Patricia A. MacKay

Are development and growth of pea aphids, Acyrthosiphon pisum, in North America adapted to local temperatures?

SummaryDevelopmental rate and adult weight were studied at constant temperatures from 9.8 to 27.9°C for 18 clones from each of five populations of the pea aphid, Acyrthosiphon pisum (Harris), from locations between 39 and 53°N latitude in central North America. The response of developmental rate to temperature for each clone was accurately described by a three parameter non-linear equation. Adult weight usually decreased with increasing rearing temperature, but the shape of the response to temperature varied among clones. Variation in the developmental parameters was greater among clones within populations than among populations. No consistent trends were observed in the developmental parameters or adult weights either with latitude or the long term average temperatures at the locations. We conclude that previously reported geographic variation in the developmental threshold of this species, which was attributed to local adaptation, occurred either because the clones tested were not representative of the populations or because rearing methods differed among studies. The results are discussed in relation to the hypothesis that life history traits which are temperature sensitive are adapted to local thermal environments.

Variability in life history traits of the aphid, Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.

Seasonal variation in the photoperiodic responses of a pea aphid population: evidence for long-distance movements between populations

SummaryThe purpose of the study was to quantify long distance movements in populations of pea aphid, Acyrthosiphon pisum (Harris), by estimating origins and distances travelled by immigrants into a southern Manitoba population. A strong relationship was demonstrated between latitude of origin and photoperiods at which pea aphid populations are stimulated to produce the diapause stage (Smith 1987). Therefore, the approach was to use photoperiodic response as a physiological marker to identify the source of immigrant aphids. The responses of 89 clones from Glenlea, Manitoba (49°38′N), sampled 5 times over 2 seasons, were measured. One sample of clones collected the first season had photoperiodic responses similar to those of a population about 300 km to the south, and significantly different from clones collected in spring of the same year at the same site. Weather analysis corroberates that the migrants were probably carried into Manitoba on a southerly flow of air during the previous 24 to 36 h.

Stability of Natural Populations of an Aphid, Uroleucon rudbeckiae, at Three Spatial Scales

Abstract Stability (temporal variability, persistence, resilience) was assessed over 8–13 years for subpopulations, populations, and regional populations of Uroleucon rudbeckiae (Fitch) (Hemiptera: Aphididae) in southern Manitoba, Canada. Contrary to expectations, natural populations of this native aphid were not more stable than those of aphids inhabiting crops. Among population parameters, prevalence (proportion of plants infested) proved more effective for quantifying temporal variability than intensity (colony size) or abundance (number of aphids per stem). The parameter “population variability” was a more effective index of temporal variability than the standard deviation of the logarithm or the coefficient of variation. Small differences in temporal variability were detected among populations that varied greatly in size. Population variability declined slightly as spatial scale increased and did not increase consistently over time. Population variability can be considered characteristic of this species in southern Manitoba, having a value of 0.648 ± 0.080 (mean ± standard deviation, n = 5, over 8–13 years) on a scale of 0–1, a high degree of temporal variability. Persistence was not related to temporal variability. Subpopulations were less persistent than populations, and one of five populations did not persist. Small populations were more likely to disappear temporarily. No resilience was detected.

Population variability and persistence of three aphid pests of potatoes over 60 years

Abstract Abundance, persistence, and variability of populations of Macrosiphum euphorbiae (Thomas), Myzus persicae (Sulzer), and Aphis nasturtii Kaltenbach (Hemiptera: Aphididae) in potato plots for intervals of 58 years (n = 1), 29 years (n = 2), 19–20 years (n = 3), and 9–10 years (n = 6) were compared. The abundance of M. euphorbiae showed no trend among decades and varied 2.4-fold, whereas that of M. persicae and A. nasturtii declined and showed 54-fold and 3700-fold variation, respectively. All three aphid species persisted through the first five decades and M. euphorbiae also persisted through the sixth (last) decade, but M. persicae and A. nasturtii failed to persist for 1 and 3 years of the last decade, respectively. Population variability (a proportion between 0 and 1) measured over a 58-year interval was high: 0.585 for M. euphorbiae, 0.771 for M. persicae, and 0.830 for A. nasturtii. During the first three but not the last three decades, population variability increased with sampling interval, owing to dramatic declines in abundance for M. persicae and A. nasturtii and one stable decade for M. euphorbiae, but no evidence of a more-time — more-variation effect was detected. Persistence was not related to population variability, but declined with abundance. Populations did not reach equilibrium, because of declining abundance for M. persicae and A. nasturtii and changes in population variability from decade to decade for M. euphorbiae. Populations of M. persicae and A. nasturtii from this crop monoculture were less stable than previously studied natural populations of a native aphid species. In contrast, the population of M. euphorbiae, a native species, had variability in a potato crop similar to that of the previously studied native species. The high population variability of M. persicae and A. nasturtii may be associated with their status as introduced species. The dynamic and species-specific characteristics of population variability require that interspecific comparisons be considered cautiously.

Measuring the performance of aphids: fecundity versus biomass

Abstract Fecundity and biomass of nine species of aphids (Hemiptera: Aphididae) feeding on six species of plants were compared to assess whether the two measures are equally effective for quantifying aphid performance. Performance was quantified by measuring both fecundity (the number of offspring born over a defined interval) and biomass (the dry mass of offspring produced) using three variables expected to affect performance: host-plant genotype, aphid genotype, and aphid density. The efficacy of the performance parameters was assessed by comparing their ability to discriminate among treatments for the three variables. Biomass usually provided a more effective measure of performance than fecundity, but for one aphid species, fecundity was more effective than biomass. Biomass of offspring is the preferred measure of performance, but biomass and fecundity should both be recorded whenever practical.

Seasonal morphometric variation in sexual and asexual, North American and Australian, populations of the aphid, Acyrthosiphon pisum

The lengths of the body and appendages of the aphid Acyrthosiphon pisum (Harris) (Homoptera: Aphididae) vary seasonally in sexual North American and asexual Australian populations. The first generation of spring aphids in North America and winter aphids in Australia have short appendages in relation to body length. Excluding this phenotype, North American and Australian aphids cannot be discriminated morphometrically. The short appendages in North America are associated with a specialized morph called a fundatrix; the short appendages of Australian aphids are caused by exposure to low temperatures during prenatal development. The same temperature‐sensitive mechanism operates in sexual and asexual North American aphids, but does not explain the short appendages of the fundatrix, which appear to arise through a separate mechanism. The short appendages are caused neither by a maternal effect from winged mothers, although such an effect exists, nor by seasonal changes in body length and allometry, nor by microevolutionary changes. The temperature‐induced shortening of appendages is a seasonal polymorphism, which mimics the short appendages seen in fundatrices. The two types of phenotypic plasticity have the same consequence in sexual and asexual populations of the same species and may be an example of convergent evolution.

Temperature Modulation of Photoperiodism and the Timing of Late-Season Changes in Life History for an Aphid, Acyrthosiphon pisum

Abstract Where winters are severe, aphids reproduce parthenogenetically and viviparously in summer, switch to sexual reproduction in late summer, and produce winter-hardy eggs by the end of the season. The role of day length and temperature in initiating seasonal changes from parthenogenetic to sexual reproduction by pea aphids, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), are described and the selection pressures that affect the timing of this transition are investigated. Over four seasons, a pea aphid clone was sampled from field cages through late summer in southern Manitoba, Canada, and reared in the laboratory to determine the phenotypes of progeny produced as the season progressed. The timing of transitions from one phenotype to another under natural day length and temperature, and the critical day lengths that caused the transitions, coincided with expectations from laboratory studies of photoperiodic responses. Males and mating females appeared later when the weather in August was warm than when it was cool. The timing of seasonal changes was adapted to minimize the physiological time to the end of the season, which maximized the number of asexual summer generations. Ambient temperature modulated the response to day length and fine-tuned the timing of sexual reproduction to adapt for annual variation in autumn weather.