Since the plants already are weakened by limited light, they have an increased susceptibility to insect damage. Especially susceptible plants include petunias, impatiens and ivies. One solution is to rotate plants every two or three weeks so that those under overhangs and on porches can by rejuvenated by increased exposure to light, rain and wind. You may also wish to drench the foliage of such plants thoroughly with a stiff spray from the hose each time you water. This disorients and kills many spider mites and washes off other sucking insects.
Scale insects present another special problem.
Ferns, particularly staghorn and other flat-leaved ferns, are highly susceptible. You can kill scales by dabbing them with a cotton swab dipped in vegetable oil. This will suffocate them. Systemic insecticies, such as Orthene, also are successful against scale insects, especially in spring when their hard outer coating becomes soft so the animals can move around and mate. It will stink up your house.