Any broad-spectrum pesticide will potentially harm unintended targets like benign insects or other organisms, including eucalyptus oil, though to what degree of harm can vary significantly depending on many factors. Scientific references we're finding about the efficacy of this ingredient usually pertain to using it as a personal repellent on the skin, not as a perimeter spray material. If anyone is using eucalyptus oil as a perimeter or yard spray, it should absolutely be as a product labeled for that use as a pesticide (likely with an EPA registration label, though sometimes very low-toxicity products are exempt from EPA registration) as opposed to one only labeled as a personal repellent.

In general, we do not recommend perimeter pesticide spraying for ticks (or mosquitoes) as it has questionable efficacy and doesn't necessarily override the need for personal protection (using on-skin or on-clothing repellents, or at least performing a thorough body check after coming indoors). Ticks might fall off their host (deer, rabbit, etc.) after the animal has crossed-over a chemically-treated perimeter and into the yard, which means the tick is then in an untreated area. There are other means by which someone can reduce the appeal of the home landscape to ticks (and their carriers), though the simplification to the landscape this may entail could have the unintended consequence of limiting biodiversity as well (including the organisms that prey on ticks or their hosts to keep populations in check). Below are some resources that might be of use; there will be a fair amount of overlap in their content:

• Cornell: Managing Ticks in the Landscape
• University of Maine: Tick Prevention
• University of Connecticut: Ticks
• Penn State: Ticks and Tickborne Disease

Miri