It would remove those less fit. I think the point of the effect only being at second generation is that it makes the lack of fitness signal weaker in these male

It would remove those less fit. I think the point of the effect only being at second generation is that it makes the lack of fitness signal weaker in these males than ordinarily sterilised males. Added to that they keep releasing the modified mosquitoes so they are more numerous (in effect more fit since they have a massive breeding advantage from the humans breeding them).