Ecological and evolutionary determinants of host species choice in mosquito vectors Issa N. Lyimo 1, 2 and Heather M. Ferguson 1, 2 1 Public Health Entomology Unit, Ifakara Health Institute, Off Mlabani Passage, PO Box 53, Ifakara, Tanzania 2 Faculty of Biomedical and Life Sciences, 120 University Place, University of Glasgow, Glasgow G12 8TA, UK Insects exhibit diverse resource-exploitation strategies, including predation, herbivory and parasitism. The eco- logical and evolutionary factors that influence the resource selection of some insects (e.g. herbivores) have been extensively investigated because of their agricul- tural importance. By contrast, there has been little inves- tigation of the selective forces that mediate host choice in haematophagous insects, despite their importance as vectors of disease. Here, we review potential determi- nants of host species choice in mosquitoes, the most important insect vectors of human disease, and discuss whether these could be manipulated to yield new disease- control strategies based on vector behavioural change. Epidemiological relevance of mosquito host species choice Insects are the most abundant and diverse group of animals [1]. Due to the massive economic impact of herbi- vorous insects on agriculture and forestry, the determi- nants of their feeding preferences have been investigated extensively [2]. By contrast, little is known about the determinants of host choice in insects that feed on vertebrate blood. These organisms transmit numerous human and animal diseases; mosquitoes have the greatest impact on public health because of their role in malaria, arboviral and filariasis transmission [1]. Although not all mosquitoes require blood for reproduction (some are capable of autogeny), the host species choice of those that do is a crucial determinant of the transmission intensity of pathogens they transmit [3] (Box 1). The ecological and evolutionary factors that may determine host choice in haematophagous mosquitoes are reviewed here. The cen- tral aim is to discuss the ultimate selective forces respon- sible for driving the evolution of host species choice and how they may be manipulated to reduce vector-borne disease. We do not focus on the proximate mechanisms used to detect and distinguish between host species or individuals (for a review, see Ref. [4]) but rather the potential ultimate evolutionary factors that have shaped these choices. Evolution of host choice A substantial body of theory and empirical research has accrued to predict how organisms should select dietary resources to maximize their fitness (see Glossary) [5]. The widespread existence of dietary specialization presents a paradox: why choose a limited variety of resources when many are available? Dietary specialization is predicted to evolve when there are trade-offs between the fitness obtained from feeding on different resources, such that net energy gain is higher when only a limited subset is consumed [6]. By contrast, generalism should evolve when there are only moderate differences in energetic gains between resources [6]. By extension to mosquitoes, generalism should evolve in environments where the chances of host encounter are low and the advantage of waiting for an optimal host is traded off against the risk of death before feeding. Conversely, specialism should arise as the frequency of encounters with favorable host species increases. The host breadth of hae- matophagous insects is probably the product of both optimal foraging on currently available hosts [7] and historical patterns of host availability, as reflected by phylogenetic congruence with their vertebrate hosts [8]. For mosquitoes to evolve generalist or specialist host- feeding strategies as predicted, there must be genetic variation for host choice. Although rarely investigated, this phenomenon has been documented in the African malaria vector Anopheles gambiae, in which divergent feeding preferences for humans or cows were generated within 5–6 generations of selection [9], and in cross-mating experiments with zoophilic Aedes simpsoni and anthropo- philic Aedes aegypti, which generated offspring of inter- mediate preference [10]. Thus, selection has the potential Review Glossary Anthropophilic: mosquitoes exhibiting a preference for blood feeding on humans. Autogeny: the ability to produce one or more egg batches without feeding on blood. Some mosquitoes are autogeneous, with energetic resources for egg production being taken from larval nutritional reserves instead of from bloodfeeding at the adult stage. By contrast, the egg production of anautogenous mosquitoes depends upon consumption of blood at the adult stage. Human blood index (HBI): the proportion of bloodfed mosquitoes in a given population sample that test positive for human blood. Fitness: this term is notoriously difficult to define and is given numerous different definitions in the literature. In this article, it is defined as the total number of offspring produced by a certain type (of phenotype or genotype) of individual relative to another type. Zoophilic: mosquitoes exhibiting a preference for blood feeding on non-human animals. Corresponding author: Lyimo, I.N. (ilyimo@ihi.or.tz) 1471-4922/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pt.2009.01.005 Available online 9 March 2009 189