MP-1315 7/78
Galls are abnormal swellings of plant tissue induced by insects, bacteria, fungi, mites and nematodes. Insect-induced galls are the most common galls in urban areas. Among the insects causing galls are certain moth caterpillars, beetles, flies, aphids and small wasps.
The gall growth provides residing insects with food and shelter during certain developmental stages. Galls have characteristic sizes, shapes and colors. In early days, when little was known about galls, naturalists named and described some galls according to plant growth patterns rather than by the insects causing them. This practice has changed because some insect species induce more than one type of gall.
Gall tissue is a plant product formed in response to a specific stimulus received from an insect. The stimulus is thought to occur either when insect eggs are deposited in host plant tissue or when eggs hatch and larvae feed on the plant (type of stimulus depends on the insect species). In either case, chemicals, which are thought to secrete from special internal glands, interact with certain plant chemicals to produce abnormal plant growth. After a brief period of cell growth, gall development stops completely. The insect lives within the gall and feeds on gall tissue during its development. Once formed, these growths discontinue utilizing host plant nutrients. Insect-induced galls are considered harmless to the growth and development of almost all ornamental plants.
The mealy-oak gall is one of the most common galls on live oak in Texas (figure 1). The gall is induced by a small wasp, Disholcaspis cinerosa (figure 2). Gall-infested live oak trees occur throughout Texas in natural and planted situations. The gall wasp also is reported from the same host in certain parts of Mexico and from western Louisiana. The following account summarizes 5 years of research on the biology, ecology and control considerations of Disholcaspis cinerosa on ornamental live oak in several Texas cities.
Disholcaspis cinerosa has 2 generations annually, each producing a different type of gall on live oak.
Asexual generation. The asexual generation develops within mealy-oak galls on branches and branchlets of host trees (figure 1). These spherically shaped galls are familiar to people who live in cities where live oaks commonly are planted as ornamentals. Galls range from 1/8 to 1 inch in diameter and are first noticed in late summer or early fall, depending on locality. When first formed, they are small, light pink to pinkish brown and the internal tissue is soft (figures 1 and 3).
Disholcaspis larvae are believed to secrete chemicals that stimulate the plant to produce the spherical swellings. From late summer to late fall (figure 4) developing Disholcaspis larvae eat plant tissues within galls. From August to mid-October, internal gall tissue is yellow-green and moist (figure 3), while during late October and November the tissue turns brown and begins to dry (figure 4). During November, pupation occurs and shortly thereafter transformation to the adult stage takes place. During December and the first part of January, adults emerge by chewing holes at gall bases (figure 1).
Emerging Disholcaspis adults of this generation are all females (figure 2) which do not require mating to lay viable eggs; hence they are asexual. Interestingly, this unique reproductive habit also is found in many common and beneficial parasitic wasps and in most aphids.
The newly emerged Disholcaspis female seeks out swollen leaf buds on live oak branches in which to deposit one or possibly two eggs per bud. Females may live 2 to 6 weeks during which time each individual may lay up to 20 viable eggs. The frequently harsh weather at this time of year appears to have little adverse effect on the egg-laying activities of most females. Females die after eggs are deposited. Eggs remain in a dormant state during the remainder of winter (January and February).
During the first winter, recently abandoned spherical galls begin to weather and in the process change from pink to grey and become dry and hard (figure 1). If left undisturbed, they may persist on host trees for several years. In essence, grey galls are inactive structures that serve to remind one of a past event. They actually assume a nonliving status about 1 month (November) before adults begin emerging from galls.
Sexual generation. As mentioned, females of the asexual generation lay their eggs in swollen leaf buds. These eggs hatch in early spring as leaf buds begin to open. Resulting larvae develop quickly in new leaf tissue, and in the process they induce the formation of a second type of gall. This new growth is beige and resembles a kernel of wheat in size and shape (figure 5). Larvae, pupae and adults complete development in a few weeks within these small, obscure galls. The sexual generation differs from the asexual generation not only in gall type but also in that the spring galls produce adults of both sexes. Sexual generation adults are about 1/3 the size of the asexual generation adults (figure 2).
Immediately after emergence, male and female wasps mate; males then die. Mated females may live up to a week during which time each individual deposits about 15 eggs in the branches of live oak. The eggs pass through a 3- to 5-month dormant period. After this period, larvae hatch and begin development in certain outer branch tissues, which leads to formation of pink spherical galls (figure 1). Thus the annual cycle of Disholcaspis is completed.
The seasonal activity of Disholcaspis may be followed partially by observing certain obvious seasonal changes in the live oak tree. Simultaneously occurring seasonal events between Disholcaspis and live oak are shown in figure 6.
Early winter (late November to early December). By early winter, freezing weather has occurred in most Texas areas. Leaf buds are swollen on most live oak trees although the old leaves have not begun to drop. These general climatic circumstances and host plant conditions coincide with the emergence of adults of the asexual generation.
Late winter to early spring (February to March). Depending on locality, most live oaks are partially leafless by March and leaf buds have begun to break open. At the bases of expanding new leaves, rapidly developing Disholcaspis larvae are found. When new leaves are about three quarters expanded, the kernel-like galls of the sexual Disholcaspis generation may be observed on certain leaf terminals. Oftentimes galled leaf terminals may be recognized since infested terminals produce extra leaves, which are clustered around the bases of galls. Once the leaves are expanded fully (late spring), most of the sexual generation Disholcaspis adults emerge and most of the egg-laying stops.
Summer to early fall. After new leaves are produced in spring (short shoots), an elongation of new branchlets takes place. Generally, the degree of shoot elongation depends on the yearly rainfall pattern. Numerous long shoots usually result when rainfall has been abundant during the year. At some time during shoot elongation, new asexual spherical galls begin forming on branches; however, no obvious tree activity pattern corresponds in time with the formation of these curious pink growths.
In summary, most of the annual life cycle of Disholcaspis can be followed by merely observing changes in seasonal development of live oak trees.
Within the same year. Seasonal history development of Disholcaspis in a given year differs from locality to locality. The difference is most noticeable between cities located in the northern versus southern parts of the state, and it is related to the general climatic conditions and associated plant growth in the respective cities. Basically, southern Texas cities experience more favorable growing conditions earlier in the year than the more northern cities. For example, populations of spring generation Disholcaspis begin development earlier in the year in Houston than in Dallas. Also, new pink spherical galls of the asexual generation always make their appearance earlier in Houston than in Dallas. However, adults of the same generation emerge from galls about the same time each year in both cities.
Year to year. In addition to difference among cities in a given year, the development of Disholcaspis populations may vary by a month or more between years in a particular city. This difference is related t plant growth. When plant growth begins “early” in a given year, this means that certain associated insects will also usually be “early.” For example, in 1974 in Dallas, live oaks begin their seasonal changes “early” in the year. The “early” signs were noted in March when trees began flushing new spring leaves. At this time, the sexual generation of Disholcaspis also was beginning to develop. In 1975, Dallas live oak trees did not begin flushing new leaves until April, which is considered “late” for that city. This “late” seasonal development of Dallas live oaks was accompanied by a corresponding “late” seasonal development of Disholcaspis.
All live oak trees are not alike in their capacity to produce Disholcaspis galls. A total of 170 live oak trees from city and shopping plazas in Dallas, Houston and San Antonio were randomly selected to examine this possibly inherited capacity. The classes, light, moderate and heavy, were established arbitrarily to rank infestations of each tree. Infestation class was determined by sampling and counting all spherical galls (asexual generation) on 12 randomly selected branches (each ½ yard in length) from each tree. Survey results are summarized below. Numbers in parenthesis indicate range in numbers of galls recorded for each class:
|
Infestation classes |
Number of trees recorded in each infestation class |
| Light (0 to 50) | 132 |
| Moderate (51 to 200) | 28 |
| Heavy (201 to 1,000) | 10 |
| Total | 170 |
The survey revealed that most trees (132 out of 170) were only lightly infested. In fat, 118 out of the 132 trees in the light group had fewer than 25 galls per sample. Of the remaining two groups, usually only the heavy group causes concern to homeowners. In this group, only 10 out of 170 trees had high numbers of galls. The survey also indicated that trees classified as heavy usually are younger and smaller (less than 10 inches in trunk diameter at breast height).
Experimental studies indicate that the gall-forming capacity of relatively nonsusceptible trees (light class) to Disholcaspis may be quite constant year after year. That is, if a tree shows little or no inclination to form galls after 2 to 4 years, it will probably never produce galls. In contrast, trees that are potentially capable of producing galls (mostly trees in the heavy class) may experience a change in infestation class over several years. In this case, a potentially susceptible tree may pass through cycles of great gall abundance interspersed with periods of few or no galls.
Long-term studies on highly susceptible trees (heavy class) suggest that a cycle of great gall abundance may last 2 to 3 years. Periods of decline in gall numbers then ensue for 1 to 3 years. At the end of the decline period, trees may not produce any Disholcaspis galls in a given year. Length of the decline period is unknown; however, it is assumed that an appropriate period must pass before the gall-forming capacity of susceptible trees returns. Because of the cyclic nature of gall production, these trees are considered potentially susceptible.
It is relatively easy for inexperienced persons to determine the state of infestation on an affected tree. In late fall-early winter, select about eight branches from the outside of oak crown (each branch should be about ½ yard in length and should be selected from a different side of tree). Remove all attached galls and divide into two groups: pink gals (new) and grey galls (old). Estimate the specific stage of the cycle by comparing the number of pink to grey galls:
Two combined factors may be responsible for the decline in Disholcaspis galls on potentially susceptible trees.
Parasitized spherical galls can be recognized easily by their smaller size. Parasitized galls generally measure 1/8 to ½ inch in diameter, while unparasitized galls are ½ to 1 inch in diameter. Disholcaspis usually are parasitized during the larval stage, at which time galls are small. A parasitized Disholcaspis larva stops producing chemicals that stimulate the tree to form gall tissue, thus stopping gall growth.
Isolated live oaks in urban environments often have greater infestations of Disholcaspis than live oaks occurring in groups. For example, young, potentially susceptible oaks planted in shopping plaza parking lots often are very heavily infested with galls. Reasons for this pattern are not fully understood. However, when oak trees are first planted, the following ecological imbalance occurs. Before introducing potentially susceptible live oaks into urban landscapes from nurseries, the trees have been carefully examined in accordance with state inspection standards and treated for any symptoms of insect infestation or disease. When these “clean” oaks are planted in urban environments, they immediately become exposed to a wide variety of insects. In the case of Disholcaspis on an isolated oak, the gall inducers arrive before their natural enemies (wasp parasites). A year or more may be required before enough parasites find the galls and begin controlling Disholcaspis. Thus, a cycle of initially high gall numbers followed by subsequent reduction in gall numbers can be expected when susceptible host plants are moved from relatively insect-free situations to isolated urban landscapes. In essence, when man isolates live oaks he is creating an island which is ecology imbalanced from the standpoint of associated insects.
In addition to the gall inducer and its parasites, the spherical gall has other interesting insects associated with it. Collectively, all of these insects are said to form a community. Like all communities (plant or animal), the community of insects associated with Disholcaspis has its own unique organization. The organization can be described by examining the various stages of gall development and the insects associated with each stage.
The presence of spherical galls of Disholcaspis usually causes concern to homeowners through fear that these abnormal swellings may increase and endanger plant health. Survey studies throughout the state indicate that trees sustaining very high gall numbers of both Disholcaspis generations, suffer no noticeable or measurable damage. It is clear, however, that during periods of gall formation, development of the respective Disholcaspis generations requires a transfer of nutrients from host tree to gall tissue. Once these developmental periods are passed, gall growth stops completely. Live oak trees seem to tolerate brief periods when tree nutrients are utilized for gall tissue formation.
The state-wide survey also revealed that not all homeowners are bothered by Disholcaspis galls. When informed of the interesting community of insects associated with galls, some homeowners with to preserve these curious, nonharmful insects. Homeowners in other parts of the United States, who are knowledgeable of gall insects associated with certain ornamental plants, seek gall-producing plants, contending that they impart an aesthetically pleasing and unique characteristic to the plants.
Numerous ornamental insect pests generally are controlled to a greater or lesser extent through use of insecticides. Strong arguments against insecticide use on either Disholcaspis or any other gall insect on live oak can be made for the following reasons (1) only about one in 17 live oaks has the capacity to produce Disholcaspis galls at levels which cause concern to homeowners; (2) potentially susceptible trees apparently pass through intervals of susceptibility so that in some years these trees will be virtually free of new galls; (3) if a tree passes through an interval of susceptibility, the period of active gall development for each Disholcaspis generation is relatively brief. Therefore, once formed, these galls do not continue to utilize host plant nutrients; and (4) economic damage has never been observed even on trees sustaining very high populations of new Disholcaspis galls. It seems, therefore, that potentially susceptible trees can tolerate occasional intervals of gall infestations. Finally, trees, which at an early age were considered potentially high gall producers, appear to lose much of their susceptibility as they age.
An important point to reemphasize is that if conditions are favorable, several parasitic wasps will provide effective long-term control against Disholcaspis. Since parasites are killed easily with insecticides, exercise care in applying any chemicals around oak trees. Untimely or careless chemical applications can cause an increase in gall numbers by virtue of eliminating the natural enemies of Disholcaspis. Once gall tissue begins to form, it is impossible to stop or reverse the growth with chemicals. Chemicals are ineffective because they do not penetrate woody growth and kill the gall-inducer inside.
Precisely timed chemical applications seem to offer a possible means for controlling these insects. However, because of the lengthy adult emergence period in December to early January and the often varying emergence period in spring, the chemical approach alone is currently unreliable. Furthermore, consider the above biological and ecological factors before making any decision to use artificial control procedures.
Nonchemical gall control methods offer only limited help. For example, spherical galls are detached easily and may be removed by hand from small infested trees, but this procedure is not practical on larger trees. In some cases, consider removing the host tree, particularly when it is newly planted.
Researchers at the Texas Agricultural Experiment Station have been growing oaks and other ornamentals from stem cuttings of superior plants for several years. Through this vegetative (asexual) reproduction method, genetic identity remains intact. In the case of Disholcaspis-induced gall formation on live oak, which is thought to be inherited, cuttings are being propogated from “apparently resistant” trees. If cutting from these trees prove, through intensive testing, to be resistant to Disholcaspis, this plant material will be propogated in quantity for distribution to the nursery industry.