The Collaborative International Dictionary
Preventively
Preventively \Pre*vent"ive*ly\, adv. In a preventive manner.
Wiktionary
preventively
adv. In a preventive way
Usage examples of "preventively".
Why this last should be, I don't know, unless it was because they couldn't afford to have it cut and singed and treated preventively as often, and therefore lost less of it.