A population dynamics perspective on effect of irrigation regimes and plant densities on greenbug abundance in grain sorghum using regression

• 1 A dataset generated from previous experiments on greenbug Schizaphis graminum (Rondani) (Hemiptera: Aphididae) response to irrigation and plant density in grain sorghum was reanalyzed using a recently‐developed mechanistic ecological model for describing aphid population density curves. The model was used to estimate seven response variables: observed peak aphid abundance, predicted peak aphid abundance, time of peak abundance, per capita birthrate, death rate coefficient, final cumulative density and duration of substantial aphid infestation across three irrigation regimes and five plant densities.
• 2 Using regression, the observed peak aphid abundance, predicted peak aphid abundance, per capita birthrate and final cumulative abundance were shown to decrease significantly, whereas the death rate coefficient and duration of the infestation were shown to increase significantly for each 100 000 plant/ha increase.
• 3 Although significant results were found for a number of variables generated from the specific data set used in the analyses, of perhaps greater importance is the potential use of these equations in future predictions of aphid population dynamics. An example of projecting population curves based on estimated peak and cumulative counts and an example of projecting population curves based on estimated birth and death rate coefficients are provided.

     

Remotely sensing arthropod and nutrient stressed plants: a case study with nitrogen and cotton aphid (Hemiptera: Aphididae)

Remote sensing can be used in combination with ground sampling to detect aphid- (Aphis gossypii Glover) infested cotton (Gossypium hirsutum L.). Changes in wavelengths in the near-infrared (NIR) have proven useful for such detection, but these changes can be confused with other factors stressing plants, such as water deficiency and nutrient status. This study was designed to test the utility of this technology to distinguish between two factors stressing plants: nitrogen deficiency and aphids. Field plots were created by applying varying rates of nitrogen to cotton at different dates in the growing season in 2003 and 2004. Subplots were created by applying disruptive insecticides, which increased aphid populations in a portion of the subplots. Airplane and satellite remote sensing data in 2003 and 2004 were supplemented with ground sampling of aphid populations in both years. Insecticide application, nitrogen application rate and date influenced aphid abundance. Cotton with higher aphid populations could be distinguished from cotton with natural aphid infestations independent of plant nitrogen status using a NIR wavelength in 2003 and a proprietary 2004 index. Complex distinctions among varying nitrogen treatments and aphid abundance were not possible using this data. In the future, possible confounding factors should be investigated from the perspective of their change on crop physiology before remote sensing can be used in an integrated pest management (IPM) program.     

Dynamics of an age‐structured population drawn from a random numbers table

I constructed age‐structured populations by drawing numbers from a random numbers table, the constraints being that within a cohort each number be smaller than the preceding number (indicating that some individuals died between one year and the next) and that the first two‐digit number following 00 or 01 ending one cohort’s life be the number born into the next cohort. Populations constructed in this way showed prolonged existence with total population numbers fluctuating about a mean size and with long‐term growth rate (r) ≈ 0. The populations’ birth rates and growth rates and the females’ per capita fecundity decreased significantly with population size, whereas the death rates showed no significant relationship to population size. These results indicate that age‐structured populations can persist for long periods of time with long‐term growth rates of zero in the absence of negative‐feedback loops between a population’s present or prior density and its birth rate, growth rate, and fecundity, contrary to the assumption of density‐dependent regulation hypotheses. Thus, a long‐term growth rate of zero found in natural populations need not indicate that a population’s numbers are regulated by density‐dependent factors.     

Stochastic modeling of aphid population growth with nonlinear, power-law dynamics

This paper develops a deterministic and a stochastic population size model based on power-law kinetics for the black-margined pecan aphid. The deterministic model in current use incorporates cumulative-size dependency, but its solution is symmetric. The analogous stochastic model incorporates the prolific reproductive capacity of the aphid. These models are generalized in this paper to include a delayed feedback mechanism for aphid death. Whereas the per capita aphid death rate in the current model is proportional to cumulative size, delayed feedback is implemented by assuming that the per capita rate is proportional to some power of cumulative size, leading to so-called power-law dynamics. The solution to the resulting differential equations model is a left-skewed abundance curve. Such skewness is characteristic of observed aphid data, and the generalized model fits data well. The assumed stochastic model is solved using Kolmogrov equations, and differential equations are given for low order cumulants. Moment closure approximations, which are simple to apply, are shown to give accurate predictions of the two endpoints of practical interest, namely (1) a point estimate of peak aphid count and (2) an interval estimate of final cumulative aphid count. The new models should be widely applicable to other aphid species, as they are based on three fundamental properties of aphid population biology.     

Nonlinear stochastic modeling of aphid population growth

This paper develops a stochastic population size model for the black-margined pecan aphid. Prajneshu [Prajneshu, A nonlinear statistical model for aphid population growth. J. Indian Soc. Agric. Statist. 51 (1998), p. 73] proposes a novel nonlinear deterministic model for aphid abundance. The per capita death rate in his model is proportional to the cumulative population size, and the solution is a symmetric analytical function. This paper fits Prajneshu's deterministic model to data. An analogous stochastic model, in which both the current and the cumulative aphid counts are state variables, is then proposed. The bivariate solution of the model, with parameter values suggested by the data, is obtained by solving a large system of Kolmogorov equations. Differential equations are derived for the first and second order cumulants, and moment closure approximations are obtained for the means and variances by solving the set of only five equations. These approximations, which are simple for ecologists to calculate, are shown to give accurate predictions of the two endpoints of applied interest, namely (1) the peak aphid count and (2) the final cumulative aphid count.     

Effects of extreme,fluctuating temperature events on life history traits of the grain aphid,Sitobion avenae

Altered temperatures affect insects’ life history traits, such as development period and fecundity, which ultimately determine population growth rates. Understanding insects’ thermal biology is therefore integral to population forecasting and pest management decision‐making such as when to utilise crop spraying or biological control. Aphids are important crop pests in temperate regions, causing considerable yield losses. The aphid thermal‐biology literature is, however, heavily biased towards the effects of rising mean temperatures, whereas the effects of fluctuating, extreme climatic events (e.g., heat waves and sub‐zero cold periods) are largely overlooked. This study assessed the effects of laboratory‐simulated heat waves and sub‐zero cold periods on the survival, development period, and fecundity of the grain aphid, Sitobion avenae (Fabricius) (Hemiptera: Aphididae: Microsiphini), in addition to assessing maternal effects on the birth weight and development period of the offspring of exposed individuals. Exposure to heat stress periods (total of 16 h at 30 °C) significantly reduced aphid fecundity and increased physiological development period (in day‐degrees) resulting in a reduced population growth rate. Cold exposure (total of 1.33 h at ?15 °C) reduced population growth rate due to an elongated development period (in days), but did not affect fecundity or physiological development period (in day‐degrees). Both cold and heat stress significantly reduced aphid survival. Maternal experience of heat stress reduced nymphal birth weight although nymphal development period was not affected by either cold or heat stress. The results suggest that including the effects of fluctuating, extreme temperature events on aphid life history in population forecast models is likely to be of great importance to pest management decision‐making. The demonstration of maternal effects on birth weight also suggests that cross‐generational effects of heat waves on population growth rates could occur.     

Generalized aphid population growth models with immigration and cumulative-size dependent dynamics

Mechanistic models in which the per-capita death rate of a population is proportional to cumulative past size have been shown to describe adequately the population size curves for a number of aphid species. Such previous cumulative-sized based models have not included immigration. The inclusion of immigration is suggested biologically as local aphid populations are initiated by migration of winged aphids and as reproduction is temperature-dependent. This paper investigates two models with constant immigration, one with continuous immigration and the other with restricted immigration. Cases of the latter are relatively simple to fit to data. The results from these two immigration models are compared for data sets on the mustard aphid in India.     

Relations among birth condition, maternal condition, and postnatal growth in captive common marmoset monkeys (Callithrix jacchus)

The present study characterizes the relations among maternal condition, litter size, birth condition, and growth in body weight for a population of common marmosets. The subjects of the study were marmosets born into a single colony between 1994 and 2001. Three sets of analyses were conducted to answer the following questions: 1) Is there a relationship between litter size, maternal condition, and birth condition? In the study population, maternal body weight, maternal age, litter size, and birth condition were related in a complex fashion. Birth weight and prenatal long‐bone growth, as reflected in knee–heel length, were both related to maternal age, with older mothers supporting higher prenatal growth. Age and maternal condition appeared to interact as determinants of long‐bone growth, as the combination of older and larger mothers resulted in significantly longer knee–heel lengths in their offspring. 2) Is there a relationship between birth condition or maternal condition and subsequent growth or final adult size? The early growth rate in this population was similar to early growth rates reported for three different marmoset colonies, suggesting that early growth may be relatively inflexible in this species. However, within this population, the variation that did occur in early growth rate was related to birth weight and maternal weight. Later growth and adult weight were related to birth weight and litter size: small twin infants displayed slower later growth rates and were smaller as adults than twins that began life at a higher birth weight, while the birth weight of triplets was not related to adult size. In these marmosets, small infants that were the result of increased litter size differed from small infants whose small birth size resulted from other factors. This reinforces the proposal that the causes of low birth weight will be relevant to the development of the marmoset as a model of prenatal environmental effects. Am. J. Primatol. 62:83–94, 2004. © 2004 Wiley‐Liss, Inc.     

Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) (Homoptera: Aphididae) in the canopy of Sitka spruce

1 Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) within the canopy of 20–25‐year‐old Sitka spruce are described based on data from two low‐altitude sites (310–420 m above sea‐level), two mid‐altitude sites (500–550 m a.s.l) and one high‐altitude site (610 m a.s.l). 2 Aphids were counted throughout the canopy on shoots representative of all needle age‐classes present at each whorl of branches. Counts were made during the middle week of each month from September to July for 4 years (1999–2003), and mean E. abietinum densities at each canopy position were calculated separately for each month and for the low‐, mid‐ and high‐altitude sites. 3 During September to November, the highest densities of E. abietinum occurred on 3–4‐year‐old needles on branches low in the canopy. Over the winter and spring, the centre of the aphid’s distribution shifted outward and upward, so that by June of the next year the highest aphid densities occurred on current and 1‐year‐old needles on branches near the top of the tree. 4 The aphid distribution was re‐set each year during July, at the time when the nutrient quality of the host was in decline and E. abietinum populations were decreasing. Aphid densities decreased less on 3–4‐year‐old needles than on current and 1‐year‐old needles, suggesting that older needles were a superior food resource at this time of year and in the autumn. However, other factors, such as higher temperatures in the upper canopy during the summer or differential mortality caused by natural enemies, could also have contributed to the change in distribution. 5 The outward and upward shift in the aphid distribution over the winter period provided no evidence that aphids at positions lower and deeper in the canopy were better insulated from freezing temperatures and had higher over‐winter survival rates. Mean air temperatures at the top and bottom of the canopy during the winter were also found to differ by only 0.1–0.2 °C. 6 The percentage of the total aphids per tree that occurred on current or 1‐year‐old needles varied widely between seasons and between sites. Consequently, sampling programmes designed to estimate total population numbers of E. abietinum have little option but to sample needles throughout the canopy, and at regular intervals during the period when the aphid is abundant.     

Plant‐mediated effects of drought on aphid population structure and parasitoid attack

The effects of predicted climate change on aphid–natural enemy interactions have principally considered the effects of elevated carbon dioxide concentration and air temperature. However, increased incidence of summer droughts are also predicted in Northern Europe, which could affect aphid–plant interactions and aphid antagonists. We investigated how simulated summer drought affected the bird cherry–oat aphid, Rhopalosiphum padi L., and its natural enemy the parasitoid wasp Aphidius ervi. Drought and, to a greater extent, aphids reduced barley ( Hordeum vulgare) dry mass by 33% and 39%, respectively. Drought reduced leaf and root nitrogen concentrations by 13% and 28%, respectively, but foliar amino acid concentrations and composition remained similar. Aphid numbers were unaffected by drought, but population demography changed significantly; adults constituted 41% of the population on drought‐treated plants, but only 26% on those receiving ambient irrigation. Nymphs constituted 56% and 69% of the population on these plants, respectively, suggesting altered aphid development rates on drought‐stressed plants. Parasitism rates were significantly lower on drought‐stressed plants (9 attacks h^?1 compared with 35 attacks h^?1 on ambient‐irrigated plants), most likely because of lower incidence of nymphs and more adults, the latter being more difficult to parasitize. Any physiological changes in individual aphids did not affect parasitoid preferences, suggesting that attacks were postponed because of drought‐induced changes in aphid demography. This study demonstrates the potential for sporadic climate change events, such as summer drought, to be disruptive to herbivore–antagonist interactions.     

Organic farming-compatible insecticides against the aphid Myzus persicae (Sulzer) in peach orchards

Abstract:  Three products allowed in organic farming (kaolin, mineral oil and insecticidal soap) as alternatives to chemical insecticides, were evaluated for the control of Myzus persicae (Sulzer) in a peach orchard. Their efficacy was compared with the neonicotinoid imidacloprid and an untreated control. The experiment was replicated for 2 years (2003 and 2004). The products were applied once in May every year and aphid infestation was recorded three to four times post-treatment. In 2004, the effect of the products on the predatory coccinellid species was also recorded. In 2003, all four products showed good control with average Abbott's efficiency ranging from 80% to 88%. In the next year, however, the products were less effective and only imidacloprid and kaolin reduced aphid population significantly compared with the untreated control, with an average Abbott's efficiency of 62% and 31%, respectively. This reduced efficiency was attributed to high rainfall occurring post-treatment. Regarding the effects on beneficial coccinellids, only imidacloprid and mineral oil reduced their numbers after application. The results showed that the three alternative products and especially kaolin are promising for aphid control in peach orchards. Kaolin has also low side-effects on beneficials and it could be used in organic peach orchards.     

Performance of clones and morphs of two cereal aphids on wheat plants with high and low nitrogen content

The great variability of the aphid life cycle and their tendency for host alternation gives rise to aphid clones and morphs. Inter‐ and intraclonal variability may be observed in the responses of aphids to various environmental factors. In this study we aimed to evaluate the influence of intrinsic factors (clone and morph type) on the intrinsic rate of increase (r_m) of the English grain aphid, Sitobion avenae (Fabricius), and the bird cherry‐oat aphid, Rhopalosiphum padi (Linnaeus). For each species four apterous clones were collected from established laboratory colonies and compared to assess their relative fitness on high‐ and low‐nitrogen wheat plants under laboratory conditions. The clones had significantly different intrinsic rates of increase on high‐ and low‐nitrogen plants. All R. padi clones had a higher intrinsic rate of increase and mean relative growth rate than S. avenae. Experiments were also conducted to compare the mean fecundity of apterous and alate morphs of S. avenae and R. padi clones on high‐ and low‐nitrogen wheat plants. On high‐nitrogen plants the apterous morphs of S. avenae clones had significantly higher mean fecundity than alate morphs. There were no significant differences between the mean fecundity of alate morphs of the same species on high‐ and low‐nitrogen plants. The results support the idea of better fitness of specific clones/morphs on certain host plants due to higher and lower intrinsic rates of increase.     

Apparent competition or apparent mutualism? An analysis of the influence of rose bush strip management on aphid population in wheat field

Abstract:  Apparent competition, mediated by a shared predator, plays a key role in conservation biological control. Appropriate agroecosystems management may favour this type of indirect interaction. In that context, our aim was to test the effect of rose bush [ Rosa rugosa (Thunb.)] strips on the building up of aphid populations and of their natural enemies in adjacent cereal habitats. Several aphid species are currently found on Rosa sp. including Metopolophium dirhodum (Walker) for which it is a primary host. Aphid predators and parasitoids may build their populations on the aphid population present on Rosa sp. and later on migrate to wheat field during the cereal aphid infestation. Moreover, the flowers of the rose bushes may provide a source of nectar and pollen to these natural enemies. Our experiment was conducted in three rose margin wheat plots with a strip of rose bushes of R. rugosa and three control plots. Plots were compared during 2 years (2003 and 2004). Aphid, parasitoid and predator densities were recorded from May to the harvest of wheat on rose bushes and in wheat. In 2003, the aphid densities were moderate but in 2004, the population of aphid was very high. Even if predator and parasitoids arrived earlier in rose margin field than in control ones, the presence of rose bushes did not influence the aphid population within the field. Metopolophium dirhodum did not seem to migrate from the rose bushes to wheat. The level of parasitism was weak in rose bushes and the natural enemy population was not different in rose margin wheat and in control plots. The causes of the lack of efficiency of this type of management are discussed as well as the high aphid population in wheat in 2004.     

Population dynamics of the bird cherry‐oat aphid,Rhopalosiphum padi (L.), during the autumn and winter: a modelling approach

1 A simulation model was developed to investigate the inter‐relationship of factors influencing the population dynamics of the bird cherry‐oat aphid (Rhopalosiphum padi (L.)) in barley crops during the autumn and winter. 2 The model incorporated algorithms describing alate immigration, development and survival of adults and nymphs, fecundity and morph determination in newly born nymphs. 3 The model was validated against pest outbreaks in barley fields in south‐east England. 4 It simulated accurately the size of the outbreaks with predictions of peak aphid populations within 20% of the observed in all but one case. Similarly, all but one of the year‐sowing date combination predictions of timing of peak abundance fell within 14 days of the observed. 5 A sensitivity analysis of the model highlighted the relative importance of various population processes in determining simulated aphid population dynamics; decreasing mortality rates of apterous nymphs by as little as 5% over the autumn and winter increased peak densities by as much as 60‐fold, whereas increasing daily temperatures by only 1 °C more than doubled peak aphid abundance. 6 The model identified our understanding of the mechanisms of aphid mortality as a limiting factor in the accurate prediction of R. padi outbreaks in the field.     

The effects of nitrogen and fungicide on cereal aphid population development and the consequences for the aphid-yield relationship in winter wheat

In a field trial, done over two seasons, nitrogen and fungicide inputs to winter wheat were varied to obtain a range of yields to study the effects on aphid population development and the aphid-yield loss relationship. In the first year, the maximum density of Metopolophium dirhodum and total aphid index were significantly higher in the plots receiving the largest amount of nitrogen but there were few other consistent effects on aphid population development. In the second year there were no significant effects of either nitrogen or fungicide on aphid population development. Mean yields were high in 1987 and 1988 (7.0 and 8.5 t/ha respectively) with less than a 2 t/ha range in either year. There were no significant effects of aphids on yield in the first year but in the second, aphids caused a significant reduction. Damage per aphid unit did not change with increase in yield. The use of this approach in determining pest-yield loss relationships is discussed.     

Estimating cladoceran birth rate: use of the egg age distribution to estimate mortality of ovigerous females and eggs

The birth rate of natural cladoceran populations can change rapidly (during 2–3 days), reflecting rapid changes in their environment. If the egg ratio is calculated on the basis of egg age distribution, the birth rate can be estimated at short sampling intervals (shorter than egg stage duration) by modified Paloheimo's (1974) formula. When female size structure and age of eggs in clutches at the beginning and the end of a sampling interval are known, death rates of ovigerous females and eggs in separate size classes can be determined and incorporated in birth rate estimates. All these methods have been employed using the data on the population of Diaphanosoma brahyurum from the lake Obsterno (North-Western Belarus) in July–August, 1992. The birth rate values computed by the proposed methods and Poloheimo's formula differed significantly in many cases. The accuracy of birth rate estimations from various calculation methods was tested using a computer simulation. The model contains the essential features of cladoceran life history: distinct egg, juvenile and adult stages, development of eggs and reproduction. The population was divided into 25 age classes, each of 1 day duration. Durations of the egg, juvenile and adult stages were set at 3, 6 and 20 days, respectively. The embryogenesis was divided into three egg stages, each of 1 day duration. Survivorship was set from 0.2 up to 1.0 for each age class. The survivorship and brood size were changed through each of five time intervals (days) that allowed to simulate an increase or reduction of population density. Fecundity, survivorship and egg stage duration remained constant during each of 5 days that assumed stability of an environment (this does not occur in nature). Nevertheless, the egg ratio, proportion of juveniles and birth rates were variable even under these circumstances. Computer simulations showed that Poloheimo's formula evaluates birth rate with the relative error of 62% and usually overestimates its values. We propose methods to decrease errors of birth rate estimations by 3.5–5.5 times.     

Estimating population birth rates of zooplankton when rates of egg deposition and hatching are periodic

Summary I present a general method of computing finite birth and death rates of natural zooplankton populations from changes in the age distribution of eggs and changes in population size. The method is applicable to cases in which eggs hatch periodically owing to variable rates of oviposition. When morphological criteria are used to determine the age distribution of eggs at the beginning and end of a sampling interval, egg mortality can be incorporated in estimates of population birth rate. I raised laboratory populations of Asplanchna priodonta, a common planktonic rotifer, in semicontinuous culture to evaluate my method of computing finite birth rate. The Asplanchna population became synchronized to a daily addition of food but grew by the same amount each day once steady state was achieved. The steady-state rate of growth, which can be computed from the volume-specific dilution rate of the culture, was consistent with the finite birth rate predicted from the population's egg ratio and egg age distribution.     

Performance of the green spruce aphid, Elatobium abietinum (Walker) on previously defoliated Sitka spruce

Abstract 1 The green spruce aphid, Elatobium abietinum, is an important defoliator of Sitka spruce in the U.K. However, it is usual for years in which high E. abietinum populations occur to be followed by a year with low aphid densities. The possibility that the performance of E. abietinum is reduced on previously infested Sitka spruce, and that this is the cause of year‐to‐year fluctuations in population density, was investigated by comparing population development and the growth rate of individual aphids on experimentally defoliated trees. 2 Separate experiments were performed to determine whether aphid performance was reduced either in the autumn immediately after defoliation in the spring, or was reduced in the spring of the next year. Different rates of initial defoliation on trees used to test aphid performance were created by artificially infesting the trees with aphids in the spring before the experiments, and varying the time of infestation. 3 Population development and the mean relative growth rate (MRGR) of individual aphids on previously defoliated and undefoliated Sitka spruce did not differ significantly in the spring of the next year. No differences were observed in the nutrient content of the 1‐year‐old needles of previously defoliated or undefoliated trees at this time. 4 In the autumn and winter immediately after spring defoliation, aphid MRGR was significantly higher on trees that had been heavily defoliated earlier in the season compared with trees that had been lightly defoliated. However, the difference in MRGR decreased over the winter period. Nitrogen, phosphorous and potassium concentrations were 9.4–12.2% higher, at the beginning of the autumn, in the current year needles of heavily defoliated trees than in the current year needles of lightly defoliated trees. 5 The experiments indicate that high populations of E. abietinum in the spring do not induce any defensive mechanisms in Sitka spruce that adversely affect subsequent generations of the aphid. By contrast, the results suggest that high spring densities of the aphid improve the nutritional quality of the current year's foliage for autumn generations.     

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.     

Effect of nitrogen fertilization on Aphis gossypii (Homoptera: Aphididae): variation in size, color, and reproduction

The effect of nitrogen fertilization on Aphis gossypii Glover color and size, fertility, and intrinsic rate of increase (rm) was studied on cotton plants. Nitrogen fertilization treatments consisted of 0, 50, 100, and 150% of the agronomic recommended level. Adult and nymph densities, as well as rm, were positively correlated with nitrogen fertilization. Aphid body length, head width, and darkness of color were recorded in populations on cotton plants fertilized with 100% nitrogen or with no nitrogen fertilization. Aphids on nitrogen-fertilized plants were significantly bigger and darker. All body size and darkness of color measurements were positively correlated with aphid fecundity. It was also found that the nutritional quality of the host plant on which the parent generation feeds has a stronger effect on the aphids than that of the quality of their own food plants. This phenomenon may dampen the effect short-term fluctuations in host plant quality have on aphid performance. Results are discussed in the context of aphid population biology, aphid-plant interactions and aphid population management.