The effect of drought stress and temperature on spread of barley yellow dwarf virus (BYDV)

1 The effect of drought stress and temperature on the dispersal of wingless aphids Rhopalosiphum padi (L.) and the pattern of spread of BYDV (barley yellow dwarf virus) within wheat plants in controlled environment chambers was quantified. Combinations of three different drought stress levels, unstressed, moderate and high stress level, and three different temperatures, 5 ± 1 °C, 10 ± 1 °C, and 15 ± 1 °C, were investigated. 2 With increased temperature there was an increase in the mean distance of visited plants from the point of release and in the number of plants visited and infected with BYDV. Drought stress had no effect on mean distance moved by aphids at any temperature or on plants infected with virus at 10 °C and 5 °C. When plants were drought stressed, the numbers of plants visited and infected were greater at 15 °C than at 10 °C and 5 °C. 3 A greater proportion of plants visited by aphids was infected with BYDV when plants were stressed than when not stressed. At 15 °C a greater proportion of these plants was infected than at lower temperatures. There was no difference between treatments in the numbers of aphids present at the end of the experiment. 4 It is concluded that drought stress and temperature are of considerable importance in virus spread.     

Three-dimensional analysis of aphid landing behaviour in the laboratory and field

The final second of the landing approach of black bean aphids, Aphis fabae, was analysed in three dimensions using video techniques. A yellow landing platform was placed upwind or downwind from aphids aggregating under a ceiling light in a laboratory wind tunnel with 10, 20, 30, 40 or 50 cm s^–1 wind speeds, and up-tunnel or down-tunnel in still air. As individual aphids flew to the platform, body orientation (assessed by direct observation) was predominantly into-wind whether the initial flight direction to the landing platform was upwind or downwind. A greater proportion showed into-wind body orientation as wind speed increased. Flight track parameters which differed significantly between wind speeds were the track length, linear start to finish distance, linearity index, horizontal ground speed, speed vertical to the ground, vertical turning rate, and horizontal turning rate. The position of the landing platform was important for track length, linear start to finish distance, horizontal ground speed, three-dimensional turning rate, horizontal turning rate, vertical turning rate, and sinuosity. As wind speed increased above 30 cm s^–1 the ground speed became more consistent and indicated considerable variation in air speed to adjust for ground speed. For the majority of aphids there was a strong preference (88%) for into-wind landings with initial upwind directed flight, while for downwind flights a significant number (55%) of insects reversed initial flight direction and landed into-wind. Field recorded landings showed that 66% of aphids landed into-wind and there was a mean bearing to the wind of 71 ± 42°, a similar finding to wind-tunnel studies.     

Altered dispersal behaviour in parasitised aphids: parasitoid-mediated or pathology?

1. Several hypotheses concerning modified dispersal behaviour in aphids parasitised by aphidiine wasps (Hymenoptera: Braconidae: Aphidiinae) were tested in the laboratory. Behavioural changes may be host-mediated, parasitoid-mediated, or a by-product of trauma and pathology. 2. Mummification site varied with parasitoid species. Pea aphids (Acyrthosiphon pisum) parasitised by Aphidius ervi, Aphidius pisivorus, Monoctonus paulensis, and Praon pequodorum mummified near the aphids’ preferred feeding sites on bean plants, but those parasitised by Ephedrus californicus often died and mummified outside the colony, away from the plants. 3. Parasitism by E. californicus had a progressive effect on the behaviour of pea aphids. Approaching death, aphids lost motor control and frequently dropped off the host plant when disturbed. Dropped aphids were unable to return to the feeding site and mummified elsewhere. The proportion of aphids mummifying outside the colony increased with mummy density. 4. Mummification site was not influenced by the presence within the same colony of aphids parasitised by different species of aphidiine wasps. 5. The evidence does not support the hypothesis that mummification site selection in E. californicus is determined by a host- or a parasitoid-mediated change in aphid dispersal behaviour. Association-specific differences in the dynamics of larval development and growth between aphidiine species provide an equally valid and possibly more general explanation of mummification behaviour.     

Alate production in an aphid in relation to ant tending and alarm pheromone

1. Winged dispersal is vital for aphids as predation pressure and host plant conditions fluctuate. 2. Ant‐tended aphids also need to disperse, but this may represent a cost for the ants, resulting in an evolutionary conflict of interest over aphid dispersal. 3. The combined effects of aphid alarm pheromone, indicating predation risk, and ant attendance on the production of winged aphids were examined in an experiment with Aphis fabae (Homoptera: Aphididae) (Scopoli 1763) aphids and Lasius niger (Formicidae: Formicinae) (Linné, 1758) ants. 4. This study is the first to investigate the joint effects of alarm pheromone and ant attendance, and also the first to detect an influence of alarm pheromone on the production of winged morphs in A. fabae. 5. After a period of 2 weeks, it was found that aphid colonies exposed to intermittent doses of alarm pheromone produced more winged individuals, whereas ant tending had the opposite effect. The effects were additive on a log scale, and ant attendance had a greater proportional influence than exposure to alarm pheromone. A tentative conclusion is that ants have gained the upper hand in an evolutionary conflict about aphid dispersal.     

Effect of parasitism on flight behavior of the soybean aphid, Aphis glycines

Many aphid species possess wingless (apterous) and winged (alate) stages, both of which can harbor parasitoids at various developmental stages. Alates can either be parasitized directly or can bear parasitoids eggs or larvae resulting from prior parasitism of alatoid nymphs. Winged aphids bearing parasitoid eggs or young larvae eventually still engage in long-distance flights, thereby facilitating parasitoid dispersal. This may have a number of important implications for biological control of aphids by parasitoids. In this study, we determined the effect of parasitism by Aphelinus varipes (Hymenoptera: Aphelinidae) on wing development and flight of the soybean aphid, Aphis glycines (Hemiptera: Aphididae). We also quantified the influence of aphid flight distance on subsequent A. varipes development. Parasitism by A. varipes was allowed at different A. glycines developmental stages (i.e., alatoid 3rd and 4th-instar nymphs, alates) and subsequent aphid flight was measured using a computer-monitored flight mill. Only 35% of aphids parasitized as L3 alatoid nymphs produced normal winged adults compared to 100% of L4 alatoids. Flight performance of aphids parasitized as 4th-instar alatoid nymphs 24 or 48 h prior to testing was similar to that of un-parasitized alates of identical age, but declined sharply for alates that had been parasitized as 4th-instar alatoid nymphs 72 and 96 h prior to testing. Flight performance of aphids parasitized as alate adults for 24 h was not significantly different from un-parasitized alates of comparable ages. Flight distance did not affect parasitoid larval or pupal development times, or the percent mummification of parasitized aphids. Our results have implications for natural biological control of A. glycines in Asia and classical biological control of the soybean aphid in North America.     

Competition and dispersal in the pea aphid: clonal variation and correlations across traits

Abstract.  1. The presence of an across-species trade-off between dispersal ability and competitive ability has been proposed as a mechanism that facilitates coexistence. It is not clear if a similar trade-off exists within species. Such a trade-off would constrain the evolution of either trait and, given appropriate selection pressures, promote local adaptation in these traits.
2. This study found substantial levels of heritable variation in competitive ability of the pea aphid, Acyrthosiphon pisum Harris (Homoptera: Aphididae), measured in terms of relative survival when reared with a single clone of the vetch aphid, Megoura viciae Buckton (Homoptera: Aphididae).
3. Pea aphids can move to new patches by either flying (longer distance dispersal) or walking (local dispersal) from plant to plant. There was considerable clonal variation in dispersal ability, measured in terms of the proportion of winged offspring produced, and ability to survive away from their host plant.
4. Winged individuals showed longer off-plant survival times than wingless forms of the same pea aphid clone.
5. There was no evidence of a relationship between clonal competitive ability and either measure of dispersal ability, although the power of the test is limited by the number of pea aphid clones used in the trial.
6. However, there was a positive correlation between clonal fecundity and the proportion of winged offspring produced. Although speculative, it is suggested that clones that are more likely to either overwhelm their host plant or attract higher numbers of natural enemies as a result of having higher fecundity are more likely to produce winged morphs.     

Wind matters: Asymmetric distribution of aphids on host plants can be explained by stems functioning as windbreaks

Understanding how abiotic factors influence organisms at present is the necessary first step to predict how species assemblages could be affected by climate change in the future. We examined how wind, a poorly studied abiotic factor, affects the distribution and abundance of two aphid species, Uroleucon aeneum and Brachycaudus cardui (hereafter black and green aphids, respectively), that live on the thistle Carduus thoermeri (Asteraceae) in a windy region of Patagonia, Argentina. First, considering the prevailing wind direction, we described the distribution of both aphid species around plant stems. Then, we performed a bi‐factorial experiment in which we cut stems with aphids to manipulate their position respect to wind (exposed/unexposed) and to control wind incidence (protected/unprotected). Finally, using the species most affected by wind, we examined possible mechanisms through which wind could affect aphids. Both aphid species were less abundant on the side of the stem exposed to wind respect to the unexposed side; and this pattern was stronger for the black aphid. When black aphids were positioned exposed to wind and without protection, their proportion changed towards the unexposed side of the stem; while green aphids showed a weaker response to wind. Laboratory experiments demonstrated that wind triggered both the detachment of black aphids and their movement towards the unexposed side of the stem. Our results showed that wind can explain the asymmetric distribution of aphids around plants and that stems can act as windbreaks. In a less windy future scenario, aphids could expand their foraging area, reaching higher infestation rates, which could affect their role in structuring ant assemblages and their status as crop pests. This work highlights the importance of testing the effects of less studied abiotic factors to fully understand how climate change could impact on the abundance and distribution of animals in the future.     

Aphid flight-track analysis in three dimensions using video techniques

Abstract. A technique is described for the three-dimensional analysis of the flight paths of small insects using two video cameras placed alongside each other with the optical axes coincident at a point some distance beyond the area of interest. The video signals were mixed and a time base introduced before recording the superimposed images from both cameras on a single VCR. With suitable lighting and a black background, flying aphids appeared on the monitor as double, bright images on a dark background. The distance between the two images was inversely proportional to the distance of the aphid from the camera lenses. A calibration grid was used to insert the correct parameters into software designed to provide the x (vertical), y (horizontal) and z (distance from the cameras) Cartesian co-ordinates for a flying insect and to calculate the distances flown, flight speed and turning parameters. The advantages of the system are that it is designed for a single VCR and monitor, provides automatic synchrony between camera signals and can examine a larger visual arena than screen-splitting methods. It operates with insects as small as aphids, and wind-tunnel studies on the black bean aphid, Aphis fabae Scop., showed that some flight parameters (for the final one-second approach to a visually attractive landing platform) differed according to whether wind was present or not. Thus, ground speed and distance moved differed significantly but turning parameters were unchanged. In addition, flight trajectory on the approach to landing depended upon initial direction of flight and the presence of wind.     

Parasitoids induce production of the dispersal morph of the pea aphid, Acyrthosiphon pisum

In animals, inducible morphological defences against natural enemies mostly involve structures that are protective or make the individual invulnerable to future attack. In the majority of such examples, predators are the selecting agent while examples involving parasites are much less common. Aphids produce a winged dispersal morph under adverse conditions, such as crowding or poor plant quality. It has recently been demonstrated that pea aphids, Acyrthosiphon pisum, also produce winged offspring when exposed to predatory ladybirds, the first example of an enemy‐induced morphological change facilitating dispersal. We examined the response of A. pisum to another important natural enemy, the parasitoid Aphidius ervi, in two sets of experiments. In the first set of experiments, two aphid clones both produced the highest proportion of winged offspring when exposed as colonies on plants to parasitoid females. In all cases, aphids exposed to male parasitoids produced a higher mean proportion of winged offspring than controls, but not significantly so. Aphid disturbance by parasitoids was greatest in female treatments, much less in male treatments and least in controls, tending to match the pattern of winged offspring production. In a second set of experiments, directly parasitised aphids produced no greater proportion of winged offspring than unparasitised controls, thus being parasitised itself is not used by aphids for induction of the winged morph. The induction of wing development by parasitoids shows that host defences against parasites may also include an increased rate of dispersal away from infected habitats. While previous work has shown that parasitism suppresses wing development in parasitised individuals, our experiments are the first to demonstrate a more indirect influence of parasites on insect polyphenism. Because predators and parasites differ fundamentally in a variety of attributes, our finding suggests that the wing production in response to natural enemies is of general occurrence in A. pisum and, perhaps, in other aphids.     

绒毛白蜡二态型果实的扩散特征

多态型果实或种子的出现对植物种群的扩散具有重要的意义。绒毛白蜡(Fraxinus velutina)的果实具有二态型特征,主要表现在果翅数量上不同,分别定义为二翅型和三翅型果实,为了比较两类果实在风传扩散时的差异,研究了两类果实的形态、果翅结构和扩散距离及扩散时长。在大型封闭地下室内,以电扇在不同速度档位产生的气流作为风源,分别从2、1.5、1m处手动释放果实,对风速为0、4.6、6.5、7.3m/s时的果实扩散距离及扩散时长进行了比较;并在此基础上对果实的形态特征与扩散特征进行了线性相关分析。结果表明:在同一高度及相同风速下,三翅型果实的水平扩散距离都极显著的大于二翅型,但其相应的扩散时长都小于二翅型。在相同情况下,三翅型的果实沉降速度显著高于二翅型。两类果实随着释放高度的增加,其扩散距离和扩散时长都相应的增加;随着风速的升高,其扩散距离及扩散时长都相应的增加。三翅型果实质量显著高于二翅型,相反,三翅型果翅长与宽都显著小于二翅型。两种翅型的果翅细胞结构都一样,细胞内部都呈现气囊状,果翅表面沿纵轴方向有流线型的纵棱。通过直线相关分析发现,翅型是对扩散距离和扩散时长影响最显著的形态特征;与果实释放高度相比,风速是影响绒毛白蜡果实扩散距离与扩散时长最显著的环境因素。绒毛白蜡三翅型果实比二翅型果实传播的远,关键在于其具有三翅,三翅对阵风瞬间响应,使得沉降速度较二翅型高,可以在瞬时风的作用下,快速传播到较远的距离。三翅型与二翅型在扩散方式上的结合增强了绒毛白蜡的生存与定殖机会。     

A controlled environment study of the effect of leaf physiological age on the movement of apterous Myzus persicae on sugar-beet plants

Apterous Myzus persicae were found to move frequently from leaf to leaf on sugar-beet plants in controlled environment conditions. It is suggested that aphid movement can be related to changes in the rate and content of translocate flow during leaf development. These changes make newly-emerged leaves nutritionally favourable to colonising aphids and make expanding leaves slowly wane in favourability during the process of ‘sink to source’ conversion leading to aphid dispersal from the leaf. Variation in temperature was not found to alter the rate of aphid movement or the period (measured in thermal time) that aphids spent on particular leaves. However, the lower temperature was found to increase the rate of aphid development, aphid size and fecundity; these effects could also be due to nutritional factors. This dispersal behaviour may be a tactic to maximise food intake by a polyphagous aphid and increase the probability that nymphs are deposited on nutritionally-favourable leaves. The implications of the interleaf dispersal of apterous M. persicae for within- and between-plant spread of beet yellows virus (BYV) and beet mild yellowing virus (BMYV) are discussed.     

The Effect of Abiotic Factors on the Male Mate Searching Behavior and the Mating Success of Aphidius ervi (Hymenoptera: Aphidiidae)

The effect of wind speed and distance from the source on the male response of the aphid parasitoid, Aphidius ervi (Hymenoptera: Aphidiidae), to a pheromone source was studied in a wind tunnel. The number of males taking flight, entering the plume and successfully reaching the source, decreased at wind speeds >50 cm/s. Furthermore, the proportion of those attempting upwind flight that fell to the ground increased with increasing wind speed. In contrast, distance from the source had no significant effect on any of the parameters examined. While male flight behavior was significantly reduced at 70 cm/s, some males walked to the source when there was a bridge connecting the pheromone source and the release platform. This suggests that ambulatory behavior could be a significant component of male mate searching in A. ervi when wind conditions are too strong for upwind flight. The possible effects of variation in atmospheric pressure on male flight behavior to the long distance pheromone, as well as to the short distance one, were also investigated. No significant effects of atmospheric pressure were observed. These findings differ significantly from those previously reported for another aphid parasitoid, A. nigripes, and the reasons for such differences are discussed.     

ON THE EVOLUTION OF DISPERSAL AND ALTRUISM IN APHIDS

How competitive interactions and population structure promote or inhibit cooperation in animal groups remains a key challenge in social evolution. In eusocial aphids, there is no single explanation for what predisposes some lineages of aphids to sociality, and not others. Because the assumption has been that most aphid species occur in essentially clonal groups, the roles of intra- and interspecific competition and population structure in aphid sociality have been given little consideration. Here, I used microsatellites to evaluate the patterns of variation in the clonal group structure of both social and nonsocial aphid species. Multiclonal groups are consistent features across sites and host plants, and all species—social or not—can be found in groups composed of large fractions of multiple clones, and even multiple species. Between-group dispersal in gall-forming aphids is ubiquitous, implying that factors acting ultimately to increase between-clone interactions and decrease within-group relatedness were present in aphids prior to the origins of sociality. By demonstrating that between-group dispersal is common in aphids, and thus interactions between clones are also common, these results suggest that understanding the ecological dynamics of dispersal and competition may offer unique insights into the evolutionary puzzle of sociality in aphids.     

Influence of drought on interactions between Rhopalosiphum padi and ground dwelling predators – A mesocosm study

Climate change will lead to extreme droughts, but it is difficult to predict how this will affect crop pests. In particular, it is unclear how interactions between natural enemies and pests will be influenced. In the field, bird cherry-oat aphids (Rhopalosiphum padi (L.)) have been observed to reside close to, or below the ground surface during dry conditions. We hypothesized that this will increase the niche overlap between R. padi and ground-dwelling predators such as carabid beetles and wolf spiders and that aphid numbers will therefore decline during dry conditions. A fully factorial mesocosm experiment was conducted testing the combined effects of drought and predator presence on aphid position and abundance on barley (Hordeum vulgare) plants. In support of our hypothesis, we found that (a) aphids moved below ground during dry conditions, (b) predators reduced aphid numbers, but only during dry conditions, and (c) predators reduced the proportion of aphids below ground in dry conditions. This increased predation effect during dry conditions was, however, compensated for by a corresponding increase in aphid performance on the plants and so the net effect of drought on aphid numbers ended up being neutral. Thus, pests can be affected by drought in complex ways via a combination top-down and bottom-up mechanisms. Predicting how pest populations will be affected by droughts in the future is thus a formidable research challenge.     

Control of Myzus persicae and Lipaphis erysimi (Hemiptera: Aphididae) by adults and larvae of a flightless strain of Harmonia axyridis (Coleoptera: Coccinellidae) on non-heading Brassica cultivars in the greenhouse

The green peach aphid [Myzus persicae (Sulzer)] and turnip aphid [Lipaphis erysimi (Kaltenbach)] are economically important pests with a worldwide distribution. We have evaluated the efficacy of releasing adults and larvae of a flightless strain of the multicolored Asian lady beetle (Harmonia axyridis Pallas) as a control measure against these aphids on plants of non-heading Brassica cultivars. Both adults and larvae of H. axyridis were observed to be effective biocontrol agents, markedly decreasing the numbers of aphids. The residence duration of adults was longer than that of larvae. The proportion of non-marketable plants damaged by aphids was lower in plots into which either adults or adults and larvae of H. axyridis had been released. These results suggest that both adults and larvae of this flightless strain of H. axyridis are effective in controlling aphids on plants of non-heading Brassica cultivars.     

Patch size of gall-forming aphids: deme formation revisited

Plants of the genus Pistacia (Anacardiaceae) serve as obligate hosts for a group of specialized gall-forming aphids (Homoptera: Fordinae). The aphids regularly migrate between the Pistacia (primary) host plants and the roots of non-specific grasses and cereals (secondary hosts). Gall density varies considerably between trees and sites. The intimate relationships between the aphids and their primary host, the natural variation of host susceptibility, and the heterogeneous geographical environment may promote local adaptation and deme formation in the aphid populations. Indeed, previous analyses of the genetic structure of the aphid Baizongia pistaciae, which forms large galls on the deciduous P. palaestina trees, suggested deme formation (Martinez et al. 2005). In this study, we analyzed the genetic structure of the B. pistaciae population at eight sites and 78 trees throughout Israel and a single population in Turkey, using two molecular markers (AFLP fingerprints and COI sequencing). The genetic distance between the Israeli populations was found to be low (D = 0.01–0.02), and there was no genetic differentiation found between any population pairs. In five of the Israeli populations, we also compared the genetic identity between aphids forming galls on the same tree and between galls on neighboring trees. The analysis indicated that the genetic identity of different galls within a tree resembles the correspondence between trees within a population. Our results showed no indication of deme formation or any hierarchical genetic substructuring within B. pistaciae populations in Israel. The extensive gene flow between aphid colonies may be explained by their dispersal abilities and the potential bridging role of the secondary hosts.     

Rearing history affects behaviour and performance of two virulent Nasonovia ribisnigri populations on two lettuce cultivars

Many aphid species have become virulent to host‐plant resistance, which limits the sustainability of insect resistance breeding. However, when this adaptation to resistant plants is associated with fitness costs for the aphids, virulence can be lost in the absence of resistant plants. For two populations of the lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae), we evaluated whether virulence to Nr‐gene‐based resistance was lost on a susceptible lettuce, Lactuca sativa L. (Asteraceae), and assessed possible costs of virulence. The feeding behaviour and performance of these aphids, reared and tested on susceptible and resistant lettuce, were investigated. The rearing plant affected feeding behaviour and performance of the aphids. Temporary reduction and long‐term loss of virulence were found. The total duration of phloem intake was shorter after being reared on susceptible lettuce and tested on resistant lettuce. In addition, one population had a lower survival on resistant lettuce after being reared on susceptible lettuce. There were also indications of fitness costs of the virulence in both populations.     

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.     

Patterns of host selection by four species of aphidiid (Hymenoptera) parasitoids: influence of host switching

Abstract.

• 1 We tested switching behaviour in four species of aphidiid parasitoids, using a two-aphid experimental system consisting of second-instar nymphs of pea aphid (Acyrthosiphon pisum (Harris)) and alfalfa aphid (Macrosiphum creelii Davis) feeding on broad beans in the laboratory.
• 2 Aphidius ervi Haliday, A.pisivorus Smith, A.smithi Sharma & Subba Rao, and Pram pequodorum Viereck showed an innate preference for pea aphid when both host species were provided in equal numbers.
• 3 Wasps encountered both aphid species equally but differed in their acceptance of alfalfa aphid. Females of A.pisivorus and P.pequodorum accepted alfalfa aphids when few pea aphids were available, but A. smithi always concentrated attacks on pea aphid. Aphidius ervi super-parasitized an increasing proportion of pea aphids as their availability declined.
• 4 Switching to the alfalfa aphid occurred in A.ervi and P.pequodorum (but not in A.pisivorus and A.smithi) under the condition of a 1:3 ratio of pea aphids:alfalfa aphids. Wasps did not switch when more pea aphids than alfalfa aphids were provided (3:1 ratio).
• 5 Alfalfa aphids were more likely than pea aphids to escape from parasitoid attack.
• 6 Switching to the most abundant host may not be adaptive in these four species of aphid parasitoids. A foraging wasp incurs a potentially higher cost in lost opportunity time when attacking (and failing to oviposit in) alfalfa aphids. In addition, alfalfa aphids may have lower host quality than pea aphids, a difference that could influence offspring fitness.

     

The influence of temperature on size as an indicator of host quality for the development of a solitary koinobiont parasitoid

The influence of aphid size on the host quality assessment and progeny performance of aphidiine parasitoids was examined using the mealy plum aphid parasitoid, Aphidius transcaspicus Telenga (Hymenoptera: Braconidae) and the black bean aphid, Aphis fabae Scopoli (Homoptera: Aphididae), as a readily acceptable alternate host. Aphid size in relation to stage of development was manipulated by rearing synchronous aphid cohorts at either 15 or 30 °C. At 15 °C, 2nd instar aphids were approximately the same size as 4th instar aphids reared at 30 °C. Cohorts of 30 aphids from each instar, reared at each temperature, were exposed to parasitism by a single parasitoid female for a period of 5 h. Overall susceptibility to parasitism did not vary between aphid cohorts, but the parasitoid response to aphid size differed significantly between rearing temperatures for both progeny sex ratio (parent female assessment of host quality) and larval growth and development (host suitability for parasitoid development). For aphids reared at 15 °C, the proportion of female progeny and emerging adult size for the parasitoid increased linearly with aphid size at the time of attack, while development time remained constant. In contrast, for aphids reared at 30 °C, the proportion of female progeny, emerging adult size, and the development time of the parasitoid all declined with aphid size at the time of attack. The contrasting responses of the parasitoid to host size for aphids reared at the two temperatures suggest that host quality is only indirectly related to aphid size among aphidiine parasitoids. The possible effects of higher temperatures on nutritional stress, obligate endosymbionts, and future growth potential of the aphids are discussed as explanations for the variation in host quality for parasitoid development.