Improving insect pest management through population genetic data:: a case study of the mosquito Ochlerotatus caspius (Pallas)

1. Rationalizing the use of chemicals in pest control programmes is a fundamental goal that is aided by the knowledge of patterns of population connectivity and historical demography. 2. In this study, we used both mitochondrial and nuclear markers to investigate the genetic structure and diversity of the northern Italian populations of Ochlerotatus caspius, a mosquito of great public health and economic impact in Mediterranean countries. 3. A substantial genetic homogeneity was found among populations, with no association of pairwise population differentiation with the geographic distribution or the environmental heterogeneity of the breeding sites. 4. On a regional scale, we hypothesize a historical demographic expansion, probably associated with late Pleistocene palaeoclimatic events. Furthermore, our results suggest that ever since the expansion event, an extensive gene flow is still the major evolutionary force shaping the overall genetic pattern observed. 5. At a local geographic scale, evidence of recent growth in the size of the population was found in several sites. The increased availability of large breeding sites offered recently by rice paddies seem to have played a major role in giving rise to the observed demographic expansions. Indeed, these sites have become more numerous and undergone changes in the management technique (increased number of dry/flood cycles), which enhanced their suitability as breeding habitats. 6. Synthesis and applications. The migration pattern among populations of Oc. caspius in northern Italy appears to be high enough to maintain an extensive genetic homogeneity. Control programmes on a small geographic scale are thus unlikely to be effective. The modern agronomic techniques and the growth of rice paddies seem to have played a role in the recent demographic history of the species in the Po plain. Therefore, satisfactory control programmes should: (i) be focused mainly on rice paddies as breeding habitats; (ii) favour agronomical techniques involving a reduced number of dry/flood cycles; (iii) be carried out on a wide (regional) geographic scale; and (iv) monitor periodically the susceptibility of populations to the compounds used in control campaigns and adopt strategies aimed to avoid the possible spread of resistance alleles.