THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Options for Controlling and Managing Tropical Soda Apple
by Brent Sellers, Jay Ferrell, Bill Overholt, and Raghavan Charudattan
For questions or comments regarding this publication contact: Dr. Brent Sellers, University of Florida, IFAS
Tropical soda apple is an invasive plant that is typically unpalatable to livestock and wildlife, except for the seed-containing fruit. Plants are armed with sharp spines, which limits grazing near the plants, effectively lowering the grazing area of a given pasture. Since a single plant can grow 3-6 feet tall and 2-6 feet wide, the potential grazing lost to a single plant can be substantial.
We have observed a significant increase in Tropical Soda Apple (TSA; Figure 1) populations over the past two years in pastures throughout Florida, and especially south Florida. While the exact cause for this is unknown, it is likely due to the two consecutive mild winters with few frost events. Since seedlings typically begin to emerge in the late fall and early winter, the lack of a significant frost (until February 20, 2015) has allowed plants to continue to grow and reproduce, further adding seeds to the soil seed bank. A light frost has been observed to cause only minor damage to TSA plants, and they tend to recover as temperatures increase. In addition to the limited or scattered frosts, increased rainfall during the past two winters compared to the previous several years has likely also contributed to the increase in TSA.
Figure 1. Typical infestation of TSA in pastures during mild winters. Photo by Brent Sellers.
There are several options for TSA management. As with any weed management program, preventing the spread of TSA is important. Seed movement should be limited as seed is the primary means of spread. One way to prevent the movement of seed is to clean equipment when leaving a TSA-infested pasture, especially mowers. Since TSA seeds can remain viable in the digestive tracts of cattle for up to six days, holding them in a small area for up to six days after removing them from a TSA-infested pasture will limit the transport of seed. However, even with this practice, seed will continue to be transported by wildlife that consume the fruit. Pay close attention to congregation areas of the pastures (hay rings, trees, etc.) as TSA plants typically invade those areas due to increased seed and limited competition from forages. Above all, prevent plants from producing fruit through some type of management program.
Management of existing plants can be obtained through herbicide applications, biocontrol efforts, or a combination of the two. In improved bahiagrass, bermudagrass, and stargrass pastures with dense infestations of TSA, broadcast applications of GrazonNext HL at 24 to 32 oz/acre plus surfactant are very effective. Chaparral at 2.5-3.0 oz/acre plus surfactant can be used for TSA control in limpograss (Hemarthria/’Floralta’). These herbicides possess postemergence control of existing plants and preemergence control of germinating seeds. Our research has shown that aminopyralid, a component of both of these herbicides, will provide residual control of germinating seedlings for approximately six months. Triclopyr is also effective, however, plants must be mowed 4 to 6 weeks prior to application and it provides no residual control of germinating seedlings.
For sparse infestations where spot-treatment is warranted mix Milestone at 1 oz/5 gallons of water or GrazonNext HL at 1 oz/1 gallon of water. Triclopyr at ½ oz/1 gallon of water could also be used. A non-ionic surfactant should be mixed with these herbicides. Glyphosate mixed at 1-2 oz/gallons of water can be reasonably effective, but is not generally recommended. Glyhposate will kill the desirable forage allowing for reestablishment of TSA from seed. Additionally, a color marking dye is beneficial when spot spraying to ensure that all plants have been treated. Be sure to spray the entire plant to the point of run-off to ensure herbicide uptake and maximum control when spot-spraying (Figure 2). Monitor treated areas monthly and treat escapes as necessary.
Figure 2. Response of TSA to Milestone and Remedy herbicides when applied to the whole plant or half of the plant. Credit: Jay Ferrell.
Biocontrol options include the TSA beetle (Gratiana boliviana) or an application of a virus called Tobacco mild green mosaic tobamovirusstrain U2 (TMGMV U2) registered as a biological herbicide under the trade name of SolviNix® LC. The TSA beetle, unless present at very high populations, does not control individual plants. However, it does reduce the overall fitness of the plant, making the plant less competitive with desirable plants. The TSA beetle has been shown to decrease plant height and diameter, as well as the number of fruits produced per plant (Figure 3). Since the beetle does not consistently control plants, it is likely not suited for dense infestations within a given pasture. However, it is well suited for areas of a pasture that are not easily accessible. The beetle’s impact is more noticeable south of Orlando. Additionally, the beetles are inactive from November through March as they enter a resting state called ‘diapause’ during which they do not feed or reproduce. When winters are mild, TSA plants grow unchecked for several months and get ahead of the beetles which only become active again in late March. During this mild winters, it would be advantageous to spot-treat TSA plants with herbicide or SolviNix so that beetles can keep up with the remaining TSA population.
Figure 3. Response of TSA plants to the TSA beetle (Gratiana boliviana; GB). Credit: Dr. Bill Overholt, UF/IFAS Indian River Research and Education Center, Ft. Pierce.
The bioherbicide, SolviNix® LC, was recently registered by the EPA as a post-emergent herbicide to control TSA. SolviNix is exempt from a tolerance requirement, so there is no need to hold the cattle from treated areas or be concerned about residue in manure, meat, or milk. The virus in SolviNix infects and kills the TSA plant, including the roots. Hence, by applying SolviNix before fruit set, it is possible to reduce or even stop further addition of seed to the soil seed bank. Additionally, SolviNix can be used safely in pastures intercropped with clovers, perennial peanut, and other forages – plants that are immune to the virus. Research has determined that a very small amount of the TMGMV U2 (around 2 g of virus/acre) provides excellent control of TSA plants (Figure 4). For SolviNix to work correctly, the plant must be injured prior to application, or be applied at very high pressures (> 80 psi). However, unlike chemical herbicides, there is no need to cover the entire plant canopy with the SolviNix spray; application to a few leaves on a plant is sufficient to allow the virus to infect and kill the plant. The virus has no soil residual activity and it is not spread by insect or other typical vectors of plant viruses. Therefore, the virus will not spread within a field from a treated plant to untreated plants. This will make it necessary to repeat applications to hit plants that were missed in an earlier application or newly emerged plants.
Presently, SolviNix is approved for use only in pastures and wooded areas that are fenced to contain grazing animals, which applies to most ranches. It is approved for application only with a backpack sprayer or hand-held herbicide wipers, which limits its use to spot treatment. The SolviNix Label and Material Safety Data Sheet as well as information on how to obtain SolviNix can be found at www.bioprodex.com .
Figure 4. Left: View of TSA plants before SolviNix application. Plants in the foreground with red paint were left untreated to mark the border of the treated site. Right: View of the treated site, on the seventh week after treatment showing nearly complete control of treated TSA plants. Credit: Raghavan Charudatan