The potential of predators in natural control of aphids in wheat: Results of a ten-year field study in two German landscapes

In the present study, we investigated the natural control of aphids by predators in wheat fields in a low (L) and high-input cropping region (H) of Germany during a 10-year period. Data for the statistical analyses were obtained from weekly after the start of aphid emergence. The mean annual aphid indices, calculated as the sum of Sitobion avenae (Fabr.), Rhopalosiphum padi (L.), Metopolophium dirhodum (Walk.)(Homoptera: Aphididae), were 30.4 and 81.5 × 103 aphid days per m2, for L and H, respectively. Nine predator fractions were analysed: Coccinella septempunctata L., adults (1) and larvae (2), Propylea quatuordecimpunctata (L.) (Coleoptera: Coccinellidae) adults (3) and larvae (4), syrphid larvae (mostly Episyrphus balteatus [De Geer] (Diptera: Syrphidae)) (5), Chrysoperla carnea Steph. (Neuroptera: Chrysopidae) larvae (6), and adult carabids (7), staphylinids (8) and spiders (9). The two sites were comparable in terms of the mean size of the overall predator community, expressed in predator units (PU): 4.9 PU/m2 (L) vs. 5.4 PU/m2 (H). Most predator fractions responded numerically to increasing aphid densities. The numerical response was strongest in syrphid larvae, scarcely detectable in adult coccinellids, and virtually non-existent in epigeic arthropods. Multiple regression models revealed indirect relationships between the weekly overall predator community densities (PU/m2) and individual predator fractions (individuals/m2) and absolute rates of aphid density increase (individuals/m2) one or two weeks after baseline. A site-independent reduction of the aphid density increase to nil (y = 0) was observed at 3.9 to 4.2 PU/m2. Consequently, the 2.7 times higher aphid density at H cannot be attributed to the presence of fewer predators or lower effects of the overall predator community or of any individual predator fraction.     

Ladybird population dynamics in potato: comparison of native species with an invasive species, Harmonia axyridis

Following the detection of the harlequin ladybird, Harmonia axyridis, in 2003 in potato crops in Belgium, a study was carried out between 2004 and 2006 on the phenology of this species compared to native species in potato. The results confirmed the success of H. axyridis, with high population levels in 2004 and 2005. In 2006, aphid populations were very low and no H. axyridis larvae were sampled in potato, but the indigenous species Coccinella septempunctata and Propylea quatuordecimpunctata were detected. A species by species comparison of the date of first larvae detection, the larvae population peak, and the difference between this peak and the aphid population peak was performed. Results showed a clear correlation between C. septempunctata and P. quatuordecimpunctata and potato aphids, with a delay of 3.5 and 6.5 days between the aphid and ladybird population peaks for the two native species. H. axyridis arrived 7–8 days after the two indigenous species and the larval peak population occurred 15.8 days after the aphid population peak. This meant that H. axyridis had to complete its larval development with very low aphid populations or even with no aphids at all. The reason for its late arrival and the possible food resources used by H. axyridis larvae are discussed.     

The importance of intraguild interactions to the combined effect of a parasitoid and a predator on aphid population suppression

Intraguild predation (IGP) occurs when consumers competing for a resource also engage in predatory interactions. A common type of IGP involves aphid predators and parasitoids: since parasitoid offspring develop within aphid hosts, they are particularly vulnerable to predation by aphid predators such as coccinellid beetles. Other intraguild interactions that include non-lethal behavioral effects, such as interference with foraging and avoidance of IGP, may also hamper parasitoid activity and reduce their effectiveness as biological control agents. In this study, we quantified mortality in and behavioral effects on Aphidius colemani Viereck (Hymenoptera: Aphidiidae) by its IG-predator Coccinella undecimpunctata L. (Coleoptera: Coccinellidae), and compared the impact of two release ratios of these natural enemies on aphid populations. Parasitoids did not leave the plant onto which they were first introduced, regardless of the presence of predators, even when alternative prey was offered on predator-free plants nearby. In 2-hour experiments, predator larvae interfered with wasp activity, and the level of aphid parasitism was lower in the presence of predators than in their absence. In these experiments, the parasitoids contributed more to aphid mortality than the predators and aphid suppression was higher when a parasitoid acted alone than in combination with a predator larva. These results were confirmed in a 5-day experiment, but only at one parasitoid:predator release ratio (4:3) not another (2:3). The over-all impact on aphid population growth was non-the-less stronger when both enemies acted together than when only one of them was present. Results indicate that for given release ratios and time scale, the negative lethal and non-lethal effects of the predator on parasitoid performance did not fully cancelled the direct impact of the predator on the aphid population.     

Interactions between the fungal pathogen Beauveria bassiana and three species of coccinellid: Harmonia axyridis, Coccinella septempunctata and Adalia bipunctata

Harmonia axyridis is a predatory coccinellid, native to central and eastern Asia. It has been available in many countries for use as a biological control agent of pest aphids and scale insects. In many of these countries, including the USA, H. axyridis has established. It is now considered an invasive alien species for a number of reasons, including its impact on functional biodiversity. Beauveria bassiana is known to be a natural mortality agent of overwintering coccinellids and is a potential candidate for the biological control of H. axyridis. In this paper we compare the susceptibility of three species of coccinellid, H. axyridis (cultures derived from Japan and UK), Coccinella septempunctata and Adalia bipunctata to infection by B. bassiana (commercial strain GHA) after exposure at three doses (10⁵, 10⁷, 10⁹ conidia ml⁻¹). The two subpopulations of H. axyridis (Japan and UK) were more resistant to B. bassiana infection than either A. bipunctata or C. septempunctata. This is exemplified by the median lethal doses at 10 days post-inoculation (LD₅₀) of 10^6.2, 10^6.0, 10^8.3, 10^9.6 conidia ml⁻¹ for A. bipunctata, C. septempunctata, H. axyridis (Japan) and H. axyridis (UK), respectively. Only doses of 10⁹ conidia ml⁻¹ resulted in mortality of H. axyridis, in contrast, 80% of C. septempunctata and 70% of A. bipunctata exposed to 10⁷ conidia ml⁻¹ of B. bassiana succumbed to infection. The fecundity (cumulative mean egg production over 10 days) of A. bipunctata and H. axyridis (UK) was also assessed. The fecundity of C. septempunctata could not be assessed because this species requires diapause prior to the onset of reproduction and these studies were on beetles that had recently eclosed (2–8 weeks). Harmonia axyridis (Japan) produced no eggs in most treatments including the control and so was excluded from analysis. High dose (10⁹ conidia ml⁻¹) inoculation reduced the fecundity of A. bipunctata to zero but egg production was similar for individuals inoculated with doses of 10⁵, 10⁷ conidia ml⁻¹ and control individuals. In contrast, all doses of B. basssiana reduced H. axyridis (UK) egg production dramatically. We discuss these results in relation to the potential for control of H. axyridis using B. bassiana.     

Comparative trials on the effects of two fungicides on a predatory mite in the laboratory and in the field

Both laboratory tests with the open glass plate method of Louis & Ufer (1995) and a field study in a vineyard were carried out to test the side effects of the two fungicides, Dithane Ultra WG75 and Polyram Combi WG70, on the predatory mite Typhlodromus pyriScheuten (Acarina: Phytoseiidae). The effects of both fungicides in the residual contact test in the laboratory were more pronounced than the actual effects obtained in the field even after multiple application of the test products. The study results suggest, that for the selected test products the current laboratory guidelines correctly triggered the field test. Methodological problems of the laboratory test due to the repellent effect of the fungicides are discussed.     

Interactions in a tritrophic acarine predator-prey metapopulation system IV: effects of host plant condition on Tetranychus urticae (Acari: Tetranychidae)

Feeding by spider mites can cause severe injury to a host plant and lead to a decreasing per capita growth rate and an increasing per capita emigration rate. Such density-dependent responses to local conditions are important in a metapopulation context because they allow the herbivores to colonize new host plants and thereby prolong the time until regional (metapopulation) extinction. In order to include density-dependent responses of the two-spotted spider mite (Tetranychus urticae) in a realistic metapopulation model, a series of greenhouse experiments was conducted with the purpose to quantify how the condition of bean plants (Phaseolus vulgaris) influences the demographic parameters of T. urticae. Plant age per se reduced the growth rate of the spider mites only slightly, whereas the growth rate declined significantly as the plants were injured by the mites. The relationships between plant condition (expressed by the plant injury index D) and the birth and loss (death + emigration) rates of the mites were quantified so as to predict population growth as a function of D. Maximum per capita growth rate (r) was estimated to be c. 0.21 day⁻¹. The growth rate is expected to be negative when D exceeds 0.8. When mites were allowed to emigrate to neighbouring plants via bridges, the per capita emigration rate increased almost exponentially with D. The proportion of eggs in the population decreased with D while the numerical ratio between immatures to adults and the sex ratio did not change with D. Overall, immatures and adults constituted 74% and 26%, respectively, of the active mites and c. 46% of the adults were males. The bridges that connected a donor plant with the surrounding recipient plants were responsible for the majority of the emigrations from donor plants. Most mites stopped after having crossed a single bridge, but a few crossed two bridges while none crossed three bridges within 24 h. The significance of the results for biological control is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interactions in a tritrophic acarine predator-prey metapopulation system III: effects of Tetranychus urticae (Acari: Tetranychidae) on host plant condition

Spider mites are serious pests on many economically important plant species, because they may reduce plant productivity and, at high mite densities, overexploit and even kill the host plants. We have conducted a series of greenhouse experiments to quantify the effects of two-spotted spider mites (Tetranychus urticae) on host plants (Phaseolusvulgaris). The average amount of chlorophyll per cm² leaf area was used as a measure of plant condition. It was shown that chlorophyll concentration decreases with plant age and intensity of spider mite feeding. Damage caused by spider mites was assessed visually, using the Leaf Damage Index (LDI) defined by, and a mathematical relationship between the visual measurements and the amount of chlorophyll/cm² was fitted to data. The relationship may serve as a short-cut to estimate overall plant injury, expressed as the relative loss of chlorophyll/cm² leaf area caused by spider mites (D). D takes values between 0 (no injury) and 1 (all leaves dead). A highly significant positive relationship between the instantaneous spider mite density and D was found, even though D is expected to reflect the cumulated density of mites (mite-days). A model of plant growth incorporating information about plant age and D predicts that plant area has a maximum when plant age is about 60 days, and that plant area decreases exponentially with an increase in D. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new species and three new combinations in Cyphellophora, with a note on the taxonomic affinities of the genus, and its relation to Kumbhamaya and Pseudomicrodochium.

Cyphellophora guyanensis sp. nov., isolated from leaf litter in French Guyana, is described and illustrated. This fungus is characterized by ampulliform to flask-shaped phialides, with a conspicuous, funnel-shaped collarette, and nearly straight to falcate or slightly sigmoid, (2−)3–6 septate conidia. Cyphellophora guyanensis is compared to the other Cyphellophora species, and the latter genus is compared to Kumbhamaya and Pseudomicrodochium. Kumbhamaya indica, P. fusarioides, and P. suttonii are transferred to Cyphellophora. The phylogenetic relationships of Cyphellophora with other genera of ascomycetes and related mitosporic fungi were inferred from the analysis of the partial nuclear ribosomal DNA SSU sequences data. The analysis supports a placement of Cyphellophora in Chaetothyriales, and probably in the Herpotrichiellaceae This revised version was published online in August 2006 with corrections to the Cover Date.     

Insect-crop interactions in a diversified cropping system: parasitism by Aleochara bilineata and Trybliographa rapae of the cabbage root fly, Delia radicum, on cabbage in the presence of white clover

Parasitism of the cabbage root fly, Delia radicum (L.) by the staphylinid Aleochara bilineata Gyllenhal and the cynipid Trybliographa rapae Westwood was examined in a cabbage monoculture and a mixed stand of cabbage undersown with white clover. Number of overwintering cabbage root fly pupae per plant was consistently reduced in the mixed stand, and the incidence of plants attacked by cabbage root fly was either reduced or not different in the mixed stand compared to cabbage monoculture. For both parasitoids, the probability of D. radicum attacked plants having at least one parasitized pupa increased with density of cabbage root fly pupae around the plant. For A. bilineata, this positive relation between presence of parasitism and host density was consistently stronger in cabbage monoculture than in cabbage undersown with clover. Location of a host plant by T. rapae was not consistently affected by the presence of clover. D. radicum attacked plants situated in the cabbage and clover mixture were found by T. rapae as easily as in cabbage monoculture. Overall, the total risk of parasitism for a cabbage root fly pupa by A. bilineata was reduced in the mixed stand compared to the cabbage monoculture, whereas the risk of parasitism by T. rapae was not consistently affected by clover. For both parasitoids, intensity of parasitism showed a variable relationship with host density on individual plants attacked by the cabbage root fly. Overall, in spite of consistently lower total density of pupae in the mixed cabbage—clover than in cabbage monoculture, the density of unparasitized pupae was reduced by the presence of non-host plants only in two of the four experiments. The results emphasize the need to include not only herbivore and crop, but also other plant species as well as natural enemies when evaluating management methods.     

Reproductive potential of Daphnia galeata in relation to food conditions: implications of a changing size-structure of the population

A field study was conducted in the highly eutrophic, biomanipulated Bautzen Reservoir to investigate the influences of food conditions on fecundity of Daphnia galeata during spring and early summer in four consecutive years. By defining a relation between body size and maximum brood size, the realised reproductive potential (RRP) of Daphnia was expressed as percentage of actual egg numbers of the population in relation to calculated maximum egg numbers. This parameter is independent of the size structure of the population. It was found to correlate generally with mean clutch size. However, deviations occurred if size at maturity (SAM) declined. Extremely low SAM values (0.8 mm) were recorded in 2 years, corresponding with increasing food concentrations. Low clutch sizes in these situations can be explained by small body sizes of ovigerous females, whereas RRP increased, thus reflecting good food conditions. From the comparison of different reproductive parameters, it is concluded that RRP is the most reliable one with regard to bottom-up influences in case of extremely variable size structure and possible predation impacts. Fecundity of D. galeata was clearly food limited during the clear-water stage. Relatively high POC-threshold concentrations of 0.26 mg C l^–1 suggest that food quality was low and may be equally important as food quantity in limiting egg production.     

Use of a continuous culture fermentation system to investigate the effect of GanedenBC30 (Bacillus coagulans GBI-30, 6086) supplementation on pathogen survival in the human gut microbiota

Single-stage continuous fermentation systems were employed to examine the effects of GanedenBC(30) supplementation on the human gastrointestinal microbiota in relation to pathogen challenge in vitro. Denaturing gradient gel electrophoresis analysis demonstrated that GanedenBC(30) supplementation modified the microbial profiles in the fermentation systems compared with controls, with profiles clustering according to treatment. Overall, GanedenBC(30) supplementation did not elicit major changes in bacterial population counts in vitro, although notably higher Bcoa191 counts were seen following probiotic supplementation (compared to the controls). Pathogen challenge did not elicit significant modification of the microbial counts in vitro, although notably higher Clit135 counts were seen in the control system post-Clostridium difficile challenge than in the corresponding GanedenBC(30)-supplemented systems. Sporulation appears to be associated with the anti-microbial activity of GanedenBC(30), suggesting that a bi-modal lifecycle of GanedenBC(30)in vivo may lead to anti-microbial activity in distal regions of the gastrointestinal tract.     

Comparing different approaches to calculate the effects of heterogeneous root distribution on nutrient uptake: a case study on subsoil nitrate uptake by a barley root system

Simulation models of nutrient uptake of root systems starting with one-dimensional single root approaches up to complex three-dimensional models are increasingly used for examining the interacting of root distribution and nutrient uptake. However, their accuracy was seldom systematically tested. The objective of the study is to compare one-dimensional and two-dimensional modelling approaches and to test their applicability for simulation of nutrient uptake of heterogeneously distributed root systems giving particular attention to the impact of spatial resolution. Therefore, a field experiment was carried out with spring barley (Hordeum vulgare L. cv. Barke) in order to obtain data of in situ root distribution patterns as model input. Results indicate that a comparable coarse spatial resolution can be used with sufficient modelling results when a steady state approximation is applied to the sink cells of the two-dimensional model. Furthermore, the accuracy of the model was clearly improved compared to a simple zero sink approach assuming both near zero concentrations within the sink cell and a linear gradient between the sink cell and its adjacent neighbours. However, for modelling nitrate uptake of a heterogeneous root system a minimum number of grid cells is still necessary. The tested single root approach provided a computational efficient opportunity to simulate nitrate uptake of an irregular distributed root system. Nevertheless, two-dimensional models are better suited for a number of applications (e.g. surveys made on the impact of soil heterogeneity on plant nutrient uptake). Different settings for the suggested modelling techniques are discussed.     

Ectomycorrhizal syntheses with Picea abies and three fungal species: a case study on the use of an in vitro technique to identify naturally occurring ectomycorrhizae

Ectomycorrhizal syntheses between Picea abies and the fungal associates Scleroderma citrinum, Boletus luridus, and Tricholoma vaccinum were carried out using Melin's Erlenmeyer flask technique. The symbioses of S. citrinum were characterized by a mantle composed of an outer prosenchymatous and an inner synenchymatous layer. The mantles of B. luridus and T. vaccinum were solely prosenchymatous. Rhizomorphs were produced in all treatments, but only in association with S. citrinum were they differentiated with additional, enlarged hyphae. All synthesized ectomycorrhizae were white or whitish to light orange and greyishorange. On large-scale root sampling in two differing Picea abies forests in Switzerland, nine out of a total of 22 morphological types of ectomycorrhizae were white or yellow in colour and were, therefore, comparable with the synthesized ectomycorrhizae. These nine natural types generally had distinct mantle features (irregular synenchyma, gelatinous matrix, cystides, thick-walled hyphae), but mostly lacked clamp connections. Synthesized ectomycorrhizae, on the other hand, lacked distinct mantle characteristics and always had clamp connections. Natural and synthesized white or yellow ectomycorrhizae did not coincide morphologically and thus identification of the fungal partners of natural symbioses by means of in vitro-synthesis with potential ectomycorrhizal fungi was not possible in the present study.