Interspecific Larval Competition Research Articles

Co-parasitization effect of Anagyrus pseudococci (Girault) and Coccidoxenoides perminutus Girault (Hymenoptera: Encyrtidae) on the vine mealybug Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae) and their intrinsic interspecific larval competition

BackgroundThe vine mealybug Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae) is a cosmopolitan species that causes economic damage to grapevines, especially in Mediterranean countries, South Africa, North and South America and Europe. In this study, the co-parasitization effect of Anagyrus pseudococci (Girault) and Coccidoxenoides perminutus Girault (Hymenoptera: Encyrtidae) on the vine mealybug Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae) and their intrinsic interspecific larval competition were investigated under laboratory conditions.ResultsIn non-choice experiments, the highest parasitism value (11.75 ± 0.87) was obtained on 2nd instar mealybug nymphs in containers with only C. perminutus, while the highest parasitism value (11.20 ± 0.59) in choice experiments was obtained on female mealybugs in containers with both parasitoids. It was determined that C. perminutus parasitized the 1st, 2nd and 3rd nymph stages, while A. pseudococci parasitized the 2nd, 3rd nymph and female stages of vine mealybug. These mean that mealybug populations can be controlled more effectively by parasitizing all mealybug stages in the environment when parasitoids are together. In intrinsic interspecific larval competition experiments on the mealybugs 2nd and 3rd nymph stages, where both parasitoids can parasitize, it was determined that C. perminutus generally won the competition, even though A. pseudococci had a three-day parasitism priority. The highest A. pseudococci emergence rate in competition trials occurred on 2nd instar mealybug nymphs when A. pseudococci had 72 h parasitism priority (36.09%). In addition, head capsule width and tibia lengths of A. pseudococci and C. perminutus adults obtained from intrinsic interspecific larval competition were measured, and it was determined that the head capsule width and tibia length of adult parasitoids generally increased as the host period progressed. In addition, it has been determined that there may be some differences in the measured characteristics of the adult parasitoids obtained due to intrinsic interspecific competition compared to the control.ConclusionsUsing these two parasitoids together in the biological control of vine mealybugs will allow parasitizing all mealybug nymph stages and females in the environment and provide more effective pest control.

Larval competition between three endemic fruit flies (Diptera: Tephritidae) of differing phylogenetic relatedness

AbstractWithin a fruit, fruit fly larvae can be subject to scramble competition, in which density‐dependent effects can influence the fitness of subsequent adults. While there is significant research on tephritid interspecific larval competition, it has been conducted in invasive situations where the species are evolutionarily novel to each other. There has been no published research investigating larval competitive interactions between naturally coexisting, endemic species. We ran laboratory‐based, intraspecific and interspecific larval competition trials involving three co‐occurring Bactrocera species of differing genetic relatedness and also measured aspects of juvenile development rate to test possible mechanisms of competitive difference. Larval density had an influence on intraspecific competition in Bactrocera tryoni, Bactrocera neohumeralis and Bactrocera jarvisi, with a decreasing percentage of pupation with increasing larval density. Interspecific competition between B. tryoni and B. neohumeralis, and between B. tryoni and B. jarvisi was influenced by the interaction between species and density. The intensity of competition between B. tryoni and B. neohumeralis was minimal but high between B. tryoni and B. jarvisi. B. jarvisi produced larger eggs and had faster initial larval growth rates than the other two species, but it took the longest time for pupation to occur. Our results conflict with theory, as the greatest competition was observed between the two more distantly related species (B. tryoni and B. jarvisi) rather than between the two most closely related species (B. tryoni and B. neohumeralis). Further, and contrary to other studies, egg size, hatch rate and larval growth rate did not provide B. jarvisi with a competitive advantage; thus, larval size does not appear to be a mechanism of larval competition between B. tryoni and B. jarvisi.

Examining the Blood-Feeding Interactions During Intra- and Interspecific Adult Competition between Aedes albopictus and Aedes aegypti -a Laboratory Study

Aedes albopictus and Aedes aegypti are striped mosquitoes with similar behavioral characteristics. They are known vectors of dangerous arboviruses and are expanding their reach globally. These species have established in specific areas of Türkiye, with Ae. albopictus expanding significantly than Ae. aegypti. While the larval competition of these two mosquito species has been studied, not much is known about their interactions during adulthood. We first investigated the effects of interspecific and intraspecific larval competition on the survival rate into adulthood and the wing length of females and males in containers of different sizes and food level conditions. This research also explored how females of these mosquitoes compete for blood meals in confined environments and its effects on blood feeding rates and female fecundity. Larval competition hinders Ae. aegypti emergence, Ae. albopictus remains resilient across varying resource levels. Notably, both species displayed increased blood feeding rates when housed together, suggesting potential facilitation or competitive avoidance strategies. Interspecific pressure caused a decrease in the number of eggs laid in mixed species cages for Ae. aegypti only. This study highlights the complex competitive dynamics between Ae. albopictus and Ae. aegypti. While larval competition appears to affect Ae. aegypti emergence. Further understanding of adult interactions is crucial for predicting their co-occurrence and effectively managing their populations, especially as Ae. albopictus shows greater adaptability and expansion within Türkiye.

Microbiota-mediated competition between Drosophila species

BackgroundThe influence of microbiota in ecological interactions, and in particular competition, is poorly known. We studied competition between two insect species, the invasive pest Drosophila suzukii and the model Drosophila melanogaster, whose larval ecological niches overlap in ripe, but not rotten, fruit.ResultsWe discovered D. suzukii females prevent costly interspecific larval competition by avoiding oviposition on substrates previously visited by D. melanogaster. More precisely, D. melanogaster association with gut bacteria of the genus Lactobacillus triggered D. suzukii avoidance. However, D. suzukii avoidance behavior is condition-dependent, and D. suzukii females that themselves carry D. melanogaster bacteria stop avoiding sites visited by D. melanogaster. The adaptive significance of avoiding cues from the competitor’s microbiota was revealed by experimentally reproducing in-fruit larval competition: reduced survival of D. suzukii larvae only occurred if the competitor had its normal microbiota.ConclusionsThis study establishes microbiotas as potent mediators of interspecific competition and reveals a central role for context-dependent behaviors under bacterial influence.8rnTpNHTJmc8sxfvPzrBu3Video

DynamAedes: a unified modelling framework for invasive Aedes mosquitoes

Mosquito species belonging to the genus Aedes have attracted the interest of scientists and public health officers because of their capacity to transmit viruses that affect humans. Some of these species were brought outside their native range by means of trade and tourism and then colonised new regions thanks to a unique combination of eco-physiological traits. Considering mosquito physiological and behavioural traits to understand and predict their population dynamics is thus a crucial step in developing strategies to mitigate the local densities of invasive Aedes populations. Here, we synthesised the life cycle of four invasive Aedes species (Ae. aegypti, Ae. albopictus, Ae. japonicus and Ae. koreicus) in a single multi-scale stochastic modelling framework which we coded in the R package dynamAedes. We designed a stage-based and time-discrete stochastic model driven by temperature, photo-period and inter-specific larval competition that can be applied to three different spatial scales: punctual, local and regional. These spatial scales consider different degrees of spatial complexity and data availability by accounting for both active and passive dispersal of mosquito species as well as for the heterogeneity of the input temperature data. Our overarching aim was to provide a flexible, open-source and user-friendly tool rooted in the most updated knowledge on the species’ biology which could be applied to the management of invasive Aedes populations as well as to more theoretical ecological inquiries.Graphical

Larval Competition Between Aedes albopictus and Aedes aegypti (Diptera: Culicidae) in Argentina: Coexistence and Implications in the Distribution of the Asian Tiger Mosquito.

Aedes aegypti (Linnaeus) and Aedes albopictus (Skuse) are worldwide vectors of dengue and yellow fever viruses. These species coexist in many countries and the biotic interactions between them can influence their abundances and distributions. In Argentina, Ae. aegypti is widely distributed in the north and center regions of the country, with temperate and subtropical climate, while both are sympatric only in the northeastern area of the subtropical region. Interspecific and intraspecific larval competition for food was evaluated to assess if their interaction influences on patterns of abundance and distribution. Finite rates of increase and survivorship for each species were estimated and the effects of mosquito density ratio and detritus availability were determined. The Lambda (λ´) index of population performance of both showed there is no competitive exclusion pattern. However, survival of Ae. albopictus was negatively affected by the presence of Ae. aegypti. These results suggest one possible explanation for the codominance pattern of both species display in rural regions of the southernmost distribution of Ae. albopictus in South America. They also show Ae. aegypti as a potential biotic barrier for the expansion of Ae. albopictus as was reported in regions of the United States.

Effects of Interspecific Larval Competition on Developmental Parameters in Nutrient Sources Between Drosophila suzukii (Diptera: Drosophilidae) and Zaprionus indianus.

Two invasive drosophilids, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) and Zaprionus indianus (Gupta) (Diptera: Drosophilidae) are expanding their geographic distribution and cohabiting grape production in the Mid-Atlantic. The ecological and economic impact of these two species within vineyards is currently unknown. Zaprionus indianus was presumably not capable of ovipositing directly into grapes because they lack a serrated ovipositor and may use D. suzukii oviposition punctures for depositing their own eggs. Therefore, an interspecific larval competition assay was performed at varying larval densities using commercial medium and four commonly grown wine grapes in Virginia to investigate the impact Z. indianus larvae may have on the mortality and developmental parameters of D. suzukii larvae. Zaprionus indianus did not affect D. suzukii mortality or development parameters even at high interspecific densities when reared in commercial medium, but it did cause higher D. suzukii mortality within grapes. Mortality was also influenced by the variety of grape in which the larvae were reared, with smaller grapes having the highest D. suzukii mortality. Presence of Z. indianus also increased development time to pupariation and adult emergence for most interspecific competition levels compared with the intraspecific D. suzukii controls. Pupal volume was marginally affected at the highest interspecific larval densities. This laboratory study suggests that competition from Z. indianus and grape variety can limit D. suzukii numbers, and the implications on D. suzukii pest management be further verified in the field.

Oviposition site attraction of Aedes albopictus to sites with conspecific and heterospecific larvae during an ongoing invasion in Medell\xedn, Colombia

BackgroundAedes aegypti and Aedes albopictus are two globally invasive vectors with similar ecological niches. Encounters between them can result in either competitive exclusion or stable co-existence, but it is unclear what drives these variable outcomes. Larval competition in favor of Ae. albopictus is a main hypothesis for the competitive exclusion of Ae. aegypti observed in some regions. However, the role of oviposition preference in determining the degree of competitive larval interactions in the field is not well understood. In this study, we used a combination of mark-release-recapture methods with ovitraps in the open-field and a semi-field cage to test whether gravid Ae. albopictus seek oviposition sites in response to the presence, species, and density of either conspecific or heterospecific Ae. aegypti larvae in the aquatic habitat. We conducted our study in Medellín, Colombia, where Ae. aegypti is a long-term resident and Ae. albopictus is a recent invader.ResultsIn the open-field and semi-field cage experiments, gravid Ae. albopictus showed strong preference for ovitraps with larvae over those without. They consistently preferred ovitraps with higher density of conspecific (Ae. albopictus) larvae and low density of heterospecific (Ae. aegypti) larvae over traps with no larvae or high density of heterospecific (Ae. aegypti) larvae. In the semi-field cage experiment, traps with low density of Ae. albopictus were not preferred more or less than any other trap, but in the open-field experiment they were preferred over traps without larvae.ConclusionsWe demonstrate, through open-field and semi-field cage experiments, that Ae. albopictus are more attracted to oviposition sites with larvae and that the combination of species and density of larvae influence attraction. This demonstrated preference could increase interspecific larval competition as Ae. albopictus actively seek containers with conspecific and heterospecific larvae. Any resulting competition with Ae. aegypti may favor one species over the other and alter the distribution or abundance of both. Because these species vary in vectorial capacity and insecticide resistance, effects of interspecific competition could ultimately impact arbovirus transmission rates and the success of vector control efforts.

The hyperparasitoid Marietta picta (Hymenoptera: Aphelinidae) mediates competitive interactions between two parasitoids of Paratrioza sinica (Hemiptera: Psyllidae): Tamarixia lyciumi (Hymenoptera: Eulophidae) and Psyllaephagus arenarius (Hymenoptera: Encyrtidae)

Both the ectoparasitoid Tamarixia lyciumi Yang (Hymenoptera: Eulophidae) and the endoparasitoid Psyllaephagus arenarius (Hymenoptera: Encyrtidae) are solitary nymphal parasitoids of Paratrioza sinica (Hemiptera: Psyllidae), a pest of goji berry, Lycium barbarum. Moreover, T. lyciumi is frequently attacked by a hyperparasitoid Marietta picta Andre (Hymenoptera: Aphelinidae) in the field. To analyze competitive interactions between T. lyciumi and P. arenarius mediated by M. picta, we evaluated interspecific host discrimination and larval competition between T. lyciumi and P. arenarius, and the effect of M. picta on their competitive interactions, under laboratory conditions. The results showed that T. lyciumi could discriminate hosts pre-parasitized by P. arenarius, but not vice versa. However, P. arenarius was a superior intrinsic competitor in multiparasitized hosts regardless the order of parasitism. Although multiparasitized hosts produced more P. arenarius and fewer T. lyciumi after exposure to M. picta, the total number of emerging T. lyciumi and P. arenarius did not decline significantly, and sex ratios did not change. Thus, larval superiority would appear to favor the establishment of newly introduced P. arenarius in the Ningxia region, while the inferior competitor T. lyciumi is able to utilize interspecific host discrimination to avoid larval competition. Competitive interactions between T. lyciumi and P. arenarius are mediated by M. picta, and the observed host–parasitoid–hyperparasitoid interactions potentially facilitate their collective coexistence in biological control of P. sinica.

Intra-host interspecific larval parasitoid competition solved using modelling and bayesian statistics

Intraguild competition is a complex phenomenon that shapes parasitoid communities. When several species of parasitoids oviposit within the same individual host, a complex phenomenon of larval competitive interaction occurs. Within the same guild there is a specialization in competitive strategies, sometimes multiparasitism is avoided, but some species are facultative hyperparasitoids/predators of their competitors.As these interactions occur within a very small host and during a brief period of time, and that direct observation is very difficult to achieve, we used an alternative methodological approach. We analyzed intraguild host competition mechanisms via the combination of a series of competitive behavioral and functional response models, thurstonian competition model and set theory. These models were fitted via a reversible-jump bayesian model selection procedure to a series of competition experiments data using larvae of three species of Gonatocerus spp. (Hymenoptera: Mymaridae), egg parasitoids of the sharpshooter Tapajosa rubromarginata (Hemiptera: Cicadellidae) as a case-of-study. This study tests the influence of intrinsic interspecific competition between inmature stages within on an individual host, and parasitoid arrival order among the three parasitoid species.The results showed that the species differed in competitive behavior, some species were better competitors than others. Individuals arriving earlier had a competitive advantage, the weaker species were able to outcompete the stronger ones if the time advantage was longer than 18 h. All the species avoided already parasitized hosts, but in different degrees. The functional response was also different, with the best competitors having shorter estimated handling times. Using this analytical approach on a conventional experimental setup, we gained insights in the mechanism of competition, both on interference and exploitation, and in terms of host selection, all in a single analysis.

How does competition among wild type mosquitoes influence the performance of Aedes aegypti and dissemination of Wolbachia pipientis?

BackgroundWolbachia has been deployed in several countries to reduce transmission of dengue, Zika and chikungunya viruses. During releases, Wolbachia-infected females are likely to lay their eggs in local available breeding sites, which might already be colonized by local Aedes sp. mosquitoes. Therefore, there is an urgent need to estimate the deleterious effects of intra and interspecific larval competition on mosquito life history traits, especially on the duration of larval development time, larval mortality and adult size.Methodology/principal findingsThree different mosquito populations were used: Ae. aegypti infected with Wolbachia (wMelBr strain), wild Ae. aegypti and wild Ae. albopictus. A total of 21 treatments explored intra and interspecific larval competition with varying larval densities, species proportions and food levels. Each treatment had eight replicates with two distinct food levels: 0.25 or 0.50 g of Chitosan and fallen avocado leaves. Overall, overcrowding reduced fitness correlates of the three populations. Ae. albopictus larvae presented lower larval mortality, shorter development time to adult and smaller wing sizes than Ae. aegypti. The presence of Wolbachia had a slight positive effect on larval biology, since infected individuals had higher survivorship than uninfected Ae. aegypti larvae.Conclusions/significanceIn all treatments, Ae. albopictus outperformed both wild Ae. aegypti and the Wolbachia-infected group in larval competition, irrespective of larval density and the amount of food resources. The major force that can slow down Wolbachia invasion is the population density of wild mosquitoes. Given that Ae. aegypti currently dominates in Rio, in comparison with Ae. albopictus frequency, additional attention must be given to the population density of Ae. aegypti during releases to increase the likelihood of Wolbachia invasion.

Testing for intraspecific and interspecific larval competition between two anurans: Hyla savignyi and Bufotes viridis

Intraspecific and interspecific competition can be important factors affecting anuran tadpoles in temporary pools. We focus on two co-occurring anuran species that are of conservation concern in Israel and may negatively interact: Bufotes viridis (Laurenti, 1768) and Hyla savignyi (Audouin, 1829). We conducted a replacement design mesocosm experiment to examine intraspecific and interspecific competition between tadpoles of these two species. Plastic tubs were assigned to five replacement design treatments: 0 Hyla + 20 Bufotes; 0 Hyla + 40 Bufotes; 20 Hyla + 20 Bufotes; 20 Hyla + 0 Bufotes; and 40 Hyla + 0 Bufotes, and a control with no tadpoles which was used to test for effects of the tadpoles on invertebrate community structure. While H. savignyi was significantly affected by intraspecific density, with smaller body mass and longer time to metamorphosis at the high-density treatment, B. viridis showed no intraspecific or interspecific density-dependent effects. However, B. viridis time to metamorphosis was negatively correlated with water temperature, while its survivorship was positively correlated with water temperature. The tadpoles affected neither abundance nor taxon richness of invertebrates. Our results suggest that stronger intraspecific competition than interspecific competition may contribute to the common co-occurrence of these two anurans in the same pools.

Assessment of Aedes albopictus (Skuse) (Diptera: Culicidae) clutch size in wild and laboratory populations.

Aedes albopictus (Skuse) is an invasive mosquito species found across the southern U.S. with range expansion into many northern states. Intra- and interspecific larval competition have been evaluated for Ae. albopictus with respect to subsequent adult size, immature and adult survivability, and its capacity to vector pathogens as an adult. However, limited data are available on egg production as related to larval rearing conditions. Because Ae. albopictus is a container-inhabiting mosquito that oviposits in resource-limited habitats, it is found under variable density-dependent conditions. Therefore, we examined the impact of specific rearing conditions on Ae. albopictus clutch size and adult body size; comparing the egg production values and wing lengths from known developmental densities to those from field-collected populations. Field populations varied significantly among collection sites in mean clutch size (23 to 46). These clutch sizes were comparable to the mean clutch sizes of females reared at the larval densities of nine (20 eggs) and three (53 eggs) larvae per 3 ml of water in the laboratory. Field populations experienced density-dependent effects impacting adult mosquito size. Mosquitoes from the four sample sites had mean wing lengths of 1.99, 2.47, 2.51, and 2.54 mm, which were less than the mean wing length of mosquitoes reared at larval densities of three larvae per 3 ml of water (2.57 mm).

Asymmetric effects of intra‐ and interspecific competition on a pond‐breeding salamander

A fundamental goal of evolutionary ecology is to understand how asymmetric competition influences phenotype expression, yet few studies have quantified the relative effects of intra‐ and interspecific competition on phenotypes. We examined the effects of intra‐ and interspecific larval competition on both discrete and continuous phenotype expression of the facultatively paedomorphic mole salamander (Ambystoma talpoideum). We manipulated the density and frequency of larval A. talpoideum and a common competitor, the spotted salamander (A. maculatum), in experimental mesocosms within a response surface design. The production of discrete ontogenetic morphs (metamorphs, paedomorphs, and overwintering larvae) was affected more by intraspecific than interspecific competition, but the effect varied between morphs. Paedomorph and metamorph production were more strongly affected by intraspecific than interspecific competition, while the production of overwintering larvae was affected by each type of competition approximately equally. Paedomorphs largely occurred only at low conspecific densities, overwintering larvae primarily occurred at high overall densities, and metamorph production peaked at intermediate densities of con‐ and heterospecifics. Density‐dependent intraspecific competition had stronger effects on body size and growth rates than interspecific competition for both metamorphs and overwintering larvae, whereas interspecific competition more strongly affected paedomorph size. Overall, these results support the hypothesis that intraspecific competition influences both continuous and discrete phenotypic expression more strongly than interspecific competition. Fluctuating strengths of each type of competition can affect variation in life history strategies by influencing discrete ontogenetic pathways via individual body sizes, leading to differential morph production and fitness within and among morphs.

Interspecific Larval Competition Differentially Impacts Adult Survival in Dengue Vectors

Mosquitoes often experience intraspecific and interspecific competition among larvae attributable to high densities and nutrient limitation, especially container mosquitoes including Aedes aegypti (L.) and Aedes albopictus (Skuse). Density-dependent effects on larvae impact adult production and adult traits that influence transmission of arboviruses. To improve our understanding of the mechanisms by which density-dependence influences transmission and identify species-specific traits, we tested the hypotheses: 1) Competitive asymmetry in favor of Ae. albopictus over Ae. aegypti translates to altered adult female survival, and 2) Ae. aegypti adult females are more resistant to life-shortening effects of low-humidity conditions than Ae. albopictus. We gauged the relative impact of inter- and intraspecific larval competition on adult survival in high- and low-humidity regimes (77 and 44% relative humidity, respectively). For Ae. albopictus, intraspecific but not interspecific competition usually reduced adult survival under both humidity regimes. For Ae. aegypti, both intraspecific and interspecific competition reduced adult survival. Ae. albopictus adult survival was minimally influenced by interspecific competition with Ae. aegypti, consistent with observations that Ae. albopictus is the superior competitor. A species comparison indicated that Ae. aegypti exhibited a survival advantage relative to Ae. albopictus under both low- and high-humidity conditions. However, similar survival of these Aedes species was observed in some cases depending on conditions experienced in both the aquatic and terrestrial environments. These results demonstrate plasticity in survival rates of dengue and chikungunya vectors and the significance of considering the influence of biological interactions during the immature stages and abiotic conditions during the adult stage.

Intraguild Interactions between Egg Parasitoids: Window of Opportunity and Fitness Costs for a Facultative Hyperparasitoid

We investigated intraguild interactions between two egg parasitoids of Nezara viridula (L.) (Heteroptera: Pentatomidae), Ooencyrtus telenomicida (Vassiliev) (Hymenoptera: Encyrtidae) and Trissolcus basalis (Wollaston) (Hymenoptera: Platygastridae), as the former has the potential to be a facultative hyperparasitoid of the latter. We assessed the suitability of N. viridula eggs for the development of O. telenomicida as a function of egg age when they were unparasitized, or had been attacked by T. basalis at different times prior to exposure to O. telenomicida females. Ooencyrtus telenomicida can exploit healthy N. viridula host eggs up to 5 days of age, just prior to the emergence of N. viridula. This window of opportunity can be extended for an additional 6–7 days through interspecific competition or facultative hyperparasitism. While there are minor fitness costs for O. telenomicida as the result of interspecific larval competition, those costs are greater with facultative hyperparasitism. In choice assays O. telenomicida females discriminated between different quality N. viridula eggs, avoiding those where their progeny would have to develop as facultative hyperparasitoids of T. basalis. Results are discussed with respect to the possible effects that the costs of intraguild parasitism might have on biological control programmes.

Inter- and Intra-Specific Density-Dependent Effects on Life History and Development Strategies of Larval Mosquitoes

We explored how inter- and intra-specific competition among larvae of two temporary-pool mosquito species, Culiseta longiareolata and Ochlerotatus caspius, affect larval developmental strategy and life history traits. Given that their larvae have similar feeding habits, we expected negative reciprocal inter-specific interactions. In a microcosm experiment, we found sex-specific responses of larval survival and development to both intra- and inter-specific larval competition. C. longiareolata was the superior competitor, reducing adult size and modifying larval developmental time of O. caspius. We observed two distinct waves of adult emergence in O. caspius, with clear sex-specific responses to its inter-specific competitor. In males, this pattern was not affected by C. longiareolata, but in females, the timing and average body size of the second wave strongly varied with C. longiareolata density. Specifically, in the absence of C. longiareolata, the second wave immediately followed the first wave. However, as C. longiareolata abundance increased, the second wave was progressively delayed and the resulting females tended to be larger. This study improves our understanding of the way intra- and inter-specific competition combine to influence the life histories of species making up temporary pond communities. It also provides strong evidence that not all individuals of a cohort employ the same strategies in response to competition.

Interspecific larval competition between two egg parasitoids in refrigerated host eggs of Riptortus pedestris (Hemiptera: Alydidae)

In a previous study, we found that soybean fields could be supplemented with refrigerated eggs of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) to enhance parasitism. As a part of a study to evaluate the effect of host egg refrigeration on parasitism, host acceptance behavior and interspecific larval competition between Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae) and Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) were studied in multiparasitized unrefrigerated and refrigerated eggs. O. nezarae showed complete host acceptance behavior when offered refrigerated host eggs that were preparasitized by G. japonicum. Adult emergence rate of O. nezarae was 43 and 74% when the interval between the first and second oviposition was 0 and 4 days, respectively, and was not different between refrigerated and unrefrigerated eggs. Refrigeration did not change host acceptance behavior of G. japonicum, but adult emergence declined from 80% in unrefrigerated eggs to 37% in refrigerated eggs that were pre-parasitized by O. nezarae on the same day. No negative effects of refrigeration on sex ratio, adult longevity, and adult size of the both parasitoids were found. Generally host egg refrigeration did not negatively affect host acceptance behavior of the both parasitoids on preparasitized eggs or larval competition between the two parasitoids in multiparasitized host eggs with exceptions in the development time and emergence rate of G. japonicum. Therefore, host egg refrigeration may not interrupt interactions between the parasitoid populations in the field.

Effect of Malathion on Larval Competition Between Aedes albopictus and Aedes atropalpus (Diptera: Culicidae)

Aedes albopictus (Skuse) and Aedes atropalpus (Coquillett) (Diptera: Culicidae) are container-dwelling mosquito species that are well established in the eastern United States. Interspecific larval competition studies have shown Ae. albopictus to be a superior competitor over many species. A laboratory experiment was conducted in artificial containers to evaluate the effects of malathion on larval interactions between Ae. albopictus and Ae. atropalpus. The survivorship of Ae. albopictus increased with increasing Ae. atropalpus densities in control but decreased with increasing Ae. atropalpus densities in the presence of malathion. Alternatively, Ae. atropalpus survivorship did not differ between control and malathion treatments. Developmental times were not affected by interspecific competition in both treatments for either species. These results show that malathion could facilitate coexistence between Ae. albopictus and Ae. atropalpus. This demonstrates how sublethal concentrations of malathion (and perhaps other pesticides with similar modes of action) can enable an inferior competitor to coexist in the same habitat with a superior competitor. This is the first report of synergistic survival of a weaker mosquito competitor in the presence of a pesticide due to condition-specific competition.

Intraguild interactions between two egg parasitoids exploring host patches

Intraguild interactions between two egg parasitoids, Trissolcus basalis (Hymenoptera: Scelionidae) and Ooencyrtus telenomicida (Hymenoptera: Encyrtidae), exploring egg masses of the Southern Green Stink Bug (SGSB) Nezara viridula (Heteroptera: Pentatomidae), were investigated in laboratory conditions by single, simultaneous and sequential host attack experiments. Mortality of N. viridula eggs was higher in simultaneous and sequential releases compared to single species releases. In simultaneous host exploitations, T. basalis females displayed an aggressive behavior against O. telenomicida females. The outcome of multiparasitism showed that interspecific larval competition was dominated by O. telenomicida regardless of the sequence in which oviposition occurred and which parasitoid was or was not simultaneously released in the patch. Finally, O. telenomicida can successfully develop in hosts already parasitized by T. basalis up to seven days earlier, acting as a facultative hyperparasitoid, so that intraguild predation (IGP) also occurred. The ecological factors that allow species coexistence and the role played by competition in biological control programs are discussed.