Temporal relationships betweenZea mays,ostrinia nubilalis (Lep.: Pyralidae) and endophyticBeauveria bassiana

The entomopathogenic fungus,Beauveria bassiana (Balsamo) Vuillemin, was applied to whorl-stage (V7) corn,Zea mays L., by foliar application of a granular formulation of corn grits containing conidia or by injection of a conidial suspension. All plants were infested with European corn borer larvae,Ostrinia nubilalis (Hübner), at the V7 (whorl), V12 (late-whorl), or V17 (pretassel) stage of plant development. Plants infested at whorl and late-whorl stages had significantly more European corn borer tunneling than did plants infested at the pretassel stage. The percentage of plants colonized byB. bassiana did not differ significantly among the whorl, late-whorl, and pretassel stages. As the plants matured,B. bassiana was isolated from different plant areas, with the pith more frequently colonized than the leaf collars. Foliar application ofB. bassiana provided immediate suppression ofO. nubilalis in those plants infested at whorl stage. The reduced efficacy ofB. bassiana at the intermediate plant stages relative to efficacy at harvest is discussed. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned.     

Dynamics of Nosema pyrausta in natural populations of the European corn borer, Ostrinia nubilalis: A six-year study

Nosema pyrausta (Paillot) (Microsporida: Nosematidae) is an obligatory intracellular parasite of the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae). This pathogen is maintained in natural populations of O. nubilalis by both horizontal and vertical transmission. The impact of N. pyrausta on fecundity of adults and survival of larvae has been well documented in laboratory and field research. In an extensive study covering a 6 year period at one site, we described the effect of N. pyrausta within O. nubilalis populations in a continuous corn following corn ecosystem. We documented the presence of the pathogen through all life stages of O. nubilalis (egg, larva, pupa, adult), by collecting throughout the crop season and examining each insect stage in the laboratory for the frequency of infection with N. pyrausta. The percentage of infection with N. pyrausta and magnitude of the O. nubilalis population fluctuated throughout generation 1 and generation 2. Both horizontal and vertical transmission played a role in maintaining N. pyrausta in the population in both generations. There were strong correlations between percentage adults with N. pyrausta and percentage larvae with N. pyrausta, and between percentage eggs with N. pyrausta and percentage larvae with N. pyrausta. There was a weak correlation between percentage adults with N. pyrausta and percentage eggs with N. pyrausta. The percentage of insects infected with N. pyrausta was always lowest in the egg.     

Effects of combining <Emphasis Type="Italic">Beauveria bassiana</Emphasis> and <Emphasis Type="Italic">Nosema pyrausta</Emphasis> on the mortality of <Emphasis Type="Italic">Ostrinia nubilalis</Emphasis>

We tested the combined effect of the fungus Beauveria bassiana and the microsporidium Nosema pyrausta on the European corn borer larvae, Ostrinia nubilalis, in the laboratory. The first instar of O. nubilalis larvae was the most sensitive to the B. bassiana infection followed by the fifth, second, third, and fourth instar (LC₅₀s were 4.91, 6.67, 7.13, 9.15, and 6.51 × 10⁵ conidia/ml for the first to fifth instars, respectively). Mortality of each instar increases positively with concentration of conidia. When B. bassiana and N. pyrausta were used in combination, mortality increased significantly in all instars. Relative to the B. bassiana treatment alone, the B. bassiana + N. pyrausta treatment decreased the LC₅₀s by 42.16%, 37.63%, 21.60%, 27.11%, and 33.95% for the first to fifth instars, respectively. The combined effects of the two pathogens were mostly additive. However, at the two highest concentrations the pathogens interacted synergistically in the first and second instar. Individuals that survived the B. bassiana and B. bassiana + N. pyrausta treatments and developed into adults had significantly shorter lifespans and females oviposited fewer eggs than non-exposed insects. The effects on the longevity and the egg production were most pronounced at high concentration of B. bassiana conidia.     

二种交配型球孢白僵菌对亚洲玉米螟的生态控制作用

利用生物种间互做关系抑制农业害虫的暴发是生物防治的重要手段。为探讨二种交配型内共生球孢白僵菌与玉米之间的互惠关系及其形成的共生体在亚洲玉米螟控制中的生态效应,以玉米为宿主植物,以球孢白僵菌孢子悬浮液进行灌根,在温室内构建了二种交配型(MAT1-1-1型,B5;MAT1-2-1型,B2)球孢白僵菌-玉米共生体,并研究了共生体对玉米的生长、对亚洲玉米螟的产卵选择和幼虫发育及其对球孢白僵菌生物学特性的影响。结果显示:通过叶片离体培养、ITS基因和交配型基因MAT检测,均能检测到白僵菌的内生定殖;MAT1-2-1型B2菌株定殖检出率高,MAT1-1-1型B5菌株在混合型接种中定殖有优势。回收后的球孢白僵菌菌落直径和毒力无显著性变化,但其产孢量都显著提高其中回收B5处理组来源菌株的产孢量提高最显著。接种过球孢白僵菌的玉米植株地上部生长速度、生物量和地下根系生物量均优于对照组,其中根系干重明显增加,而地上植株干重也相对增加。MAT1-1-1型菌株B5对共生体玉米植株地上高度促生长贡献明显;MAT1-2-1型菌株B2对共生体玉米植株地下干重增加贡献明显。总体上球孢白僵菌内生定殖对玉米地下根系生物量影响大于对地上植株生物量的影响。在产卵选择性试验中,各处理组亚洲玉米螟的产卵量显著少于对照组。共生体对亚洲玉米螟产卵具有明显的趋避作用,MAT1-2-1型菌株B2对产卵的趋避作用明显,而MAT1-2-1型菌株B5的趋避作用较弱。在人工接种幼虫的试验中,处理组回收的亚洲玉米螟幼虫存活率均显著低于对照组,其中,B5组回收幼虫的存活率最低,仅为38.33%;处理组的化蛹率与对照组差异不显著,但B5组的回收幼虫化蛹率显著低于B2组和对照组,仅为34.77%,这说明MAT1-1-1型B5菌株对玉米螟幼虫发育抑制最明显。上述结果表明,不同交配型球孢白僵菌内生定殖效率有差异,在经过内生定殖后在产孢量方面有显著性提高,两个交配型菌株在联合应用时具有协同增效作用;两个交配型菌株均能够通过内生定殖与玉米形成共生体并促进玉米植株的生长,这显示球孢白僵菌和玉米之间已经建立具有互惠关系的共生体。这种共生体通过趋避亚洲玉米螟产卵、抑制幼虫存活和降低化蛹率等方面的潜力虽然不一样,但都有助于对亚洲玉米螟的可持续生态防治,也证明了共生体的建成有效提高了玉米的生态适应性,为利用球孢白僵菌内共生性实施亚洲玉米螟防控提供了新思路。     

Nosema pyrausta infection in Macrocentrus grandii, a braconid parasite of the European corn borer, Ostrinia nubilalis

Macrocentrus grandii which develop within Nosema pyrausta-infected larvae of the European corn borer, Ostrinia nubilalis, develop direct systemic infections from the ingestion of spores at the time of larval emergence from the host. Infections adversely affect pupal development and adult longevity. Infected females are unable to transmit the microsporidian to additional corn borer hosts. Pathogen development in the parasite host appears identical to its development in the corn borer host and mature spores show no morphological differences in size or shape when observed at the ultrastructural level. The prevalence of infection in natural parasite populations is 53.8% and closely parallels the 56.7% prevalence of infection in corn borer populations. Results suggest N. pyrausta may play a significant role in limiting M. grandii populations when levels of N. pyrausta in corn borers are high.     

Mycelia and spent fermentation broth of Beauveria bassiana incorporated into synthetic diets affect mortality,growth and development of larval Helicoverpa zea (Lepidoptera: Noctuidae)

Beauveria bassiana endophytically colonises corn (Zea mays) reducing tunneling from European corn borer (Ostrinia nubilalis). Endophytic colonisation of other plants by B. bassiana has been reported, and potentially, may reduce insect feeding on these plants. We evaluated the effects on larval growth and development, and mortality of different rates of dried, ground mycelia and water-soluble metabolites from fermentation broth culture of different isolates of B. bassiana incorporated into a synthetic diet and fed to neonate bollworm, Helicoverpa zea larvae. Development was delayed, weights of larvae were lower, and mortality was high for larvae fed the highest rates (1.0 and 5.0%, w/v) of mycelia incorporated diet compared to control. Insects fed diets containing mycelia of B. bassiana isolate 11-98 had the greatest mortality. Mortality was 100% for larvae fed 5% (w/v) mycelia incorporated diet of isolate 11-98, and 61% for isolate 3-00. For insects fed low rates (0.1 to 0.5%, w/v) of mycelia incorporated diet, mortality was lower, approximately 5% for isolate 11-98, and 5 to 14% for isolate 3-00. At the 0.1% (w/v) rate of mycelia incorporated diet, development occurred at an accelerated rate, compared to fungus-free controls, indicating increased nutrition in the lowest rate fungal diet. Mortality was low for all larvae fed diets containing spent fermentation broth of B. bassiana; however, development was delayed. Insects fed the highest rate (0.5%, v/v) of spent fermentation broth-amended diet had lower pupal weights, and a greater number of days to pupation than insects fed the lowest (0.1%, v/v) rate. Insects fed the 5% (v/v) rate of spent fermentation broth of isolates 11-98 and 3-00 had the longest days to pupation.     

On-farm evaluation of Beauveria bassiana for control of Ostrinia nubilalis in Iowa, USA

Ostrinia nubilalis is (Lepidoptera: Crambidae) a severe pest ofcorn in the major corn growing areas of theUnited States. The efficacy of a Beauveria bassiana application, for season-long suppression of O. nubilalis was evaluated in 1996 and 1997 at locationsacross Iowa. Beauveria bassiana,Mycotech 726 (Mycotech Corporation, Butte, MT)formulated on corn grit granules (14–20 mesh)at 2.2 × 10⁹ conidia/g and applied with ahand-held applicator at the rate of 0.4 g/plant(8.8 × 10⁸ conidia/plant). Applicationswere made when plants were in the V7 or R1growth stage. The length of larval tunneling,percentage of plants not infested with O.nubilalis, percentage of plants with anendophyte, and yield from treated and controlplots were determined. Whorl-stage applicationof B. bassiana in 1996 resulted in asignificant reduction in centimeters oftunneling (46–55%) and the percentageplants not infested by O. nubilalis. In1997, B. bassiana caused significantreductions in larval tunneling at all locations(20–53%); however, a significant increase inthe percentage of plants not infested with O. nubilalis occurred at only one location. Treatment of plants with B. bassiana in1997 did not significantly increase thepercentage of plants with an endophyte;however, the trend, with the exception of onesite, was for a greater percentage ofendophytic plants in treated versus untreatedplots. A whorl-stage application of a granularformulation of B. bassiana was mostefficacious in reducing O. nubilalis larval damage.     

Endophytic beauveria bassiana (balsamo) vuillemin in corn: The influence of the plant growth stage and ostrinia nubilalis (hübner)

A conidial suspension of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin, was injected into corn plants at anthesis to suppress populations of the European corn borer Ostrinia nubilalis (Hübner) (Lepidoptera: Pyralidae). The fungus colonized the plants and moved, primarily upward, within the pith, possibly along with plant photosyn‐thates. Beauveria bassiana injected into the corn plants significantly reduced the amount of European corn borer tunneling. Colonization of corn plants by B. bassiana was independent of European corn borer infestation. The impact of environmental conditions on the movement and colonization of the plants by B. bassiana is discussed.     

Impact of Nosema pyrausta on field populations of Macrocentrus grandii, an introduced parasite of the European corn borer, Ostrinia nubilalis

In field populations of the European corn borer, Ostrinia nubilalis, there is a significant inverse correlation between the prevalence of corn borer infection with the microsporidian Nosema pyrausta and the prevalence of parasitism by the introduced braconid Macrocentrus grandii where infections with N. pyrausta exceed 45%. This relationship occurs geographically and from year to year. Corn borer infection with N. pyrausta, the source of infection for M. grandii, is significantly related to corn borer density in the cornfield. These findings strongly suggest that N. pyrausta has a significant adverse affect on field populations of M. grandii and may help explain the diminishing role of this and other introduced parasites as natural controls of the corn borer in the United States.     

Dissemination of entomopathogenic fungi using Busseola fusca male as vector

The stem borer, Busseola fusca (Fuller), is an important pest of maize Zea mays L. and sorghum Sorghum bicolor (L.) in eastern and southern Africa. To control this pest, biological control methods including the use of entomopathogenic fungi are being considered. The pathogenicity of one isolate of Metarhizium anisopliae (Metsch.) Sorok. and one isolate of Beauveria bassiana Bals. (Vuill.) were first tested on different developmental stages of B. fusca including eggs, neonate, 2nd and 3rd-instar larvae. Both fungal isolates were pathogenic to all the stages tested. However, differences in mortality were observed among larvae that hatched from treated egg masses. Experiments were conducted thereafter to test whether B. fusca males could serve as a vector for fungal conidia to contaminate B. fusca females and subsequently eggs and larvae. Results demonstrated that B. fusca males successfully transferred inoculum to females during copulation, which in turn transmitted it to the eggs they laid on maize plants, resulting in the decrease of leaf damages.     

Pathogenicity, formulation and storage of insect pathogenic hyphomycetous fungi tested against Diabrotica speciosa

Studies were conducted tosearch for fungal strains with potentialpathogenicity against Diabrotica speciosa(Germar) (Coleoptera: Chrysomelidae).Among sixteen fungal isolates screenedthe most virulent was a Beauveria bassiana(Balsamo) Vuillemin isolate (FHD13) thatcaused 70% mortality of D. speciosathird instar larvae. The LC₅₀ value ofB. bassiana isolate FHD13 was3.48 × 10¹⁰ conidia/ml.Different temperatures (4, 17 and 26 °C)and vegetable oils (corn, sunflower and canola)used for storage did not significantly affectviability of conidia. A pathogenicity trialagainst D. speciosa larvae performed withthe corn oil formulation (1 × 10⁸ conidia/mlof oil) caused 65% of mortality.     

Lyophilization, vacuum drying, and subsequent storage of Nosema pyrausta spores

Methods of preserving Nosema pyrausta spores were developed by subjecting laboratoryreared, nondiapausing larvae of the European corn borer, Ostrinia nubilalis, infected with N. pyrausta to different techniques of lyophilization and vacuum drying. Such material was still viable when it was stored in an airtight container and held at ?12°C for 12 months, or when it was stored in a vacuum desiceator and held at 22–24°C for 6 months.     

<Emphasis Type="Italic">Trichogramma Brassicae</Emphasis> and SLAM<Superscript>®</Superscript>, an Integrated Approach to Managing European Corn Borer and Corn Rootworms

European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae); western corn rootworm (WCRW), Diabrotica virgifera virgifera LeConte; and northern corn rootworm (NCRW), Diabrotica barberi (Smith & Lawrence) (Coleoptera: Chrysomelidae) are important pests of corn, Zea mays L., that occur simultaneously in the US Corn Belt. Areawide management strategies for northern and western corn rootworm in Iowa, Kansas, South Dakota, Illinois, and Indiana, and for the Mexican strain in Texas, use SLAM^®, which contains cucurbitacin, a feeding stimulant, and a small amount of the insecticide carbaryl. This management strategy is environmentally sound because it introduces a substantially lesser amount of chemical insecticide into the environment than prophylactic soil insecticide applications for WCRW management. To develop a management program for both corn pests, the compatibility of SLAM^® to control CRW, along with a biological agent to manage European corn borer, was investigated. Laboratory studies were conducted using the egg parasitoid Trichogramma brassicae (Bezdenko) (Hymenoptera: Trichogrammatidae). In experiments in which SLAM^® and T. brassicae were evaluated as components of a management system, SLAM^® did not affect parasitism or parasitoid emergence rates. Trichogramma brassicae displayed vigorous biotic fitness, emergence rates, sex ratios, and fecundity over the duration of the study. Data collected under these laboratory conditions indicated that SLAM^® had no adverse effect on the quality of T. brassicae, indicated by its ability to reproduce in the filial generation.     

Use of the egg parasiteEdovum puttleri [Hym.: Eulophidae] in an IPM system developed for Colorado potato beetle [Col.: Chrysomelidae] control on potatoes,Beltsville, Maryland

Edovum puttleri Grissell in conjunction with the fungal pathogenBeauveria bassiana (Balsamo) Vuillimen, a β-exotoxin ofBacillus thuringiensis varthuringiensis Berliner, and the insect growth regulator triflumuron (Alsystin 4F) were evaluated in a pest management system to control Colorado potato beetle,Leptinotarsa decemlineata (Say). Soil treatments ofB. bassiana had no significant impact on emerging Colorado potato beetle adults. Foliar applications of the β-exotoxin and triflumuron significantly reduced 1 ^st larval generation. Releases ofE. puttleri resulted in 56 % egg mass parasitization, significant reduction of 2 ^nd generation larvae, and significantly higher potato yields.Leptinotarsa texana (Schaeffer) was successfully used as an alternate host for production ofE. puttleri.      

Age-specific dose-mortality effects of Beauveria bassiana (Deuteromycotina: Hyphomycetes) on the European corn borer,Ostrinia nubilalis (Lepidoptera: Pyralidae)

Three isolates of Beauveria bassiana were evaluated under laboratory conditions for pathogenicity against the European corn borer, Ostrinia nubilalis. Dose-mortality regressions and nonparametric statistical analyses revealed significant differences in pathogenicity between isolates and across O. nubilalis instars. First instars were found to be the most susceptible of the five larval stages. Little difference was noted for instars 2, 3, and 5 within any of the three B. bassiana isolates. Fourth instar larvae were found to be the most tolerant in all cases.     

Variability in susceptibility to simulated sunlight of conidia among isolates of entomopathogenic Hyphomycetes

The influence of simulated sunlight on survival of conidia of 4 species of entomopathogenic Hyphomycetes was investigated. Conidia from 65 isolates ofBeauveria bassiana, 23 ofMetarhizium anisopliae, 14 ofMetarhizium flavoviride and 33 isolates ofPaecilomyces fumosoroseus were irradiated by artificial sunlight (295 to 1,100 nm at an ultraviolet-B irradiance of 0.3 W m^–2) for 0, 1, 2, 4 and 8 h. Survival was estimated by comparing the number of colony forming units (CFU) produced by conidia exposed to irradiation to the number of CFUs produced by an unexposed control. Survival decreased with increased exposure to simulated sunlight; exposure for 2 h or more was detrimental to all isolates tested. Overall, isolates ofM. flavoviride were the most resistant to irradiation followed byB. bassiana andM. anisopliae. Conidia ofP. fumosoroseus were most susceptible. In addition to the large interspecies differences in susceptibility to irradiation, there was also an intraspecies variation indicating that strain selection to irradiation tolerance may be important in the development of microbial control agents where increased persistence in an insolated environment is desirable.Abbreviations CFU Colony forming units - UV-B ultraviolet radiation-B     

Treatment of millet crop plant (Sorghum bicolor) with the entomopathogenic fungus (Beauveria bassiana) to combat infestation by the stem borer,Chilo partellus Swinhoe (Lepidoptera: Pyralidae)

Experiments were done to test if Beauveria bassiana can become an endophyte in sorghum and confer protection from stem borer. Four-week-old sorghum seedlings were treated with B. bassiana. The plants were examined for endophytic presence of B. bassiana, 30 and 60 days after treatment. Stem cultures from treated plants showed growth of B. bassiana. PCR amplification using fungal specific primers for a conserved region of β tubulin gene yielded identical 360 bp products from both B. bassiana and treated sorghum plants. In a subsequent experiment, B. bassiana treated and untreated (control) sorghum plants were artificially infested with stem borer (Chilo partellus) larvae 15 days post treatment and the extent of damage was compared. About 40% of the control plants developed dead heart while no plant in the B. bassiana treated plot did. In the surviving control plants, stem tunneling by shoot borer was significantly higher compared to B. bassiana treated sorghum plants.     

The effect of stem diameter on European corn borer behavior and survival: potential consequences for IRM in Bt-corn

The ability of non-crop plants to support complete development of insect pests is an important factor for determining the impact of those plants on resistance management programs for transgenic crops. We assessed the effect of one physical factor, plant stem diameter, on the ability of plants to support full development of the European corn borer (ECB), Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), the target pest of transgenic Bt-corn. In the field, European corn borer larvae were significantly more likely to tunnel and survive in plants with larger stem diameters. Larvae were 40× more likely to survive on corn, the largest plant tested, compared to many of the smaller plants. In the laboratory, larvae were more likely to survive in and less likely to abandon the largest diet-filled artificial stems that varied only in stem diameter. In conditions simulating those that an ECB larvae would encounter upon abandoning a host, larvae survived up to three weeks and were able to locate corn as a new host with a significantly higher frequency than would be expected if they were foraging randomly. These results indicate that the probability of ECB larval survival to maturity on a plant other than corn is relatively low and thus these smaller stemmed non-corn plants may not make a substantial contribution to the pool of susceptible adults. Conversely, since more mature larvae are not as susceptible as neonates, any larvae that partially develop on non-corn plants and subsequently colonize Bt-corn may not be exposed to a lethal dose of the toxin. Since some proportion of the individuals that survive could be partially resistant heterozygotes the presence of non-corn host plants could facilitate the development of resistant ECB populations.     

Field evaluation ofAllorhogas pyralophagus [Hym.: Braconidae], imported into texas for biological control of the stalkborerEoreuma loftini [Lep.: Pyralidae] in sugar cane

The external, gregarious, braconid parasite,Allorhogas pyralophagus Marsh was mass released into field cages enclosing sugar cane infested with the pyralid,Eoreuma loftini (Dyar) to examine factors affecting parasitism rates and to assess the parasite's potential contribution to borer larval mortality. Three field experiments were conducted in July, September, and December, 1984. Total parasitism was 25.2% for the study and was the single most important source of borer mortality. Borers were equally susceptible to attack in leaf sheaths, whorls, and stalks in general, but larvae tunneling deep within stalks were parasitized less. An interaction between tunneling behavior of the borer and attack strategy of the parasite was important for parasite success.A. pyralophagus is probably unable to effect complete control ofE. loftini in sugar cane becauseA. pyralophagus is physically unable to parasitize borers deep in the stalks. However, the parasite may be useful as part of an intergrated pest management program, acting in concert with other parasite species, or attacking the borer in small-stemed grasses that serve as alternate host plants.      

Host plant constituents as oviposition stimulants for a generalist herbivore: European corn borer

Oviposition response of the polyphagous European corn borer,Ostrinia nubilalis (Hübner) (Lepidoptera: Pyralidae), to chemical constituents in host plants was investigated in the laboratory using two-choice bioassays. Foliar extracts of corn (Zea mays L.), pepper (Capsicum annuum L.) and potato (Solanum tuberosum L.) were prepared using the solvents pentane, acetone and methanol. In all three host plants, chemicals soluble in pentane stimulated oviposition. In potato, chemicals extractable in acetone also elicited a positive oviposition response. When presented with a choice between pentane extracts of corn and pepper, females preferred corn. No preferences were exhibited between pentane extracts of corn and potato or pepper and potato. Pentane extracts of corn husks, tassels, silk, and corn leaves from plants at early whorl and tassel (pre-pollen shed) stages of development also stimulated oviposition. Similar extracts from plants at 2-leaf and blister (when kernels resemble blisters) stages were not stimulatory. This indicates that plant phenology affects chemically mediated oviposition response in European corn borer. The potential use of plant chemicals for management ofO. nubilalis in the field is suggested.