Parthenogenic species generally have some trick to avoid the otherwise-inevitable effects of co-evolving parasites and sudden environmental changes. Frequently,

Parthenogenic species generally have some trick to avoid the otherwise-inevitable effects of co-evolving parasites and sudden environmental changes. Frequently, they are parthenogenic only under some circumstances--there is a parthenogenic species of fresh-water fish that responds to environmental stress by shifting over to sexual reproduction. Purely parthenogenic species also have genetic tricks to play. There is a purely parthenogenic snake that is triploid (three copies of every chromosome instead of the more usual two) which likely allows it some more "freedom of expression", genetically speaking. So while you're right that sexual reproduction is a huge evolutionary benefit, parthenogenesis is such a huge benefit to the single parent (it gets 100% of its genes into every offspring instead of a mere half!) that under the right circumstances it will be strongly favoured. In the case of these termites, its only new queens that are the product of parthenogenesis, and it isn't obvious that this is the only way new queens can be produced, so the typical limits are relaxed.