Karyotype occurrence and host plants of the corn leaf aphid (Homoptera : Aphididae) in a Mediterranean region

Karyotype occurrence and host plant association of Rhopalosiphum maidis (Fi tch) were studied in Catalonia (northeastern Iberian Peninsula), a region w ith a Mediterranean climate. Populations with 2n = 10, 2n = 8 and 2n = 9 ch romosomes were found, the latter being, much less common. Aphids with the 2 n = 10 karyotype were found mainly on barley (Hordeum vulgare L.), Setaria verticillata (L.) P. Beauv., and Echinochoa crusgalli (L.) P. Beauv. It is suggested that the partial overlap of the life cycles of these hosts allows aphids with this karyotype to complete their annual cycle on local hosts. The aphids with nine chromosomes were mainly found on barley, whereas those with the 2n = 8 karyotype were found mainly on sorghum (Sorghum bicolor L. Moench.) and johnsongrass [Sorghum halepense C. (L.) Pers.]; and corn (Zea mays L.) was an unsuitable host. The role of corn as a host of R. maidis i n the region and the reasons why colonies are incapable of growing on corn are discussed. The corn leaf aphid should not be regarded as a single pest in the area studied. Karyotypes with 8 and 10 chromosomes show different bi ological features and host-plant associations. Thus, separate strategies sh ould be followed to control them on sorghum and barley, respectively. The r esults of our survey show that R. maidis cannot for the moment be considere d a corn pest in the northeast of the Iberian Peninsula.