WAGENINGEN, THE NETHERLANDS-- A small band of molecular biologists is seeking to replace the natural mosquito populations ravaging developing countries with "designer mosquitoes," genetically modified so that they are unable to transmit diseases such as malaria. But at a workshop here last week, 20 of the world's leading mosquito ecologists said that huge ecological questions remain--and it's time funding agencies, which have enthusiastically endorsed the transgenic mosquito plan, start devoting attention and money to answering them.