A variety of different species of tiny, non-stinging wasps in the family Cynipidae (gall wasps) create galls (deformations) in the leaves of oak. The good news is, while these species may at times change the appearance of oak leaves to a very noticeable extent (that is perhaps alarming to some), they very rarely if ever require management. These insects typically do not cause damage to the overall health of their host plants. In fact, they can be celebrated as a sign of biodiversity if found on their hosts. The galls caused by these insects may be different shapes, sizes, and colors and depending upon the species of insect, may impact various different plant parts. In some cases, the species of insect responsible can be identified using the characteristics of the gall they create. The insects cause the galls to form in the oak leaf tissue through a process that is not yet completely understood by science. The purpose of the gall is: to house the insect, to provide some protection from predators, parasitoids, and also (unnecessary) chemical management options, and to provide the insect with food - the galls are often highly concentrated with proteins. So these species have figured out how to tell the host plant tissues to make them a home, feed them, and protect them.
Each species will cause the formation of a gall that is specific to them. Some gall wasps have alternating generations in their life cycle - meaning a parent generation of galls may look different from its offspring or occur on different plant parts, which are then more similar to the "grandchildren" generation. This makes identifying the alternating generations of different species very difficult - but hopefully DNA analysis is helping with some of that confusion.
A few examples of cynipid gall wasps causing various types of galls on oak leaves are discussed here, but are by no means a complete list. Amphibolips confluenta is a species that causes "oak apple galls" attached to the midrib or petiole of a leaf. If sliced open, the apple galls appear filled with a spongy mass with a single larva present inside this mass in the very center in a hard cell. Once dry, the inside of the gall looks like fibers coming from the center of the gall, attached to a thin, brown, papery exterior.
The complete life cycle and the correct scientific name for Druon quercuslanigerum or the live oak woolly leaf gall maker (or wool-bearing or fuzzy gall wasp) has been a source of confusion even in recent years (Hood et al., 2018). The asexual generation of this species develops within the fuzzy, single-chambered leaf galls it creates on the underside of new leaves along the midvein of its host. Galls range in 0.7-4.0 mm in size but may be mistaken as larger if many separate galls are close together. Asexual females eventually emerge from these galls, and oviposit on buds of live oak that form catkins. Galls are formed on the catkins and the sexual generation develops along with catkin development. In a few weeks, sexual forms emerge, mate, and females lay their eggs into newly growing leaves, which produces the asexual generation. The galls produced on the catkins form on the central stalk on each catkin. These galls are also single-chambered, look like swellings, and are golden brown in color ranging from 0.5 to 2 mm. in size. Emergence holes from the sexual generation on the live oak catkins can be seen with magnification. 1-4 galls can form on a single catkin stem (Hood et al., 2018).
Dryocosmus quercuspalustris may be referred to as the roly-poly gall or the "succulent" oak gall wasp. Galls are round, light green, and sometimes speckled and found on pin oak or others in the red oak group. These galls may be formed on both the leaves and the catkins. Galls of this species are hollow, approximately 1/2 inch in diameter, and contain a white, seed like structure that rolls around within the gall, containing the single wasp larva developing within. While galls do form protective barriers for the wasps within, they do not always work. Predation has been observed on galls of this species, attributed to birds feeding on the larvae.
Neuroterus quercusverrucarum or the oak flake gall is created by a tiny wasp that is found associated with oak species in the white oak group. Small, round, fuzzy galls on the leaf undersides are formed in the spring, each containing a single wasp larva. One generation occurs per year with the once white galls turning brown or reddish-brown by the end of the season. Light green to yellow blister like bumps occur opposite the galls on the upper leaf surfaces, which may be noticed first. Eventually, these blisters also turn a reddish brown at the end of the season. Leaves can be so heavily galled that they become twisted and distorted. However, like the other leaf galls on oak, damage to the overall health of the tree does not occur.
Callirhytis seminator or the wool sower gall wasp creates galls on white oaks in the spring. Galls are rounded, fuzzy, and white/pink-red. The pink/red areas may turn brown as the season progresses If the woolly gall is pulled apart, it reveals seed-like chambers containing the individual developing gall wasps. A yet undescribed alternating generation of this species occurs.
Acraspis erinacei or the hedgehog gall wasp creates a gall on white oak. There are alternating sexual and asexual generations of this species as well. The sexual generation forms galls on leaf buds after they mate and the asexual female wasps initiate the hedgehog galls on host tree leaves.
Note that the species Neuroterus saltatorius (jumping oak gall wasp) is historically thought to be limited to the western United States (ex. California), however in recent years samples consistent with this species have been seen in Massachusetts. Either this insect has changed in distribution across the US, or those found in the east are a separate species. The jumping oak gall wasp creates a tiny, seedlike gall on the undersides of the leaves of white oak. Each gall contains a single larva, which eventually drops from the leaf to the ground, containing the larva. The larvae are capable of jumping within the gall, once they hit the ground, which may allow them to locate a safer place to overwinter. In its native range in California and other western states, populations sometimes build so much so that premature leaf loss occurs. Female wasps of the species emerge after overwintering, and lay their eggs near opening buds. Trees who break bud earlier may be more heavily impacted by the activity of this insect. This generation causes blister like galls on oak leaves, and was previously thought to be a different species of wasp. Males and females emerge from the blister like galls, mate, and females lay their eggs on the leaves of their host which hatch, with the seed-like galls forming on the leaf undersides approximately 40 days later.
These wasp-caused galls can be many different shapes and sizes, depending upon the species of wasp. They are rarely detrimental to the tree. However, some leaf galls on oaks are the alternate generation for gall insects that cause damage to stems, such as the gouty oak gall, which can be serious pests. But if the species of wasp only causes a gall to form on the leaves of its host, typically the changes to the leaf are more aesthetic in nature and require no management. They do not reduce the overall health of the host plant.
Each species of cynipid gall wasp will initiate gall formation at different times of the year. If specifics about the life cycle are known to science, this information can be helpful in monitoring for or identifying the species causing the gall to form. Many gall wasps that create changes to their host plant leaves initiate this early in the spring. Galls may not be noticed until they are fully formed or have dried out in the summer or fall.
For leaf gall forming insects, management may not be necessary and is often not practical or possible. The efficacy of removing heavily infested host plant leaves or branches may not be fully understood for each species of cynipid gall wasp, and may be very complicated depending upon whether or not there is an alternating generation, and where that occurs on the host plant.
The natural enemies of cynipid gall wasps will differ and be specific to the specific gall wasp species. Parasitoids, hyperparasitoids (parasites of the parasites of the gall wasps), and inquilines (organisms living within the galls of these species which either have no impact on the wasps, or may be natural enemies of them) as well as predators of cynipid galls wasps have been reported in the literature. However, the full scope of all of these organisms, or knowledge of all of the species associated with each species of gall wasp, is still incomplete. What is clear is that galls on trees and shrubs create fabulous microhabitats that can be incredibly complex webs of connected species. Because of this, insect galls on trees and shrubs that are not causing damage to the host plant should be preserved and celebrated (Forbes et al., 2016; Sanver and Hawkins, 2000).
Acephate (NL)
Carbaryl (L)
Dinotefuran (NL)
Emamectin benzoate (L)
Imidacloprid (L)
Pyrethrins + piperonyl butoxide (L)
Rarely requires treatment.
Active ingredients that may be applied systemically include: acephate (injection).
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.
