Context-dependent interactive effects of non-lethal predation on larvae impact adult longevity and body composition

Sex and stress modulate pupal defense response in tobacco hornworm

In insects, larval and adult defenses against predators have been well studied. However, pupal (also known as resting stage) defenses have been overlooked and not examined thoroughly. Although some pupa possess antipredator strategies such as hairs, spines, cryptic coloration, and exudation of chemicals, few studies have tested these responses and the factors affecting them. Here, we investigated the behavioral responses in tobacco hornworm Manduca sexta that pupates in soil by introducing an external stimulus using vibrations from an electric toothbrush to mimic predation. We observed that M. sexta made violent wriggling (twitching), followed by pulsating movements in response to the vibrational stimulus. Detailed examination showed that these twitches and pulsating events occurred more frequently and for longer periods of time in male pupa and were dependent on the magnitude of the stress (high and low frequency). However, when we estimated the angular force exerted by pupa using radian and angular momentum of twitches, it was found to be independent of pupal sex. A follow-up experiment on possible cascading effects of stress exposure on eclosion success revealed that low- and high-frequency stress exposure didn't cause any of the common defects in eclosed adults. Our study clearly demonstrates that the so-called defenseless pupal stage uses a wide range of measurable defense behaviors that can actively defend against predators and should be examined further-linking observed behavior with underlying mechanisms.

Predation risk effects on larval development and adult life of Aedes aegypti mosquito

Biological control is one of the methods available for control of Aedes aegypti populations. We used experimental microcosms to evaluate the effects of actual predation and predation risk by dragonfly larvae (Odonata) on larval development, adult longevity, and adult size of Ae. aegypti. We used six treatments: control, removal, variable density cues (Cues VD), fixed density cues (Cues FD), variable density predator (Predator VD), and fixed density predator (Predator FD) (n = 5 each). Predator treatments received one dragonfly larva. Cue treatments were composed of crushed Ae. aegypti larvae released into the microcosm. For the FD treatments, we maintained a larval density of 200 individuals. The average mortality of Ae. aegypti larvae in the Predator VD treatment was used as the standard mortality for the other treatments. Mosquitoes from the Predator VD and Cues VD treatments developed faster, and adults were larger and had greater longevity compared to all other treatments, likely due to the higher food availability from larval density reduction. High larval density negatively affected larval developmental time, adult size, and longevity. Males were less sensitive to density-dependent effects. Results from this study suggest that the presence of predators may lead to the emergence of adult mosquitoes with greater fitness, causing an overall positive effect on Ae. aegypti population growth rates.

The size of larval rearing container modulates the effects of diet amount and larval density on larval development in Aedes aegypti

Mass-rearing of mosquitoes under laboratory conditions is an important part of several new control techniques that rely on the release of males to control mosquito populations. While previous work has investigated the effect of larval density and diet amount on colony productivity, the role of the size of the container in which larval development takes place has been relatively ignored. We investigated the role of container size in shaping life history and how this varied with density and food availability in Aedes aegypti, an important disease vector and target of mass-rearing operations. For each treatment combination, immature development time and survival and adult body size and fecundity were measured, and then combined into a measure of productivity. We additionally investigated how larval aggregation behaviour varied with container size. Container size had important effects on life history traits and overall productivity. In particular, increasing container size intensified density and diet effects on immature development time. Productivity was also impacted by container size when larvae were reared at high densities (1.4 larva/ml). In these treatments, the productivity metric of large containers was estimated to be significantly lower than medium or small containers. Regardless of container size, larvae were more likely to be observed at the outer edges of containers, even when this led to extremely high localized densities. We discuss how container size and larval aggregation responses may alter the balance of energy input and output to shape development and productivity.

Changes in Container-Breeding Mosquito Diversity and Abundance Along an Urbanization Gradient are Associated With Dominance of Arboviral Vectors

Environmental conditions associated with urbanization are likely to influence the composition and abundance of mosquito (Diptera, Culicidae) assemblages through effects on juvenile stages, with important consequences for human disease risk. We present six years (2011-2016) of weekly juvenile mosquito data from distributed standardized ovitraps and evaluate how variation in impervious cover and temperature affect the composition and abundance of container-breeding mosquito species in Maryland, USA. Species richness and evenness were lowest at sites with high impervious cover (>60% in 100-m buffer). However, peak diversity was recorded at sites with intermediate impervious cover (28-35%). Four species were observed at all sites, including two recent invasives (Aedes albopictus Skuse, Ae. japonicus Theobald), an established resident (Culex pipiens L), and one native (Cx. restuans Theobald). All four are viral vectors in zoonotic or human transmission cycles. Temperature was a positive predictor of weekly larval abundance during the growing season for each species, as well as a positive predictor of rapid pupal development. Despite being observed at all sites, each species responded differently to impervious cover. Abundance of Ae. albopictus larvae was positively associated with impervious cover, emphasizing that this medically-important vector not only persists in the warmer, impervious urban landscape but is positively associated with it. Positive temperature effects in our models of larval abundance and pupae occurrence in container habitats suggest that these four vector species are likely to continue to be present and abundant in temperate cities under future temperature scenarios.

Modeling the effects of Aedes aegypti's larval environment on adult body mass at emergence

Mosquitoes vector harmful pathogens that infect millions of people every year, and developing approaches to effectively control mosquitoes is a topic of great interest. However, the success of many control measures is highly dependent upon ecological, physiological, and life history traits of mosquito species. The behavior of mosquitoes and their potential to vector pathogens can also be impacted by these traits. One trait of interest is mosquito body mass, which depends upon many factors associated with the environment in which juvenile mosquitoes develop. Our experiments examined the impact of larval density on the body mass of Aedes aegypti mosquitoes, which are important vectors of dengue, Zika, yellow fever, and other pathogens. To investigate the interactions between the larval environment and mosquito body mass, we built a discrete time mathematical model that incorporates body mass, larval density, and food availability and fit the model to our experimental data. We considered three categories of model complexity informed by data, and selected the best model within each category using Akaike’s Information Criterion. We found that the larval environment is an important determinant of the body mass of mosquitoes upon emergence. Furthermore, we found that larval density has greater impact on body mass of adults at emergence than on development time, and that inclusion of density dependence in the survival of female aquatic stages in models is important. We discuss the implications of our results for the control of Aedes mosquitoes and on their potential to spread disease.

In this work we examined how the environment in which juvenile mosquitoes develop affects their adult body size as measured by adult body mass. Adult size has potential impacts on mosquito behavior and the ability of mosquitoes to transmit disease. We used a combination of experimental work and mathematical modeling to determine important factors affecting adult mosquito body size. In our model, we incorporated potentially interacting aspects of the mosquito life cycle and traits that affect mosquito growth as juveniles. These aspects include body mass, density of the population, and level of available resource. We compared different models to determine the one that best describes the data. As mass at emergence is linked to the success of adult mosquitoes to produce offspring and to their ability transmit pathogens, we discuss how important influences on development and survival of young mosquitoes affect mosquito control and disease spread.

Simulated climate change, but not predation risk, accelerates Aedes aegypti emergence in a microcosm experiment in western Amazonia

Climate change affects individual life-history characteristics and species interactions, including predator-prey interactions. While effects of warming on Aedes aegypti adults are well known, clarity the interactive effects of climate change (temperature and CO₂ concentration) and predation risk on the larval stage remains unexplored. In this study, we performed a microcosm experiment simulating temperature and CO₂ changes in Manaus, Amazonas, Brazil, for the year 2100. Simulated climate change scenarios (SCCS) were in accordance with the Fourth Assessment Report of Intergovernmental Panel on Climate Change (IPCC). Used SCCS were: Control (real-time current conditions in Manaus: average temperature is ~25.76°C ± 0.71°C and ~477.26 ± 9.38 parts per million by volume (ppmv) CO₂); Light: increase of ~1,7°C and ~218 ppmv CO₂; Intermediate: increase of ~2.4°C and ~446 ppmv CO₂; and Extreme: increase of ~4.5°C and ~861 ppmv CO2, all increases were relative to a Control SCCS. Light, Intermediate and Extreme SCCS reproduced, respectively, the B1, A1B, and A2 climatic scenarios predicted by IPCC (2007). We analyzed Aedes aegypti larval survivorship and adult emergence pattern with a factorial design combining predation risk (control and predator presence–Toxorhynchites haemorrhoidalis larvae) and SCCS. Neither SCCS nor predation risk affected Aedes aegypti larval survivorship, but adult emergence pattern was affected by SCCS. Accordingly, our results did not indicate interactive effects of SCCS and predation risk on larval survivorship and emergence pattern of Aedes aegypti reared in SCCS in western Amazonia. Aedes aegypti is resistant to SCCS conditions tested, mainly due to high larval survivorship, even under Extreme SCCS, and warmer scenarios increase adult Aedes aegypti emergence. Considering that Aedes aegypti is a health problem in western Amazonia, an implication of our findings is that the use of predation cues as biocontrol strategies will not provide a viable means of controlling the accelerated adult emergence expected under the IPCC climatic scenarios.

Antipredatory Responses of Mosquito Pupae to Non-Lethal Predation Threat-Behavioral Plasticity Across Life-History Stages

Antipredatory behavioral responses tend to be energetically expensive, and prey species thus need to resolve trade-offs between these behaviors and other activities such as foraging and mating. While these trade-offs have been well-studied across taxa, less is known about how costs and benefits vary in different life-history contexts, and associated consequences. To address this question, we compared responses of the yellow fever mosquito (Aedes aegypti [Diptera: Culicidae]) to predation threat from guppy (Poecilia reticulata [Cyprinodontiformes: Poeciliidae]) across two life-history stages-larvae (data from previous study) and pupae (from this study). Pupae are motile but do not feed and are comparable to larvae in terms of behavior. To understand how physiological costs affect the threat sensitivity of pupae, we used sex (with size as a covariate) as a proxy for stored energy reserves, and quantified movement and space use patterns of male (small-sized) and female (large-sized) pupae when exposed to predation threat. We found that pupae did not alter movement when exposed to predator cues but instead altered spatial use by spending more time at the bottom of the water column. We found no effect of pupa sex (or size) on the behavioral responses we measured. We conclude that pupa behavior, both antipredatory and otherwise, is primarily targeted at minimizing energy expenditure, as compared with larval behavior, which appears to balance energy expenditure between the opposing pressures of foraging and of avoiding predation. We suggest that antipredatory defenses in metamorphosing prey are modulated by varying energetic trade-offs associated with different life-history stages.

Linking Mosquito Ecology, Traits, Behavior, and Disease Transmission

Mosquitoes are considered to be the deadliest animals on Earth because the diseases they transmit claim at least a million human lives every year globally. Here, we discuss the scales at which the effects of ecological factors cascade to influence epidemiologically relevant behaviors of adult mosquitoes. In particular, we focused our review on the environmental conditions (coarse-scale variables) that shape the life-history traits of larvae and adult mosquitoes (fine-scale traits), and how these factors and their association, in turn, modulate adult behaviors to influence mosquito-borne disease transmission. Finally, we explore the integration of physical, physiological, and behavioral information into predictive models with epidemiological applications.

Determining Temperature-Dependent Development and Mortality Parameters of the Swede Midge (Diptera: Cecidomyiidae)

The swede midge (Contarinia nasturtii Kieffer) is an invasive insect in North America whose feeding has caused a decline of over 60% of total canola acreage in Ontario, Canada since 2011. Temperature-dependent development and mortality information are important to develop an effective pest management strategy for this insect; as the most comprehensive study on C. nasturtii development was completed on populations from the United Kingdom in the 1960s, new geographically relevant information is needed. Contarinia nasturtii eggs, larvae, pupae, and adults were reared from wild populations collected from Elora, Ontario, and allowed to develop at different temperatures. Resulting development rates were fit to a series of growth models and the model with the best relative goodness-of-fit was selected to represent the given life stage. Eggs from Ontario populations developed more quickly than their UK counterparts at temperatures below approximately 17°C, but more slowly at temperatures above 17°C. The same phenomenon was observed in larvae at 20°C. Pupae from both populations had similar development rates, and adult longevity was similar as well. This information will inform the management of C. nasturtii, and may help prevent its spread to other canola-producing regions of North America.

The demographic and life-history costs of fear: Trait-mediated effects of threat of predation on Aedes triseriatus

Predators alter prey populations via direct lethality (density-mediated effects), but in many taxa, the indirect nonlethal threat of predation may be almost as strong an effect, altering phenotypically plastic traits such as prey morphology, behavior, and life history (trait-mediated effects). There are costs to antipredator defenses and the strength of prey responses to cues of predation likely depends on both the perceived level of risk and food availability.The goal of this study was to test the hypothesis that the costs of nonlethal trait-mediated interactions impacting larvae can have carryover effects that alter life-history traits, adult characteristics, and ultimately population dynamics.The effects of Toxorhynchites rutilus kairomones and chemical alarm cues on Aedes triseriatus were assessed in a two-level factorial design manipulating nutrient level (low or high) and chemical cues of predation (present or absent).Nonlethal chemical cues of predation significantly decreased female survivorship and significantly decreased female size. Females emerged as adults significantly earlier when exposed to predation cues when there was high nutrient availability. When raised in the low nutrient treatment and exposed to predator cues, adult females had 2.1 times the hazard of death compared to high nutrient-no predator cues. Females raised in the high nutrient and predator cue treatment blood fed sooner than did females from other combinations.Fear of predation can substantially alter prey life-history traits and behavior, which can cascade into dramatic population, community, and ecosystem effects. Exposure to predator cues significantly decreased the estimated cohort rate of increase, potentially altering the expected population density of the next generation.

How Do Trait-Mediated Non-lethal Effects of Predation Affect Population-Level Performance of Mosquitoes?

Non-lethal, trait-mediated effects of predation impact prey behavior and life-history traits. Studying how these effects in turn influence prey demography is crucial to understand prey life-history evolution. Mosquitoes are important vectors that claim several million lives every year worldwide by transmitting a range of pathogens. Several ecological factors affect life-history traits of both larval and adult mosquitoes, creating effects that cascade to population-level consequences. Few studies have comprehensively explored the non-lethal effects of predation and its interactions with resources and competition on larval, adult, and population traits of mosquitoes. Understanding these interactions is important because the effects of predation are hypothesized to rescue prey populations from the effects of density-dependence resulting from larval competition. Aedes aegypti larvae reared at two different larval densities and subjected to three non-lethal predator treatments were monitored for survival, development time, and adult size through the larval stages to adult eclosion, and adult females were monitored for survival and reproduction through their first gonotrophic cycle. Intraspecific competition increased larval development time, yielded small-bodied adults, and reduced fecundity in individuals exposed to predatory chemical cues as larvae. Exposure to cues from a living predator affected both body size and latency to blood feed in females. Analysis of life-table traits revealed significant effects of competition on net reproductive rate (R ₀) of mosquitoes. The interaction between competition and predator treatments significantly affected the cohort rate of increase (r) and the index of performance (r'). The index of performance, which estimates rate of population change based on the size-fecundity relationship, was significantly and positively correlated with r, but overestimated r slightly. Lack of significant effect of predator treatments and larval density on cohort generation time (T _c) further suggests that the observed effects of treatments on r and r' were largely a consequence of the effects on R ₀. Also, the significant effects of treatment combinations on larval development time, adult body size and fecundity were ultimately manifested as effects on life-table traits estimated from adult survival and reproduction.