JARS v36n2 - How Not to be Bored

How Not to be Bored
David G. Leach, North Madison, OH

One of the hardy but false legends about the cultivation of rhododendrons is that the Rhododendron Borer, Synanthedon rhododendri , attacks mainly plants of reduced vigor which have been weakened by poor growing conditions, other insects, disease or injury. Nothing could be further from the truth.
Unfortunately, statements repeated often enough acquire a patina of venerable respect, usually by being copied time after time by succeeding generations of garden writers whose inclinations run more toward convenience than observation. Articles containing erroneous information about the Rhododendron Borer appeared at least 60 years ago. Cribbing from earlier writers has a long, if not respected tradition; even the hallowed Carolus Linnaeus, in his Flora Lapponica , published in 1737, copied without attribution to the author from a book written by Henry Mundy at Oxford in 1680.
It is not true, either, that these borers invariably progress downward toward the base of the plant in their destructive tunneling. It is commonplace to find them headed upward, and there may be two or three of them in one tunnel, especially if the galleries intersect. It is not correct that only large, old branches are attacked; I have seen 18- to 24-inch plants infested by the scores in nursery rows.
The Borer emerges from eggs laid by a moth with transparent wings which appears in late May or early June. It is one of the two most destructive pests that attack rhododendron plantings, and it is among the most difficult to control. An infestation never subsides spontaneously; it only progresses, with exponential increase. There is a clear pattern of spread to adjacent plants. An unchecked invasion can be devastating in the disfigurement and casualties that it produces.
Injury shows up most clearly in the fall, when one or more of several symptoms appear. The leaves of branches with vascular circulation interrupted lose their sheen; progressively, they become pale green, then olive, and finally and emphatically, chlorotic yellow-green. They then wilt and die. If a branch is noticed at the earliest stage, it may be saved. The key is to detect any branch with even a slight difference in foliage color from the remainder of the rhododendron. Another sign is a branch on which the twigs have not made normal growth, compared with the surrounding branches; this usually results from slight to moderate injury the previous season, producing foliage which is both sparse and undersize. Still a third symptom may be only one or two branches with the discoloration of older leaves which precedes normal shedding.
Should any of these symptoms appear, a search of the limb crotches and of the ground beneath the plant will usually turn up small accumulations of fine, brunneous sawdust which has been pushed from the borer tunnels. Somewhere on the plant above the sawdust accumulations will appear perforations in the bark as if the stem had received part of a charge of buckshot; if the stem is heavily infested, it will have larger dark brown pits of irregular outline.
If the injury is only minimal, the afflicted branch may possibly be saved by probing the tunnels with a piece of wire to kill the borers, and then injecting a larvacide to eliminate any that may have been missed. Almost always, it is better to accept the disfigurement and cut off the branch below the lowest penetration of the borers; they will otherwise spread, and the branch will almost certainly die anyway. If the borers have already progressed to the point where their elimination would require cutting off all or most of the plant, leave a branch or two at the base, probe with a wire the tunnels which will be visible in the stump, and then, from an oilcan with a flexible spout, inject a mixture of a larvacide such as Dursban and water, half and half. This is the only control that has really been effective for me.
The larvae are milky white with brown heads, about three quarters of an inch long. Dr. David Nielsen, entomologist at the Ohio Agricultural Research and Development Center, states that adult moths, eggs and exposed larvae can be killed for the season with a single spray of Dursban, 2 quarts of the 2E emulsion per 100 gallons of water (4 teaspoons per gallon), at the end of May or the first week of June in northern Ohio. It has an affinity for organic matter and a very long residual effectiveness. Other sprays have been sensationally ineffective for me.
Whether or not a universal, easy and safe control under all conditions is found, hobbyists and professional growers are likely to find conventional ideas about the borers costly in terms of plant losses. Contrary to the concept that enfeebled or injured old plants are the most vulnerable, the fact is that the lustiest and fastest growing young plants are the most susceptible, possibly because the stem tissue is less dense and provides easier entry for the borers. In my fields, four lots of hybrid seedlings are conspicuously far more vigorous than any others: 'Catalgla' and 'Scarlet Blast' crossed with the white R. campanulatum SSW9107; and 'Spring Frolic' and 'Catalgla' crossed with the Edinburgh form of R. fictolacteum . These robust super-plants are not only much more susceptible to borer attack than others of comparable age, an association with superior vigor that I have previously noticed; but also, instead of being stately old specimens, not one is more than two feet tall.