The distribution and abundance of predatory arthropods overwintering on farmland

Soil samples (0·04 m^-2) were taken from December to January in 1981-82 and 1982-83 from five crop and two non-crop habitats on farmland in Hampshire to quantify the distribution and abundance of overwintering polyphagous predatory arthropods. Total numbers of overwintering polyphagous predators were greater in field boundaries, winter-sown cereals and established grassland than in the other four habitats. Many individual species overwintered in significantly higher densities in field boundaries than in any other habitat; others predominated in woodland, grassland and winter-sown cereals. The carabids Agonum dorsale and Demetrias atricapillus, probably the most important predators of cereal aphids, overwintered almost entirely in field boundaries.     

Abiotic and biotic factors influencing the winter distribution of predatory insects

Summary Various environmental factors were investigated to analyse those involved in successful overwintering and possibly overwintering site selection for Tachyporus hypnorum and Demetrias atricapillus, both important coleopteran predators of cereal aphids. The results of the study indicated food supply to be important for both predator species during the winter period, although the role of biotic factors in site selection in the autumn could not be clearly demonstrated. The winter distribution of the two species could, however, be explained well in terms of abiotic factors. It is suggested that these and other similar predator species have well-defined overwintering requirements and that these can be exploited in the management of field boundary habitats.     

Untersuchungen über das Vorkommen der Laufkäfer [Col.: Carabidae] auf Europäischen Getreidefeldern

Zusammenfassung Mit Bodenfallen wurde das Auftreten der Laufk?fer vorwiegend in Winterweizenfeldern in der Zeit von Ende Mai bis Anfang August 1973 und 1974 in Belgien, Deutschland, den Niederlanden und Schweden festgestellt. Unter den h?ufigsten Arten tratenPterostichus vulgaris undBembidion lampros in allen,Agonum dorsale in fast allen Untersuchungsgebieten gemeinsam auf. Sechzehn weitere Arten konnten als allgemein für die meisten Weizenfelder der Untersuchungsgebiete genannt werden. In fast allen Gebieten waren (gemessen an der Artenzahl) die Frühlingstiere mit Herbstbestand in der überzahl Die durchschnittliche Anazahl der pro Falle und Tag gefangenen Laufk?fer variierte von 1 bis 13 und lag im Mittel aller Gebiete bei 6 K?fern. Mit der Quadratmenthode wurden in einem norddeutschen Gebiet 6000 bis 11000 Individuen der ArtAgonum dorsale pro ha festgestellt. Es wird darauf hingewiesen, dass es lohnenswert ist, die Laufk?fer bei der Erstellung integrierter Pflanzenschutzprogramme zu berücksichtigen.

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Summary The occurrence ofCarabidae was investigated by pitfall trapping mainly on fields of winter wheat from the end of May to the beginning of August 1973 and 1974 in Belgium, Germany, the Netherlands, and Sweden. Among the most common speciesPterostichus vulgaris andBembidion lampros were found in all,Agonum dorsale in nearly all regions. Furthermore the following 16 species could be regarded as common for most of the wheat fields in the regions of investigation:Trechus quadristriatus, T. discus, Pterostichus cupreus, P. niger, Asaphidion flavipes, Clivina fossor, Loricera pilicornis, Nebria brevicollis, Harpalus pubescens, H. aeneus, Agonum mülleri, Bembidion ustulatum, Amara familiaris, Calathus fuscipes, Carabus cancellatus ardC. granulatus. In most of the regions, if numbers of species were regarded, spring breeders with an autumn generation were more numerous than spring breedens without an autumn generation, and autumn breeders. The average number of ground beetles per pitfall trap and day varied from 1 to 13 and averaged 6, if all regions were regarded. By using the square method in a Northern German region there was stated a number of 6,000 up to 11,000 individuals ofAgonum dorsale per hectare. Where a comparison was possible, no distinct difference could be stated concerning the number of carabid species between the present investigations and those having taken place ca. 20 years ago. It is emphasized, that it is worth while taking into account the ground beetles, too, when establishing integrated control programs in cereal growing.

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West-Pal?arktisch Regionale Sektion der O.I.L.B., Arbeitsgruppe Integrierte Bek?mpfung im Getreidebau

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Dem Andenken unseres Freundes und KollegenThyge Thygesen (1923–1974) gewidmet, deran der Gründung der Arbeitsgruppe beteiligt vor Abschluss der Untersuchungen verstorben ist.     

AUSWIRKUNGEN VON INSEKTIZIDBEHANDLUNGEN AUF DIE EPIGÄISCHEN RAUBARTHROPODEN IN GETREIDEFELDERN,INSBESONDERE DIE LAUFKÄFER (COLEOPTERA,CARABIDAE)

Zusammenfassung Die Ergebnisse wurden mit Bodenfallen in Getreidefeldern erarbeitet. Fenitrothion beeinflußte bei Flugzeugausbringung in normaler Dosierung (600 g/ha in 40 1 Wasser) nur die Populationsdichte von Pterostichus vulgaris und P. niger vorübergehend. Eine Spritzung mit erhöhter Dosierung (900 g/ha in 350 1 Wasser) wirkte sehr stark auf die fünf häufigsten Carabidenarten ein. Parathion-äthyl beeinflußte in normaler Dosierung (125 g/ha in 300 1 Wasser) bei einer Behandlung Anfang Juni besonders einschneidend die Frühlingstiere, Ende Juni stark die Frühlings- und die Herbsttiere. In doppelter Dosierung reduzierte es alle wichtigen Prädatorengruppen sehr deutlich. Parathion-methyl (Staub) wirkte bei Ausbringung auf feuchte Pflanzen (200 g/ha) nicht stark auf die epigäischen Räuber. Nur kletternde Arten (Staphyliniden und Agonum dorsale) wurden geschädigt. Methoxychlor wirkte als Spritzmittel (600 g/ha in 300 1 Wasser) stark auf die großen Carabiden, während die kleinen Laufkäferarten, die Staphyliniden und Spinnen nicht beeinflußt wurden. Methoxychlor-Staub, auf die feuchten Pflanzen ausgebracht (1000 g/ha), beeinflußte die epigäischen Räuber nicht. Eine Randbehandlung eines Winterrapsfeldes mit Methoxychlor-Staub störte die Besiedlung des Feldes durch Carabiden und Staphyliniden nicht.

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Summary The influence of some insecticides often used in agriculture upon the terrestrial predaceous arthropods was investigated by pitfall trapping on cereal fields of about 10 hectares each, half the area of which was treated.Fenitrothion, sprayed by aeroplane at the normal dosage (600 g/ha in 40 1 water) influenced the abundance of Pterostichus vulgaris and P. niger only, for a short period. Spraying fenitrothion at a higher dosage (900 g/ha in 350 1 water) killed a high percentage of the five most common carabid species.Parathion-ethyl at the normal dosage (125 g/ha in 300 1 water), sprayed at the beginning of June reduced numbers of spring breeders especially (e.g. Pterostichus cupreus). Sprayed at the end of June, it influenced both spring and autumn breeders (e.g. P. vulgaris). Rain, falling one day after the insecticide treatment, did not alter its effect upon the terrestrial predators. At a higher dosage (250 g/ha) parathion-ethyl reduced numbers of all predators to a large extent, but not very much more than at the normal dosage.Parathion-methyl dust, applied to moist plants (200 g a.i./ha), did not influence all terrestrial predators, but only those species which also climb up the plants (some staphylinids and Agonum dorsale).Methoxychlor emulsion, sprayed at a dosage of 600 g/ha in 300 1 water, killed the bigger ground beetles only (Carabus spp. and P. vulgaris), but not the smaller ones (Agonum dorsale and Bembidion lampros), the staphylinids and the spiders. Methoxychlor, dust, applied to moist plants (1000 g a.i./ha) did not influence the terrestrial predators. Treating the edge only (ca. 20 m) of a winter-rape field with methoxychlor dust did not prevent the colonization of the field by carabids and staphylinids. It is recommended, therefore, to treat the field edges only, which should control most cereal pests.

     

Quantifying food limitation of arthropod predators in the field

A method for quantifying food limitation of arthropod predators in the field is presented and applied to species of ground beetles (Carabidae) and sheet-web spiders (Linyphiidae) from a cereal field. Food limitation is expressed quantitatively as accumulated hunger (=starvation) by transforming 24-h food consumption at 20°C of animals newly collected in the field into days of starvation at 20°C. This is done by means of a reference curve relating 24-h food intake at 20°C to starvation periods (days) at 20°C. Such a reference curve was obtained for the carabid beetle Agonum dorsale in the laboratory. For other species the reference curve was modified with species-specific data. The procedure makes it possible to compare the feeding conditions of different species populations that are part of the same community. Hunger levels in the field for females of A.␣dorsale were equivalent to c. 15 days of starvation in early spring, c. 5 days in June, increasing to c. 10 days in late summer. Two ground beetles occurring during summer also showed increasing hunger levels from June to July, probably the result of a dry summer. Two spider species experienced a hunger level in the range of 4–8 days of starvation. Received: 15 August 1997 / Accepted: 22 January 1998     

Habitat choice of some field-inhabiting carabid beetles (Coleoptera: Carabidae) studied by recapture of marked individuals

ABSTRACT.

• 1 Habitat choice of some field-inhabiting carabid beetles was studied in the field, and included the adult-overwintering Bembidion lampros Herbst, Pterostichus cupreus L. and Agonum dorsale Pontoppidan, and the larval-overwintering Trechus secalis Paykull, Pterostichus melanarius Illiger, P.niger Schaller and Harpalus rufipes De Geer. These were compared to the forest-inhabiting and adult-overwintering species, Carabus nemoralis Müller.
• 2 Marked beetles were released in the centre of a circular enclosure which was placed on the edge between a cereal field and a wood. The direction of movement in this circle was compared with that in a control circle placed entirely in the cereal field.
• 3 In the field/wood circle, adult-overwinterers and the larval-overwin-terer H.rufipes choose to move into the cereal field. The majority of T.secalis. P.melanarius and C.nemoralis, however, moved into the wood, while P.niger exhibited no preference.
• 4 In the field circle, adult-overwinterers moved towards the more sun-exposed parts of the circle, whereas the movements of larval-overwin-terers were random.
• 5 Some species-specific characteristics considered important for habitat choice by these carabid beetles, and the relevance of these characteristics for their dispersal behaviour are discussed.

     

Predator density manipulation and its effects on populations of Rhopalosiphumpadi (Horn.: Aphididae) in spring barley

Experiments were carried out in 1981-83 to assess the impact of polyphagous predators (e.g. Carabidae, Staphylinidae and Araneae) on populations of cereal aphids in spring barley in Sweden. Barriers were erected around mid-field and field-edge plots at different times during the aphids' establishment phase in order to manipulate the predation pressure during the aphid pre-peak period; predators were emigrating from field boundaries. Rhopalosiphum padi was the dominant species of cereal aphid in 1981 and 1982, and was abundant in both years. Reduction of the predation pressure resulted in R. padi populations that were two to six times larger compared with those in unenclosed control plots. Aphid populations were much larger when the predation pressure was reduced earlier in June than when this was done later, in both years, since late erection of barriers excludes fewer predators because most had already entered the enclosed areas. In 1983, R. padi and Sitobion avenae were equally common but occurred in low numbers, and the effects of reducing the predation pressure were less obvious. Bembidion spp. (B. lampros and B. quadrimaculatum) and linyphiid spiders were the most abundant predators during the early establishment phase of aphid populations in each year, and significant inverse correlations were found between peak numbers of R. padi per shoot and numbers of predators from these taxa in 1981 and 1982. Similar relationships were found between total aphids and numbers of Staphylinidae, B. quadrimaculatum and Coccinella septempunctata in 1983. It was concluded that differences between the manipulated numbers of polyphagous predators caused the observed differences in peak population levels of R.padiin 1981,1982 and to a lesser extent, 1983. Apparent levels of parasitism and fungal disease were low during the pre-peak period. Aphid specific predators, abundant after the peak, were considered partly responsible for the decline of aphid populations each year. The results are compared with those from similar experiments elsewhere with S. avenae and Metopolophium dirhodum in winter wheat. Reasons why the effects of polyphagous predators are greater, and occur over a much shorter period of time in Sweden are discussed.     

Microcosm studies on control of aphids by generalist arthropod predators: Effects of alternative prey

Generalist predators are potential controlagents of aphids in cereal fields. Becauseaphids are low-quality prey for most generalistpredators, the availability of alternativehigh-quality prey may influence theinteractions between aphids and their predatorsby reducing the predation rate due to loweredpreference for aphids. We analysed this bytesting the ability of six generalist predators(the spiders Erigone atra (Bl.), Clubiona lutescens (Westr.)/reclusaO.P.-C., Pachygnatha degeeri Sundevall,Pardosa prativaga (L.Koch), the carabidbeetle Bembidion lampros (Herbst), andthe harvestman Oligolophus tridens (C.L.Koch)) to suppress Rhopalosiphum padi(Linné) populations in the presence orabsence of alternative prey types (fruit fliesDrosophila melanogaster (Meigen) or thecollembolan Tomocerus bidentatus(Folsom)). Experiments of 10 days duration werecarried out in a microcosm set-up. Withoutalternative prey all predators except B.lampros were able to reduce aphid populationdevelopment significantly relative topredator-free controls. The harvest spider O. tridens was the most efficient predator(<90% reduction). Presence of alternativeprey (fruit flies) had a significant negativeeffect on aphid limitation by P. prativagaand a weak positive effect in B. lampros, but did not influence the ability to reduce aphids in E. atra, Clubiona, P. degeeri and O. tridens. In addition, 24-hours' consumption experiments with adult P. degeeri and subadult C. lutescens/reclusa, using R. padi and D. melanogaster as prey types, showed markedly lower consumption rates of aphid than of fruit fly prey. The microcosm arrangement is a simple way to partly simulate the habitat complexity of an agricultural field under laboratory conditions and proved to be a useful tool for investigating complex predator-prey interactions.     

Habitat factors influencing the distribution of polyphagous predatory insects between field boundaries

The winter distributions of a range of polyphagous predators were investigated in relation to field boundary structure. The distribution of Demetrias atricapillus was significantly positively correlated with the density of cocksfoot (Dactylis glomerata) tussocks, and Forficula auricularia and Tachyporus chrysomelinus were significantly positively correlated with percentage cover of deciduous leaf litter, However, although densities of predators varied significantly between field boundaries, no further relationships were identified to explain the observed predator distribution patterns. It is suggested that detailed intensive investigations rather than the extensive approaches of the present study, are required to address questions relating to successful over wintering and selection of over wintering site.     

The responses ofPraon spp. parasitoids to aphid sex pheromone components in the field

In autumn 1991, aphid parasitoids of the genusPraon (Hymenoptera; Braconidae) were caught in water traps with lures containing synthetic aphid sex pheromone components at three sites in England and one in Germany. At two of the English sites and at the German site, the traps were placed in winter cereal fields whilst the third English site was in woodland. Three species were caught,P. volucre, P. dorsale andP. abjectum. Those caught in cereal fields were almost entirelyP. volucre, whilstP. dorsale dominated at the woodland site. Of the known aphid sex pheromone components, the most effective lure was the (+)-(4aS, 7S, 7aR)-nepetalactone. Nepetalactone traps placed at the woodland site in spring and summer caught fewPraon females, and attraction may be confined to the autumn, when sexual female aphids are present in the field. Male parasitoids did not respond to the aphid pheromones at any time, although they were caught in suction traps operated at the woodland site during the autumn. At the cereal sites height had a significant influence on the efficiency of the pheromone traps, those placed just above the crop canopy being most effective. There was no evidence that any other genus of parasitoid responded to aphid sex pheromones at these sites.     

Predation of Rhopalosiphum padi (Homoptera: Aphididae) by polyphagous predatory arthropods during the aphids' pre-peak period in spring barley

Polyphagous predators (e.g. Araneae, Carabidae and Staphylinidae), collected from spring barley fields during 1981-85, were examined by either gut dissection or a R. padi-specific antiserum in enzyme linked immunosorbent assay (ELISA) in order to detect predation of Rhopalosiphum padi during the aphids' establishment and exponential growth phases. Overall 18% of c. 3000 carabids dissected were shown to feed on R. padi during the aphids' pre-peak period. No overall relationship was found between percentage carabids with R. padi in the diet and peak R. padi densities. Relatively high proportions of Bembidion spp. (particularly B. lampros) and Pterostichus cupreus fed on R. padi during the aphids' establishment phase, and proportions of those predator taxa containing R. padi increased with increasing R. padi densities in both high and low aphid density years. P. melanarius and Harpalus rufipes mainly fed on R. padi during the aphids' exponential growth phase. Overall 11% of c. 1350 predators examined in ELISA gave positive reactions to the R. padi antiserum. Relatively high proportions of linyphiid and lycosid spiders were positive throughout the aphid pre-peak period. Several Acari, Opiliones, Trechus spp. (Carabidae), Philonthus spp. (Staphylinidae), Cantharidae and Chilopoda were positive mainly during the aphids' exponential growth phase. Sample sizes were small, however. Very few of the Bembidion spp. tested in ELISA were positive compared with those examined by gut dissection. The maximum period of R. padi protein (antigen) detection in B. lampros was related to temperature, i.e. 8.5 h at 30°C, 20.5 h at 20°C and 34.5 h at 10°C, respectively. It is suggested that the rate of R. padi protein digestion in B. lampros is faster at higher temperatures than the rate of elimination of prey solids from the guts. Several key predators (in this case B. lampros, P. cupreus and linyphiid spiders) which are abundant in spring cereal fields at a time when R. padi emigrants arrive in the crop and which feed on R. padi during the aphids' establishment phase, are identified. The results are compared with those from similar investigations elsewhere with predators of Sitobion avenae and Metopolophium dirhodum.     

Overwintering locations and hosts for onion thrips (Thysanoptera: Thripidae) in the onion cropping ecosystem in New York

Identifying locations where onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), overwinter and subsequently disperse is important for designing control strategies. In upstate New York from 2003 through 2006, potential overwintering sites in the commercial onion, Allium cepa L., cropping system were investigated early in the spring before onion seedling emergence and again late in the season after onions were harvested. Onion thrips adults were sampled directly from the soil and indirectly from the soil by using emergence cages. Sampling locations included onion field interiors and edges and areas outside of these fields, including woods. Host material sampled included onion culls; volunteer onions, which sprout from cull onions left behind after harvest; and weeds. Onion thrips adults were found in all sections of onion fields and in locations outside of onion fields, with the fewest emerging from woods. Emergence began in early May and extended into June. Peak emergence occurred during the last half of May, at which time 50-75% of the population had emerged. Adults colonized volunteer onions as early as late March and as late as mid-November. No adults were found overwintering in onion cull piles. Adults also colonized several weed species, especially pigweed, Amaranthus hybridis L., and lambsquarters, Chenopodium album L., late in the fall. Our results indicate that onion thrips adults overwinter in the soil within and near onion fields and that they probably colonize volunteer onion plants before subsequent generations infest the onion crop in the spring. Volunteer onions and weeds also provide onion thrips with a host after onions are harvested. Consequently, onion thrips management strategies should include tactics that reduce volunteer onion and weed abundance.     

Seasonal Changes in Habitat Preference by Coleomegilla maculata: Implications for Colorado Potato Beetle Management in Potato

Seasonal abundance, dispersal, and overwintering of the 12-spotted ladybird beetle, Coleomegilla maculata (DeGeer), in the mid-Atlantic states were investigated to assess the potential to manipulate the habitat of this predator to increase its impact on Colorado potato beetle, Leptinotarsa decemlineata (Say), management in commercial potato plantings. C. maculata populations were highest in corn fields through August, and overwintering aggregations were found most commonly in habitats adjacent to these fields. Although adult and larval populations of C. maculata were synchronized with first-generation potato beetle eggs and small larvae, they were concentrated in wheat rather than potato fields because most of the potato plants had not yet emerged when C. maculata adults dispersed from their overwintering sites. The subsequent generation of C. maculata tended to aggregate in corn rather than potato in late May and early June. Therefore, given the current mixture of crops as well as climatic and marketing limitations on planting date for potatoes, the potential for habitat manipulation to increase the impact of C. maculata on Colorado potato beetle management in potato in the mid-Atlantic states appears to be limited.     

Effects of position within wheat field and adjacent habitats on the density and diversity of cereal aphids and their natural enemies

The spatial structure of agricultural landscapes can have a strong impact on the distribution and diversity of insects. Here we studied the effects of within-field position (edge or center) as well as adjacent habitats on the community structure of the natural enemies of cereal aphids. Twelve agricultural sites were included in the study, with two spring wheat fields selected for each site (one adjacent to an alfalfa field, the other adjacent to a corn field). We sampled two rows per field (1 and 20 m from the edge) using pitfall trapping for ground-dwelling predators, sweep netting for leaf-dwelling predators and hand collecting of aphid mummies for parasitoids. Adjacent alfalfa areas, as opposed to corn fields, can significantly increase the abundance and diversity of leaf-dwelling predators and parasitoids near the field edges. Abundance and diversity were found significantly higher near the edges than in the centers of fields adjacent to alfalfa areas. In contrast, no significant differences were found between edges and centers of fields adjacent to corn fields. Of the fifteen most abundant species, Aphidius avenae (Haliday), A. gifuensis (Ashmead), Hippodamia variegata (Goeze) and Chrysopa sinica (Tjeder) were significantly more abundant near the edge than in the center. Being adjacent to alfalfa habitats could enhance parasitism and predator/prey ratios of leaf-dwelling predators at the edges, but has no effects on ground-dwelling predators. In conclusion, the effect of within-field position and adjacent habitats on natural enemies of agricultural pests was species specific. This should be considered for designing efficient plans of biological control.     

An agro-environmental mowing regime favors the number of inflorescences and flower-visiting insects but not ground beetles of herbaceous boundaries of arable fields

Herbaceous boundaries adjacent to arable fields can deliver ecosystem services not sufficiently provided at the field scale, as well as disservices such as increased weed pressure. The levels of services and disservices depend on the management regime implemented in these boundaries. Our study was conducted in Western France, where herbaceous boundaries of many arable fields constitute the road verge, and various mowing regimes were tested for conservation purposes. Until now, most studies of the influence of mowing regimes focused on one taxonomic group and documented contrasting results depending on the environmental context. This study aimed to compare the influence of two mowing regimes on road verges by evaluating different taxonomic groups that could potentially deliver services or disservices to adjacent arable fields. The mowing regimes used were a standard and an agro-environmental regime, the latter being characterized by one late mowing, a high mowing height and biomass removal. We monitored, during two consecutive years, seven road verges and their adjacent arable fields. Each road verge was divided into two zones of equal length, characterized by a different mowing regime. Communities of plants, flower-visiting insects and ground beetles were sampled in herbaceous boundaries and/or arable fields in the period between the mass-flowering of oilseed rape and sunflower crops. In zones where the agro-environmental mowing regime had been implemented, the average number of inflorescences and flower-visiting insects within boundaries were higher than under the standard mowing regime. However, between the two mowing regimes, no differences in weed cover within arable fields and the average number of ground beetles within boundaries and arable fields were observed. To conclude, our study suggests that in our particular environmental context, an agro-environmental mowing regime could promote pollination in adjacent arable fields without increasing weed pressure within the field.     

Rubidium marking reveals different patterns of movement in four ground beetle species (Col., Carabidae) between adjacent alfalfa and maize

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The aggregative numerical response of polyphagous predators to aphids in cereal fields: attraction to what?

We tested in a field experiment two hypotheses for why polyphagous predators aggregate at concentrations of aphids: 1) because they are attracted directly to aphids as prey, or 2) because they are attracted to alternative prey that aggregate around the honeydew produced by aphids. Small plots were established in the field with two experimental treatments, aphid addition and honey spraying, and a watersprayed control, each replicated 10 times. Arthropods were recorded by pitfall traps and sticky traps in each plot. Diptera were significantly more abundant in the honey plots. Of the predators, Agonum dorsale, “All carabids” and Philonthus sp. were most abundant in the honey plots; Tachyporus spp. and carabid and staphylinid larvae were most abundant in the aphid addition plots. It is suggested that these results reflect differences among the predators in their ability to tolerate and utilise aphids as food.     

Can polyphagous predators control the bird cherry-oat aphid (Rhopalosiphum padi) in spring cereals?: A simulation study

Predation by two carabid species,Bembidion lampros (Herbst.) andPterostichus cupreus L., on the cereal aphid,Rhopalsiphum padi L., was described using a discrete event simulation model. The results were incorporated into an aphid population growth model. The aphid model was run using actual temperatures,R. padi emigration rates, field densities and distributions from three years with distinctly different aphid population levels. Predation was incorporated at different time periods. Predation during the arrival of emigrants and establishment of aphid populations had approximately the same effect as predation over the entire aphid period, whereas predation during the 20 days prior to peak had very little effect on resulting peak populations. We conclude that a) yearly levels ofR. padi infestation are mainly determined by the volume and intensity of emigrants; b) peak levels are sensitive to changes in predator density; and c) in some cases an outbreak ofR. padi may be prevented by the action of spring active polyphagous predators alone.     

The spatial dynamics of crop and ground active predatory arthropods and their aphid prey in winter wheat

The distribution of aphid predators within arable fields has been previously examined using pitfall traps. With this technique predominantly larger invertebrate species are captured, especially Carabidae, but the technique provides no estimate of density unless mark-recapture is used. However, many other numerically important aphid predators occur in arable fields and relatively little is known about their distribution patterns nor whether they exhibit a density-dependent response to patches of cereal aphids. Identification of the most effective predators can allow management practices to be developed accordingly. In this study, the distribution of cereal aphids and their predators was examined by suction sampling within a field of winter wheat in Devon, UK, along with visual estimates of weed patchiness. Sampling was conducted on four occasions in 1999 across a grid of 128 sample locations. The distribution of 11 predatory taxa from the Carabidae, Staphylinidae and Linyphiidae was examined. Additionally, the total number of aphid predators and a predation index were used in these analyses. Carabid adults and larvae, along with staphylinid larvae showed the strongest aggregation into patches and the most temporal stability in their distribution. Other taxa had more ephemeral distributions as did the cereal aphids. The distribution of carabid larvae was disassociated from the distribution of cereal aphids for the first two sampling occasions indicating biocontrol was occurring. Other predatory groups showed both association and disassociation. Carabid larvae, Bathyphantes and total numbers of Linyphiidae showed a strong correlation with weed cover for two of the sample dates. Cereal aphids were disassociated from weed cover on three sampling occasions.     

Diet, fecundity and egg size in some polyphagous predatory carabid beetles

Egg size was measured at different rates of egg laying in three polyphagous carabid species, known to be useful predators of cereal aphids; the small Bembidion lampros Herbst and the medium-sized Pterostichus cupreus L. and P. melanarius Illiger. Variations in fecundity, as well as the ability of the medium-sized species to also build up fat reserves, were obtained when beetles were subjected to different dietary regimes consisting of aphids, or foods with a lower or higher protein content. Egg size was found to be dependent on the rate of egg laying within a species. A diet of cereal aphids appeared to be adequate for egg production in these polyphagous carabids, but female P. cupreus were unable to build up fat reserves when they ingested aphids contaminated with the aphicide pirimicarb. Beetles were able to devote resources to more and larger eggs (B. lampros), or to larger eggs and/or fat reserves (P. melanarius/P. cupreus) when given access to a carbohydrate-rich food with low protein content. The highest rate of egg laying was obtained when female P. cupreus and P. melanarius were given a more varied diet at frequent intervals; including regular shifts between unsprayed aphids, carbohydrate-rich food and protein-rich maggots. Within the varied diet treatment a negative relationship was obtained between egg size and egg number among similar-sized individuals of P. cupreus and P. melanarius; females producing the largest number also laid the smallest eggs. Egg size affected larval survival, since first instars hatching from large eggs were found to survive longer than those hatching from small eggs. The influence of differences in food intake on reproduction, maintenance metabolism, and survival of fieldinhabiting carabids is discussed.