Nov 17, 2011 – Mysterious Blobs


I live in Central Texas. On our Oak trees are BB-sized balls glued to the leaves - thousands of them per tree. I've been told these balls are from wasps. Could you please give me more data?


Yes, these are caused by tiny wasps, but they are not glued to the leaf. In fact, they are part of the leaf tissue and they are galls, most often caused by wasps in the family Cynipidae. The one you describe sounds very much like one we have so commonly in northern California, and which I sort of thought might be unique to our state. We call ours the California Jumping Gall, and this is due to the fact that it falls from the leaf in the summer and the wasp larva inside that little gall begins tossing itself back and forth, perhaps intended to cause the gall to end up within some crevice in the soil where the larva then can pupate in comfort. But, all that tossing and flipping will cause the gall to jump right off the surface, and the jump could be an inch or more high. If you can imagine thousands of these tiny little balls bouncing all over the patio and on cars and outside furniture, the result is amazing, albeit terrifying for the homeowner who has no idea what's happening.

You don't mention whether or not your galls are hopping, but if they were a species that did this you likely would be aware of it by now. However, there are hundreds of other kinds of galls on oak trees and other trees, although some of those on oaks are the most interesting. They are created when a female wasp "stings" the tissue of the leaf to implant an egg. They really are not stinging wasps, but they still use the ovipositor for this. The egg hatches and the larva begins feeding within the leaf, but as it feeds it secretes some chemicals that cause the leaf to react in a way that is specific to each kind of gall wasp, forming these enlarging and unusual eruptions of the leaf tissue. Some of them end up looking like red Hershey's Kisses candy, some like balls of fluff, some are spikey, others flattened - just an amazing array of shapes and sizes. They nearly always are attached on a leaf vein where the thicker tissue is.

They nearly always are completely harmless to the tree, particularly when they are on deciduous oaks that lose their leaves in the fall anyhow. Some kinds of galls, like spindle gall, will form on the thin stems rather than the leaf, and when these enlarge they do cause some minor die-back of the plant from that point outward, but it does not harm the tree overall. The Jumping Galls cause the greatest interest and concern due to their weird hopping and visibility to homeowners.

What can you do about them? Essentially nothing, and there is really no reason to spray pesticides all over the trees in an effort to eliminate them. It won't do much good anyhow, as the active ingredients just don't penetrate that gall, so it is a waste of toxic material. Better is to educate the customer with the trees to understand what these are and the benign nature of them, and hopefully they then will actually learn to enjoy the oddity. Not all "bugs" are a problem and not all of them need to be killed.

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