Rhododendrons are evergreen shrubs. They are part of the Rhododendron genus, which also includes azaleas. Once established, rhododendrons rarely suffer from serious insect problems, according to Clemson University. Still, the damage caused by these bugs can be very unattractive, and in young, newly planted rhododendrons, insect damage can adversely affect the shrub's health.
Two types of insects commonly damage rhododendrons: those that suck the juices from the leaves, and those that bore into the wood, consuming the bark and inner layers of the wood. While sucking insects usually do not hurt the plant, borers can cause the health of rhododendrons to decline.
Insect damage on rhododendrons is fairly easy to spot. Chewing and sucking insects leave holes in the center of the leaves or along the margins. Leaves may turn yellow or brown, and wilting or dieback may occur, according to the University of Connecticut. Sucking insects also excrete a sticky substance called honeydew, which may coat the leaves and twigs of the shrub. Damage from boring insects can include black, cracked and dead bark, and little piles of "sawdust" (which is actually excrement) that the burrowing insects leave behind.
Scale, aphids, mites and other common garden insects can infest rhododendrons, but there are other, more damaging bugs that can affect these plants. The black vine weevil (Otiorhynchus sulcatus), for example, leaves distinctive, "C" shapes along the edges of the leaves, according to Ohio State University. It is their larvae, however, that is most damaging. When they hatch, they feed on the roots, which can severely impact the plant's ability to absorb moisture and nutrients. The rhododendron borer (Synanthedon rhododendri) is another major insect pest of rhododendrons. This flying insect lays its eggs on the twigs, where the larvae hatch and immediately burrow deep into the wood of the plant. Branches may wilt and die, and a major infestation can kill a young rhododendron.
Minor infestations of insect pests can be removed by hand or with horticultural oil sprays, according to Clemson University. More serious infestations should be treated with insecticides. The insecticides should be applied at the right time, when the adults are active but have not laid their eggs. This can vary depending on the specific insect, but late June is usually a good time. Prune off wilted branches to control the spread of boring insects if you think larvae have already bored into the plant. You can also cut off any branches that seem to be infested if the insects have not spread to any other part of the plant.
Healthy shrubs are better able to withstand insect damage, so make sure your rhododendron is being cared for properly and is planted on well-draining soil. They need acidic soil and shelter from strong afternoon sunlight and winds. In addition, some cultivars are much more disease and insect pest resistant than others. Try planting Catawba hybrids (R. catawbiense), as these are the hardiness rhododendrons available, according to the University of Missouri.