Increasing Aphid Density Research Articles

Variable effects of mycorrhizal fungi on predator-prey dynamics under field conditions.

Interactions between herbivores and their predators are shaped, in part, by plant phenotype. Consequently, ubiquitous symbionts of plants below-ground, such as arbuscular mycorrhizal fungi (AMF), may influence interactions above-ground between predators and their prey by altering plant phenotype. However, the ecological relevance of below-ground organisms on predator-prey interactions under field conditions remains unclear. We assessed how AMF influence herbivore-predator interactions through a field experiment. We planted two milkweed species (Asclepias curassavica and Asclepias incarnata) provided with different amounts of AMF inoculum (zero, medium, and high) in a randomized block design. We added aphids to plants and reduced predator pressure weekly for 5weeks to evaluate effects of AMF on predator recruitment. We then allowed herbivore-predator interactions to re-establish naturally for the remainder of the season to examine whether AMF-mediated variation in predator recruitment influenced the suppression of aphid populations. Arbuscular mycorrhizal fungi availability in soils mediated interactions between predaceous aphid midge flies Aphidoletes aphidimyza and their aphid prey Aphis nerii, but the effects were plant species-specific. On A. curassavica, by mid-season, midges were recruited most strongly on plants under medium AMF availability and least on plants under high AMF availability. In contrast, each midge killed fewer aphids with increasing aphid density on medium AMF plants, but killed more aphids with increasing aphid density on high AMF plants. In combination, aphid mortality rates imposed by midges were greatest on medium AMF plants, followed by high and zero AMF plants. By comparison, on A. incarnata, the recruitment of midges was strongest on high AMF plants and weakest on medium AMF plants. AMF had no effect on the number of aphids killed per midge, relative to aphid density, so mortality rates of aphids imposed by midges mirrored recruitment. Rates of decline in aphid populations following predator recolonization were associated with midge densities, as well as lacewing and syrphid densities, which were unaffected by AMF availability. Therefore, the effects of AMF on aphid population decline were not a simple function of AMF-midge interactions. Our findings demonstrate that the availability of AMF in soils has pervasive, but complex, effects on predator-herbivore dynamics in the field.

Intraguild predation between Harmonia axyridis and Aphidius gifuensis: effects of starvation period, plant dimension and extraguild prey density

The effects of various factors on intraguild predation (IGP) between predators have been widely investigated. However, little is known about the influence of these factors on IGP between predators and parasitoids. Under laboratory conditions, we studied the effects of starvation period of predator, plant dimension, and extraguild prey density on IGP between Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) and Aphidius gifuensis Ashmead (Hymenoptera: Braconidae), two natural enemies of Myzus persicae (Sulzer) (Hemiptera: Aphididae). Both H. axyridis fourth instar and adults consumed A. gifuensis mummies. H. axyridis fourth instar and adults starved for 24 and 48 h had a higher frequency of IGP than those that were not starved. However, the number of aphids and mummies consumed by H. axyridis did not differ between the 24 and 48 h starvation treatments. Although IGP still occurred, mummy consumption decreased with increasing aphid density and plant dimension. In addition, H. axyridis consistently preferred aphids to mummies. Our results indicate that prey preference of H. axyridis, a shorter starvation period of predator (< 48 h), larger plant dimension, and higher extraguild prey density could reduce the frequency of IGP by H. axyridis on A. gifuensis, and thus decrease the negative impact of IGP on aphid control.

Aphid density and community composition differentially affect apterous aphid movement and plant virus transmission

1. Although many vector-borne pathogens are transmitted by an array of vector species, most studies do not account for the potential effects of species interactions. 2. By manipulating conspecific and heterospecific vector density in small experimental mesocosms, this study disentangled the impact of vector density and community composition on vector movement and plant virus transmission in the potato virus Y system. 3. The following predictions were tested: (i) increasing aphid density will increase aphid movement and virus transmission; (ii) adding low-efficiency vectors and thereby decreasing the average transmission efficiency of the vector assemblage will decrease virus transmission; and (iii) aphid movement and the average vector transmission efficiency will mediate the effect of aphid density and community composition on virus transmission. 4. It was found that initial density positively affected aphid movement, but had no effect on virus transmission, and that conspecific density was more important than heterospecific density. Conversely, community composition affected both aphid movement and virus transmission. These effects were driven by species identity, rather than species richness per se. 5. The results of this study emphasise the importance of accounting for vector behaviour, and analysing it within the context of the wider vector assemblage.

Outcomes of competitive interactions after a natural increment of resources: the assemblage of aphid-tending ants in northern Patagonia

Fil: Lescano, Maria Natalia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Patagonia Norte. Instituto de Investigacion En Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio de Ecotono; Argentina

Reduced wind strengthens top‐down control of an insect herbivore

Global wind speeds have decreased 5–15% over the last 30 years and are expected to continue decreasing in the future. However, little is known about how wind affects species and their interactions within communities. I experimentally tested the effects of wind on predator–prey interactions using soybean aphids and predatory multicolored Asian ladybeetles. First, I examined the direct effect of wind on aphids in a greenhouse without predators under three treatments: no wind, wind (oscillating fan), or simulated wind movement. Aphid abundances did not differ among treatments. Next, I conducted a field experiment in soybean plots assigned to either control or wind‐block treatments. Predators were more abundant in wind‐block treatments and reduced aphid abundance by 40% compared to control plots. To elucidate why wind indirectly increased aphid density in open plots, I conducted a feeding trial with ladybeetles foraging for aphids on plants that were assigned to either control or simulated wind movement treatments. Plant movement doubled the amount of time it took predators to begin consuming aphids and decreased predation rate by two‐thirds. These experiments illustrate how wind can have indirect effects on prey by altering predator behavior and show the importance of this under‐studied effect of global change.

Aphid‐induced plant volatiles affect the attractiveness of tomato plants to Bemisia tabaci and associated natural enemies

AbstractAn in‐depth understanding of plant‐mediated interactions between herbivores and their natural enemies is essential in community ecology and co‐evolution, and for developing sustainable pest management strategies. The influence of Myzus persicae (Sulzer) (Hemiptera: Aphididae)‐induced tomato plant [Solanum lycopersicum L. (Solanaceae)] volatile compounds on the olfactory responses of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), three predator species [Coccinella septempunctata L., Propylea japonica (Mulstant) (both Coleoptera: Coccinellidae), and Orius sauteri (Poppius) (Hemiptera: Anthocoridae)], two whitefly parasitoid species [Encarsia formosa (Gahan) and Encarsia sophia (Girault &amp; Dodd) (Hymenoptera: Aphelinidae)], and one aphid parasitoid species [Aphidius gifuensis Ashmead (Hymenoptera: Aphidiidae)] were examined in two‐choice tests using a four‐arm olfactometer. Tomato plants were experimentally manipulated for aphid density, duration of aphid feeding, and interval between aphid removal and the behavioral test. We also compared the systemic effects of aphid feeding on the olfactory behavior of whiteflies and natural enemies according to three relative leaf positions: the aphid‐infested leaf itself and two adjacent leaves (above and below). Bemisia tabaci were deterred by the odors of the leaves infested with aphids in all treatments. All six natural enemies were attracted to the leaves infested with aphids. Bemisia tabaci deterrence by volatiles was greater for leaves at the highest aphid density, and with increasing duration of exposure to aphid feeding. In contrast, the attraction to leaves was greater for the three parasitoids than for the three predators both with increasing aphid density and increasing duration of plant exposure to aphids. There was no significant influence of leaf position on preference of B. tabaci, the two species of lady beetles, and the aphid parasitoid. The two Encarsia species were attracted to the leaf above the aphid‐infested leaves, indicating that systemic volatiles were released by adjacent leaves. On the other hand, O. sauteri was only attracted by the aphid‐infested leaf itself. Our results showed that M. persicae‐infested host plants emitted volatiles that could inhibit the colonization by B. tabaci and also attract natural enemies of both aphids and whiteflies. The results of this study may need to be considered further for optimizing pest management methods.

低剂量杀虫剂对星豹蛛捕食效应的影响及其机理初探

PDF HTML阅读 XML下载 导出引用 引用提醒 低剂量杀虫剂对星豹蛛捕食效应的影响及其机理 DOI: 10.5846/stxb201307241940 作者: 作者单位: 山西农业大学农学院,山西农业大学研究生学院,山西农业大学农学院,山西农业大学资源环境学院,山西农业大学农学院 作者简介: 通讯作者: 中图分类号: X503.1 基金项目: 中国博士后基金（112125）；山西省科技攻关项目（20130311011-5，20120311013-4）；山西省留学基金项目（2013-重点6） The effect of low-dose of pesticide on predation of spider and its preliminary mechanisms Author: Affiliation: College of Agronomy,Shanxi Agricultural University,,College of Agronomy,Shanxi Agricultural University,College of Resources and Environment,Shanxi Agricultural University,College of Agronomy,Shanxi Agricultural University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:探讨低剂量杀虫剂对蜘蛛捕食效应的影响及其生化机理。采用药膜法，测定了低剂量吡虫啉作用下，星豹蛛和甘蓝蚜敏感性、捕食效应以及成蛛体内乙酰胆碱酯酶（AChE）、谷胱甘肽S-转移酶（GSTs）、超氧化物歧化酶（SOD）、过氧化氢酶（CAT）和中肠蛋白消化酶的活力变化。低剂量农药作用下，星豹蛛对甘蓝蚜的功能反应类型为HollingⅡ型，与对照组相比，随着猎物密度的增大星豹蛛的捕食量增加，寻找效应降低，对猎物的处理时间Th缩短，从而增强了对猎物的捕食作用；低剂量农药处理后，星豹蛛体内AChE和GSTs活性均显著低于对照组（P < 0.05），说明酶活性受到抑制，且抑制作用随吡虫啉浓度的增大而加强，随作用时间的延长而减弱；SOD，CAT和中肠蛋白消化酶活性显著增强，与对照组相比存在显著或极显著差异（P < 0.05），且随吡虫啉浓度增大和作用时间的延长而逐渐降低，最后接近对照组。低剂量杀虫剂作用下，蜘蛛体内AChE活性受到抑制，AChE敏感性降低，对神经递质乙酰胆碱的分解作用下降，使蜘蛛的兴奋性增加；GSTs、SOD、CAT等代谢酶活性发生变化，使蜘蛛的新陈代谢加速，从而刺激捕食；中肠蛋白消化酶的活性增强，提高了对猎物的消化吸收功能。总之，在低剂量杀虫剂作用下，星豹蛛通过外在的捕食行为和体内一系列酶系生理生化反应的综合作用促进蜘蛛对害虫的控制作用。 Abstract:Spiders are among the most abundant predators of insects and other invertebrates in agro- and forest ecosystems, playing an important ecological role by regulating invertebrate populations. Biological control using spiders may represent a sustainable method of controlling agricultural pest species and reducing the amount of pest damage on crops. However, pesticide application on agricultural crops may directly or indirectly impact spider foraging in the crop. In particular, pesticides may have mutagenic effects on spiders, negatively affecting the spider population abundance and transferring these effects to the next generation. Recent studies have reported that low-dose pesticides kill pests, but promote the predation of natural enemies. Therefore, revealing the mechanism behind low-dose pesticides on natural enemies is critical to improving predator ability, exploring the potential control of insect predators and coordinating bio- and chemical control.Our aim was to investigate the effect of low-dose pesticides on spider predation behavior and reveal the underlying mechanisms. Pardosa astrigera was treated with a low-dose of imidacloprid using the drug-film method. We determined the sensitivity of P. astrigera, the predation effect of P. astrigera on B. brassicae and the activity of acetylcholinesterase (AChE), glutathione S-transferase (GSTs), superoxide dismutase (SOD), catalase (CAT) and midgut proteinase. The results showed that under different concentrations of imidacloprid, the predacious functional response of spiders on aphids belonged to the Holling type. Compared with the control, the theoretical maximal consumption increased, searching efficiency decreased and the prey handling time declined with increasing aphid density. Therefore, low-dose pesticide levels resulted in enhanced predation by spiders. When P. astrigera was treated with a low-dose of pesticide, this species showed a reduction in AChE and GSTs activity compared with the control (P < 0.05), suggesting spider activity was inhibited. The inhibition was enhanced with increasing pesticide concentration and weakened with prolonged treatment. SOD, CAT and midgut proteinase activity were induced significantly at the low-dose of 1, 3 and 5mg/L(P < 0.050, but decreased gradually with increased concentration and extended treatment, with activity close to the control. After low-dose pesticide treatment, P. astrigera showed inhibited AChE activity, leading to reduced AChE sensitivity, less decomposition of AChE in nerve transfer and increased spider activity. Increased GSTs, SOD and CAT activity induced by low-dose pesticide, accelerated the spiders' metabolism, stimulating predatory behavior. Enhanced midgut proteinase activity may benefit food digestion and nutrient absorption. The low-dose pesticide regime may enhance the predatory behavior of P. Astrigera via three main pathways. First, it enhances the instant attack rate, shortens prey handling time and increases the amount of prey per day. Second, it affects physiological enzyme activity, inhibits to a degree the metabolic detoxification enzyme subject of: low doses of imidacloprid causing a reduction in AChE sensitivity of the neurotransmitter acetylcholine decomposition, resulting in higher spider excitability; increased protection enzyme, such as GSTs, SOD, CAT, function oxidases (MFOs) and esterase, resulting in the acceleration of spider metabolism, stimulating predator behavior. Third, on the physiological activity of digestive enzymes, low doses of pesticides activate protein midgut digestive enzymes, improving the digestion and absorption function of prey. In summary, certain concentrations of pesticide promote the predation function of P. astrigera against prey, changing spider behavior through a series of physiological and biochemical reactions. 参考文献 相似文献 引证文献

Bottom-up effects may not reach the top: the influence of ant-aphid interactions on the spread of soil disturbances through trophic chains.

Soil disturbances that increase nutrient availability may trigger bottom-up cascading effects along trophic chains. However, the strength and sign of these effects may depend on attributes of the interacting species. Here, we studied the effects of nutrient-rich refuse dumps of the leaf-cutting ant, Acromyrmex lobicornis, on the food chain composed of thistles, aphids, tending ants and aphid natural enemies. Using stable isotopes tracers, we show that the nitrogen accumulated in refuse dumps propagates upward through the studied food chain. Thistles growing on refuse dumps had greater biomass and higher aphid density than those growing in adjacent soil. These modifications did not affect the structure of the tending ant assemblage, but were associated with increased ant activity. In contrast to the expectations under the typical bottom-up cascade effect, the increase in aphid abundance did not positively impact on aphid natural enemies. This pattern may be explained by both an increased activity of tending ants, which defend aphids against their natural enemies, and the low capacity of aphid natural enemies to show numerical or functional responses to increased aphid density. Our results illustrate how biotic interactions and the response capacity of top predators could disrupt bottom-up cascades triggered by disturbances that increase resource availability.

Lack of strong refuges allows top-down control of soybean aphid by generalist natural enemies

Refuges have been shown to be important mediators of predator–prey interactions, and in particular, have been proposed as a potential mechanism allowing herbivore populations to reach outbreak levels. However, very little research on the role of refuges has been conducted in systems dominated by generalist predators. We investigated the existence of refuges from predation for the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) at multiple scales. This species invaded North America and in spite of previous studies demonstrating strong suppression by generalist natural enemies, its populations periodically cause significant economic losses. Using naturally occurring populations of soybean aphid and its natural enemies, we tested for the presence of A. glycines spatial and dynamic refuges at the within-field, single plant, and within-plant scale. At the within-field level, we found only weak and transient spatial patterns in aphid populations suggesting the lack of spatial refuges at this scale. Similarly, at the plant level we found no individual colonies that escaped predation and aphid suppression was 9- to 28-fold greater in comparison with caged controls regardless of initial aphid density. When high aphid populations were exposed to predation they were rapidly reduced to levels close to the average field density and showed reduced per capita growth rates, indicating an absence of dilution of predation risk at increased aphid density. Finally, we found a significant shift in the distribution of aphids to the lower portions of the plant in the presence of generalist predators, suggesting a partial refuge from predation at the within-plant scale. Overall, we found the naturally occurring community of generalist predators to exert strong top-down suppression of soybean aphid populations at multiple scales, and no evidence that the presence of refuges at the scales studied can lead to outbreak populations. The partial refuge from predation at the within-plant scale revealed in our study may have important consequences for the within-season population dynamics of A. glycines, since it may be associated with low plant quality tradeoffs, and therefore warrants further research.

Attractiveness and host suitability of winter wheat cultivars for cereal aphids (Homoptera: Aphididae)

Cereal aphids (Homoptera: Aphididae) yearly infest winter wheat occasionally leading to significant yield reduction when biotic and abiotic factors are optimal for rapid population growth. Within the host plant complex, influences of eight winter wheat cultivars on the development of cereal aphids was studied in a two-year project focussing on the most important species in terms of yield reduction. Therefore, antibiosis, aphid settlement behaviour, and the relation of infestation and yield loss were evaluated by observing the development of isolated aphids within clip cages, by estimating the natural infestation of alatae morphs, and by measuring yield parameters. The cultivar Hybnos I significantly reduced numbers of offspring of caged Metopolophium dirhodum and Sitobion avenae at seedling stages (growth stage 13, in laboratory) in comparison to other cultivars. During later stages such as shooting and flowering (in laboratory and fields, respectively), no significant differences in aphid development were observed among cultivars. During the period of immigration, alatae of Rhopalosiphum padi and S. avenae preferred to settle on Batis (on May 25 th) and Tommi (on June 10th), respectively. At other evaluations, no significant differences were found among the cultivars and aphid species. Crop yield (kg per ha) and hectolitre weight (g per ccm) were significantly reduced with increasing aphid density in all cultivars, with highest reductions in cultivar Dekan. In contrast, only some cultivars showed significant reductions in protein content with increasing aphid density, while others were tolerant and not reacting to aphid infestation with respect to protein content. In summary, no striking indications for different aphid susceptibility could be found in the set of cultivars tested, even though they differed strongly in attributes such as colour, height, yield potential and development pattern. The results are discussed in terms of cultivar selection to improve sustainability of integrated pest management and the importance of cultivar features for aphid migration and population models.

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.

Oviposition responses to patch quality in the larch ladybird Aphidecta obliterata (Coleoptera: Coccinellidae): effects of aphid density, and con- and heterospecific tracks

The effects and persistence of oviposition-deterring semiochemical cues from conspecific and heterospecific larval tracks on the oviposition rate of Aphidecta obliterata (Linnaeus) females were investigated. In addition, the effects of varying aphid prey density were considered and also whether any resulting response originated from differential nutritional status of females and/or due to aphid odour stimuli. The existence of oviposition responses to conspecific egg chemicals was also considered. Gravid A. obliterata females were deterred from oviposition by conspecific larval tracks and the effect was density dependent. Females actively avoided searching in these contaminated areas. Tracks induced a significant effect on oviposition for up to three days. Heterospecific tracks of the coccinellid Adalia bipunctata (Fabricius) or the chrysopid Chrysoperla carnea (Stephens) did not induce any oviposition response in A. obliterata females. Increasing aphid density induced increased oviposition rate in A. obliterata females. Nutritional status of females was an important factor in the relationship between aphid density and oviposition rate, but aphid associated cues (odours) were not. There was an inhibitory effect of extracts of conspecific egg-surface chemicals on oviposition by A. obliterata females. In the field, cannibalism, competition and limited food availability represent the major threats to egg and larval survival. Patch quality assessment mechanisms enable females to lay eggs at sites where offspring survival is maximized. Oviposition-deterring semiochemicals tend to promote more even distribution of predators over prey patches.

Changes in the Use of Extrafloral Nectaries of &lt;I&gt;Vicia faba&lt;/I&gt; (Leguminosae) and Honeydew of Aphids by Ants with Increasing Aphid Density

Abstract To elucidate how ants use two simultaneously present food resources, honeydew and extrafloral nectary (EFN), in relation to aphid density, we first examined ant recruitment patterns with different densities of aphids on Vicia faba L. (Leguminosae) with EFN in two field sites with different species composition of ants. Three species of ants visited plants of V. faba at each site, but two different species, Tetramorium caespitum Linnaeus (Hymenoptera: Formicidae) and Lasius niger L. (Hymenoptera: Formicidae) were the main visitors on plants of V. faba at the different sites. The numbers of ants of T. caespitum and L. niger visiting a plant increased as the density of Aphis craccivora Koch (Homoptera: Aphididae) per plant increased. The correlation between the numbers of L. niger and Aphis craccivora was higher than that between the numbers of T. caespitum and A. craccivora. Second, we examined in a laboratory how two ant species, T. caespitum and L. niger, use the honeydew of A. craccivora and V. f...

How predictable are aphid population responses to elevated CO2?

Experiments investigating the population responses of aphids to CO2 enrichment have yielded results suggesting that aphid populations will be both larger under elevated CO2 and that they will be smaller under elevated CO2. Most studies have failed to reject the null hypothesis of no difference in population sizes due to atmospheric CO2 concentration. This diversity of results has led some investigators to conclude that aphid responses are not general, and that every aphid-plant interaction may be unique and unpredictable a priori. We use a single, general, mathematical model to consider the population responses of cereal aphids to grass grown under different CO2 concentrations. The model shows that it is possible to explain any of the three observed results: larger populations, smaller populations, or no difference, and that which of these three outcomes arises may depend critically on the interaction between aphid nitrogen requirements and the nitrogen fertility of the soil. The model also shows that the qualitative results will depend on how sensitive the aphid species is to increases in its own density. Past studies have shown that aphids increase their production of winged offspring in response to increasing aphid density. The model predicts that, in general, aphid species that have lower nitrogen requirements and that are less sensitive to their own density will be more likely to have larger populations in elevated CO2 compared to ambient CO2. Differences between aphid species (and clones) in their nitrogen requirements and the strength of their density-dependent response have not been widely reported in the literature. Also, the nitrogen fertility of the soil has rarely been manipulated in experiments on aphid responses to rising CO2 levels. The model suggests that the diversity of population responses of aphids may be both understandable and predictable in the context of such an interaction.

Comparison of field population growths of three cereal aphid species on winter wheat

Population growths of three aphid species colonising winter wheat stands, Metopolophium dirhodum, Rhopalosiphum padi and Sitobion avenae, were analysed by regression method. The calculations were based on counts in 268 winter wheat plots at 3 or 7 day intervals over 10 (leaves) or 6 (ears) years. The population dynamics of a particular species differed widely between years. Density independent exponential growth of the population was most common, but its rate differed significantly between species, and for S. avenae also between populations on leaves and ears, on which the populations grew fastest. Field estimates of the intrinsic rate of increase derived from the exponential growths ranged between 0.010&amp;ndash;0.026 in M. dirhodum, 0.0071&amp;ndash;0.011 in R. padi, and between 0.00078&amp;ndash;0.0061 and 0.0015&amp;ndash;0.13 in S. avenae on leaves and ears, respectively. In the populations with the most vigorous population growth, S. avenae on ears and M. dirhodum on leaves, the rate of population increase significantly decreased with increasing aphid density. &amp;nbsp;

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.

Adult Coccinellid Activity and Predation on Aphids in Spring Cereals

Improved understanding of coccinellid activity and predation on aphids in the field could clarify their potential in aphid biological control. Our objective was to determine the influence of abiotic and biotic factors on activity and predation by adults of three coccinellid species (Hippodamia convergens Guerin-Meneville, H. tredecimpunctata tibialis (Say), and Coleomegilla maculata DeGeer) in spring cereal fields. The proportion of time coccinellids spent searching was correlated with air temperature, aphid density, and time of day. The relationship between searching and these variables differed among species. Mean walking speed ranged from 66.4 cm/min for C. maculata to 83.2 cm/min for H. tredecimpunctata and increased with temperature for all species. The frequency of short flights (<2 m) by beetles increased with temperature and decreased with increasing aphid density for all species, but the relationship of short flight frequency to these variables differed among species. The frequency of long flights (>2 m) was similar for all species and influenced by temperature and calendar date. The frequencies with which aphids were encountered and eaten were correlated with aphid density and temperature for H. convergens and H. tredecimpunctata. Despite over 250 h spent observing adult coccinellid behavior in the field, predation data were insufficient to develop a useful predation model.

Feeding Damage of Russian Wheat Aphid on Resistant and Susceptible Wheat Genotypes

Effects of Russian wheat aphid (Diuraphis noxia Mordvilko) feeding on resistant wheat (Triticum aestivum L.) plant introduction line, PI 137739 (antibiotic line), and a susceptible cuitivar, Stephens, were examined. Reduction in leaf chlorophyll content was determined after different densities (0,40 and 80) of aphids were allowed to feed for 3, 6, 9, and 11 days in leaf cages attached to seedlings. In the non-caged region of infested plants, leaf chlorophyll content was significantly higher in PI 137739 compared with Stephens. Reduction in chlorophyll content was not aphid-density or duration of aphid-feeding dependent in the non-caged regions. In the caged region, increased physical damage to leaves and significant reduction in total chlorophyll content were observed in PI 137739 compared with Stephens. Increased aphid density and prolonged aphid-feeding caused greater reduction in leaf chlorophyll content in caged sections. Increased probing of aphids because of restricted movement in the caged section and tissue unpalatibility of the resistant genotype could have caused severe chlorophyll reduction in the resistant plant introduction line, PI 137739.

Behaviour of Aphidius colemani Searching for Aphis gossypii: Functional Response and Reaction to Previously Searched Aphid Colonies

Understanding the searching strategy of Aphidius colemani Viereck (Hymenoptera: Braconidae), might gain insight into the mechanisms by which control of Aphis gossypii Glover (Homoptera: Aphididae) is obtained in glasshouses, and provide information on how to improve the biological control of aphids. The functional response of A. colemani in Petri dishes is described, as well as its behaviour on returning to a previously visited leaf disk with aphids. The number of aphids attacked during the first visit followed a sigmoid functional response. The increase of the parasitization rates was clearest at densities of up to 10 aphids/leaf disk. The duration of the first visit to a leaf disk increased with increasing aphid density. After re-entering a recently visited leaf disk, the parasitoid stayed for a much shorter time than on the first visit, and no effect of host density on the duration of the visit was observed. No evidence of patch marking was found, so the reduction of visiting times was probably caused by encounters with parasitized hosts.

How an ant decides to prey on or to attend aphids

AbstractThe following results on the behavior decision making of the antLasius nigertoward two species of myrmecophilous aphidsLachnus tropicalisandMyzocallis kuricolaon chestnut trees have been found. (1) An individual worker consistently attended only one aphid species, even if her nestmates attended other aphid species on the same tree. (2) The ants preyed less on the aphid species which they attended than on other myrmecophilous aphid species. (3) The ants preyed less on the aphids which had been attended by their nestmates, even if both aphids were the same species. (4) The ants preyed less on aphids which had provided honeydew to their nestmates. (5) The increased aphid density per ant led to an increase in the rate of predation on the introduced aphids by the ants. These results suggest that each worker ofL. nigerchooses aphid species to attend from her experience. In addition, the workers can recognize whether an aphid has been attended by their nestmates and whether an aphid has given their nestmates honeydew. Through these processes, each worker decides to attend or to prey on the aphid. As a result, they may realize efficient collective foraging dependent on aphid density per worker.