No evolutionary response to four generations of laboratory selection on antipredator behavior of Aedes albopictus: potential implications for biotic resistance to invasion

The Role of Tire Leachate in Condition-Specific Competition and the Persistence of a Resident Mosquito from a Competitively Superior Invader

(1) Background: Condition-specific competition, when the outcome of competition varies with abiotic conditions, can facilitate species coexistence in spatially or temporally variable environments. Discarded vehicle tires degrade to leach contaminants into collected rainwater that provide habitats for competing mosquito species. We tested the hypothesis that more highly degraded tires that contain greater tire leachate alters interspecific mosquito competition to produce a condition-specific advantage for the resident, Culex pipiens, by altering the outcome of competition with the competitively superior invasive Aedes albopictus. (2) Methods: In a competition trial, varying densities of newly hatched Ae. albopictus and Cx. pipiens larvae were added to tires that had been exposed to three different ultraviolet (UV)-B conditions that mimicked full-sun, shade, or no UV-B conditions in the field. We also measured Cx. pipiens and Ae. albopictus oviposition preference among four treatments with varying tire leachate (high and low) and resources (high and low) amounts to determine if adult gravid females avoided habitats with higher tire leachate. (3) Results: We found stronger competitive effects of Cx. pipiens on the population performance and survival of Ae. albopictus in tires exposed to shade and full-sun conditions that had higher concentrations of contaminants. Further, zinc concentration was higher in emergent adults of Ae. albopictus than Cx. pipiens. Oviposition by these species was similar between tire leachate treatments but not by resource amount. (4) Conclusions: These results suggest that degraded tires with higher tire leachate may promote condition-specific competition by reducing the competitive advantage of invasive Ae. albopictus over resident Cx. pipiens and, combined with Cx. pipiens' preferential oviposition in higher resource sites, contribute to the persistence of the resident species.

No Impact of Biocontrol Agent’s Predation Cues on Development Time or Size of Surviving Aedes albopictus under Optimal Nutritional Availability

Simple Summary

Aedes albopictus is a highly invasive species of mosquito that can infect humans with chikungunya, dengue, yellow fever, and Zika. Within the next few decades, this mosquito species is predicted to invade South East England. Cyclopoid copepods are small crustaceans that have previously been used as biocontrol agents due to their high efficiency at killing small early instar mosquito larvae. We assessed the effect of Megacyclops viridis, a copepod species local to South East England, on the survival and traits of mosquito larvae exposed to these predators during the larger late instar stage. Our experiment was designed to measure the impact of copepod predation on both the development time and adult body size of Ae. albopictus. These traits can shape mosquito population dynamics and disease transmission. While we found that copepod attacks cause a small increase in late instar mortality, our methods did not detect a significant difference in either development time or size between the predator and control treatments. The lack of a strong sublethal effect on these traits supports the use of M. viridis copepods as biocontrol agents against Ae. albopictus in the UK. This information may be useful for guiding public health measures that aim to prevent outbreaks of mosquito-borne disease.

Abstract

Cyclopoid copepods have been applied successfully to limit populations of highly invasive Aedes albopictus mosquitoes that can transmit diseases of public health importance. However, there is concern that changes in certain mosquito traits, induced by exposure to copepod predation, might increase the risk of disease transmission. In this study, third instar Ae. albopictus larvae (focal individuals) were exposed to Megacyclops viridis predator cues associated with both the consumption of newly hatched mosquito larvae and attacks on focal individuals. The number of newly hatched larvae surrounding each focal larva was held constant to control for density effects on size, and the focal individual’s day of pupation and wing length were recorded for each replicate. Exposing late instar Ae. albopictus to predation decreased their chances of surviving to adulthood, and three focal larvae that died in the predator treatment showed signs of melanisation, indicative of wounding. Among surviving focal Ae. albopictus, no significant difference in either pupation day or wing length was observed due to copepod predation. The absence of significant sublethal impacts from M. viridis copepod predation on surviving later stage larvae in this analysis supports the use of M. viridis as a biocontrol agent against Ae. albopictus.

How can mortality increase population size? A test of two mechanistic hypotheses

Overcompensation occurs when added mortality increases survival to the next life-cycle stage. Overcompensation can contribute to the Hydra effect, wherein added mortality increases equilibrium population size. One hypothesis for overcompensation is that added mortality eases density dependence, increasing survival to adulthood ("temporal separation of mortality and density dependence"). Mortality early in the life cycle is therefore predicted to cause overcompensation, whereas mortality later in the life cycle is not. Another hypothesis for overcompensation is that threat of mortality (e.g., from predation) causes behavioral changes that reduce overexploitation of resources, allowing resource recovery, and increasing production of adults ("prudent resource exploitation"). Behaviorally active predation cues alone are therefore predicted to cause overcompensation. We tested these predictions in two experiments with larvae of two species of Aedes. As predicted, early mortality yielded greater production of adults, and of adult females, and greater estimated rate of population increase than did later mortality. Addition of water-borne predation cues usually reduced browsing on surfaces in late-stage larvae, but contrary to prediction, resulted in neither significantly greater production of adult mosquitoes nor significantly greater estimated rate of increase. Thus we have strong evidence that timing of mortality contributes to overcompensation and the Hydra effect in mosquitoes. Evidence that predation cues alone can result in overcompensation via prudent resource exploitation is lacking. We expect the overcompensation in response to early mortality will be common in organisms with complex life cycles, density dependence among juveniles, and developmental control of populations.

Emerging Themes in Our Understanding of Species Displacements

The displacement of a species from a habitat by actions of another is the most severe outcome of interspecific interactions. This review focuses on recent developments in the understanding of (a) ecological mechanisms that lead to displacements, (b) how outcomes of interspecific interactions are affected by the context of where and when they occur, and (c) impacts of displacements. Displacements are likely to escalate as their primary initiating factors-the spread of non-native species and environmental change-continue at unprecedented rates. Displacements typically result from interactions of multiple mechanisms, not all of which involve direct competition. Various biotic and abiotic factors mediate these mechanisms, so variable outcomes occur when the same species interact in different environments. Though replacement of one species by another has particular relevance to pest management and conservation biology, the cascading effects that displacements have in managed and natural systems are critical to understand.

Larval mosquito habitat utilization and community dynamics of Aedes albopictus and Aedes japonicus (Diptera: Culicidae)

Aedes albopictus (Skuse) and Ae. japonicus (Theobald) are important container-inhabiting mosquitoes that transmit disease agents, outcompete native species, and continue to expand their range in the United States. Both species deposit eggs in natural and artificial containers and thrive in peridomestic environments. The goal of our study was to examine the types and characteristics of containers that are most productive for these species in the northeastern United States. In total, 306 containers were sampled in urban, suburban, and rural areas of New Jersey. Multiple biotic and abiotic factors were recorded in an attempt to identify variables associated with the productivity of each species. Based on pupal abundance and density of container types, results showed that tires, trash cans, and planter dishes were the most important containers for Ae. albopictus, while planter dishes were the most important containers for Ae. japonicus. Container color (black and gray), material (rubber), and type (tires) were correlated with species presence for Ae. albopictus and Ae. japonicus. These factors may play a role in the selection of oviposition sites by female mosquitoes or in the survival of their progeny. Differences in species composition and abundance were detected between areas classified as urban, suburban, and rural. In urban and suburban areas, Ae. albopictus was more abundant in container habitats than Ae. japonicus; however, Ae. japonicus was more abundant in rural areas, and when water temperatures were below 14 degrees C. Our results suggest many variables can influence the presence of Ae. albopictus and Ae. japonicus in container habitats in northeastern United States.

Seasonal photoperiods alter developmental time and mass of an invasive mosquito, Aedes albopictus (Diptera: Culicidae), across its north-south range in the United States

The Asian tiger mosquito, Aedes albopictus (Skuse), is perhaps the most successful invasive mosquito species in contemporary history. In the United States, Ae. albopictus has spread from its introduction point in southern Texas to as far north as New Jersey (i.e., a span of approximately 14 degrees latitude). This species experiences seasonal constraints in activity because of cold temperatures in winter in the northern United States, but is active year-round in the south. We performed a laboratory experiment to examine how life-history traits of Ae. albopictus from four populations (New Jersey [39.4 degrees N], Virginia [38.6 degrees N], North Carolina [35.8 degrees N], Florida [27.6 degrees N]) responded to photoperiod conditions that mimic approaching winter in the north (short static daylength, short diminishing daylength) or relatively benign summer conditions in the south (long daylength), at low and high larval densities. Individuals from northern locations were predicted to exhibit reduced development times and to emerge smaller as adults under short daylength, but be larger and take longer to develop under long daylength. Life-history traits of southern populations were predicted to show less plasticity in response to daylength because of low probability of seasonal mortality in those areas. Males and females responded strongly to photoperiod regardless of geographic location, being generally larger but taking longer to develop under the long daylength compared with short day lengths; adults of both sexes were smaller when reared at low larval densities. Adults also differed in mass and development time among locations, although this effect was independent of density and photoperiod in females but interacted with density in males. Differences between male and female mass and development times was greater in the long photoperiod suggesting differences between the sexes in their reaction to different photoperiods. This work suggests that Ae. albopictus exhibits sex-specific phenotypic plasticity in life-history traits matching variation in important environmental variables.

Behavioral differences of invasive container-dwelling mosquitoes to a native predator

Aquatic prey show behavioral modifications in the presence of predation-risk cues that alleviate their risk from predation. Aedes albopictus (Skuse), Aedes japonicus (Theobald), and Culex pipiens L. are invasive mosquitoes in North America, and their larvae are prey for the native mosquito predator, Toxorhynchites rutilus (Coquillett). Ae. albopictus and Ae. japonicus are recent invaders, whereas Cx. pipiens has been in the United States for >100 yr. In the presence of predation-risk cues from Tx. rutilus larvae, Cx. pipiens larvae increased the time spent resting at the surface (least risky behavior) more than the other prey species. Ae. japonicus larvae increased resting at the surface of the containers more than Ae. albopictus larvae in the presence of predation-risk cues. Cx. pipiens larvae spent more time motionless at the surface even in the absence of predation-risk cues when compared with the other species, indicating that Cx. pipiens larvae are the least vulnerable prey. As compared with the other prey species, Ae. albopictus larvae exhibited more high-risk behaviors both in the presence and absence of predation-risk cues, indicating that they are the most vulnerable prey. Ae. albopictus is the superior competitor; however, predation by Tx. rutilus larvae may prevent competitive exclusion by Ae. albopictus and promote coexistence among the three prey species.