Crouching tiger, hidden trouble: urban sources of Aedes albopictus (Diptera: Culicidae) refractory to source-reduction

Abstract

Our ultimate objective is to design cost-effective control strategies for Aedes albopictus, the Asian tiger mosquito, an important urban nuisance and disease vector that expanded worldwide during the last 40 years. We conducted mosquito larval surveys from May through October 2009 in the City of Trenton, New Jersey, USA, while performing intensive monthly source-reduction campaigns that involved removing, emptying, or treating all accessible containers with larvicides and pupicides. We examined patterns of occurrence of Ae. albopictus and Culex pipiens, another urban mosquito, among different container types by comparing observed and expected number of positive containers of each type. Expected use was based on the relative frequency of each container type in the environment. Aedes albopictus larvae and pupae were found significantly more often than expected in medium volumes of water in buckets and plant saucers but were rarely collected in small volumes of water found in trash items such as discarded cups and cans. They were also absent from large volumes of water such as in abandoned swimming pools and catch basins, although we consistently collected Cx. pipiens from those habitats. The frequency of Ae. albopictus in tires indicated rapid and extensive use of these ubiquitous urban containers. Standard larval-based indices did not correlate with adult catches in BG-Sentinel traps, but when based only on Ae. albopictus key containers (buckets, plant saucers, equipment with pockets of water, and tires) they did. Although we found that only 1.2% of the 20,039 water-holding containers examined contained immature Ae. albopictus (5.3% if only key containers were counted), adult populations were still above nuisance action thresholds six times during the 2009 mosquito season. We conclude that in urban New Jersey, effective source reduction for Ae. albopictus control will require scrupulous and repeated cleaning or treatment of everyday use containers and extensive homeowner collaboration.

Figure 1. Aerial map of the study site and 12 BGS trapping sites within a 50 m radius (red circles/sampling units).

We aimed to place each BGS trap within one of the cells of a square grid with 200 m sides. When that was not possible we placed the BGS trap in a nearby cell. Orange highlighted parcels indicate parcels with at least one container positive for immature Ae. albopictus during the month of August 2009. The four parcels highlighted in yellow were the only ones never inspected during the study.

Figure 2. Number of wet containers and container type examined each month (May-October) in the study site during larval surveys in 2009.

Containers were classified as “disposable” if intended for one-time use, “non-disposable” if intended for long-time term or repeated use, “movable” if easily displaced by an average adult and “non-movable” if otherwise.

Figure 3. Daily temperature, humidity, and rainfall data from the Trenton weather station (Mercer-Trenton Airport, TTN) during the 2009 mosquito season.

Gray line with circles: temperature, dark grey line with arrows: humidity, black columns: precipitation.

Figure 4. Seasonal shift in the number of containers positive for Ae. albopictus (black line) and Cx. pipiens (grey dashed line) during the larval surveys.

Time stage 1 = May/June, 2 = July, 3 = August, 4 = September/October.