Acta Tropica 109 (2009) 118–123 Contents lists available at ScienceDirect Acta Tropica journal homepage: www.elsevier.com/locate/actatropica Effect of temperature and inter-specific competition on the development and survival of Anopheles gambiae sensu stricto and An. arabiensis larvae Matthew J. Kirby ∗ , Steve W. Lindsay School of Biological and Biomedical Sciences, Durham University, Science Laboratories, South Road, Durham DH1 3LE, UK article info Article history: Received 25 February 2008 Received in revised form 15 July 2008 Accepted 17 September 2008 Available online 1 November 2008 Keywords: Larval development Temperature Inter-specific competition Anopheles gambiae sensu stricto Anopheles arabiensis abstract The two major African malaria vectors Anopheles gambiae sensu stricto and An. arabiensis are sibling species that occupy different climatic niches but are frequently found in the same larval habitats. Differences in survival and development of the aquatic larval stages of these species at different temperatures may help explain adult distribution. The development time from first instar larva to adult at constant water temperatures (25, 30 and 35 ◦ C) was measured in these two species when reared together in the same container (ratio 1:1) and separately. Survival to adult was highest in both species reared at 25 ◦ C and decreased with increasing temperature. More adult An. gambiae s.s. were produced at 25 ◦ C than An. ara- biensis (80% interquartile range (78–88) versus 68% (63–78)) but this situation was reversed at 35 ◦ C (7% (3–17) versus 33% (27–32)). The survival of An. gambiae s.s. when reared alone was similar to that when reared in the presence of An. arabiensis. In marked contrast An. arabiensis suffered reduced survival when raised with An. gambiae s.s. at 30 ◦ C (20% (7–57)) than when reared independently (57% (45–72)). Mean age at eclosion and adult size decreased for both species with increasing water temperature, how- ever An. arabiensis larvae developed at a slower rate and resulted in larger adults than An. gambiae s.s. throughout. The apparent greater production of An. arabiensis at high water temperatures and An. gam- biae s.s. at lower water temperatures may in part explain the spatial and temporal distribution of the two species. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Water temperature is critical to key life-fitness parameters of the aquatic larval stages of many insect species (Baba and Takaoka, 1991; Joshi, 1996; Tun-Lin et al., 2000). Behaviour, survival, rate of development, emergence time, adult size and gender are all to some degree affected by temperature (Hadi and Takaoka, 1995; Ivanova, 1940; Lyimo et al., 1992; Pritchard and Mutch, 1985). Of these factors, survival to adult and development rate probably have the most influence on the recruitment of new individuals to insect populations, and are therefore the most important considerations in studies of insect population dynamics. Insect larvae, like all organisms, survive within a range of tem- peratures defined asthe critical thermal maximum and minimum. The limits of this range depend on the conditions to which the species as a whole has been exposed during its evolution, and those of an individual during its lifetime (Cossins and Bowler, 1987). Tropical and sub-tropical mosquito larvae are necessarily warm- ∗ Corresponding author. Tel.: +44 1913341266; fax: +44 1913341289. E-mail address: m.j.kirby@durham.ac.uk (M.J. Kirby). tolerant and cold-sensitive. Survival in such larvae tends to be highest between 20 and 30 ◦ C and declines rapidly either side of this range (Bayoh and Lindsay, 2003; Shelton, 1973; Tun-Lin et al., 2000). It should be noted however that the thermal range for pro- cesses such as growth and reproduction is usually narrower than that for survival (Cossins and Bowler, 1987). Insects must accu- mulate enough degree days to complete their life cycle (Stacey and Fellowes, 2002). A general trend of accelerated development and/or decreased time to emergence with increasing temperature, within minimum and maximum thermal limits, has been observed in blackflies (Diptera: Simulidae) (Baba and Takaoka, 1991; Hadi and Takaoka, 1995), stone flies (Plecoptera: Nemouridae) (Hogg and Williams, 1996), midges (Diptera: Chironomidae) (Frouz et al., 2002) and in several mosquito species (Diptera: Culicidae) (Mottram et al., 1994; Tun-Lin et al., 2000)) including Anopheles gambiae sensu stricto Giles (Bayoh and Lindsay, 2003; Lyimo et al., 1992). An. gambiae s.s. and An. arabiensis Patton are the principal vectors of malaria in Africa, though An. gambiae s.s. is considered the more important vector because of its greater anthropophily (White, 1974). It has been shown that An. arabiensis dominates in drier conditions while An. gambiae s.s. thrives in wetter conditions (Lindsay et al., 1998); nevertheless the species are sympatric across 0001-706X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.actatropica.2008.09.025