Ecological relationships between non-cultivated plants and insect predators in agroecosystems: the case of Dittrichia viscosa (Asteraceae) and Macrolophus melanotoma (Hemiptera: Miridae)

Species of the genus Macrolophus (Hemiptera: Miridae) are thought to be effective predators in reducing the numbers of several pests in vegetable crops. These predators are omnivorous as in addition to prey they also utilize plant sap for growth and development. Populations of these predators build in non-crop host plants and provide inoculum that augments natural control of insect pests in adjacent crops. However, to enhance their effectiveness in crops requires knowledge of their trophic relationships with host plants. In this study, the ecological relationships between the predator Macrolophus melanotoma (Costa) ( =  M. caliginosus Wagner) and its most important natural host plant Dittrichia viscosa L. (W. Greuter) (Asteraceae) were investigated in the laboratory and in field studies. A 2-year field study of M. melanotoma populations on D. viscosa was made using the percentage of plants infested by C. inulae as a measure of aphid prey abundance. The field studies revealed that M. melanotoma populations were present throughout the year on D. viscosa reaching highest numbers in June and July despite very low levels of aphid infested plants. Laboratory life table studies were used to compare the survival and reproduction of the predator on D. viscosa leaves alone and leaves plus aphid prey (Capitophorus inulae (Passerini)). Predators reared on D. viscosa leaves plus aphid prey had an average developmental time of 16.73 days, fecundity was 69.55 eggs/female and the intrinsic rate of population increase was 0.0614/day. When fed only leaves, the developmental time was 21.13 days, fecundity was 10.80 eggs/female and the intrinsic rate of population increase was 0.0229/day. The results of the two studies suggest an important role for D. viscosa in conserving and augmenting M. melanotoma in agro ecosystems, and in the development of natural control augmentation strategies in vegetable crops.     

Cost-benefits of reduced aphicide usage on dwarf hops susceptible and partially resistant to damson-hop aphid

Soil‐applied imidacloprid at full (125 g a.i. ha^?1) and half approved doses gave levels of control of damson‐hop aphid, (Phorodon humuli), similar to that provided by foliar spray(s) of tebufenpyrad on the aphid‐susceptible dwarf hop cvs First Gold and Herald. On those cultivars, aphid control was unreliable on plots treated with quarter dose imidacloprid and was generally no better than on untreated plants. Aphids were virtually eliminated from the leaves by the end of July each year in all treatments consistent with the action of natural enemies. Aphid contamination of cones reflected the numbers on foliage at flowering time, but varied widely between years. Yields and percentage α‐acids content of dried hops were unaffected by the numbers of aphids on leaves early in the season and in cones at harvest, but aphid contamination reduced the economic values of crops by as much as 80%. Few P. humuli colonised the partially aphid‐resistant breeding line 23/90/08 before their numbers were regulated and consistent with natural enemy activity. Yields, percentage α‐acids content, and commercial value of harvested cones were similar in all treatments on 23/90/08 whether or not plants were treated with aphicides. The commercial risks posed by P. humuli preclude substantial reductions in aphicide usage on aphid‐susceptible dwarf hop cultivars, but future cultivars expressing a similar level of partial resistance to aphids as 23/90/08 should not need treatment with aphicides.     

Effects of Elatobium abietinum on nutrient fluxes in Sitka spruce canopies receiving elevated nitrogen and sulphur deposition

1 In a field acid mist simulation experiment, Sitka spruce (Picea sitchensis) was sprayed with different pollutant treatments: N, NH₄NO₃; S, Na₂SO₄; NS Acid, NH₄NO₃ + H₂SO₄ and control, no spray. Treatment effects on the abundance of the green spruce aphid Elatobium abietinum and honeydew production were assessed. In addition, needles were sampled for phyllosphere micro‐organisms. In a manipulative experiment, shoots were established and maintained as with or without E. abietinum infestation in order to determine the effects of infestation on needle loss and throughfall nutrient fluxes. 2 Aphid numbers were highest during the end of May and early June, with almost twice as many needles infested in the NS Acid treatment compared with the other treatments. Honeydew production was not affected by the treatments. 3 On infested shoots, increasing numbers of yellowing and dead needles were recorded above the throughfall collectors as the season progressed. The numbers of dead needles falling into the collectors were significantly higher beneath infested shoots. There were strong positive correlations between aphid numbers above the throughfall collectors, the number of yellowing and dead needles on the shoots and the number of needles in the funnels of the throughfall collectors. Litter production was more affected by aphid number than by pollutant treatment. 4 Bacteria, yeasts and filamentous fungi were more prolific on infested needles and treatment effects on colony forming units (CFUs) were most pronounced in the NS Acid treatment. 5 Fluxes of inorganic nitrogen beneath infested shoots were generally lower than beneath uninfested shoots. This effect was more pronounced in those treatments that supplied N i.e. N, NS Acid. The combination of aphid infestation and N‐addition exerted the strongest influence on nutrient fluxes. The fluxes of potassium and of organic carbon (DOC) were higher beneath infested shoots in all treatments, through most of the survey period.     

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.     

Action of mephosfolan on anthocorid predators of Phorodon humuli

In glasshouse and field studies, soil drenches of the systemic insecticide mephosfolan were applied to hop plants to control the aphid Phorodon humuli, and to study the toxicity to anthocorid predators. The mortality of Anthocoris nemorum and A. nemoralis eggs in the leaf tissues increased with the dosage of mephosfolan while A. confusus eggs, which were laid in the petioles and veins, appeared to be unaffected at the dosages used, up to 2 g a.i./plant. Mortality of A. nemorum eggs, which were laid mainly in the leaf margins, was higher than in A. nemoralis eggs, which were laid mainly in the interveinal areas. For both species mortality was highest for the eggs laid in the leaf margins, and susceptibilities appeared similar. It is suggested that mephosfolan becomes unevenly distributed within the leaves, being most concentrated in the leaf margins and least concentrated in the veins and petioles. No species showed any preference when offered mephosfolan-treated and untreated leaves for oviposition, and the oviposition rates were unaffected by various mephosfolan dosages. Oviposition rates, larval mortality and duration of larval development were unaffected when specimens of each species were reared from eclosion to maturity on a diet of either mephosfolan-killed or heat-killed P. humuli.     

Role of terpenes from aphid-infested potato on searching and oviposition behavior of Episyrphus balteatus

To cope with pathogen and insect attacks, plants develop different mechanisms of defence, in both direct （physical and chemical） and indirect ways （attractive volatiles to entomophagous beneficials）. Plants are then able to express traits that facilitate ＂top-down＂ control of pests by attracting herbivore predators. Here we investigate the indirect defence mechanism of potato plants by analyzing the volatile patterns of both healthy and aphid- infested plants. Important changes in the emitted terpene pattern by the Myzus persicae infested host plant were observed. Using Solid Phase MicroExtraction （SPME） and GC-MS, the （E）-fl-farnesene （EBF） appeared to be emitted by aphid-infested potato and not by healthy plants. To assess the infochemical role of these volatile releases after aphid damage on the aphidophagous predators Episyrphus balteams, the hoverfly foraging behavior was assessed using the Observer 5.0 software （Noldus, Wageningen, The Netherlands）. Aphidfree potato plants were also used as a control volatile source in the predator behavioral study. While aphid-infested plants induced efficient searching and acceptation behaviors leading to egg-laying, no kairomonal effect of healthy potato plants was observed, leading to longer immobility durations and shorter searching periods in the net cage. High oviposition rate of E. balteatus was observed when aphid-infested potato was used （mean of 48.9 eggs per laying and per female）. On the other hand, no egg was produced by the hoverfly on healthy aphid-free plants. The E. balteatus foraging and reproductive behaviors according to the volatile emission from aphid-infested plants are discussed in relation to the potential use of active infochemical molecules in integrated aphid pest management.     

The effects of the invasive Argentine ant (Linepithema humile) and the native ant Prenolepis imparis on the structure of insect herbivore communities on willow trees (Salix lasiolepis)

Abstract 1. We examined the relative effects of the invasive Argentine ant, Linepithema humile, and a common native ant, Prenolepis imparis, on the community of herbivorous insects occurring on willow trees, Salix lasiolepis in Northern California, U.S.A. 2. Using paired control and treatment branches from which we excluded ants and other non‐volant predators, we found that effects of Argentine ants on the herbivore community were generally similar to those of P. imparis. Argentine ants and P. imparis suppressed the damage by skeletonising insects by 50%, but had little effect on most other external‐feeding or internal‐feeding guilds. 3. The abundance of aphids was 100% greater in the presence of Argentine ants, but there was no effect on aphid numbers in the presence of P. imparis. Late season aphid numbers were substantially higher in the presence of Argentine ants, but not P. imparis. 4. The effects of Argentine ants on skeletonising insects and aphids combined with the overwhelming abundance of Argentine ant workers, suggests that they may have substantial, but often overlooked, effects on the herbivore communities on other plant species in or near riparian habitats in which they invade.     

On the demography of Rattus sordidus colletti in monsoonal Australia

Results of a mark-release study of Rattus sordidus colletti (Gould) on sub-coastal, treeless plains in the monsoonal north of the Northern Territory of Australia are given for 5 years. R. s. colletti is the dominant component of the small mammal fauna of these plains, with only small numbers of Melomys spp. and Planigale maculata also occurring. Two classes of peak densities were observed. Localized peaks resulted from non-breeding adult R. s. colletti refuging onto the marginally higher levees during flooding of the plains, and also from refuging into a lower-lying area at the end of a dry-season drought. Generalized peak densities resulted from peaks in reproductive effort. In 1972 and 1974, reproduction was confined to the period immediately following the monsoons, but in 1973 breeding continued throughout the dry season, following unseasonal rain in June. Thus, generalized peak densities were observed in one year at the beginning of the dry season, and in another at its end. The relatively shallower flooding and mildness of the 1972–3 wet season resulted in commencement of breeding earlier in 1973 than in 1974, while rapid severe flooding of the plains in December 1974 resulted in mass mortality and failure of the refuging populations of adults to recolonize the lower plains in 1975. R. s. colletti remained extremely rare until November 1976, when the study was terminated. The good conditions which allowed reproduction during the dry season of 1973 were reflected in rapid growth rates at that time, compared with rates approaching zero in the 1974 dry season. R. s. colletti is the most fecund form of Australian Rattus species. This high fecundity appears to compensate for the restriction, by dry-season aridity and wet-season flooding, of the breeding season in most years to a short period immediately following the monsoons. In unusual years with rainfall during the dry season, the high fecundity gives R. s. colletti populations the capacity to reach very high densities.     

Incidence of Pentalonia nigronervosa coqueral (homoptera: Aphididae) and associated predators on banana plants in Egypt

The incidence of banana aphid, Pentalonia nigronervosa Coqueral and the coccinellid predators populations on banana plants cultivated at the Agricultural Research Centre Farm in El-Kanater El-Khayria, Qualiobia Governorate, Egypt (about 30 km North Cairo) had been studied. The population of this insect pest was relatively high during the cold and wet months (October, November, December, January and February), and found at its lowest level during the summer months. On the other hand, the numbers of aphid were higher on the mother plants than on the suckers. On mother plants, the population density was much higher on the lower parts of the plant than on the upper parts. The growth rate of aphid population infesting the mother plant of banana in season 2001, increased during the period from January to March and decreased from April to June. The population growth rate then increased from July to November then decreased in December. The same trend could be applied for the growth rate of aphid population in season 2002. At high growth rate it is advisable to use chemical control. The effect of weather factors on the population density of the banana aphid; the correlation between the total number of aphid and temperature or R.H. had been worked out. The coccinellid beetles were found on the banana plants during the warm period from March to August.     

The ecology of Lake Nakuru (Kenya)

Summary Consumer biomass and spatial distribution in the equatorial alkaline-saline Lake Nakuru were studied from 1972–1976. These data will provide the basis for estimating feeding and production rates and for quantifying energy flow at the consumer level. Two of the main consumers, the Lesser Flamingo (Phoeniconaias minor) and the Soda Tilapia (Sarotherodon alcalicus grahami), were covered by earlier papers. The biomass of the only planktonic crustacean, the copepod Lovenula (=Paradiaptomus) africana was very high (1.5 gDW (dry weight)·m^-3, mean in 1972/73) in comparison with other tropical lakes. Lovenula was absent in 1974 and 1975, and at very low levels (0.1–0.4 gDW·m^-3) in part of 1976. Among the rotifers Brachionus dimidiatus dominated in 1972/73 (0.2 gDW· m^-3), but was outnumbered by B. plicatilis throughout most of 1974 to 1976 (mean total rotifer biomass 1.4 gDW· m^-3, peak densities 7 gDW·m^-3); during high salinity periods (>20) Hexarthra jenkinae occurred in very low numbers. For short periods rotifers can be the dominant species of L. Nakuru. Aquatic heteroptera (four species) played a minor role: they contributed 0.4% to total consumer biomass in 1972/73; in 1974–1976 the lake had no aquatic heteroptera. Benthic biomass (0.4 gDW·m^-2) was within the range of other tropical lakes, it consisted almost exclusively of Leptochironomus deribae. Bird counts of the twelve most important species are given for the years 1972–1974: Pelecanus onocrotalus accounts for 90% of the biomass (0.44 gDW·m^-3, mean 1972/73) with peak densities of almost 20,000 birds.-The consumer organisms covered by this and the two preceding papers represent >99% of L. Nakuru's consumer biomass. Population dynamics of various consumer species are discussed.     

Oviposition preferences of Maculinea alcon as influenced by aphid (Aphis gentianae) and fungal (Puccinia gentianae) infestation of larval host plants

Abstract 1. The influence of infestation of the larval host plant Gentiana cruciata on the egg‐laying preferences of the xerophilous ecotype of Alcon Blue butterfly (Maculinea alcon) was studied in a semi‐dry grassland area (Aggtelek Karst Region, Northern Hungary). 2. We examined whether oviposition patterns of females differed when G. cruciata stems were uninfested compared with when they were infested by an aphid (Aphis gentianae) or a rust (Puccinia gentianae) species. 3. Females laid more than 90% of their eggs on fertile, uninfested G. cruciata stems, although these stems comprised only ～ 50% of the total stems available. Stems infested by aphids were similar to uninfested ones in properties that had a strong correlation with egg numbers, and yet there were significantly fewer eggs on infested stems than on intact ones. 4. Females never laid eggs on parts of Gentiana stems infested by aphids, and the presence of Lasius paralienus ants, which have a mutualistic interaction with Aphis gentianae, did not increase the repulsive effect of aphids. Infection of Gentiana by Puccinia did not influence the egg‐laying behaviour of females, even though the flowers and buds of infested stems exhibited a delayed development. 5. Aphid infestation can influence butterfly oviposition patterns through both direct and indirect effects. The presence of aphids directly excluded oviposition, but our data also indicated the possibility of an indirect effect of aphid infestation. Stems that had no aphids at the last egg counting, but were infested prior to it, had significantly fewer eggs than those that were never infested.     

Rise and fall of cotton aphid (Hemiptera: Aphididae) populations in southeastern cotton production systems

The impact of natural enemies on cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), populations in cotton, Gossypium hirsutum L., production systems in the southeastern United States was evaluated over 3 yr in irrigated commercial cotton fields. Fungal epizootics caused by the entomopathogen Neozygites fresenii (Nowakowski) Batko reduced aphid numbers to subthreshold levels in 1999, 2000, and 2001 and occurred consistently in early to mid-July in all 3 yr. Scymnus spp. were the most abundant aphidophagous predators, although other coccinellid species and generalist predators such as spiders, fire ants, heteropterans, and neuropterans also were present. Studies using arthropod exclusion cages demonstrated little impact of predators or parasitoids on aphid populations before fungal epizootics. Arthropod natural enemies were most abundant after epizootics and may have suppressed aphid populations late in the season. Seed cotton yield, and lint quality were not affected by aphicide applications in any year of the study. Implications of these findings for aphid management in the southeastern United States are discussed.     

Quantifying the impact of polyphagous invertebrate predators in controlling cereal aphids and in preventing wheat yield and quality reductions

Exclusion barriers were used to manipulate numbers of polyphagous invertebrate predators so that their impact on cereal aphids and consequently wheat yield and quality could be examined. Experiments were conducted within the framework of the LINK Integrated Farming Systems Project which allowed comparisons to be made between the integrated and conventional farming systems under examination on a study farm in Hampshire, UK. Only in 1995 were the numbers of aphids per tiller, the aphid peak and rate of increase to the peak significantly greater in the exclusion areas where the density of polyphagous predators had been reduced. The maximum increase in aphids as a result of excluding polyphagous predators was 31%, which was equivalent to 130 aphid days. However, the polyphagous predators did not reduce the number of tillers infested. The relatively low impact of polyphagous predators was attributed to the aphid population phenology and greater effects may have been found had aphids infested the crops earlier in their development. Sowing date was shown to govern the time over which a crop may be susceptible to yield loss from aphids, with later-drilled crops being more susceptible to late-summer aphid infestations. Aphid numbers rarely affected grain yield but were found to be related to some grain quality parameters, but reducing polyphagous predators had no direct impact on grain yield or quality even where the aphid burden increased. The peak period of activity and density differed between the species of Carabidae, Staphylinidae and Araneae consequently influencing their relationship with the aphids. Some negative correlations were found between these groups of polyphagous predators and aphids. Species composition and abundance differed between fields thereby influencing the level of aphid predation. The exclusion barriers were most effective at reducing numbers of Carabidae although numbers of Staphylinidae and Araneae were also reduced. The consequences for Integrated Crop Management are discussed.     

Life history characteristics of Orius insidiosus (Say) fed Aphis glycines Matsumura

The soybean aphid, Aphis glycines Matsumura, is a new invasive pest of soybeans throughout most of the soybean production areas of North America. Field studies have demonstrated that the indigenous predator, Orius insidiosus (Say), is an important natural enemy of the soybean aphid early the soybean crop season. Because soybean aphid is newly introduced into North America, the life history characteristics of predators fed this aphid are not known. In laboratory assays, we measured the survival, development, longevity and reproduction of O. insidiosus fed 1, 3, 6 or 12 seconds to third instars of soybean aphid. O. insidiosus nymphal development decreased from 34.0 to 21.4 days as the number of soybean aphid nymphs provided increased from 1 to 6 aphid nymphs daily. Stage-specific mortality was highest at 68% for first instar O. insidiosus nymphs fed 1 soybean aphid nymph per day. Adult longevity (43.9 days) and fecundity (49.7 eggs per female) was highest for O. insidiosus fed 6 soybean aphid nymphs daily, but longevity (23.5 days) and fecundity (10.1 eggs per female) declined for adults fed 1 soybean aphid nymph daily. The intrinsic rate of increase of O. insidiosus ranged from 0.048 to 0.133. Compared to other prey species, soybean aphid is an adequate prey item for O. insidiosus. Our results suggest that O. insidiosus will be most effective in suppressing soybean aphid population growth in the initial phase of the aphid’s colonization of soybeans.     

Intraguild interactions and aphid predators: biological efficiency of Harmonia axyridis and Episyrphus balteatus

Introductions of the harlequin ladybird Harmonia axyridis into Belgium for aphid biological control have been followed by declines in native aphid natural enemies. We first examined, in laboratory and field conditions, the impact of larval mobility of either H. axyridis or Episyrphus balteatus, the most abundant native hoverfly in central Europe, on aphid suppression. The hoverfly larvae consumed more aphids (Megoura viciae) in a short term, whereas the ladybird ones were more efficient in a long term. We second investigated the intraguild interactions between H. axyridis and E. balteatus larvae and adults. In larva pairings between the two species in laboratory microcosms containing aphid (M. viciae) infested broad bean (Vicia faba) plants, H. axyridis had an intraguild predation (IGP) advantage over the hoverfly. When conspecific larvae were paired together on aphid‐infested plants, no cannibalism between them was detected. The presence of either H. axyridis or E. balteatus larvae on aphid‐infested plants negatively influenced the ovipositional behaviour of H. axyridis and E. balteatus females; lower numbers of laid eggs were recorded compared to control treatment. Moreover, eggs laid by E. balteatus females were also dropped as victims of predation mainly by H. axyridis larvae. Our results suggest that while the exotic ladybird was more efficient in aphid biological control, larvae and eggs of the native hoverfly species face increased IGP by H. axyridis, which would contribute, as a consequence, to the decline in E. balteatus population following invasion.     

Impact of intraguild predation by adult Harmonia axyridis (Coleoptera: Coccinellidae) on Aphis glycines (Hemiptera: Aphididae) biological control in cage studies

The soybean aphid, Aphis glycines Matsumura, has become a principal arthropod pest of soybean in the U.S. since its first detection in 2000. This species threatens soybean production through direct feeding damage and virus transmission. A diverse guild of insect predators feeds on soybean aphid in Michigan including the exotic coccinellid Harmonia axyridis, the native gall midge Aphidoletes aphidimyza and the native lacewing Chrysoperla carnea. In addition to feeding on A. glycines some members of this guild may also engage in intraguild predation. These interactions may produce positive, negative, or neutral impacts on A. glycines biological control. We explored the impact of intraguild predation on soybean aphid population dynamics by comparing aphid populations in microcosms with either A. aphidimyza larvae or C. carnea larvae alone, with both a H. axyridis adult and either A. aphidimyza or C. carnea larvae, and without predators. When H. axyridis was present with larval A. aphidimyza or C. carnea, the lady beetle acted as an intraguild predator. However, intraguild feeding did not result in a release of aphid populations compared with microcosms containing only the intraguild and aphid prey. A similar result was found in field cages. Cages allowing large predators had reduced numbers of A. aphidimyza and C. carnea larvae but also significantly fewer aphids compared with predator exclusion cages. Thus, in both lab and field studies the direct impact of H. axyridis on A. glycines overcame its negative impact as an intraguild predator. Together, these studies indicate that while the exotic H. axyridis does act as an intraguild predator and may contribute to local declines in A. aphidimyza and C. carnea, it is also currently important in overall biological control of A. glycines.     

Implications of Russian wheat aphid, <Emphasis Type="Italic">Diuraphis noxia</Emphasis>, falling rates for biological control in resistant and susceptible winter wheat

The restriction of aphid reestablishment onto plants by epigeal predators represents a critical component of integrated pest management. To further realize the potential that these predators might have in control programs, it is necessary to quantify such behavior as aphid falling rate to reveal the number of aphids that are available as potential prey. This study calculated the falling rate of the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Sternorrhyncha: Aphididae), and tested whether this aphid more likely fell from wheat plants that differed between flat leaf architecture versus those with furled leaves. Specifically, the hypothesis was tested that a resistant wheat line (flat leaves) will have a higher aphid falling rate than a susceptible closely related line (furled leaves). The experiment was performed at Fort Collins and Akron, Colorado, USA, from May through July, 2008. Aphids were sampled from infested wheat rows to estimate aphid density, and sticky traps were used to capture falling aphids and to measure falling rate. Falling rates ranged from 0.7 to 69.5% in Fort Collins and from 1.4 to 59.5% in Akron. The falling rate of D. noxia was more influenced by plant growth stage than aphid densities, with the highest falling rate occurring after wheat senescence. Wheat plants with flat leaf architecture did not significantly increase aphid falling rate. Diuraphis noxia falls at a higher rate at lower aphid densities, which is when epigeal predators could have their greatest biological control impact.     

The development of an action threshold for cabbage aphid (Brevicoryne brassicae) in oilseed rape in the UK

Replicated small plot field experiments were done at two sites growing winter oilseed rape (ADAS Boxworth, Cambridgeshire and ADAS High Mowthorpe, North Yorkshire) and two sites growing spring oilseed rape (ADAS Bridgets, Hampshire and ADAS Rosemaund, Herefordshire) to investigate the effect of cabbage aphid (Brevicoryne brassicae) on crop yield and quality. All four sites were included in the first 2 yr of the experiment in 1994 and 1995 but only those with winter oilseed rape were continued into the final year in 1996. Plots were artificially inoculated with cabbage aphids at either five aphid 4 m^-2 or 5 aphids 16 m^-2 or left uninoculated to become naturally infested. In 1995 and 1996 the naturally infested treatment was omitted. Sprays of the aphicide pirimicarb at GS 3.3, 3.7, 4.5, 4.9 and 5.5 were used to manipulate aphid populations. Once a plot had been treated at a target growth stage it was sprayed on all subsequent occasions to prevent recolonisation. Aphid numbers were assessed prior to each spray date and their effect on the crop measured in terms of yield of seed and oil and glucosinolate content. Artificial inoculation of aphids was often successful in establishing different populations of the pest at a range of growth stages. Results showed that cabbage aphid sometimes reduced both crop yield and quality. Yield responses to insecticide treatment tended to be larger in spring oilseed rape than in winter oilseed rape mainly because it became more heavily infested at an early growth stage. Tentative thresholds are proposed for control of the pest in both winter and spring oilseed rape. It is stressed that cabbage aphid is a sporadic pest and rarely likely to reach these threshold levels in field crops.     

The effect of augmenting grain aphid (Sitobion avenae) numbers in a field of winter wheat in spring on the aphid's abundance in summer and its relevance to the forecasting of outbreaks

Sitobion avenae was introduced into areas within a field of winter wheat in Norfolk in mid-May 1985, when the indigenous population was negligible. Aphid numbers in these areas increased for the first 3 wk and declined for the next 4. A second S. avenue introduction into the same areas and into previously uninfested areas was carried out 5 wk after the first introduction, when the crop was at ear emergence complete (G.S. 59). This increased aphid numbers in the previously uninfested areas, but numbers in the previously infested areas continued to decline to below the level in control areas during the subsequent 2 wk. Populations in all areas then increased rapidly to a maximum, followed by a rapid final decline. Monitoring of the aphid and natural enemy populations, and caging aphids onto the crop, revealed that the principal cause of the population decline around G.S. 59 was predation by syrphids. The relevance of these findings to the problem of forecasting S. avenae outbreaks is discussed, and a general explanation for cereal aphid outbreaks is put forward.