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We’ve had quite a few email questions about a malady that’s hitting our pin oaks, particularly west of I-270 and along the 40 corridor. Huge golf-ball to nectarine-sized galls are proliferating on oaks, and if you’re observant you’ve noticed the galls getting continually worse in affected trees, and also spreading from tree to tree. In addition to the pin oak, shingle oaks are favored by this INSECT pest…it is not a disease. The vast majority of these galls are “horned oak galls” and a second closely-related species is the “gouty oak gall”. The culprit is a small wasp (Callirhytis cornigera), about 4 mm long, called a “cynipid wasp.” The wasps are rarely seen! Here’s a link to a digital picture of the cynipid wasp causing gouty oak gall: Click here. Here are two great shots of the galls we’re talking about.
There are numerous cynipid wasps that cause a wide range of galls on oak leaves, too. The galls are caused when the cynipid larvae secret a hormone into the oak tissue, triggering the formation of the mass of tissue that makes the gall. Developing larvae feed inside of the gall tissue until they mature into adults.
The cynipid wasps causing the either horned oak or gouty galls have a complex life cycle, and this complex life cycle is the reason that chemical control methods are limited in effectiveness. With horned oak and gouty oak gall, the first generation of wasps will emerge in the early spring from the existing woody galls. Those adults fly to developing oak buds and leaves, where they will lay new eggs on the underside of the leaf directly in the leaf veins. The leaf galls are not particularly noticeable for this species. "THE Turf Guys" suggest that these developing larvae should be your target for controlling this pest with insecticides). The larvae in the leaf galls emerge as adults in approximately 3 months. This second generation of adult wasp lays their eggs directly into the twig tissues. Twig galls then begin developing into the large, recognizable mass, reaching maximum size in about two years. These galls provide food and shelter for the developing larvae.
When branches develop numerous galls they can actually die off, because the expanding gall girdles the stem, interfering with the transport of water and sugars. Heavy infestations can lead to the decline of the tree, and can actually cause significant loss of crown tissue. Therefore, it is very important to keep your trees well watered and fertilized! Use the trickle irrigation method to help the tree avoid drought stress, particularly during from July through late September. If you’re following THE Turf Plan® you’re providing the minimum of N, P and K to the root zone of the oak to keep it sufficiently fed. Additional deep-root feeding, or slow release nitrogen in the mulched zone around the base of the tree is warranted.
Various insecticide treatments can provide effective control, but it can be very expensive. The problem with most insecticides is that they have to be timed perfectly in an attempt to control the larvae stage (for systemic products) or the adult stage (for contact insecticides). Spraying the entire crown of large pin oaks is costly, and the insecticide has to be timed virtually perfect with the emergence of an adult population for a foliar spray to be effective. Most arborists prefer to inject systemic insecticides into the root flair (base) of trees; as a general rule of thumb the injections need to be made at about every 12 inches around the tree’s base. Again, timing is everything! The injections need to be made early enough that the insecticide has a chance move (translocate) upwards into the crown of the tree, typically reported as long as 3 to 4 weeks. A third method of insecticide application is to apply a systemic insecticide into the root zone of the afflicted tree, either by pressure injection (professional arborist) or a root drench (professional arborist or home owner). This route is probably the most cost effective of those offered by licensed arborists, but remember the TIMING requirement. Using systemic insecticides won’t control the first generation of wasps that emerge from galls that are on the tree now. The typical homeowner is best suited to make a “root drench application” with imidacloprid insecticide, timed right at or soon after bud break.
The decision to treat your pin oaks for oak gall is influenced by the value of the afflicted tree…if it’s a magnificent shade tree, providing your home with afternoon shade it’s probably worth the cost of treatment. No matter what your plan of attack on this destructive wasp, be sure to water and fertilize your large trees, starting in mid-to late June when rainfalls become less frequent.
Contact your favorite arborist IMMEDIATELY, to talk to them about treatment options. If you wait till April to call the arborist, you’re too late.
Another issue complicating insecticide usage is that you can’t really ever stop treating for oak galls…it will require treatment EVERY year. And even if you decide to engage in a chemical control program, your neighbor may choose to do nothing. Thus, this insect pest is here to stay.
As mentioned above, there is one active ingredient that the homeowner has access to, imidacloprid, which is used extensively by professional arborists (as root flair injections, root zone injections and foliar sprays). You may recognize this active ingredient as the same one that is in Mallet®, Merit® or GrubEx® branded products. It is the same one that we promote for the June 15th application (on 0-0-7 fertilizer). This versatile product is available to homeowners in many different forms, and THE Turf Plan® is recommending a concentrated formulation of the insecticide, Dominion™ 2L Termaticide/Insecticide. This product contains 21.4% active ingredient, and is a great value because most competitive products only contain ~1.5% of the insecticide. It can be mixed with water and applied as a root drench for pin oaks and shingle oaks, to suppress the development of horned oak and gouty oak galls (along with the control of nearly two dozen other pests, including emerald ash borer, flatheaded borers, aphids, scales, lacebugs, Japanese beetles, leafminers, mealybugs and thrips).
The product label recommends a dose of 6 mL per every inch of tree trunk diameter (“diameter at breast height, or DBH”). DBH is the diameter of the tree trunk 4.5 ft high. Use a flexible tape measure and determine the tree’s circumference. For example, the pin oak in Jeff’s front yard has a circumference of 85 inches.
Divide the circumference by pi (3.14) to determine diameter. Sticking with the example of Jeff’s pin oak:
To treat this tree according to the product label, 5.5 oz of product is required. The label states to use at least 2 gallons of water to effectively distribute the concentrate completely around the tree’s base, but this is no where near enough…Turf Guys think you should mix every ounce of concentrate in 2 gallons of water. The larger the tree, the more water you’ll need. For Jeff’s tree, we’d mix 1 oz of concentrate (2 TBL) in a 5 gallon bucket, and apply that solution around the base of the tree, from the root flare, to about 5 ft out from the tree trunk. We’d repeat this four more times, with 2 TBL of concentrate mixed into 2 gallons of water each time. Be sure to stir the concentrate solution in the water, before pouring on the root flair and root zone.
Make the treatments on pin oaks about a week before bud burst. Imidacloprid will last the entire season in the leaf tissue of the tree. If everything goes right we hope to have enough imidacloprid in the leaf tissue to kill the developing larvae. The adult females won’t be controlled at any point in the life cycle…controlling the larvae that develop in the leaf tissue is the goal. In addition, if there is enough imidacloprid in the crown of the pin oak, it is possible that it can accumulate in the developing woody galls of the third generation, too. Anything we do to reduce the population of the larvae will help the tree. Make only one application per year.
Recall from the earlier paragraphs that this wasp has a complex life cycle with multiple generations. This treatment won’t control the first generation of adults that emerge from existing woody galls, because the adults will not be eating treated plant tissue. If dosed and timed appropriately, imidacloprid in the sap stream may/should reduce the population of developing larvae of the second generation of wasps (which develop on the underside of the oak leaves). Imidacloprid is systemic, and must be ingested in sufficient quantities by each larvae, to kill it. It is plausible that larvae developing in the new galls during the summer would be adversely affected by the insecticide, because systemic insecticides accumulate in growing tissues. The only way to know for sure is to use this product for 2 to 3 years in a row, to determine if the gall concentration is reduced on the new twigs. With all pesticides, ALWAYS READ AND FOLLOW LABEL DIRECTIONS.
If you’re still with us, you’re probably interested in this problem to justify a few more minutes of your time. Suppose you go through the trouble and expense of using this product, and you’ve done everything right…while you may be successful in killing or reducing the second generation larvae in your tree, there are usually dozens of pin oaks in your neighborhood, meaning wasps from other trees can still come to your tree and lay eggs in the stems. That said, is it worth it? We think your large pin oaks are valuable, and the least you should do is try the root drench of imidacloprid. Treating the pin oak in Jeff’s yard costs ~$10.00. The cost of removing a huge pin oak easily exceeds $1,000.
If you want even more detail on this particular issue, here’s a quick synopsis of the situation from Dr. Chris Starbuck, Associate Professor of Horticulture at the University of Missouri. Click here
Here’s what is probably the best technical paper (very detailed) on the biology of the horned oak and gouty oak galls. Click here
Additionally Dominion 2L controls dozens of pests in the lawn and garden. For control of common lawn pests (beetles that cause white grubs, cutworms, craneflies and cinchbugs) apply 0.5 TBL/M (17 ml, or a generous half-tablespoon per 1,000 sq ft). If rainfall doesn’t occur within 24 hours, water the product in to help it penetrate the thatch layer and move it into the rootzone. When applied as a broadcast application (hose end or sprayer) the maximum labeled rate is 1.6 pts per acre per year, or 17 ml/M/year. In other words, you can only make one application per season.
Dominion 2L can also be applied as a foliar spray to control many of those tough pests that destroy our ornamental plants, including, but not limited to, Japanese beetles, aphids, mealybugs, thrips and whiteflies. Mix 1.5 TBL of Dominion 2L per 100 gallons of water and spray plant foliage. It can be used for control of labeled pests on virtually all ornamental plants, including indoor plants. We recommend that you include KALO DRI™ with all foliar applications, to increase the effectiveness of the product.
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