The control of mosquitoes is becoming increasingly challenging because climate change and global trade favor the spread of invasive mosquito species (Roiz et al. 2008, Schäfer and Lundström 2009) and strongly increase the associated risk of vector-borne diseases (Weaver and Reisen 2010). Most strategies for mosquito control are based on the use of insecticides. However, intensive use of insecticides has unwanted effects on non-target species (Suma et al. 2009, Mommaerts et al. 2010) and increases the risk of target species developing resistance (Akiner et al. 2009, Melo-Santos et al. 2010). Furthermore, treated populations can recover after application of the insecticide (Seleena et al. 1999). As a consequence, integrated pest management, using biological antagonists either alone or in combination with insecticides, has drawn increasing interest (Gurr et al. 2003, van den Berg and Takken 2009, Gentz et al. 2010).
Strategies for mosquito control that are based on biological methods have been used since the early 20th century (Legner and Sjogren 1984). In particular, the use of predators (e.g., Gambusia affinis, Dystiscidae, Odonata) has been investigated intensively (Bence 1988, Blaustein et al. 1995, Fincke et al. 1997, Kumar and Hwang 2006, Beketov and Liess 2007). However, difficulties in colonization and the management of predators, as well as a lack of synchrony between the life cycles of predator and prey, have impeded their deployment (Bay 1974, Kindlmann and Dixon 2001, Kumar and Hwang 2006). As a consequence, we have changed our focus to the use of natural competitors (i.e., crustaceans) to control mosquito larvae. Many crustacean species show similar biotope preferences (i.e., early colonization of temporary ponds) and similar feeding behavior (i.e., filter feeding) to that shown by mosquito larvae (Williams 2006). Given that crustaceans are found in nearly all types of pond, they could be used as agents for mosquito control without disturbing the natural biotope fauna. Previous field investigations found that competing communities of crustaceans had a negative impact on populations of mosquito larvae (Chase and Knight 2003, Beketov et al. 2010, Meyabeme Elono et al. 2010). In semi-field experiments, Cladocerans (e.g., Daphnia magna) negatively affected mosquitoes (i.e., Culex pipiens or Anopheles quadrimaculatus) by reducing the amount of oviposition (Duquesne et al. 2011), increasing time to pupation (Knight et al. 2004, Stav et al. 2005), and reducing total abundances of mosquito larvae (Knight et al. 2004, Duquesne et al. 2011). However, the negative effect of Cladocerans was only found for well-established populations, that is, Cladoceran populations that had developed for more than one week before colonization by mosquito larvae (Knight et al. 2004, Stav et al. 2005, Duquesne et al. 2011). Hence, under field conditions, control of mosquito larvae by crustaceans will be limited during the initial phase of community development, when abundances of crustaceans are still low. Thus, the use of insecticides may still be required to control mosquito larvae until populations of crustaceans have become established. In this respect, the use of the biological insecticide Bacillus thuringiensis israelensis (Bti) in combination with the introduction of a crustacean community might be an appropriate integrated approach to mosquito management. Indeed, Bti has no negative effect on crustacean populations (Rivière et al. 1987, Becker et al. 1992). However, its ability to eliminate mosquito larvae is only temporary (Boisvert and Boisvert 2000).
On the basis of the findings mentioned above, we hypothesized that the long-term effects of crustaceans will strengthen the impact of the insecticide, owing to additive and complementary effects of these two stressors on larval populations of mosquitoes. In the present study, this hypothesis was evaluated under field conditions by attempting to eliminate mosquito larvae that had colonized ephemeral ponds by either introducing only natural competitors, applying Bti alone, or applying Bti in combination with introduced natural competitors. The aim of the study was to investigate whether the integrated control approach was more effective and sustainable than the common strategy for mosquito control, which involves Bti treatment alone.