The mid‐season crash in aphid populations: why and how does it occur?

Abstract. 1. Aphid populations on agricultural crops in temperature regions collapse over a few days from peak numbers to local extinction soon after mid‐summer (e.g. mid‐July in the U.K.). The populations recover 6–8 weeks later. There is anecdotal or incidental evidence of an equivalent mid‐season population crash of aphids on grasses and forbs in natural vegetation. 2. The ecological factors causing the mid‐season population crash of aphids include a decline in plant nutritional quality and increased natural enemy pressure as the season progresses. Extreme weather events, e.g. severe rainstorms, can precipitate the crash but weather conditions are not a consistent contributory factor. 3. The population processes underlying the crash comprise enhanced emigration, especially by alate (winged) aphids, depressed performance resulting in reduced birth rates, and elevated mortality caused by natural enemies. 4. Mathematical models, previously applied to aphid populations on agricultural crops, have great potential for studies of aphid dynamics in natural vegetation. In particular, they can help identify the contribution of various ecological factors to the timing of the population crash and offer explanations for how slow changes in population processes can result in a rapid collapse of aphid populations.     

Do natural enemies really make a difference? Field scale impacts of parasitoid wasps and hoverfly larvae on cereal aphid populations

1. Naturally occurring predators and parasitoids are known to reduce the abundance of pest invertebrates in arable crops, yet current treatment thresholds do not account for such a contribution to pest management.
2. In the present study, we provide evidence for the presence of natural enemies correlating with a subsequent reduction in pest population growth.
3. The abundance of cereal aphid pests and two key aphidophagous natural enemies, parasitoid wasps (Aphidiinae) and hoverfly larvae (Syrphinae), was assessed at field boundaries and interiors in southeast England.
4. The highest rate of aphid population growth was associated with locations where no natural enemies were found. The presence of either Aphidiinae wasps or predatory Syrphinae larvae was associated with a reduction in the rate of aphid population growth, irrespective of location within the field, and overall aphid population growth was negatively correlated with increasing natural enemy abundance.
5. The results of the present study indicate that natural enemies contribute significantly to pest control, and provide further evidence supporting the use of management strategies for promoting natural enemies in agro‐ecosystems.
6. Aphid predators and parasitoids make an important contribution to aphid pest control within cereal fields, and thresholds for insecticide application should account for this to avoid unnecessary treatments.

     

Does sex-biased dispersal account for the lack of geographic and host-associated differentiation in introduced populations of an aphid parasitoid?

Host recognition and use in female parasitoids strongly relies on host fidelity, a plastic behavior which can significantly restrict the host preferences of parasitoids, thus reducing the gene flow between parasitoid populations attacking different insect hosts. However, the effect of migrant males on the genetic differentiation of populations has been frequently ignored in parasitoids, despite its known impact on gene flow between populations. Hence, we studied the extent of gene flow mediated by female and male parasitoids by assessing sibship relationships among parasitoids within and between populations, and its impact on geographic and host‐associated differentiation in the aphid parasitoid Aphidius ervi. We report evidences of a high gene flow among parasitoid populations on different aphid hosts and geographic locations. The high gene flow among parasitoid populations was found to be largely male mediated, suggested by significant differences in the distribution of full‐sib and paternal half‐sib dyads of parasitoid populations.     

Does genetic diversity limit disease spread in natural host populations?

It is a commonly held view that genetically homogenous host populations are more vulnerable to infection than genetically diverse populations. The underlying idea, known as the 'monoculture effect,' is well documented in agricultural studies. Low genetic diversity in the wild can result from bottlenecks (that is, founder effects), biparental inbreeding or self-fertilization, any of which might increase the risk of epidemics. Host genetic diversity could buffer populations against epidemics in nature, but it is not clear how much diversity is required to prevent disease spread. Recent theoretical and empirical studies, particularly in Daphnia populations, have helped to establish that genetic diversity can reduce parasite transmission. Here, we review the present theoretical work and empirical evidence, and we suggest a new focus on finding 'diversity thresholds.'     

The existence of LSP-1βS in Drosophila melanogaster natural populations in two northern states

LSP-1^S is present in Michigan and Massachusetts Drosophila melanogaster natural populations. Its frequency, 10%, is significantly higher in an East Jordan, Mich. (latitude, 45.10° N), population than in East Lansing, Mich. (latitude 42.44° N), or Hadley, Mass. (latitude, 42.21° N), populations, where it averages 3% at each location. The average frequency of LSP-2^S is more comparable, 6, 5, and 7% at East Jordan, East Lansing, and Hadley, respectively. LSP-1^F variants are also present. A total of 342 single third-instar larvae was scored for LSP-1 autosomal variants, and 323 for LSP-2 variants. Each larva represented a newly established isofemale line from collections at East Jordan in 1981 and 1983, East Lansing in 1982, and Hadley in 1981, 1982, and 1983. Within localities, frequencies of hemolymph protein variants did not differ significantly between years. Proteins 9, 10, 11, and 15 correspond to the LSP-1, , and triplet and LSP-2 polypeptide in D. melanogaster. Our results together with those of Singh and Coulthart [(1982). Genetics 102:437] indicate that D. melanogaster populations in north temperate climates maintain considerable genetic heterogeneity for the larval hemolymph proteins.     

The polymorphism of the genes/enzymes involved in the last two reductive steps of monolignol synthesis: what is the functional significance?

The polymorphism of genes and enzymes involved in the last two steps of monolignol synthesis is examined in the light of recent data coming from genomic studies and mutant/transformant analyses. The two catalytic activities considered--cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD)--are encoded by small multigene families. While some degree of diversification can be noted at the sequence level, it is often difficult to use this information to assign substrate specificities to each member of a gene family. Expression profiles, however, suggest for both CAD and CCR the existence of two sub-families: one devoted to developmental lignification, and the other involved in the synthesis of defence-related compounds.     

Drivers of host plant shifts in the leaf beetle Chrysomela lapponica: natural enemies or competition?

1. The leaf beetle, Chrysomela lapponica, originally uses the salicyl glucosides (SGs) of its host plants to sequester salicylaldehyde, which serves as a defence against generalist enemies but attracts specialist enemies. However, some populations of C. lapponica have shifted to SG‐poor hosts, and their secretions do not contain salicylaldehyde. 2. In was suggested that beetles shift to SG‐poor hosts to escape from specialist enemies. To test this hypothesis, we compared field mortality between two populations of C. lapponica that were associated with SG‐rich willow, Salix myrsinifolia (Kola Peninsula and Finland) and two populations that fed on SG‐poor willows, S. glauca (Ural) and S. caprea (Belarus). 3. Mortality from generalist enemies was significantly higher in Belarus than in three other populations, whereas mortality from specialists did not differ among populations. A specialist predator (syrphid fly larvae, Parasyrphus nigritarsis) and specialist parasitoids (phorid flies, Megaselia spp.) were attracted to the secretions of larvae reared on both SG‐rich and SG‐poor hosts. 4. Feeding on leaves of S. caprea and S. myrsinifolia both previously damaged by leaf puncturing and by the larvae of potentially competing species Chrysomela vigintipunctata, decreased the weight and prolonged the development of C. lapponica. 5. Thus, populations of C. lapponica that have shifted to SG‐poor willow species did not obtain enemy‐free space because specialist enemies have developed adaptations to herbivores that switched to a novel host plant. We suggest that in some populations host plant shift was favoured by interspecific competition with the early season SG‐using specialist, C. vigintipunctata.     

Assessment of aphid lethal paralysis virus as an apparent population growth‐limiting factor in grain aphids in the presence of other natural enemies

The influence of aphid lethal paralysis virus (ALPV), Rhopalosiphum padi virus (RhPV), natural enemies and fungal infection on the population growth of Rhopalosiphum padi and Sitobion avenae in the wheat fields of the Western Cape Province of South Africa was investigated at two sites. Time‐specific life tables were compiled for R. padi at one site. During the logarithmic phase of the development of R. padi aphids, natural enemies were not present in high numbers and the apparent large‐scale mortality observed appeared to be due to other causes. During the decline phase of this aphid population, the population size was reduced by 49%. This reduction coincided with a calculated high mortality of 70 aphids per plant. A dramatic decline in R. padi numbers and a high incidence of ALPV present in the aphid population was experienced during this period. Virus assays were carried out by double‐antibody sandwich enzyme‐linked immunosorbent assay (DAS‐ELISA) and an indirect immunofluorescent technique. Entomophthorales‐type fungal infection of aphids also reached its highest level during the decline phase, but at a later stage than ALPV infection, with a calculated level of 21 aphids per plant. This suggested that the presence of ALPV limited population development in R. padi. Similar results were obtained with S. avenae.     

Interspecific hybridization in natural sturgeon populations of the Eastern Black Sea: the consequence of drastic population decline?

The eastern part of the Black Sea and its tributaries are suitable habitats for several sturgeon species, among which Acipenser gueldenstaedtii, A. stellatus, A. nudiventris, A. persicus, A. sturio, and H. huso are well documented. However, different threats have led these species to a dramatic decline, all of them are currently listed as Critically Endangered, and some Locally Extinct, in that area. We tested 94 wild sturgeon samples from the Black Sea and Rioni River by analyzing the mitochondrial Control Region and nuclear markers for hybrid identification. The data analyses (1) assessed mitochondrial diversity among samples, (2) identified their species, as well as (3) indicated instances of hybridization. The data collected, besides confirming a sharp decrease of catches of Beluga and Stellate sturgeon in recent years, also revealed four juvenile hybrids between Russian and Stellate sturgeon, providing the first evidence of natural interspecific hybridization in the Rioni. The present communication raises concerns about the status of sturgeon species in this area and underlines the urgent need for conservation programs to restore self-sustaining populations.

     

The physiological significance of 11β-hydroxysteroid dehydrogenase in the rat lung

We have examined the interconversion of cortisone (E) and cortisol (F) in rat lung homogenate and microsomal fraction and in the isolated rat lung perfused with Krebs bicarbonate solution containing 4.5% albumin. In the perfused lung the apparent K_m was 5.1 μM E and the V_max was 9nmol·g⁻¹ · min⁻¹. The ability of the lung to reduce E to F was enhanced both by 7 days prior exposure of the rat to an ambient temperature of 2°C and by starvation of the rat for 3 days. The activity was inhibited by adrenalectomy and castration of 7 days duration. Whereas little steroid oxidation occurred in the perfused lung, preparations of lung homogenatcs and microsomal fraction readily reduced or oxidised the 11-position of the corticoid molecule depending on the preponderance of either NADPH or NADP, respectively. We conclude, that the predominance of the reductive reaction in the whole rat lung under physiological conditions reflects the very active pentose-phosphate shunt in the lung, which produces NADPH. We suggest that this ability of the lung to activate E to F may exert a fine control over the arterial concentrtion of unbound, physiologically active, 11-hydroxylated steroid.     

Spiders in mofette fields—Survival of the toughest in natural carbon dioxide springs?

In mofette fields, natural carbon dioxide springs, organisms have to stand extreme CO₂ concentrations up to 100%. These hostile conditions are spatially small-scaled and further influenced by earth tides, wind and temperature. The present project investigated the influence of increased atmospheric CO₂ concentration on spiders as representatives of above-ground organisms by means of pitfall traps in three mofette fields, differing in habitat conditions in the Plesná valley, eastern Cheb Basin, Czech Republic.Among the 71 recorded spider species four were rarely found in the Czech Republic. A canonical correspondence analysis revealed significant influences of environmental parameters on the spider assemblages. Two groups of spiders are clearly distinguishable, one being positively influenced by humidity and the second by temperature. A cluster analysis showed distinct and congruent results: spider assemblages of pitfall traps at spots with a mean CO₂ concentration above 7.6% grouped close together and this grouping was independent of site. At >7.6% CO₂ significantly fewer individuals and species were found in comparison to areas with lower CO₂ concentration. Between 2.5 and 10% CO₂, spiders indicated increased CO₂ concentrations much more sensitively than endogeic organisms (Nematoda, Collembola) in a nearby mofette field. Unlike in nematodes, collembolans and plants, no mofettovageous or mofettophilous spiders were detected. In contrast to humidity, CO₂ concentration and temperature, the vegetation cover was not among the factors, which significantly influenced spiders. This is explained by the fact that mofettophilous plants occurred at spots where almost no spiders could live. In a field experiment, most Pardosa pullata males tested passed a 30 cm long corridor with increased carbon dioxide concentration. These results and that of pitfall traps showed that relatively large and wandering specimens respectively were able to transit moderately hostile spots. Further experiments are necessary to find out if there is any active avoidance of high-CO₂ areas by spiders.     

Does insecticide resistance alone account for the low genetic variability of asexually reproducing populations of the peach-potato aphid Myzus persicae?

The typical life cycle of aphids includes several parthenogenetic generations and a single sexual generation (cyclical parthenogenesis), but some species or populations are totally asexual (obligate parthenogenesis). Genetic variability is generally low in these asexually reproducing populations, that is, few genotypes are spread over large geographic areas. Both genetic drift and natural selection are often invoked to account for this low genetic variability. The peach-potato aphid, Myzus persicae, which encompasses both cyclical and obligate parthenogens, has developed several insecticide resistance mechanisms as a consequence of intense insecticide use since the 1950s. We collected asexually reproducing M. persicae from oilseed rape and examined genetic variability at eight microsatellite loci and three insecticide resistance genes to determine whether their genetic structure was driven by drift and/or selection. We identified only 16 multilocus microsatellite genotypes among 255 individuals. One clone, which combined two insecticide resistance mechanisms, was frequently detected in all populations whatever their location over a large geographical area (the northern half of France). These unexpected findings suggest that drift is not the unique cause of this low variability. Instead, the intensification of both insecticide treatments and oilseed rape cultivation may have favored a few genotypes. Thus, we propose that selective pressures resulting from human activities have considerably modified the genetic structure of M. persicae populations in northern France in a relatively short period of time.     

Diversity in the circumsporozoite protein of Plasmodium vivax: Does it matter?

Since the gene encoding the malarial circumsporozoite surface protein was characterized a decade ago, the corresponding protein has been considered an important vaccine candidate. Victoria Mann, Michael Good and Allan Saul here discuss molecular variation in the circumsporozoite surface protein of Plasmodium vivax in this context. There is still doubt about the degree and importance of polymorphisms in non-repetitive regions of the molecule. The degree of polymorphisms and data on naturally occurring protective responses suggests there has been minimal immunological pressure; the authors contend that antigenic diversity is unlikely to be a major factor in the use of this antigen as a vaccine for P. vivax.     

Visual awareness: the eye fields have it?

Whether a stimulus is perceived is suggested to depend on the amount or strength of the sensory signal. This idea has received support from a recent study which found that, when neural activation in the frontal eye fields was boosted by magnetic stimulation, the observers' detection performance on a visual task improved.     

Does the natural diet influence the intestine's ability to regulate glucose absorption?

Summary Two fish species (rainbow trout and common carp) that differ in natural diet also exhibit differences in the adaptive flexibility of their intestinal nutrient transport mechanisms in response to changes in dietary nutrient composition. When carp ingested a feed that was 24% glucose by weight, there was an increase in both the intestinal length and rates of nutrient absorption, particularly for glucose, when compared to carp fed an isonitrogenous diet devoid of digestible carbohydrate. As a result, the intestine's uptake capacity (nmol of glucose and proline absorbed per min per g body weight) was higher in carp fed the 24% glucose feed. Due to the greater increase in glucose uptake, the ratio of glucose uptake relative to proline (G/P ratio) was higher in carp fed the 24% glucose. Thus, carp are able to adapt to the quantity, and apparently also to the type, of digestible carbohydrate in the diet. In contrast, glucose uptake by trout was unresponsive to the quantity of dietary carbohydrate. Insted, the elevated glucose paradoxically resulted in a greater uptake capacity for proline and a lower G/P ratio. Hence, the adaptive capabilities of the intestinal nutrient transport processes are matched to the potential variation in the carbohydrate content of the natural diet.Abbreviation PEG polyethylene glycol     

The 3-D model: Does time plasticity represent the influence of tableting speed?

The objective of this study is to test the hypothesis that time plasticity (parameterd from 3-D modeling) is influenced by tableting speed. Tablets were produced at different maximum relative densities (ϱ_rel,max) on an instrumented eccentric tableting machine and on a linear rotary tableting machine replicator. Some 3-D data plots were prepared using pressure, normalized time, and porosity according to Heckel. After fitting of a twisted plane, the resulting parameters were analyzed in a 3-D parameter plot. The materials used were dicalcium phosphate dihydrate (DCPD), spray-dried lactose, microcrystalline cellulose (MCC), hydroxypropyl methylcellulose (HPMC), κ-carrageenan (CAR), and theophylline monohydrate (TheoM). The results show that tableting speed especially influences the parameterd (time plasticity) of the 3-D model for plastically and viscoelastically deforming materials such as MCC, HPMC, CAR, and TheoM. For more plastically deforming materials such as MCC, HPMC, and TheoM, a subtle influence on ω is also visible. The stages of higher densification are affected more than the stages of lower densification. Brittle materials such as DCPD exhibit no influence of tableting speed. The influence of speed on spray-dried lactose is minor. The results are valid for data obtained from an eccentric tableting machine and also for data from a linear rotary tableting machine replicator. Thus, the empirically derived parameter time plasticityd really represents the influence of time.     

Origin of an insect outbreak: escape in space or time from natural enemies?

Initiation of insect outbreaks is poorly understood, and may involve sporadic events that temporarily release insect populations from predation or parasitism. While studying a declining outbreak of the western tussock moth (Orgyia vetusta) on bush lupine (Lupinus arboreus), we witnessed the onset of a new tussock moth outbreak, separated by 1,000 m in space and 2 months in phenological timing from the original population. This new population underwent explosive growth for 2 years and then collapsed because of a massive die-off of lupines. We tested whether during its growth phase, this new outbreak benefited by escaping in either space or time from the natural enemies attacking the original population. In experimental populations on single bushes, we compared predation and parasitism at the sites of the new and the old outbreak. At the site of the old outbreak, we compared predation and parasitism early and late in the season. Parasitism was significantly lower and population growth significantly higher at the new outbreak site than the old one. Neither seasonal timing, predator exclusion, nor their interaction significantly affected survival at either site. Thus the new outbreak appeared to escape in space from parasitism. These results corroborate our previous experimental findings, which suggest that as predicted by theory, the interaction between the tussock moth and its parasitoids can produce large-scale spatial patterning in population densities.     

EMG sensor location: Does it influence the ability to detect differences in muscle contraction conditions?

Even though it is well known that electromyography (EMG) characteristics are influenced by electrode placement it is common to use a single pair of sensors per muscle for EMG. This study was designed to determine if the ability to distinguish between contraction conditions was influenced by sensor location. Subjects (n = 10; 27+/-5.3 years; 82+/-13.4 kg; 178+/-7.1 cm) completed six elbow flexor conditions: three isometric contraction intensities (100% maximum effort, 80%, 50%) and three isotonic contraction intensities (heavy weight, 80% and 50% of the weight). Three pairs of electrodes were placed centrally, medially and laterally on the biceps brachii belly in line with the muscle fibers. Isometric contractions were held for 5s, with the middle 3 s analyzed. Isotonic exercises included five repetitions of elbow flexion-extension, with the middle three repetitions analyzed. Average EMG (EMG(AVG)), root mean square EMG (EMG(RMS)) and mean power frequency (MPF) were calculated for each extracted data set. Dependent variables were analyzed using 2 (contraction type) x 3 (intensity) repeated measures ANOVAs per sensor. EMG(AVG) was influenced by the interaction between contraction type and intensity for all sensors (p < 0.05). EMG(RMS) as well as MPF were influenced by the interaction between contraction type and intensity for the lateral and central leads (p < 0.05) but not the medial leads (p > 0.05). Different conclusions could have been reached from the same experiment due to different sensor locations. These differences were primarily related to comparing contraction types (i.e., isotonic vs. isometric).     

Pyroglutamate-Aβ: Role in the natural history of Alzheimer's disease

The accumulation of amyloid-beta (Aβ) peptides is believed to be a central contributor to the neurodegeneration typically seen in Alzheimer's disease (AD) brain. Aβ extracted from AD brains invariably possesses extensive truncations, yielding peptides of differing N- and C-terminal composition. Whilst Aβ is often abundant in the brains of cognitively normal elderly people, the brains of AD patients are highly enriched for N-terminally truncated Aβ bearing the pyroglutamate modification. Pyroglutamate-Aβ (pE-Aβ) has a higher propensity for oligomerisation and aggregation than full-length Aβ, potentially seeding the accumulation of neurotoxic Aβ oligomers and amyloid deposits. In addition, pE-Aβ has increased resistance to clearance by peptidases, causing these peptides to persist in biological fluids and tissues. The extensive deposition of pE-Aβ in human AD brain is under-represented in many transgenic mouse models of AD, reflecting major differences in the production and processing of Aβ peptides in these models compared to the human disease state.