just leave them..they wont hurt your plants, and when the adults hatch, your plants might catch them!
but if you really dont want the larva in the trays, just dump the trays, rinse them out, and then fill them up with new water..
my water trays get pretty gunky after awhile, mostly algae, but there are whole citys of critters living in there too!
every few weeks I let the trays almost dry out (so as not to dump and waste precious rain water!)
then take all the trays and rinse them out good with the hose, maybe even rub with them down with an old sponge or rag..(make sure you dont use any sponges with that "antibacterial" crap embedded in them..I hate all the antibacterial junk we have to have in EVERY product nowdays..dont people realise that only make bacteria WORSE?!
)
but I digress..
clean out the trays, put them back, fill them with new water and they are sparkly cleen!
personally, I dont think the trays every really *need* to be cleaned..the gunk doesent hurt anything, it just looks bad..and i suppose if you let it go *too* far it would probably start to stink!
so cleaning out is fine..I do mine maybe once a month or so..only 2 or 3 times in a season..
Scot