Strain specific differences in intraspecific competition in Aedes albopictus (Diptera: Culicidae)

Assessing temperature-dependent competition between two invasive mosquito species

Invasive mosquitoes are expanding their ranges into new geographic areas and interacting with resident mosquito species. Understanding how novel interactions can affect mosquito population dynamics is necessary to predict transmission risk at invasion fronts. Mosquito life-history traits are extremely sensitive to temperature, and this can lead to temperature-dependent competition between competing invasive mosquito species. We explored temperature-dependent competition between Aedes aegypti and Anopheles stephensi, two invasive mosquito species whose distributions overlap in India, the Middle East, and North Africa, where An. stephensi is currently expanding into the endemic range of Ae. aegypti. We followed mosquito cohorts raised at different intraspecific and interspecific densities across five temperatures (16-32°C) to measure traits relevant for population growth and to estimate species' per capita growth rates. We then used these growth rates to derive each species' competitive ability at each temperature. We find strong evidence for asymmetric competition at all temperatures, with Ae. aegypti emerging as the dominant competitor. This was primarily because of differences in larval survival and development times across all temperatures that resulted in a higher estimated intrinsic growth rate and competitive tolerance estimate for Ae. aegypti compared to An. stephensi. The spread of An. stephensi into the African continent could lead to urban transmission of malaria, an otherwise rural disease, increasing the human population at risk and complicating malaria elimination efforts. Competition has resulted in habitat segregation of other invasive mosquito species, and our results suggest that it may play a role in determining the distribution of An. stephensi across its invasive range.

The effect of inbreeding and larval feeding regime on immature development of Aedes albopictus

The fundamental approach to the biological control of Aedes albopictus requires the mass rearing of mosquitoes and the release of highly competitive adults in the field. As the fitness of adults is highly dependent on the development of immatures, we aimed to identify the minimum feeding regime required to produce viable and competitive adults by evaluating three response parameters: development duration, immature mortality, and adult wing length. Our study suggests at least 0.60 mg/larva/day of larval diet composed of dog food, dried beef liver, yeast, and milk powder in a weight ratio of 2:1:1:1 is required to maximize adult fitness. With standardized protocols in mass rearing, intensive studies can be readily conducted on mosquito colonies to facilitate comparisons across laboratories. This study also evaluated the differences in response of laboratory and field strains under different feeding regimes. We found that strain alone did not exert substantial effects on all response parameters. However, the field strain exhibited significantly lower immature mortality than the laboratory strain under the minimum feeding regime. Females and males of the laboratory strain had longer wing lengths under nutritional constraint due to the higher mortality that resulted in reduced interactions with the remaining larvae. Meanwhile, the field strain exhibited heterogeneous duration of immature development compared with the laboratory strain. The disparities demonstrated by the two strains in this study suggest the effect of inbreeding surfaced after a long term of laboratory colonization. Despite the trade-offs resulting from laboratory colonization, the competitiveness of the laboratory strain of Ae. albopictus is comparable to the field strain, provided the larvae are fed optimally.

Pesticide-Induced Stress in Arthropod Pests for Optimized Integrated Pest Management Programs

More than six decades after the onset of wide-scale commercial use of synthetic pesticides and more than fifty years after Rachel Carson's Silent Spring, pesticides, particularly insecticides, arguably remain the most influential pest management tool around the globe. Nevertheless, pesticide use is still a controversial issue and is at the regulatory forefront in most countries. The older generation of insecticide groups has been largely replaced by a plethora of novel molecules that exhibit improved human and environmental safety profiles. However, the use of such compounds is guided by their short-term efficacy; the indirect and subtler effects on their target species, namely arthropod pest species, have been neglected. Curiously, comprehensive risk assessments have increasingly explored effects on nontarget species, contrasting with the majority of efforts focused on the target arthropod pest species. The present review mitigates this shortcoming by hierarchically exploring within an ecotoxicology framework applied to integrated pest management the myriad effects of insecticide use on arthropod pest species.

Insecticide-mediated shift in ecological dominance between two competing species of grain beetles

Competition is a driving force regulating communities often considered an intermittent phenomenon, difficult to verify and potentially driven by environmental disturbances. Insecticides are agents of environmental disturbance that can potentially change ecological relationships and competitive outcomes, but this subject has seldom been examined. As the co-existing cereal grain beetle species Sitophilus zeamais Motschulsky and Rhyzopertha dominica F. share a common realized niche, directly competing for the same resources, they were used as models in our study. Intraspecific competition experiments were performed with increasing insect densities and insecticide doses in additive and replacement series using various density combinations of both beetle species maintained on insecticide-free or -sprayed grains. Insecticide-mediated release from competitive stress was not observed in our study of intraspecific competition in grain beetles. The insecticide enhanced the effect of insect density, particularly for the maize weevil S. zeamais, further impairing population growth at high densities. Therefore, insecticide susceptibility increased with intraspecific competition favoring insecticide efficacy. However, the effect of insecticide exposure on competitive interaction extends beyond intraspecific competition, affecting interspecific competition as well. Sitophilus zeamais was the dominant species when in interspecific competition prevailing in natural conditions (without insecticide exposure), but the dominance and species prevalence shifted from S. zeamais to R. dominica under insecticide exposure. Therefore, high conspecific densities favored insecticide efficacy, but the strength of the relationship differs with the species. In addition, the insecticide mediated a shift in species dominance and competition outcome indicating that insecticides are relevant mediators of species interaction, potentially influencing community composition and raising management concerns as potential cause of secondary pest outbreaks.

Larval competition between An. coluzzii and An. gambiae in insectary and semi-field conditions in Burkina Faso

Competition in mosquito larvae is common and different ecological context could change competitive advantage between species. Here, larval competition between the widely sympatric African malaria mosquitoes, Anopheles coluzzii and Anopheles gambiae were investigated in controlled insectary conditions using individuals from laboratory colonies and under ambient conditions using wild mosquitoes in a semi-field enclosure in western Burkina Faso. Larvae of both species were reared in trays at the same larval density and under the same feeding regimen in either single-species or mixed-species populations at varying species ratios reflecting 0%, 25%, 50% and 75% of competitor species. In the insectaries, where environmental variations are controlled, larvae of the An. coluzzii colony developed faster and with lower mortality than larvae of the An. gambiae colony (8.8±0.1 days and 21±3% mortality vs. 9.5±0.1 days and 32±3% mortality, respectively). Although there was no significant effect of competition on these phenotypic traits in any species, there was a significant trend for higher fitness of the An. coluzzii colony when competing with An. gambiae under laboratory conditions (i.e. lower development time and increased wing length at emergence, Cuzik's tests, P<0.05). In semi-field experiments, competition affected the life history traits of both species in a different way. Larvae of An. gambiae tended to reduce development time when in competition with An. coluzzii (Cuzick's test, P=0.002) with no impact either on mortality or size at emergence. On the other hand, An. coluzzii showed a significant trend for reduced larval mortality with increasing competition pressure (Cuzick's test, P=0.037) and production of smaller females when grown together with An. gambiae (Cuzick's test, P=0.002). Our results hence revealed that competitive interactions between larvae of the two species are context dependent. They further call for caution when exploring ecological processes using inbred laboratory colonies in this system of utmost medical importance.