Dye another day: the predatory impact of cyclopoid copepods on larval mosquito Culex pipiens is unaffected by dyed environments

摘要

The control of vectors of parasitic diseases and arboviruses urgently requires the development of innovative measures to enhance efficacies. Cyclopoid copepods have been successful in the predatory biological control of mosquitoes, with marked per capita feeding rates bolstered by high abundances and suitability for container‐style habitats (Marten and Reid 2007, Cuthbert et al. 2018a,b). Indeed, their application to bodies of water has resulted in the complete eradication of dengue fever from entire communities (Kay and Nam 2005). However, mosquito oviposition is highly selective in response to predators, with many mosquito species capable of detecting and avoiding predatory cues within environments fostering high densities of aquatic predators (see Vonesh and Blaustein 2010). Thus, predator cues may have strong trait‐mediated consequences that may affect effective mosquito biocontrol, and so developing measures to counteract such cues is of importance for population management. The use of pond dyes is popular in urban areas where mosquitoes proliferate, and is increasingly applied to enhance reflection and as a means of reducing the growth of algae in waterbodies (Ortiz‐Perea and Callaghan 2017). The application of black pond dye has been shown to be a particularly strong oviposition attractant for Culex mosquitoes, whereas other colored dyes have no effect (Ortiz‐Perea and Callaghan 2017, 2018). The strength of attraction to black dye is strong enough to reverse predator avoidance behaviors by mosquitoes (Cuthbert et al. 2018b). Thus, exploration of the use of black pond dye synergystically with predatory biocontrol agents is warranted. In particular, it is necessary to identify biocontrol agents that are not reduced in their efficacy due to pond dye effects, such as interference with the visual predation capacity of such agents. Here, we assess the impact of commercial black pond dye on the functional responses (FRs; rate of resource intake under varying densities) of two predatory cyclopoid copepods, Macrocyclops albidus and Megacyclops viridis, towards larvae of C. pipiens, an efficient vector of West Nile virus (Di Sabatino et al. 2014). Macrocyclops albidus and M. viridis were collected from ponds within the Glastry Clay Pits, Northern Ireland (54°29’18.5”N, 5°28’19.9”W) using a polypropylene dipper. These copepods were transported in source water to Queen’s University Marine Laboratory and maintained in insectary conditions (25 ±2o C, 50‐60% RH, 16:8 light:dark regime) to stimulate proliferation. Gravid females were isolated from samples with respect to each species and used to initiate pure cultures in accordance with the available literature (Marten and Reid 2007). Following first generation emergence of nauplii, originating females were dissected and identified to species. Separate copepod cultures were initiated in 10‐liter tanks, and fed ad libitum with the protists Paramecium caudatum and Chilomonas paramecium (Sciento, Manchester, England). These protists were cultured in 2‐liter flasks containing 1 liter of dechlorinated tap water and autoclaved wheat seeds ad libitum in insectary conditions. The prey, newly hatched Cx. pipiens, were obtained from the same laboratory where a colony had been maintained. Adult mosquitoes were kept in 32.5 × 32.5 × 32.5 cm cages and blood fed using defibrinated horse blood through a membrane feeding system (Hemotek Ltd, Accrington, England). Pads of cotton soaked in 10% sucrose solution were provided for sustenance of the mosquito colony. Egg rafts were extracted regularly from cages and were placed into 3‐liter larval bowls, with hatched larvae fed ad libitum using ground guinea pig pellets (Pets at Home, Newtownabbey, Northern Ireland) until pupation. To derive FRs, we supplied prey densities of 2, 4, 6, 8, 15, or 30 newly hatched 1st instar Cx. pipiens (1.1 – 1.3 mm) to adult female M. albidus or M. viridis (respective body lengths excluding caudal setae: 1.6 – 1.8 mm and 2.0 – 2.3 mm) over a 6 h experimental period during light conditions (n = 4 per experimental treatment). We starved non‐ovigerous adult female copepods individually for 24 h before the experiment to standardize levels of hunger. Experiments were undertaken in arenas of 42 mm diameter containing 20 ml of dechlorinated tap water from an aerated source. Treatments either had 0.3 g/liter black liquid pond dye added, in line with the manufacturer’s recommendations (Dyofix, Leeds, England), or remained undyed, in a fully factorial design, with the three factors being ‘dye presence/absence,’ ‘predator species,’ and ‘prey supply level.’ The source of dye was also continuously aerated to eliminate any variability in dissolved oxygen concentrations between treatments. Prey were allowed to settle for 2 h in the assigned dye treatment before the addition of predators in a fully randomized array. After 6 h, the predators were removed and the remaining prey alive were counted to derive those killed. Co