You can control how fast Google crawls your site in Google Webmaster Tools.
Crawl rate for your site determines
the time used by Googlebot to crawl
your site on each visit. Our goal is
to thoroughly crawl your site (so your
pages can be indexed and returned in
search results!) without creating a
noticeable impact on your server's
bandwidth. While most webmasters are
fine using the default crawl setting
(i.e. no changes needed, more on that
below), some webmasters may have more
specific needs.
Googlebot employs sophisticated
algorithms that determine how much to
crawl each site it visits. For a vast
majority of sites, it's probably best
to choose the "Let Google determine my
crawl rate" option, which is the
default. However, if you're an
advanced user or if you're facing
bandwidth issues with your server, you
can customize your crawl rate to the
speed most optimal for your web
server(s). The custom crawl rate
option allows you to provide Googlebot
insight to the maximum number of
requests per second and the number of
seconds between requests that you feel
are best for your environment.
Googlebot determines the range of
crawl rate values you'll have
available in Webmaster Tools. This is
based on our understanding of your
server's capabilities. This range may
vary from one site to another and
across time based on several factors.
Setting the crawl rate to a
lower-than-default value may affect
the coverage and freshness of your
site in Google's search results.
However, setting it to higher value
than the default won't improve your
coverage or ranking. If you do set a
custom crawl rate, the new rate will
be in effect for 90 days after which
it resets to Google's recommended
value.
You may use this setting only for root
level sites and sites not hosted on a
large domain like blogspot.com (we
have special settings assigned for
them). To check the crawl rate
setting, sign in to Webmaster Tools
and visit the Settings tab. If you
have additional questions, visit the
Webmaster Help Center to learn more
about how Google crawls your site or
post your questions in the Webmaster
Help Forum.
Other then that you would probably need to create your own filtering system that sniffs out their user agents and either allows or denies search engine bots based in their user-agent. But that would only affect decreasing their frequency.
