The effect of travel time on oviposition behavior and spatial egg aggregation: experiments with Drosophila

A higher degree of spatial egg aggregation is often observed in environments where resource patches are more sparsely distributed. This suggests a higher probability of species coexistence when resource distribution is sparse. However, it is still unclear how the degree of spatial egg aggregation increases. I propose a model to explain this phenomenon, which assumes that (i) egg load (the number of mature eggs in ovaries) increases in the travel period between resource patches and (ii) the retention of eggs in the ovaries is harmful (egg load pressure). With these assumptions, a female would lay accumulated eggs on arrival at a new resource patch, resulting in a higher degree of spatial egg aggregation. Laboratory experiments with three drosophilid species, Drosophila simulans Surtevant, Drosophila auraria Peng, and Drosophila immigrans Sturtevant, support the model. This study provides evidence that host availability affects the spatial egg aggregation via egg load.     

Sex ratios when helpers stay at the nest

This study investigates the evolution of the sex ratio (parental investment in sons) when breeding adults are supported by help provided by nonbreeding individuals of one sex. The study also assumes that the helping sex remains on its natal site to compete for the opportunity to breed, whereas the nonhelping sex disperses. Two kin-selection models are presented, both of which incorporate the age structure found in many natural populations where such helping occurs. The first model assumes that helpers increase the survival of their parents. The second model assumes that helpers are indiscriminant: a helper chooses to increase the survival of a random pair of adults breeding on its natal patch. In both models, sex ratios are not always biased toward the sex that provides the most help. When helpers do not discriminate (second model), the direction of sex-ratio bias is determined solely by the size of the benefit of helping behavior. When this benefit is small, sex-ratio evolution is primarily influenced by local resource competition and sex ratios are biased toward the nonhelping (dispersive) sex. If the benefit of help is large enough, the effect of local resource competition is reduced and sex-ratio bias favors the helpful sex. When helpers help only their parents, the same qualitative relationship exists between the direction of sex-ratio bias and the benefit of helping. In this case, however, the direction of sex-ratio bias is also influenced by the size of the social group, mortality, and which individual (mother or father) controls the sex ratio. This study also investigates a sex-ratio conflict that exists between mates. Helping behavior of nonbreeders can act to alleviate the disparities between the optimal sex ratio from the perspective of a mother and that from the perspective of a father. This consequence of helping has not been previously recognized.     

Shifting preference between oviposition vs. host-feeding under changing host densities in two aphelinid parasitoids

Destructive host-feeding is common in hymenopteran parasitoids. Such feeding may be restricted to host stages not preferred for oviposition. However, whether this is a fixed strategy or can vary according to resource levels or parasitoid needs is less clear. We tested the trade-off between host feeding and oviposition on two whitefly parasitoids under varying host densities. Females of two aphelinid parasitoids, Eretmocerus hayati and Encarsia sophia were exposed to nine different densities of their whitefly host, Bemisia tabaci, in single-instar tests to identify their functional response. Mixed-instar host choice tests were also conducted by exposing whiteflies at four densities to the parasitoids. We hypothesized that the parasitoid females can detect different host densities, and decide on oviposition vs. host-feeding accordingly. The results showed that both Er. hayati and En. sophia females tended to increase both oviposition and host-feeding with increased host density within a certain range. Oviposition reached a plateau at lower host density than host-feeding in Er. hayati, while En. sophia reached its oviposition plateau at higher densities. At low densities, Er. hayati parasitized most on first and second (the optimal ones), and fed most on third nymphal instars (the suboptimal one) of the whitefly host as theory predicts, while at high densities, both parasitism and host-feeding occurred on first and second instars which are preferred for oviposition. En. sophia parasitized most on third and fourth (the optimal ones), while fed on first instars (the suboptimal one) at low densities, and utilized third and fourth instars for both at high densities. In conclusion, oviposition vs. host-feeding strategy of parasitoid females was found to vary at different host densities. The balance between reserving optimal hosts for oviposition or using them for host-feeding depended on parasitoid life history and the availability of host resources.     

COMPARISONS OF OBSERVED PHYLOGENETIC TOPOLOGIES WITH NULL EXPECTATIONS AMONG THREE MONOPHYLETIC LINEAGES

Three null models have been proposed to predict the relative frequencies of topologies of phylogenetic trees. One null model assumes each distinguishable n-member tree is equally likely (proportional-to-distinguishable-arrangements model). A second model assumes that each topological type is equally likely (equiprobable model). A third model assumes that the probability of each topological type is determined by random speciation (Markov model). We sampled published phylogenetic trees from three major groups of organisms: division Angiospermae, class Insecta, and superclass Tetrapoda. Our sampling was more restricted than previous studies and was designed to test whether observed topological frequencies were distinguishable from those predicted by the three null models. The pattern of evolution reflected in five-member phylogenetic trees is different from predictions of the equiprobable and Markov model but is indistinguishable from the proportional-to-distinguishable-arrangements model. This indicates that 1) speciation (and/or extinction) is not equally likely among all taxa, even for small phylogenies; or 2) systematists' attempts at reconstructing small phylogenies are, on average, indistinguishable from those expected if they had merely selected a tree at random from the pool of all possible trees. The topology frequencies were not different among the three groups of organisms, suggesting that factors shaping patterns of speciation and extinction are consistent among major taxonomic groups.     

Risk-Sensitive Foraging, Fitness, and Life Histories: Where Does Reproduction Fit into the Big Picture?

Risk-sensitive foraging may occur whenever feeding success hasnon-linear effects on fitness. Models of sensitivity to variationin amount of food obtained have concentrated on foraging inorder to survive even though feeding during growth, migration,or reproduction can have strong and non-linear effects on fitness.I illustrate how risk-sensitive foraging for reproduction coulddiffer from risk-sensitive foraging for survival using two simplemodels. Each model assumes that organisms must accumulate somethreshold amount of resources before they can reproduce. Inthe first model, additional resources above the threshold leadto increased reproductive success. Here variance in feedingsuccess can be advantageous even when the mean gain would alloworganisms to reproduce. In the second model, early breedingis superior to late breeding because recruitment rate declinesover time. Here a symmetrical distribution in foraging ratesresults in a skewed distribution of breeding times. Despitethis, variance in feeding success may be advantageous even whenthe seasonal decline in recruitment is linear. The advantageof risk is much larger if foragers can switch freely betweenfeeding options. These two models are united by "the bankruptcyeffect" because all failures to reproduce carry the same consequences.These simple models suggest that risk-prone foraging may belikely when factors other than survival are important for fitness.Finally, I advocate using a mix of modelling techniques, notethe implicitly risk-sensitive nature of stochastic dynamic programming,and speculate on potential connections between risk-sensitiveforaging and theories of life histories in variable environments.INTRODUCTION visit restaurants that provided twice     

Costs and benefits of host feeding in the parasitoid wasp Trichogramma turkestanica

Females of the parasitoid wasp Trichogramma turkestanica Meyer (Hymenoptera: Trichogrammatidae) generally host feed after ovipositing on the first egg of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) they encounter. We measured the impact of host feeding on the fecundity and longevity of females, in absence of host or food, and on the fitness of their progeny. We also determined if the frequency of host feeding is influenced by the humidity level at which T. turkestanica females developed. Host feeding increased egg production by 70% but decreased female longevity. This impact of host feeding on the longevity of females is probably due to the allocation of carbohydrates to egg production at the expense of somatic maintenance. Humidity did not influence the occurrence or duration of host feeding. The size of individuals developing in eggs on which females host fed was smaller, indicating that their fitness was affected.     

Reproductive senescence and dynamic oviposition behaviour in insects

We explore the relationship between clutch size behaviour and the likelihood of egg limitation in insects with parasitoid-like life histories. We compare the incidence of egg limitation in insects that produce clutch sizes consistent with maximizing fitness per host (i.e. the single host maximum, or Lack clutch size) with insects exhibiting two types of dynamic behaviour. In the first case, insects produce clutch sizes according to a rule that specifies a negative relationship between egg complement and clutch size; in the second case, clutch size is determined by both egg complement and the host encounter rate using a dynamic state-variable model. The analyses are an extension of models presented by Rosenheim (1996), which predicted a strong positive relationship between the risk of egg limitation and the host encounter rate in the absence of dynamic oviposition behaviour. Our results are consistent with Rosenheim's, but they show that dynamic behaviour can greatly reduce th e risk of egg limitation and weaken the relationship between the host encounter rate and egg limitation.     

Reproduction now or later: optimal host-handling strategies in the whitefly parasitoid Encarsia formosa

We developed a dynamic state variable model for studying optimal host‐handling strategies in the whitefly parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae). We assumed that (a) the function of host feeding is to gain nutrients that can be matured into eggs, (b) oögenesis is continuous and egg load dependent, (c) parasitoid survival is exponentially distributed and (d) parasitoids encounter hosts randomly, are autogenous and have unlimited access to non‐host food sources to obtain energy for maintenance and activity. The most important prediction of the model is that host feeding is maladaptive under field conditions of low host density (0.015 cm^?2) and short parasitoid life expectancy (maximum reproductive period of 7 d). Nutrients from the immature stage that can be matured into eggs are sufficient to prevent egg limitation. Both host density and parasitoid life expectancy have a positive effect on the optimal host‐feeding ratio. Parasitoids that make random decisions gain on average only 35% (0.015 hosts cm^?2) to 60% (1.5 hosts cm^?2) of the lifetime reproductive success of parasitoids that make optimal decisions, independent of their life expectancy. Parameters that have a large impact on lifetime reproductive success and therefore drive natural selection are parasitoid life expectancy and the survival probability of deposited eggs (independent of host density), the number of host encounters per day (when host density is low) and the egg maturation rate and number of host types (when host density is high). Explaining the evolution of host‐feeding behaviour under field conditions requires field data showing that life expectancy in the field is not as short as we assumed, or may require incorporation of variation in host density. Incorporating variation in walking speed, parasitised host types or egg resorption is not expected to provide an explanation for the evolution of host‐feeding behaviour under field conditions.     

Importance of host feeding for parasitoids that attack honeydew-producing hosts

Insect parasitoids lay their eggs in arthropods. Some parasitoid species not only use their arthropod host for oviposition but also for feeding. Host feeding provides nutrients to the adult female parasitoid. However, in many species, host feeding destroys an opportunity to oviposit. For parasitoids that attack Homoptera, honeydew is a nutrient‐rich alternative that can be directly imbibed from the host anus without injuring the host. A recent study showed that feeding on host‐derived honeydew can be an advantageous alternative in terms of egg quantity and longevity. Here we explore the conditions under which destructive host feeding can provide an advantage over feeding on honeydew. For 5 days, Encarsia formosa Gahan (Hymenoptera: Aphelinidae) parasitoids were allowed daily up to 3 h to oviposit until host feeding was attempted. Host feedings were either prevented or allowed and parasitoids had ad libitum access to honeydew between foraging bouts. Even in the presence of honeydew, parasitoids allowed to host feed laid more eggs per hour of foraging per host‐feeding attempt than parasitoids that were prevented from host feeding. The higher egg‐laying rate was not compromised by survival or by change in egg volume over time. In conclusion, host feeding can provide an advantage over feeding on honeydew. This applies most likely under conditions of high host density or low extrinsic mortality of adult parasitoids, when alternative food sources cannot supply enough nutrients to prevent egg limitation. We discuss how to integrate ecological and physiological studies on host‐feeding behavior     

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.

     

Effect of host feeding on life history traits of <Emphasis Type="Italic">Tamarixia radiata</Emphasis>, parasitoid of the Asian citrus psyllid, <Emphasis Type="Italic">Diaphorina citri</Emphasis>

The parasitoid Tamarixia radiata (Hymenoptera: Eulophidae) is being used for the biological control of the Asian citrus psyllid (ACP), Diaphorina citri (Hemiptera: Liviidae). The parasitoid is strongly synovigenic, as it is born with very few mature eggs. Synovigenic insects need to feed on host haemolymph to mature additional eggs, and are able to resorb mature eggs to allocate resources toward maintenance. We investigated the effect of host feeding on parasitism, longevity and egg load dynamics, and estimated egg maturation and resorption rates. Although host feeding does not increase survival or longevity, it results in increased parasitization rates when parasitoids are seven days old, and that a single host meal leads to an average gain of three eggs. We discuss the importance of these data to predict the foraging and parasitization behavior of T. radiata in the field, and to potentially improve current efforts to control ACP.     

Estimating resource preferences of a native bumblebee: the effects of availability and use–availability models on preference estimates

Identifying resource preference is considered essential for developing targeted conservation plans but, for many species, questions remain about the best way to estimate preference. Resource preferences for bees are particularly difficult to determine as the resources they collect, nectar and pollen, are challenging to estimate availability and collection. Resources are traditionally measured at the flower or inflorescence level, but these measures of availability do not correspond to the resources actually used by bees. Additionally, it is unclear as to whether common models including availability are appropriate for bees which may target resources regardless of available quantities. Here we first compare two common hypotheses of resource use – the ‘random use hypothesis’ and the ‘linear preferences hypothesis’ – using three different measures of availability (pollen, flower and inflorescence) – to determine if one measure of availability was better for understanding bee pollen use. Next, the superior model using availability was compared to a novel model of bee pollen use the ‘target use hypothesis’. This model assumes that bees target some resources regardless of how much of each resource is available (but assuming resources are present at a site), and thus models preference without availability data. Of the models including availability, the linear preference model using inflorescence availability best explained the pollen use data. This suggests that bumblebee pollen use is non‐random and that cues to identify and locate resources (i.e. display size and quantity) may be more important than the quantity of the resource available (i.e. pollen availability). Additionally, in most cases the target use model explained the data equal to or better than the other models suggesting bee resource use may be better modeled without measured availability data compared to linear models. These results could be important for expanding resource use analysis of bees that are difficult to quantify availability.     

Diadegma mollipla parasitizing Plutella xylostella: host instar preference and suitability

Oviposition decisions (i.e., host selection and sex allocation) of female parasitoids are expected to correspond with host quality, as their offspring fitness is dependent on the amount and quality of resources provided by a single host. The host size model assumes that host quality is a linear function of host size, with larger hosts believed to contain a greater quantity of resources, and thus be more profitable than smaller hosts. We tested this assertion in the laboratory on a solitary larval–pupal parasitoid Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae) developing on three instars (second–fourth) of one of its hosts, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). In a no‐choice test, parasitism levels and sex ratio (i.e., proportion of female progeny) were significantly high in hosts attacked in the second instar followed by third then fourth instars. However, the few parasitoids that completed a generation from the fourth instars did so significantly faster than conspecifics that started development in the other two instars. In direct observations, however, the parasitoids (i) randomly attacked the various host instars, (ii) spent a similar period examining the various host instars with their ovipositors, (iii) subdued all three host instars with about the same effort, and (iv) no statistical differences were observed in the attack rates on the three host instars. In a choice test, the females parasitized significantly more third instars followed by second then fourth instars. However, total parasitism in this experiment was 43% lower compared to parasitism of only second instars in the no‐choice test. No significant differences were detected in progeny sex ratios. In both choice and no‐choice tests, significantly more fourth instars died during the course of the experiments than second instars, while third instars were intermediate. The higher parasitism of third than second instars in the choice test indicates that the females perceived larger hosts as higher quality than smaller hosts, despite their lower suitability for larval development.     

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.     

Bottom-up derivation of population models for competition involving multiple resources

This paper provides first-principles derivations of population models for competition involving multiple resources with different competition types, based on resource partitioning between individuals. The following two cases are investigated. The first is the case in which the resource competed for and its competition type change depending on life stages from scramble to contest competition, or from contest to scramble competition. The second is the case in which individuals compete for two resources simultaneously with scramble and contest types, respectively. In both cases, population models are derived analytically, and in particular, the Hassell model is derived in the second case. The nature of reproduction curves and the stability properties of three population models derived are compared with each other. These models provide three representative models for competition involving both scramble and contest types.     

Dose-dependent schistosome-induced mortality and morbidity risk elevates host reproductive effort

Parasitism changes the host environment and may influence resource allocation between reproductive effort and somatic maintenance. We characterized the impact of dose-dependent schistosome exposure and/or infection establishment on intermediate host survival and reproduction. Four matched groups of Biomphalaria glabrata snails were individually exposed to increasing doses of Schistosoma mansoni parasites, with a fifth control group remaining unexposed. Increased mortality was observed amongst both snails infected and also those snails exposed to the parasite but within which infection did not establish, although only exposed but uninfected snails showed a dose-dependent increase in mortality. Snails also facultatively altered their reproductive output in response to parasite exposure: egg mass production decreased with increasing parasite dose in patently infected snails, whilst, in contrast, exposed but uninfected snails demonstrated a positive association between egg mass production and parasite dose in the post-patent period. These results uniquely suggest an exposure-dose-dependent post-patent fecundity compensation occurring in relation to the risk of future parasite-associated mortality.     

Growth models of cultures with two lipid phases. I. Substrate dissolved in dispersed phase

Mathematical models which can be used to describe batch growth in fermentations with two liquid phases are developed for systems in which the growth limiting substrate is dissolved the dispersed liquid phase. In the models, the possibilities of growth occurring at the surface of the dispersed phase and in the continuous phase are considered. It is assumed that the composition of the dispersed phase is such that substrate utilization from it causes little of no change, in interfacial area. Three special cases are examined. In the first, it is assumed that all growth occurs at the surface of the dispersed phase. In the second and third, it is assumed that growth occurs both at the interface and in the continuous phase. The second case assumes that substrate equilibrium is continuously established between the two phases while the third assumes substrate consumption in the continuous phase is limited by rate of transport of substrate to that phase. Comparison of the first model with available experimental data shows good agreement between model and data.     

Linking reproductive and feeding strategies in the parasitoid Ibalia leucospoides: does feeding always imply profit?

Resource acquisition and allocation to different biological functions over the course of life have strong implications for animal reproductive success. Animals can experience different environmental conditions during their lifetime, and this may play an important role in shaping their life-history and resource allocation strategies. In this study we investigate larval and adult resource allocation to reproductive and survival functions in the parasitoid wasp Ibalia leucospoides (family Ibaliidae). The pattern of larval resource allocation was inferred from the relationship between adult body size and ovigeny index (OI; a relative measure of investment in early reproduction determined as the ratio between the initial egg load and the potential lifetime fecundity); and adult resource allocation was explored through the influence of adult feeding on reproduction, maintenance and metabolism, in laboratory experiments. Food acquisition by this parasitoid in the wild was also examined. The relationship between size and OI was constant, suggesting no differential resource allocation to initial egg load and potential lifetime fecundity with size. This finding is in line with that predicted by adaptive models for the proovigenic egg maturation strategy (OI = 1). Despite of this, I. leucospoides showed a high OI of 0.77, which places this species among the weakly synovigenic ones (OI < 1). Adult feeding had no effect on post-emergence egg maturation. However, wasps extended their lifespan through feeding albeit only when food was provided ad libitum. Although the information we obtained on the feeding behaviour of free-foraging wasps is limited, our results suggest that food intake in the wild, while possible, may not be frequent in this parasitoid. We discuss the results relative to the environmental factors, such as reproductive opportunities and food availability, which may have driven the evolution of larval and adult pattern of resource allocation in parasitoids.     

Mallard resource selection trade‐offs in a heterogeneous environment during autumn and winter

Animals select resources to maximize fitness but associated costs and benefits are spatially and temporally variable. Differences in wetland management influence resource availability for ducks and mortality risk from duck hunting. The local distribution of the Mallard (Anas platyrhynchos) is affected by this resource heterogeneity and variable risk from hunting. Regional conservation strategies primarily focus on how waterfowl distributions are affected by food resources during the nonbreeding season. To test if Mallard resource selection was related to the abundance of resources, risks, or a combination, we studied resource selection of adult female Mallards during autumn and winter. We developed a digital spatial layer for Lake St. Clair, Ontario, Canada, that classified resources important to Mallards and assigned these resources a risk level based on ownership type and presumed disturbance from hunting. We monitored 59 individuals with GPS back‐pack transmitters prior to, during, and after the hunting season and used discrete choice modeling to generate diurnal and nocturnal resource selection estimates. The model that classified available resources and presumed risk best explained Mallard resource selection strategies. Resource selection varied within and among seasons. Ducks selected for federal, state and private managed wetland complexes that provided an intermediate or relatively greater amount of refuge and foraging options than public hunting areas. Across all diel periods and seasons, there was selection for federally managed marshes and private supplemental feeding refuges that prohibited hunting. Mallard resource selection demonstrated trade‐offs related to the management of mortality risk, anthropogenic disturbances, and foraging opportunities. Understanding how waterfowl respond to heterogeneous landscapes of resources and risks can inform regional conservation strategies related to waterfowl distribution during the nonbreeding season.