Healthy plants with plenty of water is the best plan. Do not try to grow plants unsited to the site, or your city's location. Replace them with some plant that is more suitable. Make an agreement with your customer to use neem oil, dormant oil, BT, rotenone, pyrithrins and so forth. And agree with your customer's go ahead to use a low risk chemical if you must--synthetic pyrithroid, or something from the EPA's "low risk" list.