Answer: Healthy, vigorous plants will withstand insect attacks, and it's really the best thing you can do to prevent insect problems. If your plants are visibly stressed, forcing them to grow with fertilizer at this point would just stress them further. Also, plants don't utilize fertilizer during dormancy, so wait until next spring. Right now, layer some compost around the root zone to decompose and add nutrients to the soil slowly. Next spring, just as new growth is to start, apply a balanced fertilizer, e.g., 10-10-10, according to the lable instructions. Water thoroughly and deeply before and after applying the fertilizer. Apply it out to the plant's drip line, where new roots are.
For future reference, malathion is not generally recommended as a control for whiteflies. Usually natural enemies of whiteflies -- ladybugs, pirate bugs, tiny wasps -- keep their numbers in check, but an insecticide like malathion kills these "good bugs," too. Less drastic measures work fairly well as long as you practice them often, even daily. A strong blast of water from the hose often works. Spray underneath leaves where they hang out.
Whiteflies are attracted to the color yellow. You can purchase or make yellow "sticky" traps from yellow cardboard smeared with petroleum jelly. They fly to it and get stuck.
Soapy water sprays are another possibility. Use 1 to 2 tablespoons of liquid detergent soap per gallon of water. Use regular strength soap, rather than concentrated. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. Spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Spray early in the morning before the sun heats up.Next on my list would be an insecticidal soap spray.
The insecticidal soaps are made from plant-derived fatty acids and target soft-bodied insects. There's really no way you can target the bad guys without fallout on the good guys. If you can regularly monitor and tolerate some damage to your plants, over time Mother Nature strikes a balance, with the beneficials coming in to control the bad guys. I hope this information helps!
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