The rhododendron stem borer is a native species of longhorned beetle that is also sometimes called the azalea stem borer. This insect is found throughout the range of its host plants, and the adult beetles are similar in appearance to the dogwood twig borer. Adults are approximately 1/2 inch to just over that in length, with yellowish-brown heads and thoraxes (also described as copper colored), with two black spots on the thorax. The hardened wings (elytra) are yellow-gray with dark margins. Adults may be active from June through August. By late June or early July, the adult female beetle lays her yellow eggs (4 mm in size) in new shoots, a few inches below the bud. The eggs are laid between two rows of holes chewed by the female that are approximately 1/2 inch apart. Eggs hatch and the immature beetles (larvae) bore into the host plant twigs, where they feed through the end of summer. It is thought the larval life stage overwinters through two years. In the first year overwintering likely occurs in the host plant stem. Feeding resumes in the next growing season, and the insect spends a second winter in the roots of the host plant. The winter is spent in a "winter cell" created in the tunnel by plugging an area with frass in the tunnel both above and below where the insect is located. Overall, the larva travels from the twig, through the trunk, and down to the roots of the host plant. Along this path, holes in the bark of the host may be found where the larva has chewed an opening, out of which frass (excrement) is expelled. By the second season, the larvae are quite large and capable of pushing out piles of sawdust-like frass that then accumulate on the soil beneath the host plant. Full grown larvae are yellowish white in color and may be up to an inch in length and legless. Pupation is thought to occur in the spring (April or May), however the exact timing as well as a detailed description of this life stage is unknown.
The rhododendron (azalea) stem borer feeds on the leaves, lower twigs, stems, and even the roots of its host plants. Adult beetles will feed on the undersides of host plant leaves, particularly on the midvein. The feeding from the adults can cause callus tissue to form, which leads to abnormal leaf curling. Adult feeding is primarily an aesthetic issue. Signs of larval feeding include sawdust-like frass, broken and dead terminal shoots and twigs, and exit holes in the bark. Smaller sized host plants may break at the base if the infestation is high enough and significant damage is caused by the feeding larvae. The larval life stage does the most significant damage to the host. Egg laying by adult females may cause flagging of branches on hosts such as blueberry and deciduous azaleas. This sort of immediate wilting of the branches may not occur on other hosts, such as rhododendron. Once larval feeding begins, wilting (flagging) may occur on rhododendron. In blueberry, the canes may be killed.
Look for signs of adult emergence and feeding from mid-May through July on susceptible hosts. Monitor for adult feeding on the undersides of leaves which may cause abnormal leaf curling. Search for egg laying sites on twigs, particularly new shoots, in late June or early July.
Prune out and destroy any infested twigs as soon as they are noticed. Look for egg laying locations on new shoots from approximately late June through July. Remove and destroy those twigs as soon as possible, to prevent the egg from hatching and the larva from boring into the stem of the host plant. Pruning should occur below the lowest hole in the bark used by the larvae to expel their frass. Examine the cut material to make sure the larva is found within the section that was removed. In areas where the infested section cannot be pruned out or removed, historical reports indicate that a thin, flexible wire (such as bailing wire) can be inserted into the hole and used to kill the larva within the tunnel. Holes used by the larvae to expel frass are oriented upward, and as such cannot be used for the "wire technique" because they will not allow one to reach the larva, which will be found below that hole.
A small, hymenopterous parasite of the larval life stage of the rhododendron stem borer is noted in the literature, however the species of the parasitoid was, at the time, unknown. Five out of ten larvae may be parasitized by this natural enemy (Driggers, 1929). Later, Woolwine et al., 1996, reported that Driggers (1929) was likely refering to Temelucha (formerly Cremastus) forbesi and T. grapholithae. In northwestern South Carolina, a Neocatolaccus spp. parasitoid of the larvae was also collected (Culin and Gorsuch, unpublished data). Also in South Carolina, additional parasitoids of the rhododendron stem borer were collected: Heterospilus spp., Neocatolaccus spp., and Euderus spp. (Woolwine et al., 1996).
Azadirachtin (NL)
Bacillus thuringiensis subsp. aizawai (L)
Beauveria bassiana (NL)
Bifenthrin (NL)
Bifenthrin+imidacloprid (L)
Chlorpyrifos (larvae) (N)
Chromobacterium subtsugae (NL)
Cyantraniliprole (NL)
Emamectin benzoate (L)
Flonicamid+cyclaniliprole (N)
Tau-fluvalinate (NL)
Imidacloprid (L)
Isaria (paecilomyces) fumosoroseus (NL)
Lambda-cyhalothrin (L)
Neem oil (NL)
Permethrin (L)
Spinosad (NL)
Active ingredients that may be applied systemically include: azadirachtin (injection), cyantraniliprole (soil drench, soil injection), emamectin benzoate (injection), imidacloprid (soil drench), and neem oil (soil drench).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
