommendations are based on chemical insecticide applications. Microbial control agents such as the entomopathogenic nematode, Steinernema carpocapsae (Weiser) and the fungus Beauveria bassiana (Balsamo) Vuillemin occur naturally in southeastern U.S. pecan orchards and have shown promise as alternative control agents for C. caryjae. Conceivably, the chemical and microbial agents occur simultaneously within pecan orchards or might be applied concurrently. The objective of this study was to determine the interactions between two chemical insecticides that are used in commercial C. caryae control (i.e., carbaryl and cypermethrin applied below field rates) and the microbial agents B. bassiana and S. carpocapsae. In laboratory experiments, pecan weevil larval or adult mortality was assessed after application of microbial or chemical treatments applied singly or in combination (microbial + chemical agent). The nature of interactions (antagonism, additivity, or synergy) in terms of weevil mortality was evaluated over 9 d (larvae) or 5 d (adults). Results for B. bassiana indicated synergistic activity with carbaryl and antagonism with cypermethrin in C. caryae larvae and adults. For S. carpocapsae, synergy was detected with both chemicals in C. caryae larvae, but only additive effects were detected in adult weevils. Our results indicate that the chemical-microbial combinations tested are compatible with the exception of B. bassiana and cypermethrin. In addition, combinations that exhibited synergistic interactions may provide enhanced C. caryae control in commercial field applications; thus, their potential merits further exploration.     

Movement of adult pecan weevils Curculio caryae within pecan orchards

1 The pecan weevil Curculio caryae (Horn) (Coleoptera: Curculionidae) is an indigenous pest of pecan Carya illinoinensis (Wangenh.) K. Koch, in North America. Understanding the movement of this pest from the orchard floor to host trees could lead to pest management practices that exploit weevil behaviour and thus reduce insecticide application to the entire orchard canopy. Furthermore, no information exists on diel periodicity of pecan weevil movement. 2 Movement of adult pecan weevils crawling and flying to the host trunk, flying to the host canopy, crawling within the host canopy and flying between host trees was studied using four types of passive traps over four seasons. Each type of trap was used to capture weevils at different locations on or near the tree and to discriminate flying versus crawling behaviour. 3 More pecan weevils crawl to the trunk than fly and a proportion of the population flies directly from the orchard floor into the pecan canopy. The majority of this movement occurs at dusk. 4 The vertical distribution of weevils was generally uniform throughout the canopy but more weevils were captured in suspended traps nearest tree tops, rather than traps near the ground, when flying between trees and this was significantly so for two of 4 years. 5 The results of the present study are contrary to previous reports suggesting that most adult pecan weevils fly to the pecan trunk after emergence from the soil; however, our results did indicate that a proportion of the population flies directly from the orchard floor into the pecan canopy and thus would circumvent strategies that attempt to control weevils moving up the trunk.     

The potential for enhanced fungicide resistance in Beauveria bassiana through strain discovery and artificial selection

Our objectives were to determine the (1) natural variation in fungicide resistance among Beauveria bassiana strains, (2) potential to increase fungicide resistance in B. bassiana through artificial selection, and (3) stability of virulence in selected B. bassiana strains. Fungicides included dodine, fenbuconazole, and triphenyltin hydroxide, which are commonly used in pecan and other horticultural crops. Comparison of seven B. bassiana strains indicated some are substantially more resistant to fungicides than others; a commercial strain (GHA) was less resistant than all wild strains isolated from pecan orchards. Artificial selection resulted in enhanced fungicide resistance in the GHA strain but not in a mixed wild strain. Removal of selection pressure for three passages did not reduce the enhanced fungicide resistance. Sub-culturing with exposure to fungicides did not affect the GHA strain's virulence to pecan weevil, Curculio caryae, larvae, whereas fungicide exposure increased virulence in a mixed wild population of B. bassiana.     

Adult polyphagous coleopterans overwintering in cereal boundaries: winter mortality and susceptibility to the entomopathogenic fungus Beauveria bassiana

Autumn-collected Bembidion lampros and Agonum dorsale were kept outdoors under semi field conditions to determine winter mortality. On three occasions (autumn, mid-winter and late winter) sub-samples of the population were incubated in the laboratory at room temperature and mortality was recorded. Generally the mortality was low in autumn and mid-winter, not exceeding 5%. A more pronounced post-winter mortality of up to 30% was observed at the end of hibernation. Only a negligible number were infected by the entomopathogenic fungi Beauveria bassiana and Paecilomyces farinosus. Infection experiments with B. bassiana isolates from Sitona lineatus, Tachyporus hypnorum and three species of ground beetles showed a similarly low susceptibility of B. lampros and A. dorsale to B. bassiana and, although Tachyporus spp. showed a distinct mycophagy and a significantly higher mortality, this could not be related to mycosis.     

Avoidance of a generalist entomopathogenic fungus by the ladybird, Coccinella septempunctata

Fungal entomopathogens are ubiquitous within the environment and susceptible insects are predicted to be under strong selection pressure to detect and avoid virulent isolates. Beauveria bassiana is an entomopathogenic fungus with a wide host range including coccinellids. Seven-spot ladybirds, Coccinella septempunctata, overwinter predominantly in leaf litter and B. bassiana is one of their major mortality factors during winter in temperate regions. Behavioural assays were conducted to assess the ability of adult C. septempunctata to avoid lethal densities of B. bassiana conidia in soil or on leaves. Further assays considered avoidance by C. septempunctata of mycosed (B. bassiana) C. septempunctata cadavers compared with uninfected C. septempunctata cadavers or in vitro B. bassiana. Treatments in any bioassays entirely avoided by C. septempunctata were regarded as censored data, to overcome the difficulties associated with zeros in log-ratio analyses. Male and female C. septempunctata avoided contact with leaf surfaces and soil inoculated with B. bassiana and mycosed cadavers. The ability of C. septempunctata to detect and avoid B. bassiana conidia is an adaptation that undoubtedly increases survival and ultimately fitness. We predict that such behavioural responses are widespread and driven by the high cost of fungal infection to a host.     

Evaluation of synergistic interactions between the Colorado potato beetle (Coleoptera: Chrysomelidae) pathogen Beauveria bassiana and the insecticides, imidacloprid, and cyromazine

Laboratory studies investigated the interaction between the fungal entomopathogen Beauveria bassiana (Balsamo) Vuillemin and sublethal doses of the insecticides imidacloprid and cyromazine when applied to larvae of the Colorado potato beetle, Leptinotarsa decemlineata (Say). When second instars were fed potato leaf discs treated with sublethal doses of imidacloprid and a range of doses of B. bassiana, a synergistic action was demonstrated. Similar results were observed when larvae were sprayed directly with B. bassiana conidia and immediately fed leaf discs treated with imidacloprid. No synergistic interaction was detected when larvae were fed leaf discs treated with sublethal doses ofimidacloprid 24 h after application of B. bassiana conidia to larvae. However, a synergistic interaction was detected when larvae were fed leaf discs treated with imidacloprid and sprayed with B. bassiana conidia 24 h later. Although sublethal doses of both imidacloprid and the triazine insect growth regulator (IGR) cyromazine prolonged the duration of the second instar, only imidacloprid interacted with B. bassiana to produce a synergistic response in larval mortality. In leaf consumption studies, the highest dose of B. bassiana tested promoted feeding in inoculated second instars. Feeding was inhibited when larvae were fed foliage treated with sublethal doses of imidacloprid and significantly reduced when fed foliage treated with a sublethal dose of cyromazine. Starvation of larvae for 24 h immediately after B. bassiana treatment produced a similar result to the combined treatment of B. bassiana and imidacloprid and increased the level of mycosis when compared with B. bassiana controls. Imidacloprid treatment affected neither the rate of germination of B. bassiana conidia on the insect cuticle nor the rate at which conidia were removed from the integument after application. The statistical analysis used to detect synergism and the possible role of starvation-induced stress factors underlying the observed synergistic interactions are discussed.     

Inhibition of Beauveria bassiana Proteases and Fungal Development by Inducible Protease Inhibitors in the Haemolymph of Galleria mellonella Larvae

Injection of zymosan or dead yeast cells enhanced the inhibitory activity against exocellular Beauveria bassiana proteases in the cell - free haemolymph of Galleria mellonella larvae . Pre - injected larvae exhibited no decreased mortality after subsequent injection with living B. bassiana blastospores but survived for a prolonged time before death . Increased levels of protease inhibitors in the haemolymph were also observed after injection of B. bassiana proteases . In contrast , no enhanced inhibitory activity against B. bassiana proteases was detected in infected larvae when mycosis was initiated with conidia which enabled the fungus to invade host larvae through the integument in a natural manner . B. bassiana proteases were not completely inhibited by the addition of cell - free haemolymph . Protease inhibitors obtained after heat and trichloroacetic acid precipitation of cell - free haemolymph were added to the protein medium of B. bassiana to study the effect on its growth in vitro. Enriched fractions from pre - injected larvae delayed fungal growth in comparison with fractions from untreated larvae , suggesting that delayed mortality of immunized G. mellonella larvae infected with B. bassiana is due to enhanced levels of protease inhibitors . A non - virulent form of the same strain exhibited reduced capacity to release proteases in vitro. The results strongly suggest that the capacity of insects to release inhibitors against fungal proteases influences their susceptibility against entomopathogenic fungi .     

Potential of Beauveria bassiana (Hyphomycetes: Moniliales) for controlling the white pine weevil, Pissodes strobi (Col., Curculionidae)

Abstract:  Studies were conducted under laboratory conditions to document the potential of Beauveria bassiana conidia applications for controlling the white pine weevil, Pissodes strobi (Peck). A screening test including six B. bassiana isolates allowed us to demonstrate that CFL (Centre de Foresterie des Laurentides) and IP-CPB (Île Perrault-charançon du pin blanc) were the most virulent isolates among the ones tested, with percentages of mortality after 3 weeks of 73% and 85.5%, respectively. These two B. basiana isolates were applied either onto soil or branch sections to compare the effectiveness of these potential control strategies. Greater than 75% mortality was observed within 3 weeks for both modes of application using suspensions of B. bassiana at a concentration of 1.0 × 10⁸ (soil application) and 1.0 × 10⁹ conidia/ml (branch application). The results demonstrate for the first time that B. bassiana is an effective entomopathogen against P. strobi .     

Effect of Beauveria bassiana and Metarhizium anisopliae (Deuteromycetes) upon the coffee berry borer (Coleoptera: Scolytidae) under field conditions

The effect of three strains of the fungus Beauveria bassiana (Balsamo) Vuillemin and two strains of Metarhizium anisopliae (Metschnikoff) Sorokin upon the coffee berry borer, Hypothenemus hampei (Ferrari), was studied in three coffee farms at different altitudes (450-1,100 m above sea level) in Soconusco, Chiapas, Mexico. The maximum average percentage mycosis varied according to altitude. At 450 m asl (El Rincon) mycosis was 14.3% for B. bassiana and 6.3% for M. anisopliae; at 880 m asl (Santa Anita) mycosis was 40.6% for B. bassiana and 12.6% for M. anisopliae, and at 1,100 m asl (Alpujarras) 33.9% for B. bassiana and 22. 1% for M. anisopliae. The effect of fungal mycosis through time was not significant (P > 0.01) in any of the farms, but there was a significant difference between the strains of the fungus (P < 0.01); the best strains being Bb25 and Ma4 at the lower altitude, Bb26 and Ma4 for the middle altitude and Bb26 and Ma4 at the higher altitude. Environmental factors such as temperature, relative humidity and rain were not correlated with the percentage mycosis caused by B. bassiana and M. anisopliae. However, in the case of B. bassiana there was a significant, positive correlation (P < 0.01) between the infestation levels of the pest and the mycosis response of the entomopathogen.     

Characterization of a newly discovered Beauveria bassiana isolate to Franklimiella occidentalis Perganda, a non-native invasive species in China

In this study, a new virulent Beauveria bassiana isolate (B. bassiana-CYT5) had been identified as a new member of the species B. bassiana. The B. bassiana-CYT5 isolate was compared with four other virulent B. bassiana isolates and found to be highly infectious and virulent against the Franklimiella occidentalis Perganda. The B. bassiana-CYT5 could approximately 93.08% mortality of F. occidentalis 6 days post inoculation in the concentration of 1×10(8) conidia/mL. The phylogenetic tree based on ITS and partial sequence of elongation factor 1-alpha (EF1-alpha) indicated that B. bassiana-CYT5 isolate was in a cluster of B. bassiana. Furthermore, B. bassiana-CYT5 isolate demonstrated high heat tolerance (60-100% relative germination) between 1-h and 2-h exposure at 37 °C, 38 °C, 39 °C and 40 °C, respectively. So our results suggested that B. bassiana-CYT5 isolate could be a new efficient biocontrol agent against F. occidentalis.     

Effects of Perennial Peanut (Arachis glabrata) Ground Cover on Nematode Communities in Citrus

The effects of perennial peanut (Arachis glabrata) ground cover on the nematode community in a citrus orchard were examined. Samples were taken from two different ground cover treatments (perennial peanut or bare ground) at each of three distances from the tree trunk. Richness, measured as total numbers of nematode genera per sample, and total numbers of nematodes were greatest in the perennial peanut treatment (P < 0.05). Abundance of many genera of bacterivores, fungivores, and omnivores were increased by the perennial peanut ground cover. Total numbers of plant parasites were greater in perennial peanut treatments on three of the five sampling dates (P < 0.05), mainly due to trends in numbers of Mesocriconema. Distance from a tree trunk and the interaction of ground cover treatments and proximity to a tree trunk were most influential for Belonolaimus and Hoplolaimus. Although differences among treatments were observed for nematode genera and trophic groups, ecological indices were not consistently sensitive to treatments. Among several ecological indices evaluated, richness was most often affected by ground cover treatment.     

Influence of Pesticide Use on the Natural Occurrence of Entomopathogenic Fungi in Arable Soils in the UK: Field and Laboratory Comparisons

The spectrum and abundance of entomopathogenic fungi in agricultural soil receiving different pesticide applications were evaluated . Seven small field plots within a barley crop were selected . Each plot had received a different pesticide treatment at slightly higher than the field rate each year for the previous 12-19 years . The field plots received either benomyl (fungicide) , triadimefon (fungicide) , aldicarb (insecticide) , chlorfenvinphos (insecticide) , glyphosate (herbicide) , all five of these pesticides or no pesticides at all (control) . Soil sampled from each plot was baited with Galleria mellonella larvae at either 18 or 26 C . Five species of entomopathogenic fungi infected these larvae . Beauveria bassiana was the dominant species , and the only species for which infection levels were high enough to be analyzed statistically . Significantly fewer G. mellonella larvae became infected with B. bassiana in soil treated with benomyl than in other treatments . This deleterious effect was confirmed in in vitro experiments where benomyl inhibited fungal growth significantly . Chlorfenvinphos also reduced the proportion of G. mellonella becoming infected with B. bassiana slightly (although not statistically significantly) . This effect was much greater in in vitro experiments in which chlorfenvinphos inhibited fungal growth significantly . B. bassiana- induced mortality was significantly greater in G. mellonella larvae placed in soil treated with triadimefon than in other treatments . In vitro studies , however , demonstrated that triadimefon inhibited rather than stimulated growth . A similar anomaly between field and laboratory observations was seen in the case of aldicarb . There was no significant difference in the number of G. mellonella larvae found infected with B. bassiana in soil treated with aldicarb than in the control soil . However , in the in vitro studies , aldicarb stimulated fungal growth at all concentrations except 10 times the field rate . In general , significantly fewer larvae became infected with B. bassiana at 26 C than at 18 C in all treatments . Pesticides may have a direct impact on the natural occurrence , infectivity and population dynamics of entomopathogenic fungi . They also affect other macro - and microorganisms in the soil which may interact with the ento mopathogenic fungi . In combination with climate and soil variables , these effects are difficult to separate . Although any suppression of these fungi may be detrimental to their capacity as natural enemies , this study has demonstrated that pesticides used under field conditions are unlikely either to kill all the entomopathogenic fungi present in the treated area or to limit their recolonization . It seems likely , therefore , that pesticides have the potential to be used in conjunction with mycoinsecticides in integrated pest management systems .     

Effect of exposure method to Beauveria bassiana and conidia concentration on mortality, mycosis, and sporulation in cadavers of Chilo partellus (Lepidoptera: Pyralidae)

The effects of exposure methods, conidial concentrations, and temperature on mortality, mycosis and sporulation in second instar Chilo partellus cadavers infected by Beauveria bassiana was investigated in laboratory studies. Larvae directly sprayed with conidia, exposed to conidia-treated leaves, and dipped into conidial suspension resulted in high mortality (98-100%). The longest LT50 (3.5 days) and days to mortality (2.6 days) were observed in the treated-leaves exposure method. The shortest LT50 (1 day) and days to mortality (1 day) were recorded for the dipping method. With increasing conidial concentrations, there were decreasing LT50 and days to mortality. Larvae exposed to treated leaves and larvae directly sprayed with conidial suspensions produced high mycoses in cadavers. Exposure of larvae to treated-leaves resulted in high sporulation. At lower concentrations of conidia, both mycosis and sporulation in cadavers were high. The optimum temperature for mycosis was 20 and 15 degrees C for sporulation.     

Changes in soil bacterial community triggered by drought‐induced gap succession preceded changes in soil C stocks and quality

The aim of this study was to understand how drought‐induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought‐induced die‐off, is being replaced by Holm‐oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and ¹³C solid‐state Nuclear Magnetic Resonance (CP‐MAS ¹³C NMR) to soils within areas of influence (defined as an surface with 2‐m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close‐chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below‐ground before above‐ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r‐strategic bacteria) further gives indications of how drought‐induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils.     

Cover crops influence soil properties and tree performance in an organic apple (Malus domestica Borkh) orchard in northern Patagonia

Little is known about the effects of cover crops on soil properties in organic orchards. To fill this gap, this work aimed at examining the effects of several cover crops on soil fertility, nitrate dynamics, populations of nematodes and tree performance in an organic orchard of apple cv. Royal Gala/EM 26 planted in 1994 at 4 × 2 m. In 1999 the following treatments were randomly applied to the inter-row spaces of 20 tree rows each: permanent cover of alfalfa (Medicago sativa L.) plus fescue (Festuca arundinacea Schribn.), Alfalfa/fescue (AF); permanent cover of strawberry clover (Trifolium fragiferum L.), Strawberry clover (SC); seeding of common vetch (Vicia sativa L.), Vetch (V); and Control (C) (natural vegetation of grasses and legumes with the soil disked twice in late winter, which is the traditional management system used by growers in this region). The cover crops were mowed 3 or 4 times during the growing season and the clippings were left on the ground for decomposition. The trial was conducted in the northern Patagonia region of Argentina in a sandy loam soil with a pH of 7.6 and initial organic matter content of 1.5%. A commercially available certified organic fertilizer containing 5% total N, 2% total P and 4% total K was added annually at a rate of 1.0 kg per tree in equal amounts to each treatment in a radius of 0.4 m around the trunk. After 6 years, soil organic matter in the top soil was 31, 27.9, 23 and 18.6 g kg⁻¹ for SC, AF, V and C treatments, respectively. Total soil N followed the same trend. Apple leaf N declined steadily in all treatments especially in AF and C from year 3 to 5 with values below 1.8% and it was therefore necessary to increase the rate of organic fertilizer. Nitrate concentrations remained under 7.5 mg kg⁻¹ during the winter months in all treatments and increased in spring and summer. A sharp increase in soil nitrate, up to 100 ppm, was observed in late spring in V due to rapid biomass decomposition after maturation of the common vetch. Populations of nematodes especially bacterivores and herbivores increased from September (late winter) to March (fall) in the V treatment. Significant differences were observed when we compared the effects of treatments on the entire nematode population. Tree growth as measured by trunk cross sectional area and canopy volume was significantly lower in the C treatment. Apple yield was also lower in the C treatment in years 5 and 6 while no differences were found among the seeded cover crops. Thus, our soil management treatments did affect soil properties, tree growth and yield. Disking is not a recommended practice because it may decrease the concentration of soil organic matter and leads to poor tree vigor that corresponds to low fruit bearing potential. However, even with the use of permanent cover crops the addition of organic fertilizers is needed in order to sustain yield and tree vigor.     

Interaction effects between Beauveria bassiana and imidacloprid against Thrips tabaci (Thysanoptera: Thripidae)

The onion thrips, Thrips tabaci Lindeman is a major pest of many horticultural crops in many parts of the world. The entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin and imidacloprid are used for the control of the onion thrips. The potential interactions between the fungus and the chemical against the onion thrips adults were evaluated in a laboratory and two greenhouse bioassays. The laboratory bioassay was a leaf dip bioassay for a mixture of B. bassiana and imidacloprid. The first greenhouse bioassay was a topical application of a mixture of the fungus and the chemical while the second bioassay was a drenching application of imidacloprid and a topical application of B. bassiana. In all bioassays, the combined application resulted in higher percentage mortalities than either B. bassiana or imidacloprid alone. In the laboratory bioassay, the highest mortality of 97% was achieved by using B. bassiana at the field rate with imidacloprid at 1/10th the field rate relative to 88%, 94%, 21% for B. bassiana alone, imidacloprid alone and the control, respectively. In the first greenhouse bioassay, the highest mortality of 80% resulted from the combined application of B. bassiana and imidacloprid at field rates compared with 55%, 75%, and 22% for B. bassiana alone, imidacloprid alone and the control, respectively. In the second greenhouse bioassay, the combined application of both control agents at field rates resulted in 85% mortality compared with 52%, 83%, and 18% for B. bassiana alone, imidacloprid alone and the control, respectively. The interaction effect between the two control agents was additive except when B. bassiana at 1/2 field rate was mixed with imidacloprid at 1/10th field rate in the laboratory bioassay, where the interaction was antagonistic. The above results indicate that combining imidacloprid with B. bassiana might reduce the rate of the insecticide application while enhancing the efficacy of the biological control agent.     

Virulence of entomopathogenic nematodes to pecan weevil larvae, Curculio caryae (Coleoptera: Curculionidae), in the laboratory

The pecan weevil, Curculio caryae (Horn), is a key pest of pecans in the Southeast. Entomopathogenic nematodes have been shown to be pathogenic toward the larval stage of this pest. Before this research, only three species of nematodes had been tested against pecan weevil larvae. In this study, the virulence of the following nine species and 15 strains of nematodes toward fourth-instar pecan weevil was tested: Heterorhabditis bacteriophora Poinar (Baine, HP88, Oswego, NJ1, and Tf strains), H. indica Poinar, Karunakar & David (original and Homl strains), H. marelatus Liu & Berry (IN and Point Reyes strains), H. megidis Poinar, Jackson & Klein (UK211 strain), H. zealandica Poinar (NZH3 strain), Steinernema riobrave Cabanillas, Poinar & Raulston (355 strain), S. carpocapsae (Weiser) (All strain), S. feltiae (Filipjev) (SN strain), and S. glaseri (Steiner) (NJ43 strain). No significant difference in virulence was detected among nematode species or strains. Nematode-induced mortality was not significantly greater than control mortality (in any of the experiments conducted) for the following nematodes: H. bacteriophora (Baine), H. zealandica (NZH3), S. carpocapsae (All), S. feltiae (SN), S. glaseri (NJ43), and S. riobrave (355). All other nematodes caused greater mortality than the control in at least one experiment. Heterorhabditis megidis (UK211) but not H. indica (original) displayed a positive linear relationship between nematode concentration and larval mortality. Results suggested that, as pecan weevil larvae age, they may have become more resistant to infection with entomopathogenic nematodes.     

Mortality of Colorado potato beetle (Leptinotarsa decemlineata) after sublethal stress with the CryIIIA delta-endotoxin of Bacillus thuringiensis and subsequent exposure to Beauveria bassiana

Acute or chronic sublethal exposure of Colorado potato beetle larvae to the CryIIIA delta-endotoxin of Bacillus thuringiensis Berliner did not significantly (P > 0.05) alter their subsequent susceptibility to Beauveria bassiana (Balsamo) Vuillemin. During the period of exposure to B. bassiana there was continued mortality from previous exposure to delta-endotoxin, and B. bassiana also caused significant mortality. Acute and chronic exposure to delta-endotoxin significantly prolonged larval development. The weights of prepupae and adults were significantly reduced by exposure to delta-endotoxin, with the greatest effect being from chronic exposure. Separation of the manifestations of stress in time (feeding vs soil stages) and space (toxin damage to the insect gut vs fungal penetration of the cuticle and activity in the hemocoel) may have precluded alteration of insect susceptibility to infection by B. bassiana. Endemic populations of B. bassiana are not expected to influence the development of resistance in the Colorado potato beetle to the delta-endotoxin of B. thuringiensis.     

设施土壤生态环境特点与调控

论述了设施土壤生态环境的特性主要是土壤养分富集 ,土壤板结 ;土壤酸化、次生盐渍化和连作障碍严重 ;土壤产生有害气体 ;土壤微生物区系变化很大。这些特点主要是盲目大量施肥 ,土壤长期连续利用 ,土壤环境密闭 ,单一化栽培 ,用肥种类不合理及某些化学物质积累造成的。利用生物、农业耕作、地上环境和工程措施调控相结合 ,是调控设施土壤生态环境的有效方法 ,土壤生物学的应用将是改善设施土壤生态环境新的研究方向。