Tropical Soda-Apple (Invasive Exotic Plants of North Carolina)

Tropical Soda-Apple

Solanum viarum

Summary ⁴

Solanum viarum, the tropical soda apple, is a perennial shrub native to Brazil and Argentina with a prickly stem and prickly leaves. The fruit is golf-ball-sized with the coloration of a watermelon. It is sometimes considered an invasive species in the lower eastern coastal states of the United States and recently on the Mid North Coast of Australia.

Impacts and control ⁵

More info for the terms: cover, fire management, fitness, invasive species, natural, prescribed fire, reclamation, restoration

Impacts: In Florida, tropical soda apple is an invasive, nonnative plant that is a major agricultural pest and a serious threat to wildlands (Mullahey 1997 personal communication cited in [52]). Since its introduction to Florida in the 1980s, it spread rapidly throughout the state, infesting an estimated 1.2 million acres (500,000 ha) by 1995. Its estimated annual rate of spread between 1990 and 1995 was 117% in Florida and 35% in other southeastern states. Its rapid spread in the Southeast has been attributed to its tremendous reproductive potential and highly effective seed dispersal (see Regeneration Processes) [27]. It is not considered invasive in its native range, presumably because it has native biological controls [28,57].

Due to its rapid spread, tropical soda apple is on the Federal Noxious Weed List [94]. In Florida, tropical soda apple was listed in 1994 on the state's noxious weed list as a Category I invasive, making it unlawful to introduce, have in one's possession, move, or release it in the state [26,67]. Category I invasive species in general are "nonnative species that have invaded natural areas and are displacing native plants or are disrupting natural community structure and function" [39]. Tropical soda apple's greatest impact in Florida has been to bahia grass pastures, where it displaces the shade-intolerant nonnative grass [27,34,66,103] and can eventually form monocultures [27]. When it establishes in wooded areas, it prevents cattle from seeking shelter from the sun [27,103], presumably because its spines restrict livestock movement [66]. As of 2009, tropical soda apple was estimated to infest nearly 1 million acres (400,000 ha) of pasture in Florida [85].

It is unclear what impacts tropical soda apple has in states outside Florida, but research in Mississippi indicates it has the potential to become a threat [11]. Georgia declared tropical soda apple a "Public Nuisance" for its impacts to the agricultural and horticultural industries [42]. The Tennessee Exotic Pest Plant Council has given tropical soda apple a ranking of 1. Plants in this category pose a severe threat in Tennessee because of their invasive tendencies and their ability to spread easily into native plant communities and displace native vegetation [89].

Tropical soda apple's ecological impacts have not been well documented [45], and little has been reported on its impacts to wildlands. It is assumed that tropical soda apple displaces native plants and interferes with natural ecosystem processes [23,26,66]. However, it has been suggested that on uncultivated rangelands and wildlands, tropical soda apple may not form dense monocultures (Mullahey personal communication cited in [34]). Eight years after its introduction to North America, tropical soda apple was reported in 20 natural areas from as far north as Alachua County, Florida (Florida EPPC 1996 cited in [52]). In Florida, it is estimated that wooded areas comprise 10% of the total land infested by tropical soda apple [67]. The spines on tropical soda apple can degrade wildlife habitat by creating physical barriers for animal movement, especially in large infestations [26]. Tropical soda apple is reported in numerous native plant communities in North America and, based on its ability to adapt to a wide range of environmental conditions, it has the potential to establish in many other communities throughout its current and predicted range 2.

Because tropical soda apple may invade phosphate-mine reclamation sites, it may interfere with restoration efforts [3]. Tropical soda apple's increased fecundity with increased phosphorus levels (see Seedling emergence and plant growth) has led researchers to conclude that soils high in phosphorus may be predisposed to invasion by tropical soda apple [17].

Economic impacts: Tropical soda apple is a serious economic threat to the beef cattle industry. If left uncontrolled, it can infest a pasture in 1 to 2 years [67], reducing pasture productivity and lowering stocking rates [70]. Surveys taken in 1993 indicated that Florida's beef cattle producers attributed $11 million dollars in production losses to tropical soda apple. Losses were associated with lower carrying capacity of pastures and heat stress on cattle that could not access shade [67]. Based on field tests in Florida, 20 tropical soda apple plants/acre of pasture could result in an estimated revenue loss of $49/acre due to a decrease in bahia grass yield [66]. Restrictions on the interstate movement of livestock, seed, hay, soil, and manure may have additional economic impacts to the beef cattle industry [34].

Because it interferes with crop production, tropical soda apple is a threat to the vegetable crop industry [26,27]. It also acts as an alternative host for the potato fungus Alternaria solani [34] and 6 viruses [56] that cause economic damage to vegetable crops of the Solanaceae family [56]. Tropical soda apple may be a host to bidens mottle virus, which causes damage to crops outside of the Solanaceae family [6]. Additionally, numerous crop pests utilize tropical soda apple as an alternative host [34]. Although no estimates of monetary losses to this industry were found, one review predicted that tropical soda apple "will" have major economic impacts to agriculture [23].

Control: As of this writing (2009), research is ongoing. The University of Florida's West Florida Research and Education Center and the Center for Aquatic and Invasive Plants provide information on tropical soda apple control.

Tropical soda apple is difficult to control due to its extensive root system, which can generate new shoots, its prolific seed production (see Regeneration Processes), and its tendency to form large patches. The sharp prickles act as barriers, limiting access to infested sites [23]. Complete eradication may be unlikely [69]. Efforts to control tropical soda apple may be most effective if the entire plant, including its immature and mature fruit and its root system, is removed [12,24]. Effective weed control strategies need to account for a soil seed bank lasting approximately 3 to 12 months (see Seed banking) [63]. To prevent reintroduction of seed from neighboring uncontrolled populations of tropical soda apple, repeat control measures and follow-up monitoring may be required [23,85].

Invasive plant control is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [54]. In all cases where invasive species are targeted for control, the potential for other invasive species to fill their void must be considered regardless of what control method is employed [9]. Efforts to control tropical soda apple would be enhanced if more were known about ecosystem responses to tropical soda apple invasion in wildlands.

Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.

Prevention: Regulations and collaborative efforts have been initiated to prevent the spread of tropical soda apple to areas outside its current (2009) North American range. Since it is on the Federal Noxious Weed List, the interstate movement of tropical soda apple plants or products containing tropical soda apple is prohibited [93]. Additionally, Vermont, Massachusetts, Kansas, Minnesota, Arizona, California, and Oregon have placed restrictions on tropical soda apple transportation and/or have listed it as a potential noxious weed or pest plant in their state [13]. In 1994, numerous southern and mid-Atlantic states formed the Tropical Soda Apple Task Force to develop effective strategies to control and limit the dissemination of tropical soda apple. As of 2009, a set of best management practices was being developed to help control tropical soda apple and slow its spread [48,71]. The tropical soda apple Research and Extension Group was organized in 1993 at the University of Florida to facilitate communication among the various researchers and extension programs involved with tropical soda apple control [67].

Florida cattlemen and sod farmers have been targeted for cooperative participation in programs and/or regulation related to cattle transport activities in an effort to slow the spread of tropical soda apple [71]. The Tropical Soda Apple Task Force collaborates with industry representatives to find cooperative approaches to slow the spread of tropical soda apple. As of 2009, no official regulatory policies had been issued in Florida restricting cattle movement as it pertains to tropical soda apple spread; however, other states may have regulations concerning the acceptance of cattle from Florida [48]. At the time of this writing (2009), Florida Department of Agriculture and Consumer Services was charging a fee to certify sod as tropical soda apple-free [71].

Various other methods have been suggested to prevent the spread of tropical soda apple by seed. Control methods that reduce tropical soda apple fruit production [63] or seed dispersal, such as mowing, may help prevent the spread of tropical soda apple (see Physical or mechanical control). Cleaning off all equipment and vehicles that have been used on sites infested with tropical soda apple may help to reduce the spread of tropical soda apple seed [64]. Researchers studying tropical soda apple seed viability in the digestive tracts of cattle concluded that cattle grazing on tropical soda apple-infested lands be held in small feedlots that are tropical soda apple-free prior to transporting them to noninfested pastures. Cattle removed from tropical soda apple-infested sites for at least 1 week prior shipping are less likely to spread tropical soda apple seed through their feces [10]. A laboratory study indicated that there was potential to prevent the spread of viable tropical soda apple seed by feeding cattle herbicide-treated feed [53]. The authors, however, did not indicate how this treatment could influence the health of cattle or consumers of beef.

Cultural control: Following greenhouse and field tests, Akanda and others [1] suggested that maintaining a thick cover of grass may help reduce emergence of tropical soda apple seedlings in pastures.

Physical or mechanical control: For small patches, tropical soda apple seedlings and mature plants can be pulled or dug out, but care must be taken to remove the entire root to prevent regrowth [102]. Mechanical procedures such as mowing or plowing may be inappropriate in wildlands.

For established populations and large patches of tropical soda apple in pastures, maintaining plants in a fruit-free condition may limit seed dispersal [63,69]. Moderate tropical soda apple control (83%) has been achieved by mowing plants 3 consecutive times at 60-day intervals, preferably before they set fruit. Improved control was achieved (93-100%) when mowing was done in combination with an herbicide application (see Integrated management) [65]. Since tropical soda apple plants typically regrow and produce fruit in approximately 70 to 80 days after being cut [65], mowing every 50 to 60 days is recommended to prevent seed dispersal [65,67,69]. Because laboratory tests showed that tropical soda apple seed from fruits <0.4 inch (1.0 cm) in diameter did not germinate, Bryson and Byrd [11] suggested that mowing or handpicking tropical soda apple when fruits are <0.4 inch in diameter may help reduce seed dispersal and subsequent seedling establishment.

A greenhouse study suggests that control strategies that keep tropical soda apple seed at the soil surface (e.g., mowing) or at depths >4 inches (10 cm) (e.g., plowing) could substantially reduce tropical soda apple seedling establishment 69.

Since new shoots can regenerate from root fragments, tilling is not recommended for mechanical control of tropical soda apple [69].

Biological control: Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls [98,104]. Tu and others [92] provide background information and considerations for biological control of invasive species in general in their Weed Control Methods Handbook. Additionally, the University of Florida's West Florida Research and Education Center and the Center for Aquatic and Invasive Plants provide information on biological control of tropical soda apple.

Efforts to identify an effective biological control for tropical soda apple were initiated in the early 1990s. Researchers, mostly affiliated with the University of Florida, have been studying a variety of insects, fungi, bacteria, and viruses for their potential to control tropical soda apple. As of this writing (2009), researchers had released a South American beetle for biological control for tropical soda apple, and other South American insects were being evaluated [58].

Gratiana boliviana was approved for field release in 2003. Since then, more than 220,000 of the leaf beetles have been released throughout Florida and parts of Alabama, Georgia, and South Carolina [48,58,97]. Populations of the beetle have established on several of these sites [58]. In 2007, G. boliviana was released in Jasper County, Texas, to control tropical soda apple [48,97]. Since its release in 2003, G. boliviana has spread as far north as Rhea County, Tennessee, at 35.6 ° N latitude [31]. After evaluating G. boliviana's generational turnaround and tolerance to cold, Diaz [31] concluded that the northern extent of this beetle's range in the United States would be near 32 to 33 ° N latitude.

Initial evaluations of G. boliviana's release reported extensive (20-100%) defoliation on tropical soda apple by the beetle [58]. One review reported that this beetle chews holes in the upper leaves of tropical soda apple, substantially reducing the weed's survival [25]. A later review indicated that G. boliviana has not typically controlled individual plants unless the beetles were present in "very high" numbers. The beetle may, however, reduce the number of fruits and/or reduce a plant's overall fitness. The review also suggested that G. boliviana is better suited for control of small infestations of tropical soda apple than large or remote infestations [85]. No nontarget effects had been reported within 3 years of the beetle's release [58].

Tobaviruses are being evaluated for their potential to control tropical soda apple [77]. In the greenhouse, tropical soda apple plants inoculated with Tobacco mild green virus died within 14 days of inoculation. Researchers concluded that Tobacco mild green virus was lethal to tropical soda apple and should be further evaluated for possible use in integrated management of tropical soda apple [21]. Overholt and others [77] investigated the influence of Tropical soda apple mosaic tobamovirus on the beetle G. boliviana, and also tested transmission of the virus by the beetle. Under laboratory conditions, there was no evidence of viral transmission by G. boliviana; however, the virus reduced the effectiveness of G. boliviana as a biological control agent on tropical soda apple, which may limit its use for integrated management.

Chemical control: Research on chemical control of tropical soda apple was ongoing as of 2009. See West Florida Research and Education Center and the Center for Aquatic and Invasive Plants databases for information on chemical control of tropical soda apple.

Herbicide treatments have and continue to be developed to control tropical soda apple and are being used in the field to reduce tropical soda apple populations and slow its spread. In the greenhouse, tropical soda apple was controlled 90 days following herbicide treatments [33]. Field tests in Florida indicated that herbicide applied in April could reduce tropical soda apple cover from 11% to 98% in 50 to 335 days, depending on the type of herbicide applied [36]. Seed production may be stopped if herbicide is applied prior to tropical soda apple flowering [1]. Combining herbicides with a growth regulator that prevents flowering, such as maleic hydrazide, may extend the number of days that herbicide controls tropical soda apple [33]. While control may be achieved with one application of herbicide, repeat herbicide applications may be necessary for herbicides that lack preemergence control [36,84,85]. Herbicides must be translocated to the roots of tropical soda apple to kill root buds; otherwise, root sprouting occurs. Results from a greenhouse study suggest that translocation of herbicide to the roots of tropical soda apple is optimized in September and October in southern Florida [69].

Many chemicals have been studied to control tropical soda apple, especially for large infestations in pastures. Reports from the University of Florida Extension service indicate that "excellent" chemical control may be achieved if herbicide is applied correctly. As of this writing (2009), aminopyralid and triclopyr were recommended for control of tropical soda apple [35,84,85]. Herbicide treatments developed for tropical soda apple control have been designed for specific circumstances, and each has strengths and weaknesses. Therefore, it is important to understand the benefits of these herbicides to achieve maximum effectiveness [35]. A few publications contain specific information on the use of these and other herbicides to control tropical soda apple: [84,85]. Trenholm [91] provides a detailed analysis of nutrient accumulation in tropical soda apple that may help to identify optimal herbicide application timing.

Herbicide treatments developed for pasture use may not necessarily be appropriate for oak hammock areas [1] or other wildlands. Because imazapyr usually stays within the top 20 inches (50 cm) of soil and may not affect mature oak trees, one study recommended its use to control tropical soda apple in oak hammocks. Lowering herbicide rates from those recommended for pasture use may help to reduce risk to immature oak trees [1].

Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [14]. See the Weed Control Methods Handbook [92] for considerations on the use of herbicides in wildlands and detailed information on specific chemicals.

Integrated management: As of this writing (2009), mowing in combination with herbicide treatment was being recommended as one method to control tropical soda apple [61,85]; however, mowing-herbicide treatment may be difficult and/or inappropriate in wildlands, and is likely limited to pastures.

Herbicide treatment following frost may control tropical soda apple. In Florida, over 95% of tropical soda apple plants were controlled within 120 days following various herbicide treatments that were applied to regrowth 60 days following a heavy frost. Effectiveness of this integrated approach may be influenced by herbicide concentration and timing of application [60]. Ferrell and others [37] investigated the potential to use Tobacco mild green mosaic tobamovirus in conjunction with herbicide for tropical soda apple control. The tobavirus was mixed with various synthetic herbicides and rubbed on the leaf surface of tropical soda apple plants. On the average, mixtures containing the tobavirus reduce tropical soda apple by 81% compared to the control rate from herbicide alone [37].

Sources and Credits

(c) J. Jeffrey Mullahey, University of Florida, Bugwood.org, some rights reserved (CC BY), http://commons.wikimedia.org/wiki/File:Solanum_viarum_fruit.jpg
(c) Más & Lugo-Torres, some rights reserved (CC BY-NC-SA), https://collections.nmnh.si.edu/services/media.php?env=botany&irn=10384906
(c) Más & Lugo-Torres, some rights reserved (CC BY-NC-SA), https://collections.nmnh.si.edu/services/media.php?env=botany&irn=10384909
Adapted by Kate Wagner from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Solanum_viarum
Public Domain, http://eol.org/data_objects/24630247

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