Because I'm heavily involved in digital mapping (GIS), I keep a digital map of my property that indicates every scrape I find each year, and the date I found it. Looking through the last couple of years, here's the earliest scrape I've found each year, and the date (in parenthesis) scraping really exploded:
2003: Oct. 10 (Oct. 22)
2004: Oct. 2 (Oct. 27)
2005: Oct. 1 (Oct. 1)
2006: Sept. 2 (Sept. 30)
2007: Oct. 29 (Nov. 4)
2008: Sept. 27 (Sept. 27)
2009: Sept. 25 (Oct. 1)
2010: Sept. 29 (Oct. 21)
2011: Oct. 5 (Oct. 6)
2012: Sept. 30 (Sept. 30)
Notice what a huge range exists for the earliest to latest dates of both first scrape found and when scraping started in earnest; almost two months difference.