Size‐induced phenotypic reaction norms in a parasitoid wasp: an examination of life‐history and behavioural traits

The amount of resources available during development often affects body size, causing phenotypic variation in life‐history traits and reproductive behaviours. However, past studies have seldom examined the reaction norms of both life‐history and behavioural traits versus body size. We measured the phenotypic plasticity of several life‐history (age‐specific egg load, egg size, longevity) and behavioural (oviposition rate, host marking rate, walking speed) traits of the egg parasitoid Telenomus podisi Ashmead (Hymenoptera: Scelionidae) in response to body size variation. We predicted that life‐history traits would show more evidence of size compensation than behavioural traits, resulting in fewer positively‐sloped size versus trait reaction norms among the former. As predicted by life‐history models, smaller wasps appear to shift resource allocation towards early‐life reproduction, having a similar egg load to large individuals 9 days after emergence. Surprisingly, longevity was unaffected by body size. However, egg size, the number of offspring produced during oviposition bouts, and the rate of subsequent egg synthesis were greater for larger individuals. In addition, as predicted, the reaction norms of behavioural traits versus body size were all positively sloped. Thus, despite possible adaptive compensatory plasticity of life‐history traits by small individuals, behavioural constraints directly related to body size would contribute to maintaining a positive size–fitness relationship.     

Income resources and reproductive opportunities change life history traits and the egg/time limitation trade‐off in a synovigenic parasitoid

1. Synovigenic parasitoids emerging with no or only a few mature oocytes could not rely on only capital resources, but also need to acquire income resources. Income resources in nature can either contribute to egg maturation as a food resource and/or create unpredictability in realised reproductive opportunities for synovigenic parasitoids. Therefore, we hypothesised such resources could affect life history traits and the risks of egg/time limitation in synovigenic parasitoids. 2. Using the Ovigeny Index, we investigated the effects of various host availability levels (unavailable, limited, and unlimited availability) and non‐host foods (water and honey) on life history traits and on the occurrence of egg/time limitation in Eretmocerus hayati, a predominant parasitoid on Bemisia tabaci. 3. The Ovigeny Index of Er. hayati was 0.28, which suggested it was a typical synovigenic species. Both host availability levels and non‐food type had major effects on life history traits of this parasitoid, but the availability of hosts for both feeding and reproduction was the key factor. Meanwhile, egg/time limitation was encountered by all wasps and its intensity varied with host availability levels. 4. Our results confirmed that the income resource and reproductive opportunity played a central role in shaping the life history and risks of egg/time limitation of a synovigenic parasitoid.     

Intraspecific variability in egg maturation patterns and associated life‐history trade‐offs in a polyembryonic parasitoid wasp

1. Life‐history theory predicts a trade‐off between the resources allocated to reproduction and those allocated to survival. Early maturation of eggs (pro‐ovigeny) is correlated with small body size and low adult longevity in interspecific comparisons among parasitoids, demonstrating this trade‐off. The handful of studies that have tested for similar correlations within species produced conflicting results. 2. Egg maturation patterns and related life‐history traits were studied in the polyembryonic parasitoid wasp, Copidosoma koehleri (Hymenoptera: Encyrtidae). Although the genus Copidosoma was previously reported to be fully pro‐ovigenic, mean egg loads of host‐deprived females almost doubled within their first 6 days of adulthood. 3. The initial egg‐loads of newly emerged females were determined and age‐specific realised fecundity curves were constructed for their clone‐mate twins. The females' initial egg loads increased with body size, but neither body size nor initial egg load was correlated with longevity and fecundity. 4. The variation in initial egg loads was lowest among clone‐mates, intermediate among non‐clone sisters and highest among non‐sister females. The within‐clone variability indicates environmental influences on egg maturation, while the between‐clone variation may be genetically based. 5. Ovaries of host‐deprived females contained fewer eggs at death (at ～29 days) than on day 6. Their egg loads at death were negatively correlated with life span, consistent with reduced egg production and/or egg resorption. Host deprivation prolonged the wasps' life span, suggesting a survival cost to egg maturation and oviposition. 6. It is concluded that adult fecundity and longevity were not traded off with pre‐adult egg maturation.     

Effect of host deprivation on egg quality,egg load,and oviposition in a solitary parasitoid,Chetogena edwardsii (Diptera: Tachinidae)

Solitary parasitoids are limited to laying one egg per host because larvae compete within hosts. If host encounter rate is low, females should not increase the number of eggs/host in response. The tachinid fly, Chetogena edwardsii,was used to evaluate the effect of host deprivation on egg accumulation, oviposition behavior, and egg quality in a solitary parasitoid. Females deprived of hosts for 2– 7 days accumulate about 1 day's supply of eggs. Egg output of deprived females once hosts are restored does not differ from that of control females. Deprived females retain one egg in the uterus where it undergoes embryogenesis. Maggots emerging from retained eggs are more likely to survive in hosts molting in 40 h or less after receipt of an egg than are maggots emerging from eggs fertilized shortly before oviposition. Egg retention is a consequence of host deprivation that permits females to broaden the range of hosts they can exploit to include soon-to-molt hosts and possibly multiply parasitized hosts.     

Influence of egg load and host size on host-feeding behaviour of the parasitoid Aphytis lingnanensis

Abstract.

• 1 Direct behavioural assays were used to investigate the influences of host size and parasitoid egg load on the decision to host feed versus oviposit made by the parasitoid Aphytis lingnanensis Compere. Egg load was manipulated without concurrent influences on the history of host contact by exploiting size-related variation in fecundity and by holding parasitoids at different temperatures to vary the rate of oocyte maturation.
• 2 Host feeding comprised a series of feeding bouts, separated by renewed probing of the scale insect body. Successive feeding bouts were progressively shorter, suggesting that hosts represent ‘patches’ yielding resources at a decelerating rate.
• 3 Parasitoids were significantly more likely to host feed on smaller hosts and oviposit on larger hosts.
• 4 Neither egg load nor the treatment variables (parasitoid size and holding temperature) exerted significant influences on the decision to host feed versus oviposit on second instar (low quality) hosts.
• 5 The failure to observe an effect of egg load on host-feeding decisions was not simply a reflection of the parasitoids being entirely insensitive to egg load; significant effects of egg load on parasitoid search intensity and clutch size decisions were observed.
• 6 Parasitoids developing on second instar (low quality) hosts experienced high levels of mortality late during development and yielded very small adults.
• 7 The discord between these experimental results and predictions regarding the importance of egg load underscores the need for additional work on the proximate basis for host-feeding decisions and the nutritional ecology of insect parasitoids.

     

Maintenance of body‐size variation and host range in the orange‐tip butterfly: evidence for a trade‐off between adult life‐history traits

1. The evolution of host range and preference in phytophagous insects is driven by a female's oviposition choice impacting her offspring's fitness. Analysis of the fitness of progeny on different host plants has commonly been restricted to the performance of immature stages. However, since host use can affect adult size, it is important to measure the ongoing effects of host choice on the resulting imagines. 2. The orange‐tip butterfly, Anthocharis cardamines, shows a strong preference for two host plants in Britain, Alliaria petiolata and Cardamine pratensis, which affect body size. Whilst females exhibit a strong positive size–fecundity relation, the impact of body‐size alteration is unknown in males. In this study, fitness effects of host plant choice for male A. cardamines were examined. 3. Males reared on C. pratensis were smaller and emerged earlier than those reared on A. petiolata, and early‐season males were smaller than late‐season ones in the field. Interestingly, regression analysis indicated that the earlier emergence of small males was a host‐mediated rather than a size‐mediated effect. Small size was associated with reduced male dispersal in a semi‐isolated wild population over a 3‐year period. 4. It is proposed that the earlier emergence associated with C. pratensis has evolved in response to depressed dispersal in isolated/semi‐isolated populations associated with this patchily distributed host. We suggest that adult life‐history traits are important for the maintenance of host range in this species, and offer a critique of Courtney's earlier hypothesis that host range is maintained by time‐limited oviposition behaviour.     

A large‐scale latitudinal pattern of life‐history traits in a strictly univoltine damselfly

1. Variation in thermal conditions and season length along latitudinal gradients affect body size‐related traits over different life stages. Selection is expected to optimise these size traits in response to the costs and benefits. 2. Egg, hatchling, larval and adult size in males and females were estimated along a latitudinal gradient of 2730 km across Europe in the univoltine damselfly Lestes sponsa, using a combination of field‐collection and laboratory‐rearing experiments. In the laboratory, individuals were grown in temperatures and photoperiod simulating those at the latitude of origin, and in common‐garden conditions. 3. The size of adults sampled in nature was negatively correlated with latitude. In all populations the females were larger than the males. Results from simulated and common‐garden rearing experiments supported this pattern of size difference across latitudes and between sexes, suggesting a genetic component for the latitudinal size trend and female‐biased size dimorphism. In contrast, hatchling size showed a positive relationship with latitude, but egg size, although differing between latitudes, showed no such relationship. 4. The results support a converse Bergmann cline, i.e. a negative body size cline towards the north. This negative cline in body size is probably driven by progressively stronger seasonal time and temperature constraints towards the higher latitudes and by the obligate univoltine life cycle of L. sponsa. As egg size showed no relationship with latitude, other environmental factors besides temperature, such as desiccation risk, probably affect this trait.     

Size‐related life‐history traits in geometrid moths: a comparison of a temperate and a tropical community

1. Comparative studies on insect life histories are facilitated by the increasing availability of reliable phylogenies but are hampered by the scarcity of comparable data. Fortunately, morphological proxies of some life‐history traits can be measured on preserved specimens. 2. This study compared values of size‐related life‐history traits among a tropical (Ugandan) and a temperate (Estonian) assemblage of geometrid moths. 3. A comparative analysis based on an originally derived phylogeny revealed that tropical moths were, on average, larger than temperate ones. Tropical moths also had somewhat lower relative abdomen masses than temperate ones. This indicates that the tropical rather than the temperate moths tend to use an income (rather than capital) breeding strategy. Nevertheless, no difference was found in a related index of pro‐ovigeny. When body size was accounted for, tropical moths were found to lay smaller eggs than temperate ones. 4. The differences between the two compared areas are consistent with selection on higher mobility of the moths imposed by the more diverse tropical vegetation. Relatively larger eggs of temperate moths may constitute an adaptation to overcome the presumably stronger quantitative defences of their host plants. 5. Overall, however, we conclude that the differences in ecologically relevant size‐related traits are relatively low among moth assemblages of a tropical and a temperate forest region, indicating that these environments may not impose radically different selective pressures on insect life histories.     

Comparative life‐history traits in a fig wasp community: implications for community structure

1. Whether life‐history traits can determine community composition and structure is an important question that has been well explored theoretically, but has received scant empirical attention. Life‐history traits of a seven‐member community of galler and parasitoid fig wasp species (Chalcidoidea), developing within the inflorescences (syconia) of Ficus racemosa (Moraceae) in India, were determined and used to examine community structure and ecology. 2. Gallers were pro‐ovigenic (all eggs are mature upon adult emergence) whereas parasitoids were synovigenic (eggs mature progressively during adult lifespan). Initial egg load was correlated with body size for some species, and there was a trade‐off between egg number and egg size across all species. Although all species completed their development and left the syconium concurrently, they differed in their adult and pre‐adult lifespans. Providing sucrose solutions increased parasitoid lifespan but had no effect on the longevity of some galler species. While feeding regimes and body size affected longevity in most species, an interaction effect between these variables was detected for only one species. 3. Life‐history traits of wasp species exhibited a continuum in relation to their arrival sequence at syconia for oviposition during syconium development, and therefore reflected their ecology. The largest number of eggs, smallest egg sizes, and shortest longevities were characteristic of the earliest‐arriving galling wasps at the smallest, immature syconia; the converse characterised the later‐arriving parasitoids at the larger, already parasitised syconia. Thus life history is an important correlate of community resource partitioning and can be used to understand community structure. 4. This is the first comprehensive study of life‐history traits in a fig wasp community. The comparative approach revealed constraints and flexibility in trait evolution.     

Development and survival of Telenomus lobatus, a parasitoid of chrysopid eggs: effect of host species

Telenomus lobatus Johnson & Bin (Hymenoptera: Scelionidae), previously known only from the eggs of a berothid species, emerged from field-collected eggs of the chrysopid Mallada macleodi. In the laboratory, host species differentially influenced development, survival, and size of the parasitoid. Females parasitized between 55 and 65% of the available eggs of Chrysoperla (C. carnea, C. harrisii, and C. rufilabris), and these yielded 98 to 100% adult parasitoids within an average of 13 days. In contrast, eggs of Chrysopa were parasitized less frequently, (C. chi: 50%; C. oculata: 16%; C. quadripunctata: 1.3%) and relatively few (C. chi: 5.6%; C. oculata: 21%) or no (C. quadripunctata) parasitoids survived to adult emergence. Preimaginal development of T. lobatus in eggs of C. chi and C. oculata was approximately two days longer than in eggs of Chrysoperla spp. Although eggs of Meleoma dolicharthra were parasitized infrequently (approx. 16%), they yielded 100% adult parasitoids. Anomalochrysa maclachlani eggs were parasitized at low rates (1.8%) and parasitoid survival was intermediate (40.7%). Among all species, size of the host egg was positively related to the size of the emerging parasitoid adult.

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Résumé T. lobatus Johnson & Bin (Hym. Scelionidae) connu jusqu'ici uniquement comme parasitoïde des ufs de Berothidae, a étè obtenu d'ufs du chrysope, Mallada macleodi récoltés dans la nature. Au laboratoire, l'espèce de l'hôte influe sur le développement, la survie et la taille du parasitoïde. Les femellles ont parasité 55 à 65% des ufs disponibles de Chrysoperla (C. carnea, C. harrisii, C. rufilabris) et ceux-ci ont donné 98 à 100% de parasitoïdes adultes en 13 jours en moyenne. Par contre, les ufs de Chrysopa étaient moins souvent parasités,-C. chi: 50%; C. oculata: 16%; C. quadripunctata: 1,3%-, et relativement peu de parasitoïdes avaient survecu jusqu'à l'émergence,-C. chi: 5,6%; C. oculata: 21%; C. quadripunctata: 0. Le développment préimaginal est approximativement 2 jours plus long dans les ufs de C. chi et C. oculata que dans ceux de Chrysoperla spp. Bien que les ufs de Meleoma dolicharthra aient été peu parasités,-environ 16%-, 100% ont donné des parasitoïdes adultes. Les ufs de Anomalochrysa maclachlani ont été peu parasités,-1,8%-, et le taux de survie a été moyen: 40,7%. Pour toutes les espèces, la taille de l'uf de l'hôte était liée positivement à la taille du parasitoïde obtenu.

     

Effects of host plant on life‐history traits in the polyphagous spider mite Tetranychus urticae

Studying antagonistic coevolution between host plants and herbivores is particularly relevant for polyphagous species that can experience a great diversity of host plants with a large range of defenses. Here, we performed experimental evolution with the polyphagous spider mite Tetranychus urticae to detect how mites can exploit host plants. We thus compared on a same host the performance of replicated populations from an ancestral one reared for hundreds of generations on cucumber plants that were shifted to either tomato or cucumber plants. We controlled for maternal effects by rearing females from all replicated populations on either tomato or cucumber leaves, crossing this factor with the host plant in a factorial design. About 24 generations after the host shift and for all individual mites, we measured the following fitness components on tomato leaf fragments: survival at all stages, acceptance of the host plant by juvenile and adult mites, longevity, and female fecundity. The host plant on which mite populations had evolved did not affect the performance of the mites, but only affected their sex ratio. Females that lived on tomato plants for circa 24 generations produced a higher proportion of daughters than did females that lived on cucumber plants. In contrast, maternal effects influenced juvenile survival, acceptance of the host plant by adult mites and female fecundity. Independently of the host plant species on which their population had evolved, females reared on the tomato maternal environment produced offspring that survived better on tomato as juveniles, but accepted less this host plant as adults and had a lower fecundity than did females reared on the cucumber maternal environment. We also found that temporal blocks affected mite dispersal and both female longevity and fecundity. Taken together, our results show that the host plant species can affect critical parameters of population dynamics, and most importantly that maternal and environmental conditions can facilitate colonization and exploitation of a novel host in the polyphagous T. urticae, by affecting dispersal behavior (host acceptance) and female fecundity.     

Effect of host availability on reproduction and survival of the parasitoid wasp Trichogramma minutum

Abstract.

• 1 We tested the hypothesis that females of the egg parasitoid, Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae), could adjust their fecundity schedule according to host availability and that there was a negative correlation between reproduction and survival in these wasps.
• 2 Newly-emerged females were provided with an unlimited or limited number of hosts in the first trial and with either unlimited, limited or zero hosts in the second trial.
• 3 When hosts were unlimited, wasps had the highest rate of reproduction in the first day, which decreased dramatically thereafter. When hosts were limited, wasps from the two trials differed in their response. In Trial I, females with limited hosts had lower first-day fecundity than, and the same subsequent-day fecundity as, those with unlimited hosts. However, in Trial II, females with limited host had a lower first-day but a higher subsequent-day fecundity than those with unlimited hosts. This indicates variation in Trichogramma's ability to shift its fecundity schedule in response to host availability.
• 4 There was a positive (rather than a negative) correlation between reproduction and survival. Wasps that oviposited (in host-unlimited treatment) had greater longevity than those that could not (in host-unavailable treatment).
• 5 The sex ratio of the progeny produced by wasps in both host-unlimited and limited treatments shifted gradually from a female to a male bias as the wasps aged.
• 6 We consider the ability of parasitoids to adjust their fecundity schedule as an adaptation to changing host resources and discuss our findings with regard to theories of life history evolution.

     

Evolution of larval competitiveness and associated life‐history traits in response to host shifts in a seed beetle

Resource competition is frequently strong among parasites that feed within small discrete resource patches, such as seeds or fruits. The properties of a host can influence the behavioural, morphological and life‐history traits of associated parasites, including traits that mediate competition within the host. For seed parasites, host size may be an especially important determinant of competitive ability. Using the seed beetle, Callosobruchus maculatus, we performed replicated, reciprocal host shifts to examine the role of seed size in determining larval competitiveness and associated traits. Populations ancestrally associated with either a small host (mung bean) or a large one (cowpea) were switched to each other's host for 36 generations. Compared to control lines (those remaining on the ancestral host), lines switched from the small host to the large host evolved greater tolerance of co‐occurring larvae within seeds (indicated by an increase in the frequency of small seeds yielding two adults), smaller egg size and higher fecundity. Each change occurred in the direction predicted by the traits of populations already adapted to cowpea. However, we did not observe the expected decline in adult mass following the shift to the larger host. Moreover, lines switched from the large host (cowpea) to the small host (mung bean) did not evolve the predicted increase in larval competitiveness or egg size, but did exhibit the predicted increase in body mass. Our results thus provide mixed support for the hypothesis that host size determines the evolution of competition‐related traits of seed beetles. Evolutionary responses to the two host shifts were consistent among replicate lines, but the evolution of larval competition was asymmetric, with larval competitiveness evolving as predicted in one direction of host shift, but not the reverse. Nevertheless, our results indicate that switching hosts is sufficient to produce repeatable and rapid changes in the competition strategy and fitness‐related traits of insect populations.     

Multiple host use by the egg parasitoid Trissolcus basalis (Wollaston) in a soybean agricultural system: biological control and environmental implications

1 Species of bugs (Hemiptera: Pentatomidae and other families) that are parasitized by the egg parasitoid Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) in the soybean agricultural system on the Darling Downs in south-eastern Queensland, Australia, are reported. The degree to which eggs of each bug species are used by T. basalis is quantified, which allows assessment of the role of these alternative host species in the biological control of the green vegetable bug Nezara viridula (L.). 2 Egg masses of nine species of pentatomid bugs and one unidentified bug species were collected. Parasitism rates of egg masses of all species averaged 50–70% but were significantly lower for the more important pest species, including the green vegetable bug, than for some of the agriculturally less important species. 3 Trissolcus basalis emerged from nearly all species collected and was the major parasitoid to emerge from most species. A number of native species were parasitized heavily by T. basalis and parasitism of such species may enhance biological control of the green vegetable bug, but may also pose environmental concerns. Parasitism of predatory bugs by T. basalis may have a negative impact on the biological control of other pests, especially lepidopterous pests. 4 Although the impact of T. basalis on native and predatory host species was not directly quantified, these host species remain abundant and therefore do not appear to be affected adversely by such high rates of parasitism.     

Effects of Pieris host species on life history parameters in a solitary specialist and gregarious generalist parasitoid (Cotesia species)

Host specificity and host selection by insect parasitoids are hypothesized to be correlated with suitability of the hosts for parasitoid development. The present study investigates the correlation between host suitability and earlier studied host-finding behaviour of two closely related braconid larval parasitoid species, the generalist Cotesia glomerata (L.) and the specialist C. rubecula (Marshall) (Hymenoptera: Braconidae). We compared the capability of both parasitoid species to parasitize and develop in three Pieris host species, i.e. P. brassicae (L.), P. rapae (L.) and P. napi (L.) (Lepidoptera: Pieridae). In laboratory experiments, we measured the effect of host species on fitness parameters such as survival, development, sex ratio and size of parasitoid progeny. The results show that C. glomerata is capable of developing in the three host species, with significant differences in parasitoid survival, clutch size and adult weight among Pieris species. The host range for development was more restricted for C. rubecula. Although C. rubecula is physiologically able to develop in P. brassicae larvae, parasitoid fitness is negatively affected by this host species, compared to its most regular host, P. rapae. A comparison of the present data on host suitability with earlier studies on host-searching behaviour suggests that the host-foraging behaviour of both parasitoid species not only leads to selection of the most suitable host species for parasitoid development, but also plays a significant role in shaping parasitoid host range.     

Intraspecific divergence and evolution of a life‐history trade‐off along a successional gradient in Hawaii's Metrosideros polymorpha

The importance of environmental gradients in the diversification of long‐lived tree species is poorly understood. Two morphologically distinct varieties of the endemic Hawaiian tree, 'ōhi'a lehua (Metrosideros polymorpha), are the canopy dominants at alternate extremes of a successional gradient formed by the recurring disturbance of lava flows on east Hawai'i Island. The maintenance of these varieties despite hybridization may be due to disruptive selection at either end of the successional gradient. To test this hypothesis, seeds from three, replicate monotypic stands of each variety on east Hawai'i Island were germinated and the resulting seedlings grown under four combinations of light and nitrogen levels in a greenhouse, and at early‐ and late‐successional field sites. Growth and survivorship measures revealed differential fitness of these varieties in high‐ and low‐light environments in the greenhouse with corresponding differential fitness in early‐ and late‐successional field sites. Unique light‐by‐nitrogen interaction effects on growth were observed in each variety, and only the late‐successional variety appeared to be nitrogen limited. These two varieties exhibit the classic plant life‐history trade‐off between fast growth in high light and high survivorship in shade, but notably within a single tree species. These findings strongly implicate a role for Hawaii's striking environmental heterogeneity in the emergence of at least two endemic forms of this woody genus.     

Selection on life‐history traits and genetic population divergence in rotifers

A combination of founder effects and local adaptation – the Monopolization hypothesis – has been proposed to reconcile the strong population differentiation of zooplankton dwelling in ponds and lakes and their high dispersal abilities. The role genetic drift plays in genetic differentiation of zooplankton is well documented, but the impact of natural selection has received less attention. Here, we compare differentiation in neutral genetic markers (F_ST) and in quantitative traits (Q_ST) in six natural populations of the rotifer Brachionus plicatilis to assess the importance of natural selection in explaining genetic differentiation of life‐history traits. Five life‐history traits were measured in four temperature × salinity combinations in common‐garden experiments. Population differentiation for neutral genetic markers – 11 microsatellite loci – was very high (F_ST = 0.482). Differentiation in life‐history traits was higher in traits related to sexual reproduction than in those related to asexual reproduction. Q_ST values for diapausing egg production (a trait related to sexual reproduction) were higher than their corresponding F_ST in some pairs of populations. Our results indicate the importance of divergent natural selection in these populations and suggest local adaptation to the unpredictability of B. plicatilis habitats.     

Covariance modulates the effect of joint temperature and food variance on ectotherm life‐history traits

Understanding animal performance in heterogeneous or variable environments is a central question in ecology. We combine modelling and experiments to test how temperature and food availability variance jointly affect life‐history traits of ectotherms. The model predicts that as mean temperatures move away from the ectotherm's thermal optimum, the effect size of joint thermal and food variance should become increasingly sensitive to their covariance. Below the thermal optimum, performance should be positively correlated with food–temperature covariance and the opposite is predicted above it. At lower temperatures, covariance should determine whether food and temperature variance increases or decreases performance compared to constant conditions. Somewhat stronger than predicted, the covariance effect below the thermal optimum was confirmed experimentally on an aquatic ectotherm (Daphnia magna) exposed to diurnal food and temperature variance with different amounts of covariance. Our findings have important implications for understanding ectotherm responses to climate‐driven alterations of thermal mean and variance.     

Effects of host larval stage preferences and diet on life history traits of Diadegma mollipla,an African parasitoid of the Diamondback Moth

Knowledge of the biological attributes of parasitoids plays a fundamental role in improved management of their pest hosts. Diadegma mollipla is a major indigenous parasitoid of the Diamondback Moth (DBM), Plutella xylostella (Linnaeus), in Eastern Africa. Two laboratory experiments were conducted to determine the DBM larval stage preferences of D. mollipla and their influence on life history traits and the effect of different diets (20% honey, 20% sugar solution, water or nothing) on adult parasitoid longevity. In the first study, 60 DBM larvae (15 each of larval stages L1–L4) were exposed to either one adult parasitoid female or four competing females in choice experiments. When DBM larvae were exposed to one parasitoid, in terms of parasitism, L1 and L2 were found to be the most preferred stages, followed by L3 and then L4. Sex ratio (females/males) increased with host larval stage from 1.3 to 3.1. Pre-imaginal development time was longer in L1 than in the other three stages. When exposed to four parasitoids, no significant differences in parasitism rates and progeny production were obtained between DBM larval stages. However, sex ratio increased from 0.8 in L1 to 4.1 in L4. Developmental time was longest in L4, intermediate in L1 and L3, and shortest in L2. In the diet experiment, honey and sugar were found to significantly increase longevity for D. mollipla adults of both sexes. Overall, combined adult male and female longevity was 24.5, 9.9, 2.2 and 1.9 days when fed on honey, sugar, water or nothing, respectively.