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California's Amazing Jumping Galls

California is known for its legendary "jumping frogs of Calaveras County," made popular by author Samuel L. Clemens (also known as Mark Twain), but few people have heard of California's famous "jumping galls." Although they superficially resemble a diminutive version of Mexico's famous "jumping beans," they are really quite different. Both jumping galls and jumping beans move about by the activities of a small, energetic wasp or moth larva inside of them; however, it is the origin of their jumping container that is very different.

When WAYNE'S WORD received a sample of jumping galls from northern California, our staff did some investigating, and sure enough, these amazing little galls behave very much like Mexican jumping beans. This remarkable discovery is discussed in an article by F.A. Leach (Natural History Vol. 23, 1923). The minute, globose galls are only 1-1.5 mm in diameter, about the same diameter as the head of an ordinary straight pin. It is easy to see how casual observers could mistake these minute hopping objects for fleas. In fact, we have had so many people curious about the size of these tiny hopping spheres that we have included the following WAYNE'S WORD exclusive comparison of a jumping gall with a human flea (Pulex irritans). Including its extended legs, the flea is slightly larger.

The papery galls are shiny, light brown in color and resemble tiny little spheres (similar to the animated gif image at the top left of this page). They are about the same size and shape as the tiny one-seeded drupelets (nutlets) inside the fleshy syconium of an edible fig (Ficus carica), another amazing story about wasps that live inside figs. These nutlets are formed by wasp pollination and provide the nutty flavor and crunch in your fig newtons. See the nutlets inside following fig newton:

Jumping galls are attached to the leaves of several native oaks in California's Sacramento Valley, including the valley oak (Quercus lobata), blue oak (Q. douglasii ) and Oregon oak (Q. garryana). During favorable years, the undersides (and uppersides) of each leaf contains dozens of galls. When multiplied by the hundreds of thousands of leaves per tree, this accounts for the millions of galls that fall to the ground beneath these oaks, like a shower of tiny BB-shaped bird seed. In fact, some valley residents become dismayed when the galls cover their patios, sidewalks and driveways. Each gall is inhabited by a tiny cynipid gall wasp appropriately named the "jumping oak gall wasp" (Neuroterus saltatorius), formerly named Cynips saltatorius. Since the gall consists of a single cavity or chamber occupied by a single wasp it is termed monothalamous by cecidologists (people who study galls). Since the galls break away from the leaves, they are called "detachable galls." When the minute galls fall to the ground they begin hopping about like fleas. Like jumping beans, the larva inside is active during the summer months, but ceases its activity by late summer and fall when it changes into a pupa. And like jumping beans under jumping bean shrubs, the sound of thousands of jumping galls in the leaf litter beneath oaks resembles the patter of rain drops falling on dry leaves.

The mysterious origin of strange growths on the stems and leaves of plants have intrigued naturalists for centuries. Generally called galls, these tumorous (neoplasmic) outgrowths develop from rapid mitosis and morphogenesis of plant tissues and come in an astounding array of colors shapes and sizes. Galls may be smooth, spiny or fuzzy, and resemble everything from marbles and ping-pong balls to dunce caps, saucers and sea urchins. Almost half of all the known galls encountered in America occur on oak trees (Quercus). Although galls may be caused by a variety of insects, most of the oak galls are caused by a single family of gall wasps, the Cynipidae. Different species of cynipid gall wasps produce their own peculiar galls. The galls provide food and a brooding structure for the wasps in one of nature's most fascinating relationship between a tree and an insect.

Gall formation begins when a female gall wasp injects her eggs into a bud, leaf or stem. To accomplish this feat she uses the pointed egg-laying device on her abdomen called an ovipositor. [The stinger in many larger wasps is actually a modified ovipositor used to inject venom into the prey.] Upon hatching from the eggs, the hungry larvae begin feeding on the host tissue surrounding them. The plant's defensive reaction to this intrusive mechanical or chemical irritation is to isolate the toxins or activities of the invader in a tough, tumorous mass of tissue called a gall. In a sense, the plant is reacting to an intruder imbedded in its tissues. Ironically, in doing so the plant provides food and shelter for the developing ravenous larvae. After completing their growth and metamorphosis, often many months later, the adult wasps escape by chewing a circular exit tunnel through the wall of the gall. The precise mechanism by which different species of wasps produce such remarkably unique galls is still being debated by cecidologists (people who study galls).

The initiation of insect galls is typically associated with oviposition by the adult female (e.g. sawflies, gall wasps and beetles) or by the feeding of early larval stages (e.g. midges, moths and aphids). Depending on the type of insect and whether it has chewing or piercing-sucking mouthparts, a variety of salivary fluids may be injected into the plant tissue. Salivary fluids of bugs (hemipterans) and aphids (homopterans) may include amino acids, auxins (IAA) and various plant digesting enzymes such as pectinases, cellulases and proteases. The precise mechanism by which these chemicals induce cell division and morphogenesis is very complicated and varies with different species and even with different types of plant tissue. An increased concentration of certain plant hormones in the gall tissue may also be important for the development of some galls. A fascinating summary of research on insect galls is presented in Biology of Insect-Induced Galls, edited by J.D. Shorthouse and O. Rohfritsch, 1992.

The inflated, papery Mexican jumping beans that roll around by seemingly perpetual motion are not galls. They are actually the sections (carpels) of seed capsules of the jumping bean shrub (Sebastiana pavoniana) which is native to rocky desert slopes and arroyos in the states of Sonora and Chihuahua, Mexico. They also grow in rugged canyons of the Sierra de la Laguna in the Cape region of Baja California. By late summer, capsules of the Mexican jumping bean shrub have separated into 3 sections, each section splitting open and ejecting its single seed. Some of these sections fall to the ground and begin "jumping" or rolling around. The jumping sections contain a robust moth larva (Laspeyresia saltitans) that has eaten the seed tissue inside. After consuming the seed tissue within the capsule section, the robust, yellowish-white larva has the peculiar habit of throwing itself forcibly from one wall to the other, thereby causing the jumping movements of the carpel. It is these-hollowed out sections or carpels containing a fat little moth larva that are sold as jumping beans. Just as pineapples are not apples and peanuts are not nuts, jumping beans are not beans. [A bean is actually a seed that is produced in a pod or legume.] Perhaps jumping beans should be called "jumping carpels" or "jumping capsule sections."

Jumping bean shrubs typically bloom during the spring and summer months, and this is when the female moth lays her eggs on the green, immature capsules (ovaries) of the female flowers. When the eggs hatch, the tiny, immature larvae bore into the young capsules and begin feeding on the developing seed tissue inside. Only seeds without larvae will reach maturity and be viable; and luckily for the plant, not all capsules are infested with moth larvae. The developing capsules are not galls, they are not produced in response to the larvae embedded in their tissue. They are genetically programmed structures that will develop with or without the moth larvae.

Jumping galls actually turn and flip about on the ground as a result of the periodic, rapid, jerking movements of the larvae inside. According to F.A. Leach (1923), they can jump over one centimeter vertically and twice as far horizontally. WAYNE'S WORD has confirmed these jumping records, which are quite amazing considering that the larva is only 1 mm long. To really appreciate this feat, imagine a hollow, spherical container of tough, lightweight cardboard just large enough for you to squeeze and contort your body into. Then imagine jumping horizontally about 20 feet while inside your spherical container. A Mexican jumping bean can only jerk and roll about (or barely jump at all), even though the larva inside is many times larger. According to Ron Russo (Plant Galls of the California Region, 1979), all this jumping activity probably results in the downward direction of the galls into the leaf litter under oak trees. This behavior may serve to move the galls out of the hot, dry air, or to hide the tiny galls from the keen eyes of seed-eating or insect eating birds (or other possible predators). It is crucial to the tiny wasp larvae inside to avoid being eaten, at least until they have a chance to reach adulthood and pass on their genes. Even still, there are other perils awaiting the defenseless gall wasps inside their papery galls. According to Ron Russo, a high percentage of them are parasitized by other minute wasps, and will never pass on their genes. Of course many will fall on concrete or asphalt and will be walked on, driven over, or swept and blown away by people who like clean roads and walkways.

By late summer or early fall most of the jumping galls stop jumping. According to Ron Russo (1979), those that bear a singular, circular exit hole are usually parasitized by a different wasp species that has escaped from the gall. The larva inside each gall that escaped predation gradually undergoes metamorphosis and changes into a pupa. The pupa overwinters inside its protective gall until the following spring. According to S.S. Rosenthal and C.S. Koehler (Annals of the Entomological Society of America, Vol. 64, 1971) during a 20 day period between March and the first of April, adult wasps emerge from the galls. The emerging wasps are entirely parthenogenetic winged females, shiny black wasps that can lay viable eggs without fertilization by males. Their flight period coincides with the time of bud opening on nearby valley oak trees. The female wasps deposit their eggs in these newly opened buds by inserting their ovipositor (egg-laying device) into the soft tissue. The newly deposited eggs develop into larvae within another type of monothalamous, integral (nondetachable) leaf gall shaped like an oval blister about 1 mm wide and 2 mm long. When fresh, these galls are green but turn brown by late April and May. The blisterlike swellings occur on both the upper and lower surfaces of the leaves. The remarkable wasp generation that emerges from these galls is bisexual, with winged males and females. The sexes are clearly distinguishable, but not as distinctive as male and female fig wasps.

The exact timing of the emergence of male and female wasps from the blister galls is truly remarkable. According to S.S. Rosenthal and C.S. Koehler (1971), more than half of the adults emerge between 4:45 a.m. and 6:30 a.m., with the remainder exiting throughout the morning. All of the females seem to leave by 1:30 p.m. In addition, S.S. Rosenthal and C.S. Koehler (1971) discovered that none of the adults lived more than 24 hours after emergence. It is this short-lived bisexual generation that mates with each other, thus providing the vital genetic variability in the population. The gravid (pregnant) females deposit (oviposit) their fertilized eggs into the oak leaves, thus giving rise to the spherical jumping galls of summer and the jumping larvae inside. Because the bisexual generation of wasps emerging from the blister galls is so different from the unisexual (all female) generation emerging from the jumping galls, it was once classified as a different species Neuroterus decipiens; however, we now know that both types of gall wasps are simply alternating generations of the same species. In accordance with the biological rules of priority, the older name Neuroterus saltatorius is retained.

When WAYNE'S WORD received a sample of jumping galls from the University of California at Davis Agricultural Extension Service during August of 1997, many of the galls already had small female wasps emerging from circular exit tunnels. Although the wasps all appeared to be parthenogenetic females, this would not be the correct time for the female generation of Neuroterus saltatorius to be emerging. This late in the summer there wouldn't be any fresh oak buds for them to lay their eggs in, and the valley oak leaves would soon be falling. In addition, the wasps did not exactly match the detailed species description for Neuroterus saltatorius given by F.R. Leach (1923). For example, the antennae, tarsus, and thorax appeared to be slightly different from the description and illustration by F.R. Leach. [In the true jumping gall wasp, the first two antennae joints are conspicuously swollen, and the abdomen is more rounded or globose. When emerging it tends to break open its gall, rather than making a small circular exit tunnel as in the "bogus jumping gall wasp."] The emerging wasps observed in August were an entirely different species, one of the opportunistic parasitic wasps (bogus jumping gall wasps) that apparently inhabit a large percentage of California's jumping galls.

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