Food quality effects on instar‐specific life histories of a holometabolous insect

1. It is a long‐standing challenge to understand how changes in food resources impact consumer life history traits and, in turn, impact how organisms interact with their environment. To characterize food quality effects on life history, most studies follow organisms throughout their life cycle and quantify major life events, such as age at maturity or fecundity. From these studies, we know that food quality generally impacts body size, juvenile development, and life span. Importantly, throughout juvenile development, many organisms develop through several stages of growth that can have different interactions with their environment. For example, some parasitoids typically attack larger instars, whereas larval insect predators typically attack smaller instars. Interestingly, most studies lump all juvenile stages together, which ignores these ecological changes over juvenile development.
2. We combine a cross‐sectional experimental approach with a stage‐structured population model to estimate instar‐specific vital rates in the bean weevil, Callosobruchus maculatus across a food quality gradient. We characterize food quality effects on the bean weevil's life history traits throughout its juvenile ontogeny to test how food quality impacts instar‐specific vital rates.
3. Vital rates differed across food quality treatments within each instar; however, their effect differed with instar. Weevils consuming low‐quality food spent 38%, 37%, and 18% more time, and were 34%, 53%, and 63% smaller than weevils consuming high‐quality food in the second, third, and fourth instars, respectively. Overall, our results show that consuming poor food quality means slower growth, but that food quality effects on vital rates, growth and development are not equal across instars. Differences in life history traits over juvenile ontogeny in response to food quality may impact how organisms interact with their environment, including how susceptible they are to predation, parasitism, and their competitive ability.

     

Number of flagellar segments and moulting in the amphipod Gammarus pulex

SUMMARY. The number of flagellar segments on the antennules of Gammarus pulex was quantitatively related to body size in animals from natural populations and to moulting and instar number of isolated animals grown from birth in the laboratory. The number of segments was 5.0 at birth increasing to means of 19–20 in females and 21–22 in males at sexual maturity or c. 5 mg wet wt reached after nine or ten moults. Addition of segments to the paired flagellae of juveniles was not regular and the number of segments cannot be used to identify particular instars of individuals or to determine their age. The mean interval (days) between moults progressively increased in the later juvenile instars and was exponentially related to moult number, age and the number of flagellar segments; the moult interval is also exponentially related to these variables in adults.     

Phenotypic plasticity and constancy of life-history traits in laboratory clones of Daphnia magna Straus: effects of neonatal length

1. The phenotypic constancy of four laboratory Daphnia magna clones in fitness-related life-history traits, such as age and clutch size at maturity, was studied among consecutive experimental runs in differing food environments.
2. A significant part of the observed clonal and genetic-by-environmental variation in age and clutch size at maturity was explained by experimentally uncontrollable variations in neonatal body length.
3. Despite food availability, neonatal length determined the number of instars invested to maturity and thus maturation age. Clonal differences in neonatal length and thus in maturation instar occurrence across environments explained most of the clonal variability observed in maturation age.
4. Although interclonal differences in clutch size existed, most of the phenotypic plasticity observed for clutch size was mediated by clonal differences in neonatal length.
5. Clonal differences in neonatal length and in the occurrence of maturation instars across environments dramatically affected the body length of instar IM-2 where provisioning of eggs take place. Since clutch size is determined from clutch mass and clutch mass was strongly related to the body length of instar IM-2, clonal differences across environments in body length of instar IM-2 mirrored clonal differences across environments in clutch size.
6. The results reported in the present study show that maternally mediated traits such as neonatal length affect how genotypes respond to different environmental selection regimes (genetic-by-environmental interaction). Future research needs to focus on the effects of neonatal length on the heritability or genetic variation of the reaction norms, since prediction of the response to selection is a key research objective in quantitative genetic studies.     

The effect of temperature on maturation threshold body-length in Daphnia magna

Aquatic invertebrates are usually larger at maturity when water temperatures are lower. For the freshwater cladoceran Daphnia, it has been suggested that a threshold size must be attained to initiate maturation, which results two instars later in the deposition of eggs into the brood chamber. This threshold size is believed to temperature on maturation threshold body-length in Daphnia magna. Daphnids were raised from birth to maturity under three constant-temperature regimes (12°C, 16°C, 22°C), and two food-level conditions. Animals were measured daily, and a body-length based maturation threshold determined for each individual. We demonstrate that mean maturation threshold length is negatively correlated with ambient water temperature. Further, daphnids with a larger threshold length tended to be larger at maturity. A maturation threshold linked to body length suggests that reduced variation in size at maturity is adaptive, even at the cost of additional variation in instar number or age at maturity.     

Reduction of Delia radicum attack in field brassicas using a vertical barrier

According to life‐history theory, longer development time may result in bigger adults. However, reaction norms describing age and size at maturity often follow an L‐shaped form. This relationship is attributable to the simple notion that slowly growing individuals may not lengthen their development excessively after the maturation decision has been made, for example, when development is time limited in seasonal environments. In arthropods, growth occurs within instars, and thus the optimal growth strategy might be mediated by the phenotypic adjustment of instar numbers. We studied the relationship between age and size at maturity of a lichen‐feeding moth, Eilema depressum (Esper) (Lepidoptera: Arctiidae: Lithosiinae), and the variability of instar numbers in relation to achieved adult body mass and time used for maturation. A positive relationship between age and size at maturity was found across developmental pathways and a negative one within the developmental pathways. Directly developing larvae had higher growth rates, attained smaller pupal mass, and passed fewer instars than larvae maturing after overwintering. Host quality did not affect whether larvae matured during the remaining or the next season. High variation in the number of instars together with variable growth rates indicates high plasticity in adaptation to varying environmental conditions. Our results also confirm previous results that instar number variability may be a key characteristic mediating age and size at maturity in insects.     

Chaoborus crystallinus predation on Daphnia pulex: can induced morphological changes balance effects of body size on vulnerability?

Juvenile Daphnia pulex form neckteeth in reponse to chemicals released by predatory Chaoborus crystallinus larvae. Formation of neckteeth is strongest in the second instar followed by the third instar, whereas only small neckteeth are found in the first and fourth instar of experimental clones. Predation experiments showed that body-size-dependent vulnerability of animals without neckteeth to fourth instar C. crystallinus larvae matched the pattern of neckteeth formation over the four juvenile instars. Predation experiments on D. pulex of the same clone with neckteeth showed that vulnerability to C. crystallinus predation is reduced, and that the induced protection is correlated with the degree of neckteeth formation. The pattern of neckteeth formation in successive instars is probably adaptive, and it can be concluded that neckteeth are formed to different degrees in successive instars as an evolutionary compromise to balance prediation risk and protective costs.     

Factors affecting the reproductive success of the crab spider Misumenoides formosipes: the covariance between juvenile and adult traits

Summary To examine the importance of covariance between stages in traits related to foraging, we quantified the relationships between reproductive success and sizerelated variability in weight gain in juvenile and adult instars of the crab spider Misumenoides formosipes (Araneae: Thomisidae). Prereproductive weight and fecundity are both highly correlated with carapace width, a linear measure of size which does not change within an instar. In field populations, adult females with larger carapaces gain more weight and are more likely to reproduce than females with smaller carapaces. The growth rate of spiders fed ad libitum in the laboratory is unrelated to size, suggesting that size-related differences in the field are due to variation in prey-capture success. Adult females with a carapace width less than 3.4 mm comprised 22% of the population, but were never found to reproduce. Of the individuals that did reproduce, a 17% increase in carapace width resulted in a 100% increase in fecundity. Juvenile stages must be examined to understand adult foraging and reproductive success, because the net weight gained by juvenile instars determines adult size. The final weight gained by spiders in the antepenultimate and penultimate instars explained nearly all the variation in carapace width in the penultimate and adult instars, respectively. We found that constraints on foraging in late juvenile stages are different from the adult stage. Penultimate foraging behavior differs from that of adults, because of constraints on foraging in the period preceding ecdysis. Additionally, in both late juvenile instars, carapace width had little or no effect on the final weight gained within the instar suggesting that factors that affect foraging are different between the juvenile and adult stages. These analyses stress the fact that to fully understand the effects of foraging on reproductive success, we must examine stage-specific constraints throughout an organism's life history.     

Developmental polymorphism in a Newfoundland population of the hemlock looper, Lambdina fiscellaria (Lepidoptera: Geometridae)

The hemlock looper [Lambdina fiscellaria (Guenée)], a widespread and highly polyphagous Geometridae, is considered one of the most economically important defoliators of North American coniferous forests. Variations in the number of larval instars between geographic populations of this species have been previously reported in the literature. However, whether such developmental polymorphism occurs within a given population is unknown. In this study, we report the presence of both four and five larval instar individuals within a population of hemlock looper in Newfoundland when reared on balsam fir. For both sexes, the majority of individuals reared on balsam fir shoots went through four larval instars, but more than one third of the females (35.3%) went through five larval instars. Females with four larval instars developed faster and had smaller pupal weight than females with five larval instars. However, a growth-related index (weight gain per unit of time) was similar for the two ecotypes (four or five larval instars). No significant difference was observed between the two ecotypes in terms of reproductive capacity (fecundity and egg size). We also found significant differences in life history traits between males and females. Results indicate that developmental polymorphism, in this case, the variation in the number of larval instars, might provide some adaptive attributes that allowed exploitation of a broader ecological niche.     

Cotesia plutellae parasitizing Plutella xylostella: Host-age dependent parasitism and its effect on host development and food consumption

The braconid Cotesia plutellae(Kurdjumov) (Hymenoptera: Braconidae) is amajor solitary, larval endoparasitoid of thediamondback moth, Plutella xylostella(L.) (Lepidoptera: Plutellidae). Parasitism oflarvae of different host instars and fourdevelopmental ages of the 4th instar ofthe pest was examined. The effects of hostinstar at initial parasitization on thedevelopment, survival, size and fecundity ofthe parasitoid were determined in thelaboratory at 25 °C. The effects ofparasitism on host development and foodconsumption were investigated at 28 °C.Cotesia plutellae could parasitize larvaeof all four instars of P. xylostella, butpreferred 2nd and 3rd instars. In achoice test, the relative parasitism indicesfor 2nd, 3rd and 4th instarswere 0.37, 0.39 and 0.24, respectively.Parasitism decreased sharply with increasinghost age in the 4th instar and approachedzero in host larvae that had gone beyond 37%of 4th stadium. The development time andthe final adult size of the parasitoid variedwith the host instar at initial parasitization.Parasitoids with initial parasitism in the4th instar hosts had the shortestdevelopment time, followed by those in the3rd instar, and then by those in the2nd instar. Parasitoids startingparasitism in 2nd instar hosts weresmaller in body size than those starting in the3rd or 4th instar. However, resultantfemales starting parasitism in 3rd instarhosts had the highest fecundity. Parasitizedlarvae exhibited longer development time andincreased food consumption compared withunparasitized ones. This study presents thefirst record that a solitary parasitoidregulates host behavior leading to an increasein food consumption by the host.     

Persistent maternal identity effects on life history traits in<Emphasis Type="Italic"> Daphnia</Emphasis>

The aim of the present study was to examine the magnitude and persistence of maternal effects in Daphnia, in particular maternal identity effects. I studied life history traits of a single clone of Daphnia galeata born to 40 different mothers belonging to three age groups. Maternal identity had large effects on offspring traits, that is, identically treated clonal females differed substantially in respect to the traits of their offspring, including size at birth, age at maturity, and number of second generation offspring. The effects of maternal identity on these traits were largely independent of maternally induced differences in offspring size, indicating that maternal effects were mediated through offspring quality. Maternal age also affected offspring traits: older mothers gave birth to larger offspring which matured earlier, were larger and more fecund, and survived better until maturity. Individuals which were larger at birth also had a better chance of survival. Contrary to expectation, I found little evidence that maternal identity or maternal age had any influence on their offsprings response to fish kairomones.     

Life history and body size evolution in Holopedium gibberum Zaddach (Crustacea, Cladocera)

SUMMARY. 1. Body size, development, sheath size and relative egg size were examined in two populations of Holopedium gibberum Zaddach. The populations were chosen because of differences in juvenile versus adult mortality.
2. The population subject to food limitation and predation from icthyoplankton (high juvenile mortality) exhibited large size at birth and maturity, large sheath size, and eggs which were relatively large compared to the adult. In contrast, individuals in the population exposed to predation by golden shiner (high adult mortality) were smaller at birth and maturity, had a smaller sheath, and produced relatively small eggs.
3. I contrast body size and relative egg sizes of these H. gibberum populations with literature values for all Cladocera, and propose a general hypothesis for the evolution of relative egg size in Cladocera.     

Interpopulation variation in growth and life-history traits of the introduced sunfish, pumpkinseed Lepomis gibbosus, in southern England

We examined attributes of growth and reproduction in 19 populations of pumpkinseed (Lepomis gibbosus) introduced into southern England in order to: (i) assess variability of these traits in a northern European climate; (ii) assess inter‐relationships among these variables; and (iii) compare these attributes with populations from other parts of Europe where pumpkinseeds have been introduced. Growth rates varied considerably among populations, but juvenile growth rates and adult body sizes were generally among the lowest in Europe. Mean age at maturity ranged from 2.0 to 3.9, and was strongly predicted by the juvenile growth rate (earlier maturity with faster juvenile growth). Other population parameters that also displayed significant negative associations with mean age at maturity were gonadosomatic index, body condition, and adult body size (total length, TL at age 5). Mean TL at maturity and the adult growth increment showed no significant associations with any of the other growth or life‐history variables. Pumpkinseed populations in England matured significantly later than those introduced into warmer, more southerly areas of the continental Europe. All of these data suggest that a combination of cool summer temperatures and resource limitation is the cause of slow growth, small adult body size and delayed maturity relative to introduced populations on the European mainland.     

Can Developmental and Behavioral Characteristics of the Multicolored Asian Lady Beetle Harmonia Axyridis Explain its Invasive Success?

Major impacts of biological invasions are widely recognized and underscore the need to understand the relation between life-history traits of invasive species and the invasion process. Growth of juveniles and adult survival of invasive species are key factors in invasion process. Life-history traits that increase juvenile fitness including increased rates of development and behavioral characteristics that facilitate competitive success such as increased predator efficiency and foraging ability may explain invasiveness of a species. Invasion of Harmonia axyridis Pallas (Coleoptera: Coccinellidae) in North America provides an opportunity to investigate life-history traits of juveniles of an invasive species. Here, we evaluate both developmental and behavioral traits that may explain the success of H. axyridis by comparing it to an ecologically similar indigenous species Coleomegilla maculata lengi Timberlake (Coleoptera: Coccinellidae). Three points may contribute to the invasiveness of H. axyridis. First, development of H. axyridis was faster during the 2nd larval instar than C. maculata, a characteristic that may reduce vulnerability at young instars. Second, H. axyridis reached the 4th instar more rapidly than C. maculata. The 4th instar of H. axyridis was also characterized by higher predation efficiency with increased voracity, lethal contact and search efficiency of pea aphids Acyrthosiphon pisum. Finally, surprisingly, a 5th larval instar occured in 33% of the individuals of H. axyridis and was characterized by the same developmental time, but with increased voracity and weight gain compared to 4th larval instars, suggesting an increased fitness of these individuals. These developmental characteristics coupled with increased predation efficiency and behavioral characteristics enhanced the juvenile growth and predatory abilities of this species and may contribute to the invasive ability of H. axyridis.     

Inter- and intra-specific differences in the number of larval instars in British populations of 24 species of stoneflies (Plecoptera)

1. Ontogenetic changes during the life cycle of aquatic insects are important not only in life‐history studies but also in evaluating food‐web structure. They require information on the growth and number of larval instars but such information is lacking for many species, including Plecoptera. Therefore, the chief objectives of the present study were to determine inter‐ and intra‐specific differences in the number of larval instars in British populations of 24 species of stoneflies, to test Dyar’s hypothesis that growth followed a geometric progression, and to synthesise this information with previously published values for four British species. 2. Larvae were reared at constant temperatures in the laboratory from eggs from 63 populations (one to six populations per species). First instars from each population were divided into three batches and each batch was reared at one of three constant temperatures. For each species, the rearing temperature and source population had no significant effect on the mean size of each larval instar. 3. The relationship between the geometric mean length of each instar and instar number was well described by an exponential equation (P < 0.001, r² > 0.9 for all species), thus supporting Dyar’s hypothesis. Only one species, Brachyptera risi, had the same number of instars for males and females (12–13). For the other 15 herbivorous species and the four smaller carnivorous ones, the number of instars was higher for females than males (range 11–16 for males, 12–17 for females). The larger size of the females was due to their additional instars, not a sex difference in growth rates. In contrast, there was a clear growth separation of the sexes after the 9th or 10th instar for the four largest carnivores. The number of larval instars was highest for these four species (range 16–19 for males, 18–23 for females), and females were much larger than males. 4. A multiple regression equation with data from the present and previous studies (n = 27) showed that variability in the mean length of the first instar and the maximum number of larval instars for each species accounted for 88% and 91% of the variability in the mean length of the final instar for males and females, respectively. 5. Values for Plecoptera in other countries were in general agreement with those in the present study, especially in the same families. Two old, but widely quoted, high values are doubtful. The present study and four previous ones provide a sound basis for ontogenetic studies on 28 species of Plecoptera and their role in aquatic ecosystems.     

Phenotypic plasticity in age and size at maturity and its effects on the integrated phenotypic expressions of life history traits of Cardamine flexuosa (Cruciferae)

We analyzed variation in phenotypic plasticity of life history traits between two Cardamine flexuosa populations based on differences in plasticity of age and size at maturity. C. flexuosa (Cruciferae) is a facultative, vernalization-sensitive, long-day annual, and its phenology and the phenotypic expressions of many life history traits are largely controlled by photoperiod and vernalization in natural populations. We used plants from two populations which differed in their responses to chilling and photoperiod treatments. The timing of developmental processes was changed by controlling temperature and photoperiod regimes in growth chambers. Plasticity in size at maturity was analyzed as changes in a growth trajectory using two parameters, age at maturity (Δt) and growth rate (k). Both traits showed plasticity, but differences between the populations were found mostly for Δt. Distinctive differences in size at maturity of individuals in the two populations were mainly due to different amounts of plasticity in Δt. Variations in plasticity of nine other life history traits and their associations to age and size at maturity were also analyzed. Variation for eight of the traits can be described, at least in part, as a function of age and size at maturity for both populations, and most of the variation in the total number of seeds was explained by age and size at maturity. Only age at maturity had any effect on changes in resource allocation. The nine life history traits were integrated through associated character expressions with age and size at maturity. Changes in the association between a trait and age and/or size at maturity were rather conservative compared to changes in the plasticity of a trait between the two populations. Associations with age and size at maturity are mostly explicable in terms of inherent relationships in the developmental processes, and they may limit the ecological range expansion and the adaptive evolution of plasticity in C. flexuosa. The negative correlation between reproductive allocation and age at maturity can be a cost of delaying maturation in C. flexuosa.     

Preference and consumption of Macrolophus pygmaeus preying on mixed instar assemblages of Myzus persicae

This study examines the effects of changes in the prey frequency and abundance on prey selection among the four instars of Myzus persicae by the predator Macrolophus pygmaeus under laboratory conditions. The central hypothesis was that M. pygmaeus will become more selective as prey density increases. It was also observed that M. pygmaeus can occasionally abandon a prey item that had already been killed (non-consumptive prey mortality). It was assumed that the frequency of this behavior would increase with the prey size and prey density. For these purposes prey selection was evaluated by simultaneously presenting all instars of M. persicae to the predator in equal proportions and at increasing densities. M. pygmaeus showed a higher predation rate and a higher preference for smaller prey instars at all prey densities. However, if the predation rate by the predator is expressed in terms of biomass consumed, then biomass gain was higher when feeding on the larger instars of M. persicae. The prey selectivity was indicated by the total prey mortality (consumptive plus non-consumptive prey mortality) as well as by the non-consumptive prey mortality, was associated with relatively high prey densities, depending on the prey instar. Therefore, we argued that the predatory impact of M. pygmaeus on the various instars of the aphid depends not only on prey traits but also on their relative abundance in a patch. Observed decreases in biomass gain from larger prey were likely the result of high prey availability at densities before saturation, which might have caused confusion in the predator’s prey selection.     

Shift in life-history strategy from reproduction to defense with colony age in the galling aphidHemipodaphis persimilis producing defensive first-instar larvae

Colony defense in some aphids is performed by sterile soldiers but in others by monomorphic larvae of a specific instar stage. This paper, focusing on a galling aphid Hemipodaphis persimilis with monomorphic defensive first instars, examined the mechanism by which the proportion of defenders is regulated in the colonies. Demographic analyses showed that the ratios of first instars (defenders) were kept constantly high (58% on average) from mid June to late September. High proportions of first instars could be explained by consistently high birth rates (birth rate hypothesis) or by a prolonged duration of the first instar stage (instar span hypothesis). With the progress of colony age, the mature-embryo content of apterous adults, used as an index of the birth rate, decreased and the proportions of advanced instars increased. These results did not support the birth rate hypothesis. By contrast, calculation of a newly proposed index, the molting rate, showed that the duration of the first-instar stage was short in incipient galls but became longer with colony age. The duration of other instar stages was always kept short. These results corroborate the instar span hypothesis and suggest that the prolongation of the first-instar stage is an adaptive mechanism by which the defender ratio is kept high in mature colonies where the birth rate is declining. The frequency of aggressive behavior in first instars increased from incipient to mature galls. Seasonal changes in the instar span and aggressiveness of first instars suggest that in H. persimilis colonies there is a strategic shift from the reproductive to defensive phase with colony age.     

The interaction of season length and development time alters size at maturity

An end-of-season penalty, with late-maturing individuals being smaller than early-maturing individuals, has been observed in a variety of univoltine terrestrial arthropods. The current study extends these observations, utilizing multiple populations of a single sexually dimorphic species to examine the ecological correlates and fitness consequences of late maturation at a small size. The orb-weaving spider, Nephila clavipes, inhabits a broad range of habitats that vary from mild to strong seasonality. Because males mature several instars earlier than females, they can reach maturity much earlier in the growing season. Within a cohort, I found that female size at maturity was negatively correlated with timing of maturation in strongly seasonal sites. At a less seasonal site, there was no correlation between female size and timing of maturation within a cohort. In most populations studied, male size was not correlated with the timing of maturation within a cohort. Within populations in strongly seasonal sites, late-maturing females had reduced fecundity. The probability of copulation, survivorship from maturity to first clutch, clutch size relative to female size, and the number of possible clutches were all reduced with delayed maturation. The probability of pre-reproductive death for late-maturing females was strongly affected by stochasticity in the timing of the end of the growing season. Received: 30 December 1998 / Accepted: 1 September 1999     

Indirect effect of planktivorous fish on the growth and reproduction of Daphnia galeata

Both the size of Daphnia galeata primiparae and the average egg volume were smaller in individuals reared from neonata in water inhabited for the preceding 24 hours by Y-O-Y roach when compared with control animals reared in the same water with the exclusion of fish. The smaller size of experimental primiparae resulted from smaller increments in some instars and from the earlier maturation of about two-thirds of those that matured in the 4th instar, while the rest of the experimental animals and all the controls matured in the 5th instar. There were larger clutches and smaller eggs in experimentals when compared with the similar sized controls, but the difference hasn't proved to be statistically significant.     

Comparative analysis of the late embryogenesis of Sida crystallina (O.F. Müller, 1776) and Diaphanosoma brachyurum (Lievin, 1848) (Crustacea: Brachiopoda: Ctenopoda)

The embryonic development of two ctenopods Sida crystallina and Diaphanosoma brachyurum has been investigated by observing living embryos removed from the female brood pouches. The sequence of morphological changes was analysed, as was the time at which the activity of certain organs began. The timing of these events at 21–22 °C is documented for both species. During embryogenesis four membranes are cast off. The growth in length of embryos began only after the shedding of the external egg envelope. Growth rates throughout embryogenesis are documented. A new outline of ctenopod embryogenesis is proposed. It includes four stages (= instars) demarcated by the shedding of membranes (= moults), as is commonly accepted for juvenile and mature animals in ctenopods and other crustacean groups. These instars are well distinguished morphologically. Their characteristics are presented. Differences in the duration of embryogenesis in Sida and Diaphanosoma are explained by greater extension of the first, and especially the fourth, instars of Sida, while the duration of the second and third instars are approximately the same in both. A list of instar features is given which may be used to determine the instar (and approximate age) of embryos in the broud pouch of ctenopod females from natural populations. This revised version was published online in July 2006 with corrections to the Cover Date.