Woody stem gall forming insects, unlike many of the leaf gall forming insects, can cause injury to their host plants. Most leaf-galling species are of aesthetic interest only, but in the case of the twig and stem galling insects, galls can severely deform the tree or even kill the tips of branches beyond the gall. Examples of such stem gall forming insects include the horned oak gall which forms because of the activity of Callirhytis quercuscornigera, a cynipid gall wasp, and the gouty oak gall formed by another wasp in the same genus, C. quercuspunctata. Galls of either species can be up to 2 inches in diameter and may form large masses that extend over 1.5 foot branch sections. The galls themselves are solid and woody, and multiple cells (where the larval wasps are found) are in the center. The main difference in appearance of the two galls is that in the second or third year (from when the gall-making wasp's eggs were first laid), horns develop in the horned oak gall that stick out of the surface of the gall. These are formed when the larvae are nearly fully developed. Each horn is the site of where a single adult wasp will exit the gall. The life cycle and biology of both of these insects are complicated. The biology and life cycle of the gouty oak gall wasp (Callirhytis quercuspunctata) may not be fully understood. The life cycle of the horned oak gall wasp is summarized below.
The horned oak gall (Callirhytis quercuscornigera, described from observations in NY) emerges as tiny adult female wasps in May and June from their fully developed twig galls. These females will lay their eggs on the undersides of host plant leaves, in larger leaf veins. Following egg hatch, the larvae begin to cause blister-like galls to form in the veins that are oblong in shape. These galls are noticeable from late May through June. By early July, this new generation of adult wasps emerges, and both males and females are present. This gall stage takes approximately 3 months to occur (Buss and Potter, 2001). These individuals mate, and the females lay fertilized eggs in the young twigs of their oak hosts. The galls formed by this generation occur in the following spring and it takes 2 or more seasons for this generation of wasps (in the woody galls) to mature (Johnson and Lyon, 1991). Buss and Potter (2001) observed the woody gall stage taking approximately 33 months to occur in Kentucky. Buss and Potter (2001) also observed that the number of wasps developing within each gall was dependent on the size of the gall and could range from 1-160 individuals. Infested branches may droop from the weight of these galls if a heavy population is present in a single tree. The woody galls left behind by the horned oak gall wasp may then be colonized by the dogwood borer (Synanthedon scitula) and up to 12-15% of 2-3 year old stem galls may contain dogwood borer larvae (Eliason and Potter, 2000).
On roses, 30 or so species of cynipid gall wasps in the genus Diplolepis initiate gall development on the leaves, stem, bud, or root of their host plant depending upon the host plant species. All of these species of wasps seem to be parthenogenetic, or able to reproduce asexually. The mossy rose gall wasp (Diplolepis rosae) causes a large (1 inch or more), round, hairy mass to form on 1 year old twigs of rose in the spring. The spiny rose gall wasp (Diplolepis bicolor) causes galls 5-10 mm in diameter to form in clusters on branches (but are made of leaf tissue) with long, reddish spines protruding from the round galls. Each spiny rose gall has a single cell inside where the larva develops. The larva begins its overwintering stage in the gall on the plant, but galls fall to the ground before the first frost occurs. Adult wasps then chew their way out of these galls to emerge in the spring.
The stem/woody galls of oak trees primarily impacts their branches and twigs. It is very difficult to obtain desired management of these insects once they have colonized a tree. Some individual trees may be more susceptible than others in any given area. Galls may eventually kill branches, as heavily infested oaks may have multiple stem galls on 80% or more of their branches. These galls can disfigure the tree or cause crown dieback.
Historically, stem galls on oaks have been a serious problem in Plymouth County, MA and Long Island, NY.
Visually monitor for the development of woody stem galls on oak twigs and branches.
On smaller trees and when safe and practical, pruning out the stem galls (before they have completed their development; ex. when still "succulent") can reduce further infestation of individual trees, especially when the population is still low. On larger trees or large numbers of trees, this may not always be practical to achieve.
Natural enemies account for high levels of mortality of Callirhytis quercuscornigera (the horned oak gall), especially during the leaf-galling generation of that insect. Buss and Potter (2001) found that 70-80% of those larvae were parasitized by Aprostocetus spp. and Pentastichus spp. (Eulophidae), Sycophila spp. and Eurytoma spp. (Eurytomidae), Ormyrus labotus, and Brasema spp. (Eupelmidae). They also observed that the chemical management options tested in that study also killed the natural enemies of the horned oak gall, and no reduction of the number of horned oak stem galls resulted from the chemical applications. In smaller stem galls especially, parasitism of the larvae by natural enemies is common; larger galls may have lower parasitism rates.
Natural enemies also impact the generation found within the woody stem galls of the horned oak gall wasp. Buss and Potter (2001) also found Ormyrus labotus and Sycophila spp., parasitoid wasps impacting the life stages within the woody galls. Adults of both of these parasitoid wasp species emerge from 2- to 3-year-old twig galls from April to May, and a second generation of the Sycophila spp. emerges from late July to early September. Several clerid beetle species and spiders were also observed predating upon various life stages of the horned oak gall in this study.
In addition to natural enemies, horned oak galls were home to a number of other insects that inhabited the galls without negatively impacting C. quercuscornigera. These included a braconid wasp, weevils, a flat-headed borer, longhorned beetle, ants, mites, springtails, and barklice. Additionally, at least two species of inquilines (insects associated with and that feed on the gall tissue but do not cause it to form) were found within the horned oak galls (Buss and Potter, 2001).
Carbaryl (L)
Dinotefuran (NL)
Emamectin benzoate (L)
Imidacloprid (L)
Pyrethrins+piperonyl butoxide (L)
Must obtain management of the stem gall forming wasps prior to gall formation for it to be successful. Because of the complicated life cycle of these wasps, and the alternating leaf and stem generations (in oak), the timing of insecticide applications can be incredibly difficult. Additionally, dead stem galls persist in trees for several years, which can make assessing whether or not insecticide applications were successful a difficult task. Researchers have attempted to manage the leaf galling generation of the horned oak gall as soon as those adult wasps emerged in the spring (at bud swell or green-tip). They looked at applying sprays of chlorpyrifos and bifenthrin and found 66-91% reduction in leaf gall density following applications. They also tried to manage these leaf galls with systemic injections of abamectin, imidacloprid, or bidrin. Injections occurred just after peak emergence of the asexual generation of wasps from the stem galls. They also looked at translaminar applications of dimethoate, acephate, abamectin, and imidacloprid to manage the leaf gall stage of the horned oak gall. Both systemic injections and translaminar applications reduced the number of leaf galls that were formed. However, the researchers also found that these treatments resulted in a severe impact on the beneficial parasitoids (natural enemies) of the leaf galling generation of gall wasps. The parasitoids alone accounted for 70% mortality of the leaf-galling generation of horned oak gall wasp on untreated trees (Buss and Potter, 2001). These researchers argue that poorly timed applications could have an unwanted effect, especially if they were not effective at managing the gall wasps while at the same time having a significant negative impact on natural enemies. Accurately timed applications made over a period of two to three years may help manage the horned oak gall wasp, however Buss and Potter (2001) call for critical evaluation of these multi-year management efforts first.
Active ingredients that may be applied systemically include: dinotefuran (soil drench), emamectin benzoate (injection), and imidacloprid (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.
