Overwintering of the egg parasitoid Trichogramma brassicae in Northern Switzerland

As part of a general risk assessment study weinvestigated the overwintering abilities ofTrichogramma brassicae Bezd. (Hym.: Trichogrammatidae) in Northern Switzerland. Eggs of six host species parasitized by T. brassicae were exposed under outdoor conditions every two weeksbetween 26 September and 7 November 1998. Wefound that T. brassicae is able tooverwinter successfully on six lepidopteranspecies in the families Tortricidae, Noctuidae,Plutellidae, Pyralidae and Crambidae. Between75 and 100% emergence was observed in thefollowing spring for all of the six tested hostspecies exposed on 26 September. On laterexposure dates, spring emergence decreasedsignificantly and no development of T. brassicae offspring occurred from host eggsparasitized on 7 November.Emergence of T. brassicae from eggs ofEphestia kuehniella Zeller parasitized atweekly intervals in autumn 1999 occurred in thesame year until the end of October if the eggswere parasitized by 10 September. All offspringfrom eggs that were parasitized from 17September onwards went into diapause. Springemergence of T. brassicae adults wasobserved between end of April and beginning ofMay both in 1999 and 2000. Fecundity of T.brassicae females that overwintered on E.kuehniella was not significantly different fromthe fecundity of control females that werereared without diapause under optimalconditions at 25 °C. Our resultsdemonstrate that the egg parasitoid T.brassicae is able to overwinter successfullyin Northern Switzerland and has the potentialto establish in Switzerland. As a result,potential non-target effects are not locallyrestricted but may occur on a largegeographical scale.     

Limonoids from the Meliaceae and Rutaceae reduce feeding, growth and development of Ostrinia nubilalis

Six limonoids from the Rutaceae and Meliaceae were evaluated for their effect on the European corn borer, Ostrinia nubilalis Hübner (Lepidoptera: Pyralidae). Antifeedant activity increased in the order gedunin, bussein, entandrophragmin, nomilin, cedrelone, anthothecol. When incorporated into artifical diets of neonates at 50 ppm all compounds either cuased larval mortality or growth reduction of survivors. However in a study with naive third instar larvae, only cedrelone and anthothecol were effective in reducing efficiency of conversion of ingested food at concentrations of 100 ppm or less. In sublethal trials at 10 ppm in diet, these two substances led to growth reduction of larvae and reduced female, but not male pupal weights, percent pupation, adult emergence and number of eggs per female. The results are discussed in terms of structure-activity relationships.

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Résumé On a évalué l'effet de six limonoides provenent des Rutaceae et des Meliaceae sur la pyrale du mais, Ostrinia nubilalis Hübner (Lepidoptera: Pyralidae). L'activité anti-appétante a augmenté dans l'ordre suivant: gédunine, busséine, entandrophragmine, nomiline, cédrélone, anthothecol. Lorsqu'on incorpore ces composés à un régime artificiel à raison de 50 ppm on observe soit des mortalites larvaires, soit une réduction de croissance des survivants. Cependant lors d'une étude avec des larves de 3ème âge n'ayant jamais été exposées à de telles substances, seules la cédrélone et l'anthothecol ont été effectifs, réduisant la capacité de conversion de la nourriture ingérée à des concentrations de 100 ppm ou moins. A des doses sub-létales de 10 ppm dans le régime, ces deux substances ont produit une réduction de croissance des larves, une réduction de poids des femelles mais non des mâles, une réduction du pourcentage de pupaison, de l'émergence des adultes et du nombre d'oeufs par femelle. Les résultats sont discutés en fonction des relations structure-activité.

     

Parasitism of non-target lepidoptera by mass released Trichogramma brassicae and its implication for the larval parasitoid Lydella thompsoni

The release of high numbers of the eggparasitoid Trichogramma brassicae Bezd.(Hym. Trichogrammatidae) to control theEuropean corn borer (ECB), Ostrinianubilalis Hb. (Lep.: Crambidae) in maize hasraised concerns about potential negativeeffects on native natural enemies. The nativelarval parasitoid Lydella thompsoniHerting (Dipt.: Tachinidae) is the mostfrequent and important ECB parasitoid insouthern Switzerland and can achieve highparasitism rates. Its first generation emergestoo early to find ECB larvae and must rely onalternative hosts living in natural habitatsclose to maize fields. Inundative releases ofT. brassicae coincide with theoviposition period of the alternative hosts ofthe tachinid. T. brassicae moving out ofrelease fields may attack and diminish thepopulation of these hosts, creating abottleneck situation for L. thompsoni inthe subsequent spring. Laboratory hostspecificity tests showed that the tachinid'stwo most abundant spring hosts Archanarageminipuncta Haworth (1809) (Lep.: Noctuidae)and Chilo phragmitellus Hübner (1805)(Lep.: Crambidae) are successfully parasitisedby T. brassicae females in no-choicesituations. Our extensive field surveys,however, showed that the two tested springhosts escape parasitism since their eggs arewell hidden or not attractive. Negativeeffects of inundative releases of T.brassicae on the native tachinid fly L. thompsoni, such as population densityreduction, displacement, or local extinction,are very unlikely.     

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.     

Quality and environmental factors affecting Trichogramma brassicae efficiency under field conditions

Field parasitism of the egg parasitoid Trichogramma brassicae Bezdenko (Hymenoptera, Trichogrammatidae) (synonymous to T. maidis Pint. et Voegele) on Ostrinia nubilalis Hübner was compared to four single quality parameters (walking speed, fecundity on the factitious host Ephestia keuhniella Zeller, fecundity on the natural host O. nubilalis, and life span) previously measured in the laboratory and, a quality index calculated from three of the measured parameters. A single quality parameter (fecundity on E. keuhniella) and the calculated quality index showed a correlation to T. brassicae field parasitism, for different T. brassicae populations. The number of female Trichogramma released clearly influenced parasitism but not in the same proportion for all populations tested, reflecting that numbers released can only to some extent compensate for low quality in Trichogramma. Variation indegree hours above 18°C also influenced field parasitism. When incorporating degree hours to the quality index a significant correlation R²=0.56 (P=0.01) is obtained. In this work fecundity of T. brassicae on E. keuhniella can be as good an indicator for the potential field performance of T. brassicae, as the quality index which takes into account three quality parameters. Since environmental factors can obscure the potential performance of a population, i.e., one which under optimal laboratory conditions performs well, the quality index gives only a partial indication of how the released strains will perform in the field.     

Flight initiation in the egg parasitoid Trichogramma minutum: Effects of ambient temperature, mates, food, and host eggs

Emergence, preening, and flight initiation were studied in laboratory-reared Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae). Male parasitoids emerged first and flew before females. When both sexes were present in flight cylinders, female parasitoids flew before males. Flight propensity in males was negatively related to the number of emerging females, while flight propensity in females was independent of the number of males present. Ambient temperature significantly affected the propensity and timing of flight; between 70–80% of the parasitoids flew at 25 and 30 °C while less than 4%, mostly males, flew at 20 °C. No flights were observed at 15 °C. The presence of fresh host eggs caused a reduction in the proportion of female parasitoids that flew and a delay in the time to flight for the females that did fly. The presence of food increased the flight propensity of female parasitoids, but did not affect the timing of flight. The relationship between flight behaviour and the efficiency of mass-reared Trichogramma is discussed in terms of its importance for inundative release programmes.     

The fate of immunoglobulin G fed to larvae of Ostrinia nubilalis

Since plants can be transformed genetically to produce functional antibodies, an immunological approach may be developed for controlling their arthropod pests. Specific antibodies would protect plants from arthropods if they could gain access to the pest antigen in sufficient amounts such that the normal function of the antigen is disrupted. In order to study the fate of ingested antibodies in the body of the European corn borer (ECB), Ostrinia nubilalis (Hübner), (Lepidoptera: Pyralidae), we fed the larvae on serum-containing diet. When larvae were fed on the serum-containing diet for various lengths of time between 12 and 96 h, no significant differences were noted in the immunoglobulin G (IgG) concentration in their body. Immediately after the larvae stopped feeding, the concentrations of the IgG in their midgut was about one half that of the diet itself, but it decreased significantly after 6 h and again after 18 h (about 3 and 10 fold, respectively). Immediately after the larvae stopped feeding, the concentration of the IgG in their hemolymph was about 1/500 that in the diet. The concentration of IgG in the hemolymph of ECB larvae was influenced directly by the titer of antibodies in their diet. During the first 6 h after the larvae stop feeding the concentration of IgG in their hemolymph did not decrease significantly; however, it did so after 18 h (about 6 fold). The possibility that specific antibodies will gain access to antigens in the ECB body is discussed.     

Sexual dimorphism of tarsal receptors and sensory equipment of the ovipositor in the European corn borer,Ostrinia nubilalis

Summary Sensilla on legs and ovipositor of the moth Ostrinia nubilalis were investigated by light and scanning electron microscopy. The ovipositor is composed of two papillae densely packed with medium length mechanoreceptor sensilla (MRb: 80–160 m, n=420–460). Long mechanoreceptor sensilla (MRa: 250–300 m, n=20–24) and contact chemoreceptors (CRa: 30–40 m, n=20–28) are evenly distributed at the periphery of these papillae. Legs support contact chemoreceptors (CRa), scattered among the scales. The pretarsus structure of each leg includes a single contact chemoreceptor (125 m) inserted dorsally. The fifth tarsomere bears a ventral area without scales on which contact chemoreceptors are disposed in two parallel rows (CRb: 40–60 m). A sexual dimorphism was found in the number and density of these sensilla (females: mean = 5.3, SD=1.0; males: mean = 3.3, SD=0.7), and in the size of the sensory field. The possible involvement of these sensory structures in oviposition site selection is discussed.     

Locomotor activity ofTrichogramma brassicae (Hymenoptera) under the influence of high-temperature shocks

In order to assess the effects of high temperatures prevailing during the releases ofTrichogramma brassicae Bezdenko parasitoid used against the European corn borerOstrinia nubilalis Hübner, we exposed wasps in the white pupa stage (WP) to single or double shocks (at 35 or 44°C for 6 h) in the laboratory. The locomotor activity of adults in the presence ofO. nubilalis egg masses is greater than in pure air. Single or double shocks at 35°C have little effect on the females. On the contrary, wasps exposed to single or double shocks at 44°C had a lower motility than control wasps.     

Biology ofTrichogramma ostriniae (Hym.: trichogrammatidae) reared onOstrinia nubilalis (Lep.: pyralidae) and survey for additional hosts

Trichogramma ostriniae has been identified as a candidate for biological control ofOstrinia nubilalis. As little was known about the biology of this parasitoid when reared onO. nubilalis, we undertook experiments to quantify biological parameters important to mass-rearing and ase ofT. ostriniae. When reared continuously onO. nubilalis, femaleT. ostriniae on average lived 2.7 days and produced 24 progeny. Continuous access to honey resulted in a four-fold increase in longevity and fecundity and a significant increase in the percentage of females parasitizing eggs. Rates of fecundity and parasitism decreased with age of female. Likewise, emergence rates and percentage of female progeny decreased with age of parental female.T. ostriniae successfully parasitizedO. nubilalis eggs until the blackhead stage. Most parasitism of eggs and eclosion of adults occurred during the first half of photophase. Eggs of 13 Lepidopterans were parasitized byT. ostriniae. Eggs of the Noctuidae, Pyralidae, and Plutellidae experienced higher levels of parasitism than others tested.T. ostriniae appears to be similar to other species ofTrichogramma in several respects and does not possess any characteristics that limit its potential for mass rearing and use for augmentative biological control ofO. nubilalis.     

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.     

Large-scale releases of Trichogramma ostriniae to suppress Ostrinia nubilalis in commercially grown processing and fresh market sweet corn

Abstract:  We evaluated the egg parasitoid Trichogramma ostriniae (Pang et Chen) (Hym.: Trichogrammatidae) for control of European corn borer [Lep.:Crambidae: Ostrinia nubilalis (Hübner)] in fields of commercial processing sweet corn in New York in 2002 and 2003. We made inoculative releases of 75 000 T. ostriniae /ha when corn was at mid-whorl and regional O. nubilalis activity had begun. Each release field was paired with a similar non-release control. The incidence of egg mass parasitism, number of stalk tunnels, incidence of ear damage and effect on insecticide spray decisions were evaluated. Parasitism of cumulative number of O. nubilalis egg masses was 51.3 ± 7.2% and 43.5 ± 5.7% (mean ± SEM) in release fields in 2002 and 2003 respectively. The incidence of ear damage by O. nubilalis was not affected by T. ostriniae in either year of the study. There was no difference in stalk damage between release fields and control fields in either 2002 or 2003. However, individual comparisons between paired release and control fields showed reduced stalk tunnelling in 10 of 19 fields, but ear damage was reduced in only two of 19 fields. Neither the number of actual insecticide sprays nor the number of decisions to spray based on sequential sampling were affected significantly by releasing T. ostriniae . In a separate but related study, where releases were conducted in commercial fresh market sweet corn and used higher rates of release, T. ostriniae had no effect on the number of insecticidal sprays. In an additional experiment using small plots of sweet corn, ear damage was similar whether plots were treated with T. Ostriniae or with insecticide. Additional research should focus on improving the timing, frequency and rate of releases.     

Changes in calling behaviour and mating success in the European corn borer (Ostrinia nubilalis), caused by relative humidity

We examined the calling behaviour of O. nubilalis virgin females, held at three different constant conditions of relative humidity from the time of emergence through their sixth night of calling, as well as the incidence of mating over time at low or high relative humidity conditions. The mean age at which females initiated calling in their life was independent of relative humidity conditions, although more variable at low humidity. On successive nights of calling, fewer virgin European corn borer females called under low compared with high humidity conditions, and those that did began later in scotophase, had fewer calling bouts, and spent less time calling. These changes in the female prereproductive behaviour were reflected on the incidence of mating which, in the first two days following emergence, was significantly less at low than at high humidity conditions. The ecological significance of these results is discussed in relation with the susceptibility of O. nubilalis to desiccation.

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Résumé Nous avons étudié le comportement d'appel des femelles vierges d'Ostrinia nubilalis maintenues sous trois conditions constantes d'humidité relative, depuis l'émergence jusqu'à la sixième nuit d'appel, ainsi que l'incidence d'accouplement d'individus placés en milieu sec ou humide pour diverses périodes de temps. Les conditions d'humidité n'ont pas significativement affecté l'âge moyen auquel les femelles ont commencé à appeler au cours de leur vie, bien qu'il ait été plus variable à 53% h.r. Les nuits subséquentes, le comportement d'appel des femelles maintenues à basse humidité a débuté plus tard dans la nuit, s'est exprimé pour une période de temps plus courte avec moins de séquences d'appel que celui de leurs congénères placés en milieu humide, et la proportion de femelles en appel a toujours été plus faible à 53% h.r. Ces changements du comportement préreproducteur des femelles se sont répercutés sur l'incidence d'accouplement qui, les deux premiers jours suivant lémergence, a été significativement plus faible à basse humidité qu'elle ne l'a été en milieu humide. Ces résultats seront discutés en relation avec l'écologie de O. nubilalis et associés à sa susceptibilité à la dessication.

     

Trichogramma platneri (Hym.: Trichogrammatidae): Host choices between viable and nonviable codling moth,Cydia pomonella,and three-lined leafroller,Pandemis limitata (Lep.: Tortricidae) eggs

In laboratory host-preference studies,Trichogramma platneri Nagarkatti (Hym.: Trichogrammatidae) females, reared on viable codling moth,Cydia pomonella (L.) (Lep.: Tortricidae) eggs, parasitized significantly more viable than nonviable codling moth eggs produced by cobalt 60 irradiated adults. This ovipositional preference was maintained when competition increased per host egg. Similarly, one, three and fiveT. platneri females consistently oviposited more eggs into viable versus nonviable host eggs. A single femaleT. platneri parasitized a similar number of viableC. pomonella and three-lined leafroller,Pandemis limitata (Robinson) (Lep.: Tortricidae) eggs. However, as the number of females per host eggs increased, a significantly larger number of codling moth versus leafroller eggs were parasitized. One, three and fiveTrichogramma females parasitized significantly moreP. limitata versus nonviableC. pomonella eggs. NonviableC. pomonella eggs deflated to less than 25% of their original volume in four and ten days when held under dry and humid conditions respectively. The blackened prepupal stage of the parasitoid generally occurred in the nonviableC. pomonella eggs four days post-parasitization whereas the parasitized viable eggs blackened zero to one day earlier. Incubation under dry or humid conditions did not appear to influence the number of parasitized eggs that were able to blacken and show evidence of parasitism. Summerland Research Centre Contribution No. 938.     

Importance of plant size, distribution of egg masses, and weather conditions on egg parasitism of the European corn borer, Ostrinia nubilalis by Trichogramma ostriniae in sweet corn

Trichogramma ostriniae (Hym: Trichogrammatidae), an egg parasitoid of the European corn borer, Ostrinia nubilalis (Lep: Pyralidae), were released into sweet corn (Zea mays L.) fields to study the effects of weather, plant size and distribution of egg masses on egg parasitism by the wasp. Sentinel European corn borer eggs were stapled onto leaves located in the upper, middle and lower third of sweet corn plants 5 to 35 meters away from the wasp release point in either a radial or grid manner. Weather conditions and plant architecture were monitored during the experiments. Logistic regression was used to analyze the data. The results indicated that percentage of eggs parasitized was negatively related to an increase in leaf area as well as an increase in distance eggs were located from the point of release of wasps. Eggs distributed on plants at different directions from the release point received different levels of parasitism. Eggs that were stapled onto leaves in the upper third of a corn plant received much less parasitism than those on the middle and lower third of the plant. Higher mean temperature adversely affected the level of parasitism during hotter times of the season and conversely, lower temperatures (<17 °C ) reduced the egg parasitism during cooler times of the season. The longer the exposure of eggs to wasps, the higher the level of egg parasitism. However, the levels of egg parasitism for 2 day's exposure were almost the same as that for 3 day's exposure due to the limited longevity and egg-laying behavior of the wasp. These results suggest that inundative releases of T. ostriniae should be made every two to three days, with multiple release points per hectare. In addition, weather conditions and plant architecture, especially temperature, plant height and leaf area must be taken into consideration to optimize levels of parasitism.     

Sex pheromone components of the cabbage armyworm, Mamestra brassicae: Isolation, identification and field experiments

Glass capillary gaschromatography coupled to a mass spectrometer has shown that in addition to Z-11-hexadecenyl acetate, hexadecyl acetate and Z-11-heptadecenyl acetate are present in pheromone glands of calling virgin females of Mamestra brassicae L. (Lepidoptera: Noctuidae, Hadeninae). Electroantennography revealed positive responses for Z 11–17: Ac and 16: Ac, while behavioural tests were only positive for Z 11–17: Ac.

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Résumé Il a été procédé à l'identification des phéromones par chromatographie en phase gazeuse et par spectrographie de masse. Outre l'acétoxy-1 hexadécène-11Z, on identifié l'acétoxy-heptadécène-11Z et l'acétoxy-1-hexadécane dans les glandes des femelles vierges de M. brassicae. L'antennographie a montré des réactions positives à l'acétoxy-hepta-décène-11Z et à l'acétoxy-1-hexadécane, tandis que seul l'acétoxyhepta-décène-11Z provoque un comportement sexuel des mâles.

     

Molecular cloning and functional expression of chitinase-encoding cDNA from the cabbage moth, Mamestra brassicae

Chitinase is a rate-limiting and endo-splitting enzyme involved in the bio-degradation of chitin, an important component of the cuticular exoskeleton and peritrophic matrix in insects. We isolated a cDNA-encoding chitinase from the last larval integument of the cabbage moth, Mamestra brassicae (Lepidoptera; Noctuidae), cloned the ORF cDNA into E. coli to confirm its functionality, and analyzed the deduced amino acid sequence in comparison with previously described lepidopteran chitinases. M. brassicae chitinase expressed in the transformed E. coli cells with the chitinase-encoding cDNA enhanced cell proliferation to about 1.6 times of the untransformed wild type strain in a colloidal chitin-including medium with only a very limited amount of other nutrients. Compared with the wild type strain, the intracellular levels of chitin degradation derivatives, glucosamine and N-acetylglucosamine were about 7.2 and 2.3 times higher, respectively, while the extracellular chitinase activity was about 2.2 times higher in the transformed strain. The ORF of M. brassicae chitinaseencoding cDNA consisted of 1686 nucleotides (562 amino acid residues) except for the stop codon, and its deduced amino acid composition revealed a calculated molecular weight of 62.7 and theoretical pI of 5.3. The ORF was composed of N-terminal leading signal peptide (AA 1-20), catalytic domain (AA 21-392), linker region (AA 393-498), and C-terminal chitin-binding domain (AA 499-562) showing its characteristic structure as a molting fluid chitinase. In phylogenetic analysis, the enzymes from 6 noctuid species were grouped together, separately from a group of 3 bombycid and 1 tortricid enzymes, corresponding to their taxonomic relationships at both the family and genus levels.