Ron Wilson

Ron Wilson

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Oak Apples, Woolly Catkins, and Leaf Blisters- Buggy Joe

Earlier this week, I received the following question through Ask Extension, “What is this? Type of "fruit" (?) found under a tree, in a Ross County, Oh wooded area. (4/30/23) It is the size of a very large grape. Object is squishy, not at all firm. Lime green color, with dark brown spots. (photo attached).”


Can you guess the ID of the object?



The lead image posted with this Alert looks very similar to the photo provided with the Ask Extension query. Of course, the “object” is a so-called oak-apple gall produced under the direction of a tiny wasp belonging to the family Cynipidae.


Oak-apple galls are so named for their resemblance to Malus fruit. The galls are a true wonder with some having surface imperfections that resemble those produced by apple pests.


There are over 50 species of cynipid gall wasps that are known to produce so-called oak-apples in North America and there are probably at least 10-15 distinct species of oak-apple gall wasps found in Ohio. Of course, those numbers are a matter of guesswork because the geographical range of cynipid wasps remains poorly understood and the numbers also imply that all species are known.


The “oak-apple” common name carries no taxonomic weight. Although wasps in the cynipid genus Amphibolips appear to dominate the oak-apple gall-makers, wasps in other cynipid genera also produce rounded, ball-like galls. Additionally, the galls may be formed from hijacked tissue located on different parts of the tree from leaf buds to reproductive structures like catkins. Most oak hosts belong to the red oak group; however, oak-apples may occasionally be found on oaks in the white oak group.


Cutting open the galls will reveal the internal structure which includes a central seed-like chamber housing a single wasp larva. The chamber may be surrounded by succulent tissue, not unlike the flesh of an apple, or you may find delicate white fibers radiating from the larval chamber.


Wasp larvae have chewing mouthparts; so, what do the gall-wasp larvae eat? They don’t eat themselves out of their house and home by consuming the gall from the inside out. Instead, the inside of the gall chamber is lined with specialized cells called nutritive tissue which is constantly being replaced as it is consumed by the gall-wasp larva. Imagine lounging in a room with pizzas constantly emerging from the walls.


Plant galls commonly change their appearance as they develop (= mature) which can present an identification challenge. Oak-apple galls range in size at maturity (= they stop expanding) from 1/2 - 2" in diameter. However, it can be difficult to determine at this time of the year whether or not the oak-apple is an inherently small gall or a large gall that’s still developing.


Once the developing oak-apple gall wasp pupates and leaves the gall-building as a newly minted wasp, the gall turns brown, and the true size of the gall is revealed. The image below shows two different oak-apple galls based on the size of the mature galls.


Woolly Catkins

The descriptively named Woolly Catkin Galls produced under the direction of the cynipid wasp, Callirhytis quercusoperator, are appearing on oaks in the red oak group in southwest Ohio. The galls arise from the catkins but will remain attached to the tree long after the normal catkins have dropped off the tree.


Carefully pulling apart the woolly matrix reveals seed-like structures housing individual gall-making wasp larvae. The gall-maker’s tiny homes are covered in dense hairs and appear to be the source of much of the woolly fibers that create the overall gall structure.


I posted an Alert earlier this month about Roly-Poly galls, produced under the direction of the cynipid gall wasp, Dryocosmus quercuspalustris, that are appearing on red oaks in Ohio. You can read the Alert by clicking this hotlink:


Roly-Poly galls may also arise from catkin tissue, so it was no surprise to find several of these galls embedded within the woolly catkin galls. However, it was surprising to see some of the roly-poly galls looking like deflated balloons suggesting the gall-making larvae were losing the battle for contested plant resources.


Blistered Leaves

So-called Oak Leaf Blister Galls produced under the direction of the cynipid wasp, Melikaiella ostensackeni, are most commonly found on red oak (Quercus rubra). The gall structures appear as irregular, hard (almost woody) protrusions on the upper leaf surface and corresponding lumpy, puckered areas on the lower surface. Slicing open the galls will reveal numerous chambers housing individual wasp larvae.


As the galls “mature,” they turn from light green to brown and are peppered with wasp emergence holes then become evident. This typically occurs in mid-to-late summer.


There are a few look-a-likes for the galls including the welt-like symptoms produced by the disease called Oak Leaf Blister caused by the fungus, Taphrina caerulescens. However, a close examination will reveal key differences in leaf symptoms. The disease produces raised patches on one leaf surface and corresponding concave areas on the opposite leaf surface.


Infections have been recorded on over 50 different species of oak belonging to both the white oak and red oak groups. The fungus overwinters as spores lodged beneath bud scales. Leaf infections occur during moist periods in the spring as new leaves expand. 


The Oak Leaf Blister Mite (Aceria triplacis) is an eriophyid mite (family Eriophyidae) that produces light green blister-like spots on the upper leaf surface and corresponding depressions on the undersides of the leaves that are filled with a hair-like growth (erineum patch). However, the depredations by the mite appear to be confined to oaks belonging to the white oak group, most commonly on burr oak (Quercus macrocarpa).


No Harm, No Foul

There are somewhere around 800 different types of arthropod galls found on oaks in the U.S. About 700 are produced by cynipid wasps. Of those, only a handful represent a serious threat to the overall health of their oak hosts. The galls highlighted in this Alert cause no appreciable harm to the overall health of their hijacked hosts; controls are not warranted.


Indeed, perhaps a healthier perspective is to simply appreciate these highly unusual plant structures brought to us by some remarkable behind-the-scenes genetic manipulation. Ephraim Porter Felt said it best in 1917 in his “Key to American Insect Galls”: “Insect galls are obvious and frequently excite surprise because of the strange form or the wonderful coloring and delicacy of structure.”

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