Modelling the dynamic spatio-temporal response of predators to transient prey patches in the field

The spatio-temporal dynamics of two aphid species ( Metopolophium dirhodum and Sitobion avenae ) and a generalist predator ( Pterostichus melanarius ) were observed in a field-scale study using a grid of 256 sampling locations with a 12-m spacing. Using Spatial Analysis by Distance Indices we demonstrate that populations show ephemeral spatial pattern at the field scale. We observed a positive, lagged beetle response to this aphid pattern; conversely, the aphids displayed a negative, lagged response to beetle spatial pattern. Examination of the local structure of the spatio-temporal dynamics revealed a strong response by the beetle population to aphid patches. The temporal structure of spatial associations between the species shows a strong correspondence with those from a conceptual model of predator–prey spatial interaction. The spatially coupled dynamics were sufficiently strong for the predator to have a negative effect on the intrinsic rate of increase of their prey.     

Monoclonal antibodies reveal changes in predator efficiency with prey spatial pattern

Spatially explicit predator–prey interactions can alter the predatory potential of natural enemies augmented through conservation biological control. To test hypotheses regarding such interactions and predatory efficiency, we used a combination of molecular techniques and mark–release–recapture to study the foraging behaviour of a generalist carabid predator, Poecilus cupreus , in response to spatial patterns of its cereal aphid prey ( Metapolophium dirhodum and Sitobion avenae ). Beetle and aphid numbers were measured across two grids of sampling locations, within which aphid spatial pattern had been manipulated to generate patchy and more homogenous distributions. Aphid consumption was measured by enzyme-linked immunosorbent assays (ELISA) of beetle gut contents, using an aphid-specific monoclonal antibody. Movement and distribution patterns suggest that P. cupreus does not aggregate at, nor instigate prey-taxis within, aphid patches. However, more than two-thirds of the 2169 P. cupreus tested by ELISA had consumed aphids and the proportion of beetles containing aphid proteins was positively related to aphid density. Against expectation, the proportion of predators feeding on aphids was greatest where prey were homogenously distributed, and this was attributed to the loss of partial refuges for prey in aphid patches. The functional value of this type of uniform foraging strategy is ideally suited to early colonization of the crop habitat, when aphid numbers are low, before populations build up and form strong spatial patterns.     

Prey selection and foraging behaviour by Pterostichus cupreus L. (Col., Carabidae) under laboratory conditions

Abstract: Several authors have indicated that carabid beetles offer potential as predators of insect pests on arable crops, but this potential is reduced by their limited ability to climb the crop plants. In the current investigation an initial laboratory experiment indicated that seven species of carabid beetles commonly found in arable fields in the UK would feed on both aphid and collembolan prey. Pterostichus cupreus L. and Bembidion guttula Fabr. were found to accept both live and dead aphids and collembolans. In further laboratory investigations P. cupreus exhibited greater consumption of the cereal aphid Metopolophium dirhodum Walker than the alternative prey species Heteromurus nitidus , Templeton (Collembola, Entomobryidae) and there was some indication of preference for the aphid prey as these were often consumed first. Significantly more handling time was required for the consumption of M. dirhodum compared to H. nitidus. The treatments where P. cupreus was given a diet of aphids or collembolans prior to the prey choice experiment, did not appear to influence the number or type of prey items consumed.
Using a four-arm olfactometer P. cupreus reacted positively to the volatiles from collembolans, but gave a less clear response to those from the cereal aphid M. dirhodum . In an artificial field arena P. cupreus readily climbed barley plants in search of M. dirhodum , perhaps as a response to volatile emissions, but this behaviour changed when H. nitidus were introduced onto the substrate of the arena. Although the current results indicated that P. cupreus has potential for the control of cereal aphids, the presence of alternative prey appeared to reduce this potential and suggest that the carabid is an opportunistic feeder.     

Prey preferences of six species of Carabidae (Coleoptera) and one Lycosidae (Araneae) commonly found in UK arable crop fields

Abstract: Laboratory experiments were used to determine the feeding preferences of six carabid beetles and one lycosid spider on aphid and collembolan prey. The first investigation used only five species of carabid Pterostichus melanarius Illiger, Pterostichus madidus F., Harpalus rufipes DeGeer, Nebria brevicollis F. and Carabus violaceus L., which had been caught most commonly in pitfall traps in the headland region of arable crop fields. When offered Brevicoryne brassicae L., Sitobion avenae F., Metopolophium dirhodum Walker and Rhopalosiphum padi L. as prey items, the species consistently consumed in high numbers was M. dirhodum . In subsequent experiments four carabids Pterostichus cupreus L., P. melanarius, P. madidus. H. rufipes and a lycosid spider Trochosa ruricola DeGeer whose distribution was shown by pitfall trapping to extend throughout the arable crop, were the chosen predators. These predators were offered a choice between M. dirhodum and entomobryid collembolans (a recognised alternative prey item) to gauge their preference between the two prey types. Both male and female P. cupreus and P. melanarius showed a significant preference for the aphid prey, while there was no significant preference displayed by the other species. The effect of temperature on the voracity of these five predators fed on M. dirhodum was investigated. There were significant differences in the number of aphids consumed by the species and sexes at the different temperatures. Regression analysis on the mean numbers of aphids eaten by each sex of the five predators, showed that in the majority of cases there was a significant increase in predation with increasing temperature. In considering the dietary preferences illustrated by these experiments, it appeared that P. cupreus and P. melanarius offered the greatest potential in controlling aphids on arable crops.     

Comparing growth patterns among field populations of cereal aphids reveals factors limiting their maximum abundance

Cereal stands in central Europe are commonly infested with three species of aphids that may become serious pests. With increasing abundance, the proportion of a particular species in the total aphid population may remain constant, suggesting a density-independent exponential growth, or the proportion can change, suggesting density-dependent constraints on growth. The constraints that affect particular species, and thus their relative abundance, were studied. The proportionality between maximum abundances of the cereal aphids was studied using a 10-year census of the numbers of aphids infesting 268 winter wheat plots. For two species their abundance on leaves and ears was compared. With increasing aphid density the maximum abundance of Rhopalosiphum padi (Linnaeus) remained proportional, but not that of Sitobion avenae (Fabricius), which was constrained by the smaller surface area of ears compared to leaves. There was no evidence of inter-specific competition. Maximum abundance of R. padi and Metopolophium dirhodum (Walker) on leaves did not change proportionally as the proportion of M. dirhodum decreased with increasing overall aphid density. This decrease was probably caused by the restricted distribution of M. dirhodum, which is confined to leaves, where space is limiting. No change in proportion between populations was detected when the average densities were below 0.54 aphids per leaf or ear. Non-proportional relationships between aphid populations appeared to be due to spatial constraints, acting upon the more abundant population. Detecting the limitation of population growth can help with the assessment of when density-independent exponential growth is limited by density-dependent factors. This information may help in the development of models of cereal aphid population dynamics.     

Seasonal dynamics of cereal aphids on Russian wheat aphid (Homoptera: Aphididae) susceptible and resistant wheats

Field experiments were conducted in 1997 and 1998 to evaluate the impact of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), on the cereal aphid complex in wheat. Two spring wheats were planted: the variety "Centennial" (Russian wheat aphid susceptible) and the advanced line IDO488 (Russian wheat aphid resistant). IDO488 incorporates the resistance found in PI 294994 into a Centennial background. Field plots were artificially infested with adult D. noxia and sampled weekly. The most abundant aphid species in 1997 were Metopolophium dirhodum (Walker), Sitobion avenae (F.), D. noxia, and Rhopalosiphum padi (L.). In 1998, the order of abundance was M. dirhodum, R. padi, S. avenae, and D. noxia. The resistant genotype had significantly fewer D. noxia than the susceptible one during both years. However, plant genotype had no significant effect on the other aphid species in either year. Both the initial density of D. noxia and plant growth stage, had a significant effect on D. noxia population development, but had no effect on the other aphid species. There was no interaction between D. noxia resistance and the population density of the other aphid species observed.     

Population dynamics of cereal aphids: influence of a shared predator and weather

1 Aphid populations may show strong year-to-year fluctuations, but questions remain regarding the dominance of factors that cause this variation, especially the role of natural enemies. To better understand the dynamics of aphid species that occur as pests in cereals, we investigated the relative influence of top-down control by a predator and weather (temperature and precipitation) on population fluctuations of three cereal aphid species.
2 From 1987 to 2005, populations of Metopolophium dirhodum , Sitobion avenae and Rhopalosiphum padi in insecticide-free stands of winter wheat were monitored in the Praha-Ruzyné region of the Czech Republic. Densities of an aphidophagous predator, the ladybeetle Coccinella septempunctata , were recorded from an overwintering site in the landscape. Weather was quantified using historical records.
3 A significant bottom-up effect of densities of aphids on those of C.   septempunctata was found, but evidence of direct top-down regulation of aphids by C.   septempunctata was only significant in the case of R.   padi . There was no significant periodicity in the dynamics of the aphid or C.   septempunctata , suggesting that there was no clear predator-prey cycle. Combinations of C.   septempunctata and weather variables could be used to explain M.   dirhodum and R.   padi per capita rate of change. There were also indications that weather directly affected peak density of M.   dirhodum .
4 We conclude that regional estimates of C.   septempunctata densities are not sufficient to determine whether aphid population dynamics are driven by predator–prey interactions. Feasibility of time series analysis as an investigative tool in aphid population dynamics studies is discussed.     

Natural enemy ravine revisited: the importance of sample size for determining population growth

Abstract. 1. The population growth of three aphid species, Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.), and Sitobion avenae (F.), on winter wheat, was analysed by regression. The calculations were based on censuses of aphids made in 268 plots at 3- or 7-day intervals for 10 years on leaves and 6 years on ears. The calculations were made separately for each plot each year, then repeated on the pooled data from all plots monitored in a year.
2. At the level of individual plots, no population growth was detected at very low densities. At high densities, the populations grew exponentially and the growth rates did not decrease with increasing aphid density.
3. Significant growth was always detected in the pooled data. These growth rates decreased significantly at the highest densities. Field estimates of the intrinsic rate of increase derived from these data ranged from 0.010 to 0.026 for M. dirhodum , 0.0071–0.011 for R. padi , and 0.00078–0.0061 and 0.0015–0.13 for S. avenae , on leaves and ears respectively .
4. The apparent lack of growth in the individual plots at low densities is attributable to small sample size. It is concluded that the natural enemy ravine in the population dynamics of cereal aphids, identified by Southwood and Comins (1976), is a consequence of low population densities at which population increase is undetectable unless very large samples are taken.     

小麦间作大蒜或油菜对麦长管蚜及其主要天敌种群动态的影响

麦长管蚜是我国冬小麦产区的重要害虫.试验探索了不同间作方式对麦长管蚜及其主要天敌的影响.结果表明,在调查期内,麦-油(油菜)间作田和麦-蒜(大蒜)间作田中麦长管蚜无翅蚜的种群密度多显著低于单作田;麦-油间作田中有较高的瓢虫种群密度和瓢蚜比;5月2日前,麦-油间作田中蚜茧蜂的种群密度高于单作田和麦-蒜间作田;5月5日后,麦-油间作田的僵蚜率和蜂/蚜比也显著高于单作田和麦-蒜间作田;麦-蒜间作田有翅蚜的种群数量较高,但瓢虫和蚜茧蜂的种群数量无明显变化.麦-油间作和麦-蒜间作均能对麦田中麦长管蚜起到较好的控制作用.     

Aspects of host—parasitoid interactions of two aphid parasitoids, Aphidius urticae (Haliday) and Aphidius uzbeckistanicus (Luzhetski) (Hymenoptera, Aphidiidae)

Abstract. 1. Various aspects were studied of two host-parasitoid interactions: Aphidius urticae parasitizing the cocksfoot aphid Hyalopteroides humilis and Aphidius uzbeckistanicus parasitizing the cereal aphid Metopolophmm dirhodum. The work formed part of a study on the population dynamics of grassland and cereal aphids in Berkshire, England.
2. A.uzbeckistanicus had a higher instantaneous attack rate but a slightly longer handling time than A.urticae. In both cases the functional response was curvilinear.
3. A.urticae was specific to H.humilis and failed to develop in M.dirhodum. A.uzbeckistanicus could develop successfully in H.humilis but the functional response was sigmoid and the progeny were less fecund than those reared on the natural host. This species showed a clear preference for M.dirhodum when supplied with mixtures of M.dirhodum and H.humilis.
4. A.uzbeckistanicus had a faster rate of development and a lower developmental threshold than A.urticae , a situation paralleled by their natural hosts.
5. The results are discussed in relation to field observations. A.uzbeckistanicus may have considerable potential for reducing cereal aphid numbers. However, its preference for M.dirhodum mitigates against a reservoir being maintained in the economically unimportant grassland aphid, H.humilis.     

植物篱对小麦蚜虫及其天敌种群的影响

采用坡地种植蓑草、苜蓿、香根草和紫穗槐等植物篱的方法,研究了其对小麦蚜虫及其天敌种群的影响.结果表明： 在20°坡度试验区,紫穗槐植物篱显著抑制了麦蚜种群的发生,且麦田拟寄生物的密度等于或显著低于香根草区.在12°坡度试验区,苜蓿和蓑草推迟了麦蚜种群发生的高峰期,蓑草作为植物篱可以显著抑制麦蚜种群数量;苜蓿区麦田拟寄生物密度显著高于对照,瓢虫种群密度等于或显著高于蓑草区.蓑草挥发物对麦长管蚜和禾谷缢管蚜均有驱避作用,而显著引诱猫蛛科1种蜘蛛;苜蓿挥发物对麦长管蚜具有引诱作用;苜蓿和蓑草挥发物对瓢虫的行为反应均无明显影响.紫穗槐和蓑草作为坡地植物篱,不仅能防护水土流失,而且有助于调控害虫及其天敌种群.     

The reactions of two cereal aphid parasitoids, Aphidius uzbekistanicus and A. ervi to host aphids and their food-plants

ABSTRACT. A Y-tube olfactometer was used to test the reactions of the hymenopteran cereal aphid parasitoids Aphidius uzbekistanicus Luzhetski and A. ervi Haliday to odours from aphids and their host plants. Only females responded to aphids but both sexes responded to plant odours. A. uzbekistanicus responded to the cereal aphids Sitobion avenae (F.) and Metopolophium dirhodum (Walker) whilst A. ervi , which has a broad host range, responded to M. dirhodum and the pea aphid, Acyrthosiphon pisum. Female A. uzbekistanicus responded to wheat leaves only but males responded to a range of plant material. Both male and female A. ervi responded to wheat and bean leaves. The failure of A. ervi to respond to either nettle aphids, Microlophium carnosum (Bukt.), or nettle leaves, despite its frequent parasitization of this aphid in the field, suggests the existence of more than one race of the parasitoid and casts doubts on the usefulness of alternative hosts as reservoirs for A. ervi in integrated control programmes. Males of both species responded to their respective females suggesting the presence of a sex specific attractant.     

Density dependence in cereal aphid populations

Long sequences of data on the incidence of cereal aphids from five European countries were analysed for evidence of density dependent processes occurring between years. Using a randomisation test (Pollard, Lakhani & Rotheray, 1987), density dependence was revealed in all (16) population censuses of Metopolo-phium dirhodum , 60% of Rhopalosiphum padi censuses (10 of 17) but only 25% of Sitobion avenae population censuses (4 of 16). Correcting for density independent effects of weather revealed the existence of significant direct density dependence in some populations censuses where it was previously undetected. The implications of density dependence in cereal aphid populations are considered.     

Plant structure and the foraging success of Aphidius rhopalosiphi (Hymenoptera: Aphidiidae)

Abstract. 1. A study of searching behaviour on wheat and host preference of Aphidius rhopalosiphi (DeStefani-Perez) is described.
2. Parasitoids divided their time equally between the leaves but spent very little time on the ear.
3. After contact with honeydew or an aphid host, parasitoids were arrested in a particular area and increased the time spent searching.
4. Aphids feeding on the ear were parasitized less successfully, since their position between the grains protected them from parasitoid attacks.
5. A.rhopalosiphi exhibited no preference for Metopolophium dirhodum (Walker) or Sitobion avenae (Fabricius), although the handling time for the latter was significantly longer; this resulted in fewer S.avenae being parasitized when it was abundant. Parasitoids did not switch between hosts in these experiments.
6. As a result of its searching behaviour, A.rhopalosiphi will encounter and parasitize M.dirhodum feeding on the leaves more frequently than S.avenae, which feeds on the ear. This will limit the parasitoid's ability to regulate populations of the cereal aphid S. avenae.     

Observations on the morphs of Macrosiphum avenae and Metopolophium dirhodum on cereals during the summer and autumn

From June to early August 1970, populations of Macrosiphum avenae and Metopolophium dirhodum on marked tillers of field barley were compared with the numbers of alatae trapped at crop height and at 12.2 m. There were always more M. dirhodum than M. avenae on the tillers. Only apterae were produced until mid-June when both alatae and apterae occurred; after mid-July only alate M. avenae were found. Until mid-June most of the flying alatae were caught at 12.2 m as they migrated from spring hosts to cereals. Thereafter, more alate M. avenae were trapped at 12.2 m than at crop level, whereas numbers of alate M. dirhodum were usually comparable at both heights. Although crop and flying populations occasionally showed temporal similarities, insufficient is known about their field distribution and the factors affecting their alate production and flight activity to interpret this relationship. In the autumn, two consecutive reproductive phases of M. dirhodum occurred on winter wheat grown in pots outdoors. Initially, apterous virginoparae and alatae, probably sexuparae, were produced, whereas only alate males appeared during the second phase. In contrast, M. avenae deposited mainly apterous virginoparae although some oviparae developed in October to lay scattered, probably infertile eggs on the tillers.     

The effect of cereal growth stages on the reproductive activity of Sitobion avenue and Metopolophium dirhodum

Apterous Sitobion avenae on oats were found to have a higher reproductive rate on the ears (5–95 nymphs per day) than on young leaves (3–78 nymphs per day), mature leaves (2–17 nymphs per day) or senescent leaves (2–08 nymphs per day). At each growth stage of the the host plant large aphids gave birth to more young than small aphids, but aphids of the same weight on different growth stages of the host plant had different reproductive rates. On wheat in the field S. avenae gave birth to more young on the ears but less on the leaves than Metopolophium dirhodum on the leaves.
The suitability of ears to S. avenae changed through the flowering and ripening stages. During the latter part of the milky-ripe stage the reproductive rate of S. avenae declined and its mortality increased. The consequences of the variations in reproduction and mortality are discussed in relation to the life history and population dynamics of S. avenae and the assessment of cereal varieties for aphid resistance.     

Effects of feeding position of the aphids Sitobion avenae and Metopolophium dirhodum on wheat yield and quality

The effects of feeding site of Sitobion avenae and Metopolophium dirhodum on grain weight, number and nitrogen content were investigated in the laboratory and in the field. When the latter species was confined to particular leaves in a growth room, grain weight reductions occurred only when the aphid fed on the flag leaf; reductions in percentage grain protein, but not grain weight, followed lower leaf feeding. Leaves which had borne aphids had a higher proportion of nitrogen at death than control leaves, but this explained little of the shortfall in grain nitrogen, which was mainly due to direct removal during aphid feeding. Multiple regressions of grain weight on aphid species/feeding site in the field confirmed the lack of grain weight effects when lower leaves were colonised; because grain size was greatly reduced by aphid feeding, percentage grain nitrogen increased with increasing aphid density in one cultivar and in others was unchanged. Successive comparison of control grain weights with those from tillers bearing an increasing range of aphid densities resulted in a significant reduction in weight with tillers bearing 21 to 25 aphid units on their ear and flag leaf combined, one unit comprising one adult or fourth-instar aphid or three of earlier instars. These and other factors affecting the nature and extent of cereal aphid damage are discussed.     

Population dynamics, disturbance, and pattern evolution: identifying the fundamental scales of organization in a model ecosystem

We used auto- and cross-correlation analysis and Ripley's K-function analysis to analyze spatiotemporal pattern evolution in a spatially explicit simulation model of a semiarid shrubland (Karoo, South Africa) and to determine the impact of small-scale disturbances on system dynamics. Without disturnities bance, local dynamics were driven by a pattern of cyclic succession, where 'colonizer' and 'successor' species alternately replaced each other. This results in a strong pattern of negative correlation in the temporal distribution of colonizer and successor species. As disturbance rates were increased, the relationship shifted from being negatively correlated in time to being positively correlated-the dynamics became decoupled from the ecologically driven cyclic succession and were increasingly influenced by abiotic factors (e.g., rainfall events). Further analysis of the spatial relationships among colonizer and successor species showed that, without disturbance, periods of attraction and repulsion between colonizer and successor species alternate cyclically at intermediate spatial scales. This was due to the spatial 'memory' embedded in the system through the process of cyclic succession. With the addition of disturbance, this pattern breaks down, although there is some indication of increasing ecological organization at broader spatial scales. We suggest that many of the insights that can be gained through spatially explicit models will only be obtained through a direct analysis of the spatial patterns produced.     

Small-scale dynamics and species richness in successional alvar plant communities

Species richness and small-scale spatial dynamics (turnover) were compared in four sites during 1990-1993 1) former alvar grassland, now young Pinus sylvestris forest, 2) clear-cut in 1990, 3) clear-cut and grazed by sheep, 4) nearby grassland It was hypothesized that in communities which show large-scale dynamics, small-scale dynamics is also more rapid to support succession Area-based species richness and shoot density increased in the cut and grazed site No significant dynamics was found in shoot-based species richness Therefore the forest was considered to be stable and the cut and grazed site large-scale dynamical (successional) Almost equally high small-scale turnover values were found in all sites, which supports the carousel model for successional communities, but means we had to reject the initial hypothesis The successional and intrinsic small-scale dynamics of the communities studied were two separate processes which acted at different scales, but the scale could not be only spatially determined, it depended also on the shoot density The overgrowing of this former alvar grassland has resulted in the decrease of shoot density and thus the decrease of species richness per unit area, but the total number of species has not changed much Consequently the possibility to restore grassland communities remains     

Joint Effects of Habitat Heterogeneity and Species’ Life-History Traits on Population Dynamics in Spatially Structured Landscapes

Both habitat heterogeneity and species’ life-history traits play important roles in driving population dynamics, yet there is little scientific consensus around the combined effect of these two factors on populations in complex landscapes. Using a spatially explicit agent-based model, we explored how interactions between habitat spatial structure (defined here as the scale of spatial autocorrelation in habitat quality) and species life-history strategies (defined here by species environmental tolerance and movement capacity) affect population dynamics in spatially heterogeneous landscapes. We compared the responses of four hypothetical species with different life-history traits to four landscape scenarios differing in the scale of spatial autocorrelation in habitat quality. The results showed that the population size of all hypothetical species exhibited a substantial increase as the scale of spatial autocorrelation in habitat quality increased, yet the pattern of population increase was shaped by species’ movement capacity. The increasing scale of spatial autocorrelation in habitat quality promoted the resource share of individuals, but had little effect on the mean mortality rate of individuals. Species’ movement capacity also determined the proportion of individuals in high-quality cells as well as the proportion of individuals experiencing competition in response to increased spatial autocorrelation in habitat quality. Positive correlations between the resource share of individuals and the proportion of individuals experiencing competition indicate that large-scale spatial autocorrelation in habitat quality may mask the density-dependent effect on populations through increasing the resource share of individuals, especially for species with low mobility. These findings suggest that low-mobility species may be more sensitive to habitat spatial heterogeneity in spatially structured landscapes. In addition, localized movement in combination with spatial autocorrelation may increase the population size, despite increased density effects.