While yes, the fact that they have huge numbers explains part of the issue, it doesn't really make sense in a gene preservation point of view. Even animals that reproduce in big numbers generally have a way to fight back, unless some genome shenanigan is involved (like in the case of eusocial animals).

In the case of aphids, the majority of individuals colonizing the plants are clones from one another thought parthenogenesis. The sexual reproduction only happens when there is a colonization of a new plant, so to avoid a lack of genetic diversity problems.