The following is information on leafminer overwintering in the landscape.
Arborvitae, Boxwood and Holly leafminers all over-winter as live larvae within foliage tissue. Pale colored lines or blotches indicate the leafminer presence in evergreen leaves. Inspect mines for live larvae and physically remove infested leaves when populations are small. Heavily infested plants can be noted in your records and scheduled for treatment next season. If soil moisture is adequate, then a soil systemic (e.g. Merit) can be applied during the fall months.
Holly leafminer mines on leaf. Photo by Daniel Herms, The Ohio State University, Bugwood.org
Information from Steven K. Rettke, Ornamental IPM Program Associate in the September 17, 2009 edtion of the Plant & Pest Advisory, Landscape, Nursery & Turf Edition, from Rutgers University http://njaes.rutgers.edu/pubs/plantandpestadvisory/2009/ln091709.pdf