Specialisation pattern of the aphid Rhopalosiphum maidis is not modified by experience on a novel host

The effect of alternative host condition on the pattern of specialisation o f the aphid Rhopalosiphum maidis (Fitch) was studied. R. maidis commonly oc curs in Chile on Johnson grass (Sorghum halepense L.) but rarely on contigu ous wheat (Triticum durum L.) crops. The performance of 23 clones of R. mai dis on S. halepense (established host) and T. durum (novel host) before and after rearing on wheat for 20 asexual generations was evaluated. Prior and after the period of conditioning on wheat no negative correlation of perfo rmance parameters between both hosts was found. Only 8 out of 23 clones sur vived the 20 asexual generations on wheat. Further, after conditioning on w heat, survival of three out of eight clones increased on wheat and three cl ones improved r(m) on Johnson grass but not on wheat. Although some genotyp es of R. maidis were able to withstand wheat stressing conditions, the cond itioning on this alternative host did not cause a decreased capacity to use Johnson grass as host. The results suggest that R. maidis performance on w heat is not genetically constrained, and also that conditioning on wheat ca nnot substantially modify this pattern.