Wow, wassup with my 'rain gutters'?

[Beachboy]

while caulking my siding of house my 8 yr old calls me over and tells me while playing "fetch" with our dog he accidentally tossed ball up into rain gutter. No problem I figure as I have 'step ladder' out already for caulking siding of house Upon standing on ladder and looking into 'rain gutter', I couldn't believe what my eyes saw!
Mosquito Larvae by the thousands if not millions were wiggling around in gutter!
Keep in mind, I've only been at this house several weeks, and rain gutter has been up less than one month too. I can only imagine what rain gutters look like in older homes?!?!

Now, is this something I should worry about, or deal with right away. Is there anything that can be done to treat 'rain gutters'? Or should I just let my filters and UV light do the work?

When should someone start to treat catchment tank with bleach or whatever