Low Aphid Densities Research Articles

The effects of insecticide dose and herbivore density on tri-trophic effects of thiamethoxam in a system involving wheat, aphids, and ladybeetles

We assess how herbivore density and insecticide dose affects the tri-trophic interactions among thiamethoxam-treated wheat (Triticum sp.), Rhophalosiphum padi and the predatory Coleomegilla maculata DeGeer. In the first experiment 2nd and 4th instar C. maculata were fed aphids reared for 24 h on wheat plants treated with sub-lethal thiamethoxam soil drenches to assess the effect of insecticidal dose on the predator. In the second experiment a constant, sub-lethal thiamethoxam soil drench was used on wheat plants, but the plants were infested with different densities of aphids. Aphids from each density treatment were then fed to C. maculata 2nd instars. Insecticide levels within wheat and aphid tissues were quantified using a competitive ELISA. In experiment one, 4th instars of C. maculata displayed no negative effects after consuming aphids from treated plants. However, 2nd instars of C. maculata ate more treated aphids than untreated aphids and larval development times varied between the untreated larvae and those receiving the low dose of the insecticide. In experiment two, the 2nd instar C. maculata displayed slower walking speeds after consuming aphids from low aphid density plants. Thiamethoxam within wheat tissue was found at higher levels with increasing dose. This insecticide was also found at higher levels in wheat plants with the lowest aphid density. Clothianidin, a toxic metabolite of thiamethoxam, was found in aphids that had fed on thiamethoxam-treated wheat, but no differences between treatments were observed. The neonicotinoid treatment altered the quality of R. padi as prey for 2nd instar C. maculata, but this depended on the aphid infestation level on the plants. In addition to revealing a potential deleterious effect of thiamethoxam systemic insecticides in wheat, our assays provide insight into the design and interpretation of toxicity assays involving systemic neonicotinoids and herbivores.

Density-dependent interference of aphids with caterpillar-induced defenses in Arabidopsis: involvement of phytohormones and transcription factors.

In nature, plants are exposed to attacks by multiple herbivore species at the same time. To cope with these attacks, plants regulate defenses with the production of hormones such as salicylic acid (SA) and jasmonic acid (JA). Because herbivore densities are dynamic in time, this may affect plant-mediated interactions between different herbivores attacking at the same time. In Arabidopsis thaliana, feeding by Brevicoryne brassicae aphids interferes with induced defenses against Plutella xylostella caterpillars. This is density dependent: at a low aphid density, the growth rate of P. xylostella was increased, whereas caterpillars feeding on plants colonized by aphids at a high density have a reduced growth rate. Growth of P. xylostella larvae was unaffected on sid2-1 or on dde2-2 mutant plants when feeding simultaneously with a low or high aphid density. This shows that aphid interference with caterpillar-induced defenses requires both SA and JA signal transduction pathways. Transcriptional analysis revealed that simultaneous feeding by caterpillars and aphids at a low density induced the expression of the SA transcription factor gene WRKY70 whereas expression of WRKY70 was lower in plants induced with both caterpillars and a high aphid density. Interestingly, the expression of the JA transcription factor gene MYC2 was significantly higher in plants simultaneously attacked by aphids at a high density and caterpillars. These results indicate that a lower expression level of WRKY70 leads to significantly higher MYC2 expression through SA-JA cross-talk. Thus, plant-mediated interactions between aphids and caterpillars are density dependent and involve phytohormonal cross-talk and differential activation of transcription factors.

Effect of prey limitation on competitive interactions between a native lady beetle, Coccinella novemnotata, and an invasive lady beetle, Coccinella septempunctata (Coleoptera: Coccinellidae).

The size and geographic distribution of Coccinella novemnotata Herbst populations have been decreasing rapidly across North America closely following the establishment, spread, and population growth of the invasive seven-spotted lady beetle, Coccinella septempunctata L. To determine whether intraguild predation and competition for prey may be partially responsible for the decline, we paired first-instar larvae of two populations of C. novemnotata (eastern and western) with first-instar C. septempunctata at low or high aphid densities. Survival of both C. novemnotata populations was significantly lower when larvae were paired with C. septempunctata, and western C. novemnotata exhibited significantly lower survival compared with the eastern population. This relationship depended on aphid density with the greatest survival of both C. novemnotata populations occurring at the high aphid density. Both male and female C. novemnotata weighed less on the day of eclosion when paired with C. septempunctata as compared with pairings with conspecifics. In a second test, C. septempunctata and C. novemnotata instars were varied at the start of the trial and C. novemnotata survival to adulthood in the presence of C. septempunctata was dependent of the instar of C. novemnotata used to initiate the experiment. C. novemnotata exhibited higher rates of survival and weighed significantly more on the day of eclosion when C. novemnotata was older than its C. septempunctata partner. These results suggest that interspecific competition including intraguild predation by C. septempunctata may contribute to C. novemnotata population declines, but that the intensity of this impact may vary across C. novemnotata populations.

Impact of plant architecture and extraguild prey density on intraguild predation in an agroecosystem

Several models and experimental studies conducted in confined environments have shown that intraguild predation (IGP) can modulate population abundances and structure communities. A number of ecological and abiotic factors determine the nature and frequency of IGP. This study examined the effect of plant architecture and extraguild prey density, and their interactions, on the occurrence of IGP between two species of ladybird, Harmonia axyridis (Pallas) and Propylea quatuordecimpunctata L. (both Coleoptera: Coccinellidae). Theoretical concepts predict that IGP levels would increase with a decrease of both extraguild prey density and plant structural complexity. We conducted a factorial experiment in an open soybean field into which coccinellid larvae were introduced in experimental plots for a period of 5 days. We tested two levels of soybean aphid [Aphis glycines Matsumara (Hemiptera: Aphididae)] density, low and high (100 and 1 000 aphids per plot, respectively), and two levels of plant complexity, low (by removing half of the branches from the soybean plants) and high (by leaving plants intact). We used species-specific molecular markers to detect the presence of P. quatuordecimpunctata in the digestive tract of H. axyridis. Molecular gut-content analysis of H. axyridis revealed that rates of IGP were higher (20%) at low aphid density than at high aphid density (<6%). Decreased plant complexity did not impact the frequency of IGP. In accordance with existing literature, this study demonstrates that IGP is amplified at low extraguild prey density. We conclude that considering environmental factors, such as extraguild prey density, is crucial to improve our ability to predict the impact of intraguild predation on community structure and, from an applied perspective, biological control.

NEEM-AZAL T/S TO CONTROL LEAFHOPPERS AND APHIDS IN GREENHOUSE SWEET PEPPER AND TOMATO

Aphids and leafhoppers can cause heavy damage to sweet pepper (Capsicum annuum) and tomato (Solanum lycopersicum). Neem-Azal T/S was therefore evaluated in greenhouse experiments for leafhopper and aphid control. In 2005, five organic greenhouse compartments were treated with Neem-Azal T/S (0.2%, 1000-1500 L water ha-1; two treatments on tomato; three treatments on sweet pepper) for control of leafhoppers; four greenhouse compartments were left as untreated controls. Leafhopper flight activity was monitored using yellow sticky traps. Leaf damage was estimated by evaluating the fifth leaf from the top of the plant on 100 plants per plot. Flight activity was reduced by 80% (Abbott’s formula; 11-20 days after the 2nd treatment) to 93% (60-75 days after last treatment). Proportion of damaged leaves was reduced by 79-82%. In 2006, one half of each of six sweet pepper glasshouse compartments (=replicate) was treated with Neem-Azal T/S (0.3%, 800 L water ha-1; three applications), the other half of each greenhouse remained untreated. The number of aphids was estimated by evaluating the fifth leaf from the top of the plant on 200 plants per plot. Three days after treatment, 303 aphids were counted per 100 leaves on untreated plants, while Neem-treated plants had lower aphid densities (24 aphids on 100 leaves). Significant differences in aphid numbers were observed 7, 14 and 21 days after treatment. Efficacy 21 days after treatment was 97.8% (Abbott’s formula). Rapid aphid development in the untreated control plots led to severe leaf distortion, stunted shoots and fruits covered in honeydew. Treated plots did not show any symptoms of aphid damage. Neem-Azal T/S was effective against aphids and leafhoppers in greenhouse sweet pepper and tomato production in Switzerland. According to observations of growers and pest management advisors, the combination of Neem-Azal T/S and natural enemies might further improve its efficacy.

The development of an economic threshold for Nasonovia ribisnigri (Hemiptera: Aphididae) on lettuce in central Spain.

This study reports economic thresholds for the lettuce aphid Nasonovia ribisnigri (Mosley), based exclusively on cosmetic damage, that is, presence or absence of aphids at harvest time. Field trials were conducted in La Poveda Experimental Farm, Madrid (Spain) during autumn (2004 and 2005) and spring (2005 and 2006). Plants were arranged in plots and just before the formation of lettuce hearts they were infested with different densities of N. ribisnigri. Two days later, half of each plot was treated with tau-fluvalinate (Klartan24AF) and the other half remained as an untreated control. Economic thresholds were obtained from nonlinear regressions calculated between the percentage of commercial plants at the end of the crop cycle for both, treated and untreated semiplots, and the different initial densities of N. ribisnigri per plant. Two criteria were used to consider a commercial lettuce plant: a conservative estimate (0 aphids/plant) and a lax one (< 5 aphids/plant). Thus, an economic threshold was established for each season and criterium. The economic thresholds that were obtained with the most and least conservative criteria were in spring 0.06 and 0.12 aphids per plant, and in autumn 0.07 and 0.13 aphids per plant, respectively. These results show that to avoid cosmetic damage, insecticide sprays are required when a very low aphid density is detected in lettuce seedlings soon after transplant.

Flowers promote aphid suppression in apple orchards

Predators and parasitoids often benefit from feeding on nectar and/or pollen, such that the addition of flowering plants should bolster natural enemies and improve biological control. Nonetheless, this conceptually-simple approach often fails to reduce pest numbers. We examined whether flowering annual plants drew more natural enemies to apples (Malus domestica) in Washington State, USA, and in turn whether this improved suppression of woolly apple aphids (Eriosoma lanigerum) on nearby trees. Initial screening of candidate flowers indicated that syrphid flies (Diptera: Syrphidae), thought to be important aphid predators, were particularly attracted to sweet alyssum flowers (Lobularia maritima). Therefore, in two subsequent field experiments we compared aphid densities on trees placed adjacent to, or relatively far from, flowering sweet alyssum. The results were striking: after one week aphid densities were significantly lower on trees adjacent to flowers than on those on control plots, and these differences were maintained for several weeks. It was unlikely that aphid decline was primarily due to syrphid predation, because lower aphid densities were observed despite few syrphid larvae being present. Rather, a diverse group of generalist-predator spiders and bugs increased significantly near sweet alyssum plantings, and may have been responsible for much of the aphid suppression that we observed. Immunomarking revealed that natural enemies regularly moved from sweet alyssum to the surrounding orchard. In summary, the floral resources that sweet alyssum plants provided attracted natural enemies and indirectly suppressed densities of woolly apple aphids, suggesting an effective means for apple growers to enhance biological control.

Aphid consumption and residence time of larvae of flightless lady beetles, Harmonia axyridis (Coleoptera: Coccinellidae), on aphid-infested plants

A flightless strain of the lady beetle, Harmonia axyridis Pallas, has been established, by artificial selection, for effective biological control of aphids. However, this biological control technique is costly because of the high cost of production of the adults. It is, therefore, important to consider the utility of the larvae in biological control. This study examined the potential for larvae of the flightless strain to be used as biological control agents compared with those of the wild-type, in terms of how they behave on aphid-infested plants. We found no differences between the strains with regard to residence time and aphid consumption on the plant for the first, second, and fourth instars, and at low and high aphid densities. In contrast, however, aphid consumption by third-instar larvae of the flightless strain tended to be lower, although they remained on the plant longer than third-instar larvae of the wild-type. Consequently, our results suggest the flightless strain does not differ from the wild-type in its ability to control aphids across all larval stages, except for the third instars. On the basis of these results, and in conjunction with the results from previous studies, we discuss the appropriateness of different larval instars for release.

A Multi-Approach Study to Delineate Interactions Between Carabid Beetles and Soybean Aphids

In recent years, the soybean aphid, Aphis glycines Matsumura, has become the most important exotic pest of soybean, Glycine max (L.) Merrill, in North America. Given the significant yield losses that are reported, considerable effort has been expended to characterize the natural enemy community associated with this pest. Several indigenous and naturalized predators have been identified as potential biological control agents, and these include carabid beetles, an abundant and important family of aphid predators. The objectives of this study were to assess the incidence of field predation by Pterostichus melanarius (Illiger), the most common carabid species in Québec soybeans, using molecular gut-content analysis, and to quantify its impact on A. glycines populations through laboratory and field cage experiments. Throughout the growing season between 16.8% (during low aphid density) and 33.7% (at times of high aphid density) of P. melanarius tested positive for aphid DNA. Furthermore, although laboratory feeding trials confirmed that P. melanarius prey on A. glycines, short-term field cage experiments failed to demonstrate a significant reduction of A. glycines populations by carabid beetles. These results suggest a relatively weak interaction between P. melanarius and A. glycines when pest densities are high, but the high predation rate when aphid densities are particularly low suggests these natural enemies may function as important early-season predators.

Towards a nutritional ecology of invasive establishment: aphid mutualists provide better fuel for incipient Argentine ant colonies than insect prey

Many potential species invasions fail before establishment. This is likely especially true for invasive Argentine ants that must overcome a severe founding bottleneck and transition from propagules that rely on protein-rich prey to massive supercolonies that dominate by consuming carbohydrate-rich honeydew from hemipteran mutualists. While this dietary shift supports the classic idea that protein fuels early colony development and carbohydrates maintain adult workers, recent evidence suggests that carbohydrates can govern initial colony establishment. In this study, we use lab experiments to show that resources from aphid mutualists had greater benefits for Argentine ant propagule survival, maintenance, growth, and worker activity rates than did prey items. These effects persisted at low aphid densities, and when colonies were otherwise starved. Moreover, prey-starved colonies did not appear to consume aphids, suggesting that carbohydrate-rich honeydew is a mechanism that facilitates colony establishment. Combined, these results support a hypothesis that the dietary shift from prey to honeydew is driven more by increased access to hemipterans after establishment, than by specific benefits of prey early in colony development. The results highlight the important role of nutritional ecology for studying invasive establishment, linking propagule success not only to the supply of food resources, but also to their quality.

Salicylic acid-mediated reductions in yield in Nicotiana attenuata challenged by aphid herbivory

Aphid herbivory decreases primary production in natural ecosystems and reduces crop yields. The mechanism for how aphids reduce yield is poorly understood as some studies suggest aphid feeding directly impedes photosynthesis, whereas other studies suggest a change in allocation of resources from growth to defense compounds reduces yield. To determine the mechanisms underlying reduced plant growth by aphids, Nicotiana attenuata plants, native tobacco, were infested with Myzus persicae ssp. nicotianae, tobacco-adapted green peach aphids, at low and high densities, and plant performance including fitness was assessed. To test the direct defense capacity of salicylic acid (SA) on aphid performance, we fed aphids an artificial diet with varying levels of SA and measured their survivorship and fecundity. There was no detectable effect of aphid herbivory on net photosynthesis, yet herbivory reduced plant growth, final biomass (43 % at high aphid density), and seed set (18 % at high aphid density) at both low and high aphid infestation levels. High-density aphid attack during the rosette and flowering stage caused an increase in SA levels, but caused only a transient decrease in jasmonic acid concentration at low aphid density. SA concentrations similar to those found in infested flowering plants decreased aphid fecundity, suggesting that SA was an effective chemical defense response against aphids. These results suggest that as aphid densities increased the proximal cause of reduced growth and yield was not reduced photosynthesis, but instead resources may have been mobilized for defense via the SA pathway, decreasing the availability of resources for building plant biomass.

Prey availability and abiotic requirements of immature stages of the aphid predator Sphaerophoria rueppellii

Aphidophagous hoverflies (Diptera: Syrphidae) are natural enemies with a high potential as biological control agents. Nevertheless, there is only one commercially available species, Episyrphus balteatus, which despite naturally occurring in the Mediterranean basin, is not the best adapted to the conditions found in its agricultural areas. Sphaerophoria rueppellii is another common hoverfly species in the Mediterranean area both in outdoor and greenhouse crops. However, little information about the biology of this species is known and this is one of the main requirements for every control-based IPM program. In this work we assess the influence of different humidity and temperature combinations on the performance (mortality, developmental time and sublethal effects) of S. rueppellii under controlled conditions. We also study the effect of aphid supply during larval stage on mortality and developmental time of immature stages and on sublethal effects on adults. The results show that this species requires high environmental humidity (higher than 60%) to complete its development. Also, S. rueppellii is able to properly develop at a wide range of temperatures (20–30°C). S. rueppellii reduces its voracity under low aphid density conditions, being able to adapt its feeding rate to the aphid availability. Mortality was not affected by food supply, while low prey availability led to longer developmental time and a decrease in larval and pupal weight and adult body size. These results show the suitability of this species as an aphid biological control agent under high humidity and temperature conditions, such as the ones inside Mediterranean greenhouses.

The functional significance of E-β-Farnesene: Does it influence the populations of aphid natural enemies in the fields?

Aphids cause much damage to Chinese cabbage in northern China. Over reliance on pesticides have large environmental and human health costs that compel researchers to seek alternative management tactics for aphid control. The component of aphid alarm pheromone, E-β-Farnesene (EβF), extracted from Matricaria chamomilla L., which attracts natural enemies in the laboratory, may have significant implications for the design of cabbage aphid control strategies. The purpose of this paper is to understand the effects of EβF on natural enemies to cabbage aphid control in Chinese cabbage fields. Ladybeetles on Chinese cabbage leaves in EβF released plots and Aphidiidae in EβF released yellow traps were significantly higher than those of in controls. No significant differences were detected in the interactions of different treatments and the two years for all natural enemies. More important, lower aphid densities were found in EβF released plots. Our results suggested that the EβF extracted from M. chamomilla L. could attract natural enemies to reduce cabbage aphids in the Chinese cabbage fields.

BIOLOGICAL CONTROL OF APHIDS BY THE PREDATORY MIDGE APHIDOLETES APHIDIMYZA IN THE PRESENCE OF INTRAGUILD PREDATORY BUGS AND THRIPS

BIOLOGICAL CONTROL OF APHIDS BY THE PREDATORY MIDGE APHIDOLETES APHIDIMYZA IN THE PRESENCE OF INTRAGUILD PREDATORY BUGS AND THRIPS

Enhancing biological control by an omnivorous lacewing: Floral resources reduce aphid numbers at low aphid densities

The availability of plant resources to omnivorous arthropod predators may have a positive, negative or negligible effect on their population densities and predation rates, depending on the availability of prey. At high prey densities, flowering buckwheat has been shown to negatively impact populations of the brown lacewing, an omnivorous predator, due to the probable increase in parasitism rate of lacewing larvae by their primary parasitoid, Anacharis zealandica. However, little is known about the effect of buckwheat flowers on this insect community at low prey densities. We used field cages to assess the effects of nectar provision by flowering buckwheat on the population dynamics of the pea aphid, the brown lacewing and its parasitoid A. zealandica in an alfalfa field, under low aphid densities in the New Zealand summer. The insects were sampled every 2 weeks with a suction device, then counted and released on each sampling date from 15 January to 15 March 2007. Buckwheat significantly increased lacewing populations and significantly decreased aphid numbers by 70% and 39%, respectively. The buckwheat had its greatest effect at the end of summer (February/March) for both these species. It had no effect on A. zealandica abundance.

Implications of Russian wheat aphid, Diuraphis noxia, 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.

Spatial distribution of flower vs. honeydew resources in cereal fields may affect aphid parasitism

The potential impact of nectar availability for biological pest control is of increasing relevance due to the disappearance of non-crop plants in intensified agricultural landscapes. However, little is known about how floral resources and their spatial distribution in and around crop fields influence cereal aphid parasitoid (Hymenoptera, Aphidiidae) survival and population dynamics. Cereal aphid parasitoids may be less dependent on flowers as they can use honeydew from their aphid hosts as an alternative food resource. However, honeydew appears to be less suitable as a food source for parasitoids when compared to nectar. Here the effects of flower distribution (nectar), aphid abundance (honeydew quantity) and honeydew quality on parasitoid life history parameters were explored using a spatially explicit, individual-based simulation model. In addition, effects of the availability of floral resources in field margins on parasitism levels in winter wheat were investigated in a field study. In the simulation study three flower distributions were compared: (i) flowers restricted to the field margin (most common in agricultural landscapes) or (ii) flowers in strips within the crop, and (iii) random flower distribution in the field. Simulations suggest that nectar feeding increases parasitoid longevity and parasitism at low aphid densities, but only in fields with randomly distributed flowers. In contrast, the spatial distribution of flowers did not affect the parasitoid longevity and parasitism rate at high aphid densities, where parasitoid feeding was restricted to the abundant honeydew resources. In simulations, parasitoid survival and the number of aphids parasitized were also affected by the nutritional quality of the honeydew, with low quality honeydew having a less positive effect on parasitoid longevity and parasitism rate. The field study did not indicate evidence of a significant relationship between flower cover in field margins and parasitism rates in wheat near the margins. This finding is compatible with results from simulations and suggests that honeydew feeding is more prevalent in cereal aphid parasitoids than nectar feeding. In conclusion, nectar may be an important factor for the potential biocontrol success of cereal aphid parasitoids, but most likely only at low aphid densities and especially when flowers are broadly distributed in the field.

Cereal aphid control by generalist predators in presence of belowground alternative prey: Complementary predation as affected by prey density

Summary Generalist predators are important antagonists of pest species in agroecosystems. Increasing populations of alternative prey through detrital subsidies is one way to foster these predators. However, alternative prey may also distract generalist predators from their prey thereby diminishing the efficiency of biological control. To develop reliable predictions for biological control, it is essential to evaluate the relative importance of generalist predators, pests, alternative prey and their respective interactions. We investigated the effects of an assemblage of generalist predators on the grain aphid Sitobion avenae in winter wheat. Treatments with 10, 100 and 1000 aphids were established inside 2 m 2 sized caged plots with control and reduced density of predators. Three weeks after the experiment started, samples were taken to estimate the size of aphid populations and those of alternative prey. Three prey taxa were significantly reduced by generalist predators: the grain aphid S. avenae (Aphidina), the click beetle Adrastus pallens (Elateridae) and the springtail Isotoma viridis (Collembola). Springtails were decreased by generalist predators independent of aphid densities, indicating complementary predation of springtails and aphids. At high aphid densities, grain aphid population peaks were decreased to the threshold level of economic damage, demonstrating efficient aphid suppression by the predator community. Click beetle numbers declined only at low and medium aphid densities. The results suggest that generalist predators preferentially fed on click beetles at low and medium aphid densities and switched to aphids at high aphid densities. Early-season predators likely had the greatest influence on aphid suppression. Our results indicate that alternative prey from the belowground system forms a substantial food resource for generalist predators, suggesting that the belowground subsystem modulates predator–prey interactions above the ground.

Reproductive success of house sparrows along an urban gradient

AbstractRecent large declines in house sparrow Passer domesticus L. populations in many European towns and cities have generated much speculation as to possible environmental causes, although empirical insights have been lacking. We measured reproductive success in a declining house sparrow population along an urbanization gradient in Leicester, England, and used mixed models to identify correlates of nesting success. A stochastic simulation approach was developed to estimate annual reproductive success when the number of breeding attempts was unknown, and to assess the sensitivity of reproductive success to variation in different environmental factors. During 2 out of 3 years, reproductive output was lower than the predicted threshold required for population stability. Years of inadequate reproduction were characterized by lower chick survival and body mass at fledging (a predictor of post‐fledging survival). Factors associated with reduced reproductive output included low ambient temperatures, extremes of rainfall, high levels of vegetable material in the diet (all affecting chick survival and body mass), low aphid densities (an important component of chick diet) and high concentrations of air pollution from traffic (affecting chick body mass). Reproductive failure linked to inadequate invertebrate availability provides a plausible demographic mechanism accounting for declines in urban–suburban house sparrow populations. Increased traffic volume and vegetation removal are candidate causal factors in these declines although the mechanism underlying the former is unclear. Increasing densities of key invertebrate prey away from major roads should be an objective of conservation management aimed at urban–suburban house sparrows. This study illustrates the utility of mixed models for the analysis of individual sibling survival and of stochastic simulation for modelling reproductive success in multi‐brooded animals.

Egg composition and reproductive investment in aphidophagous ladybird beetles (Coccinellidae: Coccinellini): egg development and interspecific variation

Abstract Most studies of insect reproductive allocation concentrate on propagule size and number and very few consider egg composition, which is likely to be equally important. In the present study, data are provided on changes in egg lipid, glycogen, free carbohydrate and protein during embryonic development of the aphidophagous ladybird Adalia bipunctata (L.) and the compositions of A. bipunctata, Adalia decempunctata and Anisosticta novemdecimpunctata eggs are compared. In A. bipunctata, egg mass, lipid and glycogen decline strongly during development and egg protein declines more weakly. Free carbohydrate declines early in egg development and increases at egg hatching. Lipid is energetically the most important developmental fuel, although approximately half of the initial egg lipid remains in the neonate larva. Across the three species, energy per unit egg mass is lowest in the least specialized species, A. bipunctata, which also has the largest eggs, and is highest in the most specialized, An. novemdecimpunctata, which has the smallest eggs. Two possible explanations for the observed pattern are discussed: (i) species laying smaller eggs may incur higher developmental costs per unit mass than species laying larger eggs and (ii) more specialized species, which reproduce at lower aphid densities, may provision neonate larvae better to facilitate location and capture of aphids.