JARS v38n3 - Pests of Rhododendrons

Pests of Rhododendrons
Arthur L. Antonelli
Extension Entomologist, WSU

Almost all plants are fed upon by insects or other animal pests, to some degree or another, and rhododendron is no exception. Most rhododendron pests are minor or sporadic in their occurrence. A few cause great harm and are nearly always present. Additionally, some pests are of major importance in one geographical area but of only minor importance in another. Rhododendron pests can be broadly categorized as to the type of damage they do to a plant. They can be classified either as chewing pests or sucking pests, or, they might be categorized as to where they feed on the plant - leaves, stems, or roots.

Chewing Pests
Root weevils: Root weevils are universally the most serious pests of rhododendrons. Several species of root weevils feed on this plant group. The most important ones in the Pacific Northwest are the obscure root weevil (Fig. 1) and the black vine weevil. Some that may be of greater importance in other areas include the clay-colored weevil, woods weevil, strawberry root weevil, and possibly others. Both the larvae and the adults damage rhododendrons as well as other plant species.

Adult obscure root weevil.
Fig. 1 Adult obscure root weevil.
Photo by unknown

Adult weevils chew on the margins of leaves, leaving characteristic notches along the edges (Fig. 6). Heavy feeding by adults causes large ragged areas rather than the typical marginal semi-circular notching.

Rhododendron leaf damage from adult root weevil.
Fig. 6 Typical appearance of a rhododendron after
adult root weevil feeding has occurred.
Photo by Arthur Antonelli

Control of adult root weevils is accomplished in several ways. Pesticides, mechanical barriers, and the selection of resistant species or hybrids provide means of reducing damage. There are effective insecticides available. Selection of chemicals should be based on local expert advice. Some prefer to use non-chemical approaches. Trapping provides one means of reducing weevil damage without using pesticides. Plastic sheeting is wrapped snugly around the lower trunk and Tanglefoot is applied to the outer surface of the plastic. The sleeve is left on only during the adult weevil feeding period. In the Pacific Northwest, the feeding period is generally from spring to late fall, but it may vary with weevil species and geographical location. Some gardeners actually apply Tanglefoot directly to the trunks without the plastic. We cannot vouch for the ultimate health of the bark from direct application until research demonstrates non-harmful effects. Tanglefoot trapping works because weevils do not fly - they climb the plants. Additionally, they generally feed only at night, dropping to the ground and hiding by day. Upon return, they are prevented from access to leaves by the sticky plastic barrier.
There are a considerable number of rhododendron species and hybrids that possess varying levels of resistance to adult weevil feeding. Partial lists of species and hybrids with weevil resistance have been published (Antonelli and Campbell, 1981). The mechanisms of resistance are of two types - mechanical and chemical. The curled leaf edge of R. williamsianum which prevents the weevil from biting the leaf edge presents a unique form of mechanical resistance (Doss, 1980). Certain species possess chemicals that repel the adult weevils (Doss, et al, 1980). Utilizing genetic resistance to root weevil feeding damage makes a great deal of sense, particularly at the onset of landscaping. In other words, why plant a known susceptible variety when resistant types are available?

Root weevil larvae.
Fig. 7 Root weevil larvae. Note that they
are legless, somewhat "C"-shaped,
and possess a brown head capsule.
Photo by Arthur Antonelli

While root weevil larvae (Fig. 7) feed on rhododendron roots, they also feed on a wide variety of other plant roots. Because of this omnivorous feeding habit, larval damage is not very often critical in the average landscape. The larvae are of extreme concern in potted plants where there is nothing available except the resident plant's roots. They even girdle or feed on the bark of the lower stem under the protection of bark or sawdust mulches (Fig. 2). There is a chemical drench registered in certain states that gives some protection. Researchers are currently investigating alternative chemicals for larval root weevil control.

Larval root weevil damage to azalea trunk.
Fig. 2 Larval root weevil damage
to lower trunk of an azalea plant.
Photo by Arthur Antonelli

Over the past year many rhododendron fanciers have inquired about the possibility of using the entomophagous (insect eating) nematode, Neoplectana carpocapsae , to control root weevil larvae. We are unaware of published data demonstrating success of this method of controlling root weevil larvae in open plantings; however, one researcher has demonstrated considerable success in controlling larvae in potted strawberry plants with N. carpocapsae (Breton stain). It is conceivable that similar results could be achieved in potted or containerized rhododendrons or azaleas.

Differences between caterpillar and root weevil 
foliar feeding.
Fig. 8 Differences between caterpillar (bottom)
and root weevil (top) foliar feeding.
Photo by S.J. Collman

Caterpillar types: A number of caterpillar species and caterpillar-like insects feed on the foliage of rhododendrons and azaleas. Many are night feeders, and thus often go undetected except for their damage. Indeed, some people misinterpret damage symptoms and inadvertently blame root weevils. To determine the identity of a night-feeding chewing pest, one should make observations at night with a flashlight. Damage from root weevil adults and that of caterpillar types is generally easy to differentiate once it has been pointed out (Fig. 8). The notching by weevils is quite different from the generally larger, "smoother" cuts of the caterpillar types. Loopers (Fig. 9), leaf-rollers, some tussock moths, and cutworms (Fig. 3), are among the more common caterpillar pests. While not true caterpillars, some sawflies also feed on leaves and cause caterpillar-like damage. The larvae of many sawflies closely resembles true caterpillar larvae.

A looper and damage to rhododendron leaf. One of the few cutworm genera that
has been positively identified as feeding on rhododendron. This one is Orthosia sp.
Fig. 9 A looper and damage to rhododendron leaf.
Photo by S.J. Collman
Fig. 3 One of the few cutworm genera that has
been positively identified as feeding on
rhododendron. This one is Orthosia sp."
Photo by Arthur Antonelli

Chemical controls are registered and effective for many of the caterpillar-like insects. In all cases, the user should consult with a local expert, and ultimately, the label of the selected chemical for details on target pests and methods of use. If only one or a few plants are involved, perhaps the worms can be picked from the plants at night or day, depending on the habits of the pest species.
There are two caterpillars that mine the leaves of rhododendrons and azaleas and they are seen repeatedly in diagnostic plant clinics. These are the azalea leaf miner and the rhododendron leaf miner. The azalea leaf miner is a small yellowish caterpillar that mines inside the leaf tissues and later rolls the leaves. The damage appears as brown blister-like mines on the leaves. The feeding of this pest may also cause premature leaf drop. The rhododendron leaf miner makes a serpentine or fairly straight mine (Fig. 4) starting at the leaf edge and eventually going vertically to, into, or across the midrib, causing all leaf tissue from that point to the tip to die.

Leaf miner damage to rhododendron.
Fig. 4 Rhododendron leaf miner. Note the
mine and the dying terminal tissue.
Photo by S.J. Collman

Chemical controls registered for leaf miners may be used if the infestation is severe. If only a few leaves are involved, squeezing the insect within its mine may decrease damage to an acceptable level. It is also advisable to rake up and destroy the leaves in the fall, as some species pupate in the leaf debris.

Sucking Pests
Aphids: Several species of aphids attack rhododendrons. They feed on the tender new leaves as they are developing. The salivary toxins of the aphids cause distortion of the leaves. Some experts believe that the degree of damage does not warrant control; others believe that control is necessary because the distortion is permanent and aesthetically unacceptable in fully mature leaves. Control efforts here would depend largely on the tolerance level of the gardener to this type of damage. Additionally, the honeydew or fecal deposits of these insects provides an excellent growth media for sooty mold fungus. Sooty mold develops on lower leaf surfaces as aphid feeding progresses.

Scales: Several species of scale insects feed upon the rhododendron complex. These include Lecanium scale, oystershell scale, azalea bark scale, and several unknown species. Mature Lecanium scales appear as tortoise-like bumps (soft) on the bark. The "bumps" may be brown or marbled brown and white. Oyster-shell scales appear, as the name depicts, as small, hard oystershells on the bark. Azalea bark scale (Fig. 5) appears as small white cottony masses on the bark. Severe scale feeding causes stress and affected plants appear thin and unthrifty. Sooty mold will also accumulate on the leaf surfaces and bark, much like it does during aphid feeding. Scales are generally always controlled with pesticides. In most cases, chemical control efforts must coincide with the emergence of the newly hatched crawlers(mobile form) of the specific scale in question, and this varies with geographical areas.

Azalea bark scale.
Fig. 5 Azalea bark scale. One has been opened to
expose the pink eggs and young crawlers.
Sooty mold is present on the bark also.
Photo by S.J. Collman

Lace bug: The rhododendron lace bug is sporadic, but where it occurs, it causes considerable unsightliness to the leaves of many rhododendron species. The damage first appears as a speckled chlorosis. Later, the leaves take on a bronzed appearance. This pest feeds on the undersides of the leaves where it deposits tar-like material (Fig. 10). This distinctive fecal deposition makes diagnosis easy, even in the absence of the lace bug. Control is necessary when damage becomes severe.

Lacebug deposition.
Fig. 10 Rhododendron lacebug.
Note the tar spots on underside of leaf.
Photo by Arthur Antonelli

Miscellaneous Pests
While they are beyond the expertise of the writer, vertebrate pests should at least be given cursory treatment if for no other reason than to create an awareness for the reader.
Moles may not include plants in their diets, however, in search of worms and insects, they may burrow through the root systems of azaleas or rhododendrons and leave them vulnerable to desiccation or other maladies of the environment. The author has experienced this on several occasions. Specialists in small animal control have indicated that trapping is the only reliable method for controlling moles and that it takes considerable practice to become proficient at it.
The mountain beaver is an unusual pest of rhododendron and is rare to nonexistent in many landscapes. However, in new housing projects that pervade forest lands, the mountain beaver may be one of the first real nuisances the homeowner encounters. This animal includes the terminals as food items along with other plant species. They will climb several feet into a bush to get at the tops of branches, and "snip" them off. This, of course, is unacceptable; so control efforts must be implemented. Trapping is very effective and success is much easier to achieve than it is for moles.

Pest control or pest prevention in rhododendrons is not necessarily difficult. All one has to remember to do an effective job is to properly diagnose the pest problem or have it done by an expert. Then, learn enough about the biology and life cycle of the pests, where they are known, so that management techniques can be effectively applied. Unfortunately, we lack adequate knowledge on certain pests that are minor or sporadic in their occurrence. There are university publications available that can help you diagnose and eliminate many of your specific problems. Seek them out and use them. A publication exists that deals with a number of rhododendron problems. (Antonelli, et al., 1984). If chemicals are used, proper timing is almost always essential. Additionally, if a chemical is used, remember to seek local expert advice. Be sure it is registered for rhododendrons, and follow the label directions and precautions.

Literature Cited
Antonelli, A.L. and R.L Campbell. 1981. Root weevil control on rhododendrons. E.B. 0970. CE-WSU., 4 pp.
Antonelli. A.L, R.S. Byther, S.J. Collman, R.R. Maleike, and A.D. Davison. How to identify rhododendron and azalea problems. E.B. 1229. CE-WSU. In press.
Doss. R.P. 1980. Investigation of the bases of resistance of selected Rhododendron species to foliar feeding by the obscure root weevil ( Sciopithes obscurus ). Environ. entomol. 9:549-552.
Doss, R.P., R. Luthi, and B.F. Hrutfiord. 1980. Germacrone, a sesquiterpene repellent to obscure root weevil for Rhododendron edgeworthii . Phytochemistry. 19:2379-2380.