Oviposition choices of dengue and chikungunya vectors in a rural area of Timor-Leste: implications for use of ovitraps in surveillance and control, based on field and laboratory experiments

Abstract Although Aedes (Stegomyia) aegypti L. (Diptera: Culicidae) is the major vector of dengue, Aedes albopictus (Stegomyia albopicta) Skuse has also been implicated in dengue outbreaks. These species are also vectors of the increasingly prevalent chikungunya virus. Larval surveys of natural and artificial containers in a small rural community in Manufahi, Timor-Leste over the past 3 years in wet and dry seasons (See Chapter 4) have found Ae. albopictus to be common, but Ae. aegypti was not detected, although this species is present in some other parts of Timor-Leste. Ovitraps (mosquito egg traps) have been widely used in monitoring Aedes mosquitoes, and lethal or autocidal ovitraps have been used as components of dengue control programs. Enhancing ovitraps with attractants or oviposition stimulants improves their efficacy, as does placing them in locations where they will be most likely to be detected by gravid females. Inexpensive, non-toxic, autocidal ovitraps constructed from recycled materials were used to compare known oviposition attractants or stimulants, singly and in combination. The attractants were: (a) a compound found in Aedes eggs (dodecanoic acid); (b) components of nitrogen, phosphorus and potassium-based (NPK) fertilizer, and (c) infusions of discarded cigarette butts. A solution of ammonium phosphate and potassium nitrate received significantly more Ae. albopictus eggs than water only (control). Dodecanoic acid and cigarette butt infusions did not receive significantly more eggs than the control; however, they attracted various other Diptera and many non-culicid larvae developed in ovitraps in which these substances were used; thus, the presence of eggs or larvae of other species may have deterred Aedes oviposition. Significantly more Aedes eggs were found in ovitraps under vegetation than in ovitraps placed inside houses or against external walls. Clear-sided ovitraps in which black mesh was placed over a black ring floating on the water surface collected significantly fewer eggs than black ovitraps with identically placed mesh and rings. Responses of gravid Ae. aegypti to various attractants were assessed by oviposition choice experiments in the laboratory, using single caged mosquitoes. Response was evaluated by comparison of egg counts, and in some cases by assessment of modification of the skip oviposition tendency. Significantly more eggs were laid in cups containing a commercial NPK fertiliser, combined ammonium phosphate and potassium nitrate solution (20 mg/L N), or dilute pepper infusion than in cups containing water only. Fewer eggs were laid in cups containing ammonium phosphate or potassium nitrate or ammonium sulphate solutions (20mg/L N) than in cups containing a combination of ammonium phosphate and potassium nitrate. No eggs were laid in cups containing concentrated pepper infusion. Approximately 14% of females did not exhibit skip oviposition at baseline. NPK solution, a solution of NPK fertiliser and pepper infusion modified the skip oviposition tendency to some extent. To be effective, ovitraps should be at least as attractive to gravid females as alternative natural or artificial oviposition sites. In the wet season, larval Ae. albopictus are frequently found in rainwater-filled broken coconut shells and experiments were conducted to assess factors affecting their attractiveness as oviposition sites . In field experiments, coconut shells containing Stage 3 and 4 larval Armigeres received significantly fewer eggs than coconut shells without larval Armigeres. Ar. malayi and Ar. milnensis had not previously been recorded in Timor-Leste, until this field study