Aedes albopictus (Skuse) is one of the most invasive species globally, and has led to rapid declines and local extirpations of resident mosquitoes where it becomes established. A potential mechanism behind these displacements is the superior competitive ability of Ae. albopictus in larval habitats. Research on the context-dependent nature of competitive displacement predicts that Ae. albopictus will not replace native Aedes triseriatus (Say) in treeholes but could do so in artificial container habitats. Ae. albopictus remains rare in temperate treeholes but less is known about how Ae. albopictus fares in artificial containers in forests. Tyson Research Center (TRC) is a field station composed of mostly oak-hickory forest located outside Saint Louis, MO. The container community has been studied regularly at TRC since 2007 with permanently established artificial containers on the property since 2013. Ae. albopictus was detected each year these communities were sampled; however, its abundance remains low and it fails to numerically dominate other species in these communities. We present data that show Ae. albopictus numbers have not increased in the last decade. We compare egg counts from 2007 and 2016 and combine larval sample data from 2012-2017.We present average larval densities and prevalence of Ae. albopictus and two competitors, Ae. triseriatus and Aedes japonicus (Theobald) as well as monthly averages by year. These data highlight a circumstance in which Ae. albopictus fails to dominate the Aedes community despite it doing so in more human-impacted habitats. We present hypotheses for these patterns based upon abiotic and biotic environmental conditions.

Egg sampling; 2007 and 2016

Eggs were collected using identical protocols during 2007 and 2016 to compare the average number of Ae. albopictus eggs laid at TRC when these communities were first studied (2007) and a decade later (2016). Fifty 500 ml black plastic cups lined with seed germination paper (thus forth “egg papers”) were attached to trees 1-2 meters from the ground and filled with 270 ml tap water and 30 ml of a 10%, by weight, hay infusion incubated for 7 days. Cups were placed along five transects, in approximately the same locations for both years. Per transect, 5 traps were placed along the forest edge on service roads (2 m wide, full canopy) 50 meters apart, and 5 were placed 50 meters into the forest. Egg papers were collected on three dates, at weekly intervals, within each of three collection periods; early June, mid-July, and late August after each paper had been in the field for four days (450 samples/year). Egg papers were incubated in an environmental chamber for 4-7 days before being placed in 0.35 g/L nutrient broth solution (Difco^TM) to stimulate egg hatching. Larvae were identified to species as third or fourth instars. No attempt was made to count total eggs laid or to identify unhatched eggs. Larvae identified as Ae. albopictus are reported as the number of Ae. albopictus eggs laid per day. We used a Generalized Linear Mixed Model (GLMM) with a zero inflated Poisson error distribution using eggs laid per day as the response variable, year and month as independent variables, and transect as a random effect (PROC GLIMMIX). The three samples within each month and the forest vs. edge samples were collapsed into “month”. We also analyzed proportion of egg papers with Ae. albopictus present using a GLMM (PROC GLIMMIX) with a binary distribution (present vs. absent) testing for effects of years, months, and interaction, with transect as a random variable. Both analyses were performed in SAS 9.4.
 

Larval sampling: 2007 and 2012-2017

To compare the larval densities and frequency of collection of common species at TRC we summarized the data collected from larval samples from 2012-2017. These data were collected from black plastic containers which always received an initial input of rainwater and oak leaf detritus. Each year of data originates from a different field experiment, with different manipulations, designed to answer research questions not directly related to this study and is repurposed here (Westby and Juliano 2017, Juliano et al. 2019, Westby et al. 2019). Details about the experimental manipulations and sampling schedule can be found in the supplemental file. Depending on the year, larval communities were either subsampled destructively or the entire community was identified and returned to its container. To standardize the data, accounting for differences in methods, we present the prevalence of each Aedes species in samples from containers by month with the years 2012-17 combined, in addition to prevalence for the dominant predator Toxorhynchites rutilus (Coquillett). We also present mean densities (larvae/liter) from containers where the focal species was present. We present the average monthly Ae. albopictus larval density for every liter of water sampled (e.g., not excluding samples where they absent) and prevalence by month for all the years including larval samples collected in 2007 (not included in the prevalence and density data above). Larval samples were collected in 2007 but volumes were not recorded.

We limited our statistical treatment to the egg sampling data collected in the two years in which we replicated field methods exactly (2007 v. 2016). For other years, the data were collected differently or there were not enough data points in each month for each manipulation in all years to include in a statistical model. We also present no data on Culex as they were rare in larval samples later in the season when we began to detect Ae. albopictus.