for pest controll, buy some cheap beer and put it out in small cans around the plant....this will keep slugs away, so make sure its small enough for them to get in...

as for yellowing and dieing leaves...it happens, most of the nutrients are going to the top of the main stems now...simply clip all old leaves and shade leaves off..

also buy some blooming nutes and take a gallon out with you once a week...

they are strong plants and will recover nicely most of the time