Galls in Plants: You Mean This is an Insect?

In the world of insects there are slightly more than 1 million species described to date, an estimate of anywhere from three to ten times that many still waiting to be discovered, and thousands of species are named every year. With this much diversity in the magic world of bugs it stands to reason that an awful lot of oddities will show up, and one of the oddities that just fascinates me is the group of plant-feeders called “Galls”.

Most people have, undoubtedly, seen galls at one time or another in their lives, but likely didn’t really know just what it was. The swellings of plant tissues create such an interesting array of designs that there just doesn’t seem to be an association with bugs causing them. As a child living in California my friends and I would commonly grab the big Oak Apples that fell to the ground under a neighbor’s huge oak tree, and toss them like baseballs. These things can be as big as a baseball, and my assumption was that they were seeds of some sort from the oak. It was many years later that I discovered they actually are created by the presence of tiny wasps that live in the tissue of the tree.

What kinds of things cause galls?

Most galls are caused by insects, although a few are created by diseases in a plant, such as a fungus or viral disease. A couple of these that are noticeable are Tongue Gall of alder trees, that looks like twisted tongues hanging down from the seed pod clusters, and Spindle Gall of oak trees, that looks like a big swelling of the twigs and can cause some dying back of terminal branches. However, the insects that cause tissue swelling and galls generally do not cause damage to the plant, although on a heavily infested tree, over time, there may be some effect on the overall health.

The most common insects creating galls are wasps, usually tiny little wasps in a family called Cynipidae, and given the rather simple name of “cynipid” or “gall” wasps. Other insects may be aphids or thrips, and their presence inside the home they have caused has been the object of a great deal of study. Thrips are minute insects that you easily would overlook in your garden, even though they can be present in huge numbers and may cause visible damage to flowers or leaves with their rasping mouths. Several species of thrips in Australia have been studied recently, and interesting discoveries have been made about the social aspect of these insects and their survival mechanisms against marauders that penetrate their fortress.

Do galls hurt the plant?

No, not really, at least not dramatically so. When someone finds galls on their cherished ornamental plants there often is a reaction that they obviously must be done away with, and spraying of pesticides to eliminate galls is done frequently. In reality, though, not only are the galls rarely causing any harm to a plant, but they also are simply not affected by chemical sprays. Product applied to the outside of the swollen tissue do not penetrate into the hidden cavities inside. Even “systemic” products applied to the soil and taken up by the roots of the tree do not seem to have any particular effect in eliminating galls from a tree, and since there is no harm being done this effort and cost can be saved.

Many of the leaf galls are created on the veins of a leaf, sometimes on a major vein that transports water and nutrients into the leaf. When this happens you might see some of the leaf tissue past that point turning brown, as it is denied sustenance, and extensive numbers of galls on the same leaf might even cause it to die and fall from the tree, but overall the tree is not harmed.

The gall is a creation of the insect, and each species of insect may cause a very distinctive kind of gall to grow. It is not entirely understood just how that happens, but it is felt that the insect must release certain chemicals into the plant tissue that cause that tissue to react and grow in a manner and color specific to the bug. The adult insects, in the case of wasps, seek out the kind of plant they need, and lay their eggs in the plants soft tissues. The larvae that result then spend their lives hidden in the expanding gall. In aphids or thrips the adult causes the plant – most often leaf tissues – to envelop them in a hollow cavity, and within this chamber they then can feed and produce offspring relatively safe from predators and the other hazards of the outside world.

Galls and Oak Trees

Perhaps it is because I have lived all my life in central California, where oak woodlands are so prevalent, but my experience tells me that oak trees are Gall Heaven. There are dozens of different kinds of galls associated with the leaves or stems of oaks. One of these seems to be peculiar to California, and every year we get calls from people wondering what they are, or jars full of them brought into our office for identification. These galls are very small – perhaps about 1/8 inch in diameter – and if someone has collected a large number in a jar it looks like popcorn popping, as the galls are all bouncing around, sometimes managing to leap almost an inch up in the air. This is the intriguing California Jumping Gall, and inside each tiny gall is the larva of a tiny wasp.

There are years when the populations of the galls are very high, and as part of their life cycle, as the larva within the gall matures, the gall falls from the tree. The larva now begins flinging itself around inside, trying to work the gall into a protected crevice of some sort where it can safely change to its pupa stage, and in the process the thousands of galls on a backyard patio are bouncing up and down in a very noticeable fashion, causing great distress to the homeowner who has no clue as to what is going on. This is simply our version of the famous “Mexican Jumping Bean”, in which a beetle larva is doing the same thing. Personally, I’m not sure how the Laws of Physics enable these things to actually leap into the air, but leap they do.

A second gall of oak trees is one I mentioned earlier, and this is the potentially huge gall known as the Oak Apple. Within each of these huge, swollen galls there may be dozens of wasp larvae, each eating its way through the swollen plant tissue it has created. These galls grow on the stems of the trees, and some trees must be particularly susceptible to them, for it is amazing how heavily populated with oak apples one tree may be, while next to it are other trees of the same variety that have virtually none. The adult wasp lays her eggs in the soft stems of the oak tree, and the reaction by the tree is to begin swelling the tissue at that point. It swells and grows around the larvae that live within it, feeding on the plant material, and the gall will be green with living tree tissues that first year, but die and turn brown the second year. These galls often hang onto the tree for several years, but eventually fall off or blow off in a windstorm, their wasp residents long ago having left the home they created.

You may find several kinds of galls all mixed together on the same leaf of an oak tree, as with the Live Oak Galls, Two Horned Galls, and the Red Cone Gall. This last one looks for all the world like red Hershey’s Kisses candies, but a careful incision into its center with a sharp knife will reveal the tiny larva of a gall wasp living comfortably inside. The presence of all these galls has no noticeable negative effect on the tree, particularly on those trees which are deciduous and will eliminate all the leaves each year anyhow, growing out a whole new batch of fresh ones each spring.

Some other curiosities

As a nature lover I enjoy poking around and discovering things, and while driving over the Sierra Nevada Mountains one time I stopped to walk around just to see what there was to see. I found some fascinating galls attached to some low-growing shrubs, and had them identified as Paper Galls. These hollow galls were sometimes almost the size of golf balls, and inside I found only a hollow interior with lots of little threads attached to the outer walls, and suspended from these threads, in the center of the gall, was the little bit of plant tissue that housed the wasp larva. What an interesting way for the plant to react, but obviously this setup was perfect for this particular wasp, for it must have been doing this for many hundreds of thousands of years.

A different species of wasp makes the Live Oak Gall, another large gall that looks similar to the Paper Gall, but instead of being hollow it is composed of very hard, woody tissue and the outer surface is covered with spines. Once again, though, a look inside reveals that this unusual growth that might be mistaken as a seed pod of some sort is actually just the home of a wasp larva, created in a specific form by the chemicals exuded by the wasp as it lives there.

As mentioned earlier, not all galls are caused by wasps. Often it may be other small insects that have adapted to a life style that allows them to create their own hiding places and food sources. Aphids are one of these alternates, and as we can see with the Poplar Gall Aphid they are very effective at this. You often may see these swellings on trees such as poplar or willow, or even on the stems of some weedy plants, and by opening the gall you can observe the family of aphids living inside, more or less protected from their enemies on the outside world

So, the next time you are wandering around in your yard with a cup of coffee, admiring the landscape and the results of your hard work, you might notice some of the many kinds of unusual galls on your plants, and know just a little more about them and where they came from.