Notes on Plant Galls and Witches' Broom
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Torrey Pines Docent Society 2019 Notes on Plant Galls and Witches’ Broom Galls are abnormal growth of plant tissue that can be induced by insects, mites, nematodes (tiny round worms), fungi, or bacteria. We find them most easily on leaves and stems, but they can occur on any other parts of a plant, down to the roots. Most gall- inducing organisms are specific with the choice of host plants. Each organism induces galls with characteristic morphology. In most cases, the growth of the gall does not harm or kill the host plant. If so, the relationship can be described as a commensalism. The gall-inducing organism is benefited by the protection and food provided by the gall housing. These notes are divided into three parts. First is a quick reference to the most common galls that are observed with plants in TPSNR. The second part goes into some of the details of the organisms. The last part is on witches’ broom, another kind of abnormal plant growth. Part I. Field Guide In this part, we cover some of the most common plants galls that can be found in TPSNR, including the Extension and the marsh trail. The identification is by the gall itself; we can hardly see the tiny gall-inducing insects. For images of these insects, we have to use websites (see Bibliography). Fig. 1. An oak apple gall on a Nuttall's scrub oak. This one is about 30 mm in diameter. 1 Oak Galls All oaks can bear galls and they may appear anywhere, from leaves, to petioles, branches, acorns, flower spikes (catkins), and even roots. The most eye-catching ones are oak apple galls on branches. They vary from about 6 mm to 40 mm in diameter. They very briefly turn an apple-like red (Fig. 1). Very soon, this fresh gall will dry up and turn brown. On the Nuttall's scrub oak (Quercus dumosa Nutt.), these galls are induced by the California gall wasp (Andricus quercuscalifornicus). Fig. 2. Half-dried oak galls induced by the Beaked Twig gall wasp on Nuttall's scrub Wasps, together with ants, bees and sawflies, oak. This one is about 12 mm in diameter. are in the insect order Hymenoptera. Within this order, gall wasps are in the family Cynipidae,1 which is quite large. There are some 800 species just in North America. These cynipid gall wasps are tiny. The adults usually grow to just several millimeters long. Thus they are also called gallflies because of the size, perhaps in comparison to the small fruit flies. Each gall-inducing species infects different parts of a plant and generates galls with characteristic size, shape, and color that also can depend on the season (more on this later with their life cycles). In fact, we usually identify an inducer by the gall characteristics. For example, another detachable gall that can be found on the scrub oak is induced by the Beaked Twig gall wasp (Disholcaspis plumbella), also a Cynipid. The galls induced by this wasp have whitish spots resembling polka dots (Fig. 2). As tempting as it may be from the outside, the inside of an oak apple gall is anything but appetizing (Fig. 3). If we cut one open, what we find are insect larvae eating up rotting plant tissue. A gall with multiple larvae with each occupying its own cavity, also called a chamber, is called polythalamous, and is monothalamous when there is just one cavity. Oak apple gall is polythalamous even though it is not clear in Fig. 3. What is evident is that the larvae have eaten up just about half of this gall. This fresh oak apple gall was still spongy and juicy (see the drippings in Fig. 3). Like the rest of the oak plant, the liquid is rich in tannic acid, which can be used to make ink (see Part II). The acid is also antibacterial and astringent, meaning that it causes the contraction of the skin and thus reduces bleeding. The Kumeyaay sliced up a fresh oak gall and applied the liquid on sores. After the larvae pupate and subsequently grow into adults, each wasp punches a hole 1 The root of the name is from the Latin cyniphes, which is derived from the Ancient Greek kníps, referring to some “stinging insects” that live under the bark of trees. 2 through the wall and emerges from the gall. The tiny holes, about 1 mm in diameter, are an indication on how small the wasps are. (See the dried galls in the Lodge display drawers.) Fig. 3. Section of an oak apple gall, roughly 30 mm diameter, from Nuttall's scrub oak. Fig. 4. A stem gall induced by the Irregular Spindle gall wasp on Nuttall's scrub oak. This one is about 25 mm long. Fig. 5. A leaf gall induced by the Midrib gall Some cynipid gall wasps induce galls that wasp on Nutall’s scrub oak. appear as swellings of stems and branches. These are called integral galls as opposed to the detachable oak apple galls. On the Nuttall's scrub oak, these oak stem galls are induced by the Irregular Spindle gall wasp, Andricus chrysolepidicola (Fig. 4). The multiple pinholes, about 1 mm in diameter, indicate that this cynipid wasp also makes polythalamous galls. While oak leaf galls are not as conspicuous as apple galls, oak leaf abnormalities are very common. On the Nuttall's scrub oak, they can be caused by the Midrib gall wasp, Neuroterus washingtonensis, another cynipid gall wasp (Fig. 5). 3 All the cynipid gall wasps mentioned here also induce galls in other white oaks, which include the Valley oak (Quercus lobata) father up north, and the California scrub oak (Quercus berberidifolia) a bit farther inland. There are other gall inducing insects, but with the local oaks, we can always start our guess with cynipid gall wasps. Coyote Brush Midge Galls On the Coyote brush, Baccharis pilularis, we often find small globular (or ball- Fig. 6. A gall, about 12 mm in diameter, shaped) galls that grow to around 10 mm in on the Coyote brush induced by the bud gall diameter at the tip of the branches (Fig. 6). midge. Often there are short leaflets protruding from them. They first appear as whitish but will turn green or even purple in time. As they turn red or purple, the surface of the gall tends to become lumpy. These galls are induced by the Coyote brush bud gall midge, Rabdophaga californica (formerly classified under Rhabdophaga). Midges belong to the suborder Nematocera of flies (order Diptera), and they are small, about 2 to 3 mm in length. They have long skinny legs and antennae like that of mosquitoes. Gall midges are in one of the Nematoceran families, Cecidomyiidae.2 They are also called gall gnats because they are tiny. Fig. 7. A sectioned Coyote brush bud gall showing multiple insect larvae (not too clear Fig. 8. A dried Coyote brush bud gall, about in the photo), each occupying its own cavity. 15 mm wide, with two smaller ones above. They all have roughly 1 mm pinholes punched through by the adult midges. 2 Cannot trace the etymology of the entire term, but cecidology is the study of plant galls. 4 Even though the bud galls are small, there are multiple larvae living inside. These galls are polythalamous. Each larva makes its own cavity. A dissected gall would show several tiny larvae inside (Fig. 7), but a more telling sign is from the number of pinholes punched out by the emerged adults in a desiccated gall (Fig. 8). Fig. 9. White sage leaf galls induced by a gall midge but here at the base of petioles. These are about 5 mm in height. Sage Leaf Galls Both the white sage (Salvia apiana) and the black sage (Salvia mellifera) may have leaf galls. They are caused by the sage leaf gall midge, Rhopalomyia audibertiae, which is also in the family of Cecidomyiidae.3 Fig. 9 shows the characteristic urn- shaped galls on a white sage, and in this particular case, the galls are at the base of the petiole instead of on a leaf. At the tip of a gall is a small opening ringed with Fig. 10. Black sage leaf galls induced by the sage leaf gall midge. tiny hairs. 3 Russo (1979) put it under Rabdophaga with the species name salviae, but it apparently is a name that is no longer used. 5 Fig. 11. More matured Black sage leaf galls induced by the sage leaf gall midge protruding from both sides of a leaf. They are about 2 mm long. Insert: closeup of dried galls. On the black sage, the galls can also appear on the petiole instead of the leaves. On the leaves, they appear as reddish blisters (Fig. 10), but sometimes they can be green. The larger ones protrude from both sides of a leaf (Fig. 11). The gall remains round on one side of the leaf, while on the other side it develops into a characteristic urn with an opening ringed with hairs. These sage leaf galls are monothalamous. A point worth noting is that they look similar but not identical on the white and black sages. Different plants have different responses to the same chemicals secreted by a gall insect. Willow Leaf Galls Similar to oaks, willows are attractive to many gall inducing insects. The easiest observable galls are the bean leaf galls, as shown here on Arroyo willow, Salix lasiolepis (Fig. 12).4 They are big and colorful. They are mostly red but sometimes can be green.