After putting my site into one domain instead of multiple subdomains, I checked my logs and noticed that search engines (particularly google) are scanning robots.txt several times a day for each subdomain. I only left the subdomains active so that people who accessed the old URLs can be automatically redirected to the proper URLs.
I have removed my subdomains (except for the one active one I'm using) from all webmaster tools accounts and I make zero references to them on my site.
Is there a way to minimize the amount of times search engines scan robots.txt on my obsolete subdomains?
Ideally, I'd like to have the search engines scan the robots.txt found on each obsolete domain only one time, and then scan all the necessary URLs on the subdomain and that's it, not scan robots.txt every few seconds.