ABSTRACT
Tropical soda apple, Solanum viarum Dunal, wetland nightshade, S. tampicense Dunal, and turkey berry, S. torvum Swartz, are considered three of Florida's most invasive plant species. These normative perennial broadleaf weeds are disrupting native plant communities in agricultural areas and natural ecosystems. The lack of natural enemies in Florida is thought to be an important factor contributing to their invasiveness. The North American leaf beetles Leptinotarsa defecta (Stal) and L. texana (Schaeffer) that attack silverleaf nightshade, Solanum elaeagnifolium Cav., a native congener of the three normative solanums, were evaluated for their potential as biological control agents. The suitability of tropical soda apple, wetland nightshade and turkey berry as host plants for the native Leptinotarsa beetles was studied in a quarantine laboratory using single plant and paired plant tests. Neonate larvae of L. defecta developed to the pupal stage only on their natural host plant silverleaf nightshade. Feeding damage on turkey berry and wetland nightshade was negligible and no feeding occurred on tropical soda apple. In contrast, development and reproduction of L. texana on the nonnative turkey berry were comparable with silverleaf nightshade. These results suggest the nonnative turkey berry may be included in the potential host range of the native silverleaf nightshade beetle L. texana.
Key Words: Biological control, weeds, Solanum viarum, S. tampicense, S. Torvum, S. elaeagnifolium, risk assessment
RESUMEN
Solanum viarum Dunal, S. tampicense Dunal, y S. torvum Swartz se consideran como tres de las especies de plantas mas invasoras en Florida. Estas malezas perennes no nativas de hoja anchor estan perturbando las comunidades de plantas en areas agricolas y ecosistemas naturales. Se piensa que la falta de enemigos naturales en Florida es un factor importante que contribuye a su habilidad para ser invasoras. Se evaluaron los escarabajos norteamericanos, Leptinotarsa defector (Stal) y L. texana (Schaeffer) que atacan las hojas de Solanum elaeagnifolium Cav., una planta nativa en el mismo genero de los tres solanums no nativos, para determinar su potential como agentes de control biologico. Se estudio si las plantas de Solanum via.rum, S. tampicense y S. torvum podrian ser hospederos adecuados de los escarabajos nativos Leptinotarsus en el laboratorio de la cuarentena usando pruebas de plantas individuales y en pares. Se desarrollaron las larvas recien nacidas de L. defector hasta la etapa de pupa solamente en su planta hospedera natural Solanum elaeagnifolium. El dano de alimentacion en el S. torvum y Stampicense fue insignificante y no se aliment6 de Solanum viarum. Al contrario, el desarrollo y la reproducion de L. texana sobre S. torvum no nativo, fue similar con los de S. elaeagnifolium. Estos resultados suguieron que se puede incluir S. torvum no nativo entre los hospederos potenciales del escarabajo de Solanum elaeagnifolium no nativo, L. texana.
Tropical soda apple, Solanum viarum Dunal, wetland nightshade, S. tampicense Dunal, and turkey berry, S. torvum Swartz, are perennial normative invasive weeds that have been identified as candidates for biological control (Cuda et al. 2002). Tropical soda apple was first discovered in Florida in 1988 (Mullahey et al. 1993, Mullahey et al. 1998), and by 1995 infested between 0.25 and 0.5 million ha of prime agricultural and nonagricultural lands (Mullahey 1996a, Mullahey et al. 1998). This invasive weed infests a variety of habitats including improved pastures, natural areas, citrus (Citrus spp.), sugar cane (Saccharum officinarum L.), sod fields, ditch banks, and roadsides. After establishing in Florida, tropical soda apple continued to expand its range into Alabama, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, Pennsylvania, Puerto Rico and Tennessee (Westbrooks & Eplee 1996, Mullahey et al. 1998). The Pennsylvania infestation has since been eradicated (Westbrooks 1998).
The foliage and stems of tropical soda apple are prickly and unpalatable to livestock. However, cattle and wildlife readily ingest the fruits and spread the seeds in their droppings. If left uncontrolled, pasture production declines and stocking rates are drastically reduced (Mullahey et al. 1993). In 1994, production losses to Florida cattle ranchers attributed to tropical soda apple infestations were estimated at US $11 million annually (Cooke 1997). Tropical soda apple also serves as a reservoir for various diseases and insect pests of solanaceous crop plants (McGovern et al. 1994a, 1994b, Medal et al. 1999). A special symposium devoted entirely to various aspects of tropical soda apple and to a lesser extent wetland nightshade's biology, ecology, environmental effects and control strategies was held in Florida in 1996 to address these emerging weed problems (Mullahey 1996b).
Wetland nightshade is a bramble-like plant with spiny tangled stems and leaves that was first reported in Florida in 1983 (Wunderlin et al. 1993, Fox & Bryson 1998). In contrast to tropical soda apple, which dominates upland sites, regularly flooded wetlands are particularly vulnerable to invasion by wetland nightshade (Wunderlin et al. 1993, Fox & Bryson 1998). The largest infestation, approximately 60 ha, occurs in southwest Florida (Fox & Wigginton 1996, Wunderlin & Hansen 2000). The ability of wetland nightshade to form dense thickets that are difficult for other species to penetrate suggests this noxious weed has the potential to invade and alter many of the state's wetland habitats thus impeding access to and use of water resources (Fox & Wigginton 1996, Fox & Bryson 1998).
Turkey berry is a large, prickly shrub that can attain heights of up to 3 m (Ivens et al. 1978). Turkey berry was first collected in Columbia Co., Florida, in 1899, and has been reported in at least nine counties throughout the state (Wunderlin & Hansen 2000, Cuda et al. 2002). This noxious solanum invades disturbed sites such as pastures, crop fields, roadsides, damp waste areas and forest clearings where it competes with desirable plants for moisture, light and nutrients. Although it is frequently cultivated as a yard plant in south Florida (Westbrooks & Eplee 1989), turkey berry is potentially poisonous to animals (Chadhokar 1976, Abatan et al. 1997), and possibly carcinogenic to humans (Balachandran & Sivaramkrishnan 1995). Turkey berry has been reported as a reservoir for Alternaria solani Sorauer (Deuteromycetes: Dematiaceae), the causative agent of wilt disease in potatoes and tomatoes (Mune & Parham 1967), and is considered one of the most invasive weeds on other continents, particularly in parts of Australia and South Africa that are climatically similar to Florida (Holm et al. 1979). In the Pacific region, turkey berry was identified as a possible target for classical biological control (Waterhouse & Norris 1987). The occurrence of this plant as an invasive weed in other countries is perhaps the most compelling evidence for predicting its eventual effect on Florida's native plant communities.
Tropical soda apple, wetland nightshade and turkey berry are currently recognized as three of Florida's most invasive nonnative plant species (FLDACS 1999, FLEPPC 1999, Langeland 2001). Although it is unclear why these exotic solanaceous plants have become weeds, the lack of hostspecific natural enemies in Florida (the introduced range) may have afforded these plants a competitive advantage over native species (Cuda et al. 2002). Tropical soda apple and wetland nightshade are native to South America (and possibly the West Indies), and Mexico, respectively (Wunderlin et al. 1993), whereas turkey berry is thought to have originated in West Africa (Ivens et al. 1978), Central or South America and the Caribbean region (Morton 1981, Waterhouse & Norris 1987), or Asia (Medal et al. 1999).
Silverleaf nightshade, Solanum elaeagnifolium Cav., is a close relative of tropical soda apple, wetland nightshade, and turkey berry that is native to the southern United States, Mexico and possibly Argentina (Goeden 1971, Boyd et al. 1983), and belongs to the same subgenus Leptostemonum as the three nonnative Solanum spp. (D'Arcy 1972, Nee 1991). Silverleaf nightshade is attacked by many insect herbivores in the southwestern United States and Mexico (Goeden 1971). Two of the most damaging insects attacking silverleaf nightshade in its native range are the defoliating beetles Leptinotarsa defects (Stal) and L. texana (Schaeffer) (Jacques 1988). Both L. defects and L. texana were released recently in South Africa for biological control of silverleaf nightshade (Olckers et al 1999), and their biologies were summarized by Olckers et al. (1995).
Silverleaf nightshade is considered the natural host plant of L. defects and L. texana (Goeden 1971, Neck 1983, Jacques 1988). This solanum defines the actual, realized or field host range of the beetles (Kogan & Goeden 1970, Cullen 1990, van Klinken 2000). Host range encompasses those plants on which an insect completes normal development in nature (Hanson 1983). However, the study by Olckers et al. (1995) demonstrated that under laboratory conditions these two beetles also developed and reproduced on other solanums that do not occur in the insects' native ranges. Similarly, Hsiao (1981) observed L. texana developed and reproduced to some extent on eggplant as well as three native plant species-S. dulcamara L., S. carolinense L. and S. rostratum Dunal. These solanaceous plants are not typically exploited by the beetles in nature but are capable of supporting some development and reproduction, and comprise what is considered the insects' potential, physiological or fundamental host range (Kogan & Goeden 1970, Cullen 1990, van Klinken 2000). Horsenettle (S. carolinense) and presumably Florida horsenettle (S. carolinense L var. floridanum Chapm.) are the only potential host plants of L. texana that are native to Florida (Wunderlin & Hansen 2000). In spite of its native status in Florida, horsenettle is listed as a troublesome weed by Hall & Vandiver (1991).
Silverleaf nightshade is adventive in Florida, occurring sporadically from the Panhandle to the Keys (Wunderlin 1982, Wunderlin & Hansen 2000). Its natural enemies L. defecta and L. texana have not spread to Florida (Jacques 1985,1988), presumably because the Gulf of Mexico is an effective barrier to insects like L. texana that are incapable of long range aerial dispersal (see Hoffmann et al. 1998). However, a computer model (CLIMEX) that uses various climatic factors to determine whether insects can colonize and persist in new geographic areas (Sutherst & Maywald 1985) predicted that Leptinotarsa beetles collected from silverleaf nightshade in the Brownsville area of south Texas could establish and persist in peninsular Florida if tropical soda apple, wetland nightshade or turkey berry were suitable host plants.
The purpose of this research was to determine whether the nonnative and invasive tropical soda apple, wetland nightshade or turkey berry are capable of supporting normal development and continuous reproduction of the North American silverleaf nightshade leaf beetles L. defecta and L. texana. If these native insects are capable of establishing `new associations' with the exotic solanums (Hokkanen & Pimentel 1984), they could be introduced into Florida for biological control of these weeds after preintroduction host specificity tests demonstrated they were safe to release.
MATERIALS AND METHODS
Collections of the silverleaf nightshade leaf beetles L. defects and L. texana were made during the months of June-October 1997 and May 2001 in Starr County, TX, USA, by personnel affiliated with the USDA-Animal and Plant Health Inspection Service, Mission Plant Protection Center, Mission, TX. Parasitoid-free colonies of L. defects and L. texana were maintained on potted silverleaf nightshade plants held in screen cages at the laboratory in Mission, TX. Egg masses of L. defecta and L. texana deposited on silverleaf nightshade were shipped via overnight mail to the Quarantine Laboratory, Entomology & Nematology Department, University of Florida after USDA, APHIS, PPQ issued an importation permit. A shipment of 138 eggs of L. defecta and 310 eggs of L. texana was received on 8 September 1997. The eggs were deposited in small masses on individual silverleaf nightshade leaves separated by species in petri dishes sealed with Parafilm(R) to prevent desiccation. The eggs were removed from the silverleaf nightshade leaves with a camel hair brush and transferred to moistened filter paper placed inside another petri dish. This procedure ensured that neonate larvae were not preconditioned by feeding on silverleaf nightshade prior to the host acceptability tests, which would bias the results of the feeding trials.
Percent survival, development time, and amount of feeding for the larval stages of both leaf beetles were measured on each test plant species. Single plant (no-choice) and paired plant (choice) host suitability tests with three replications were conducted with neonate larvae in a quarantine room maintained at a temperature of 24.0 +/- 3. 1degC, relative humidity of 66.8 +/-6.8% and a 16-h photophase. Leaves used in the experiments were obtained from potted plants fertilized with Peters(R) 20-20-20 (N: P: K) solution and maintained in a glasshouse or an outdoor shade house. In the single plant tests, five neonate larvae were transferred directly to a freshly excised leaf of each test plant. The leaf was placed inside a large covered petri dish (25.0 cm diam. by 9.0 cm depth) lined with a Seitz(R) filter disk (25 cm diam.). The filter disk was routinely moistened with deionized water to prevent the leaf from desiccating, and the leaf was replaced each day or every other day until the larvae pupated or died. Leaf consumption was measured by scanning the leaves photometrically before and after exposure to the larvae. The difference in leaf areas was assumed to be the amount eaten by the developing larvae. The single plant larval feeding and development tests were initiated in early September and completed in late November 1997.
Paired plant (choice) tests of the feeding preferences of L. texana larvae were conducted with silverleaf nightshade as the control. Four leaf disks (30 mm diam.) were punched from the base of freshly detached leaves of silverleaf nightshade and turkey berry, the test plant species that supported larval development of L. texana in the single plant trials (See Results). The leaf disks were positioned alternately by species and equidistantly around the perimeter of the same container used in the single plant trials. Ten neonate larvae were placed in the center of the container and allowed to select their food source when presented with a choice of silverleaf nightshade or turkey berry leaf disks. The amount of feeding on each test plant species in the paired comparison tests was measured by the same procedure used in the single plant trials. The paired plant (choice) larval feeding trials with three replications were initiated in mid-September and were completed by the end of December 1997 when the last larva pupated or died.
On 9 May 2001, a final shipment of 72 adults of L. texana (48 males, 24 females) was received from Texas to compare the beetle's reproductive performance on turkey berry with silverleaf nightshade, and larval feeding and development on potato tree, Solanum donianum Walpers. Potato tree is a state listed threatened species (Coile 1998), and a critical non-target plant that would be vulnerable to attack by L. texana if this insect were approved for release in Florida for biological control of turkey berry.
The beetles were equally divided among whole plants of either silverleaf nightshade or turkey berry in 3.8 liter (1 gal.) pots covered with acrylic cylinders (41 cm height x 14 cm diam.). The tops of the cylinder cages were covered with Nitex(R) (41 x 42 in. mesh) to prevent the beetles from escaping. Individual leaves with the egg masses intact were removed from the plants daily, and placed in standard petri dishes with moistened filter paper to incubate. When the larvae hatched, a maximum of 10 larvae was transferred to a plastic rectangular container (20 cm x 14 cm x 10 cm) provisioned with leaves of the same host plant from which they originated, and a piece of paper toweling to collect the frass produced by the developing larvae. Each plastic container also had a hardware cloth insert (16 cm x 10 cm x 5 cm) that served as a platform to keep the leaves from coming in contact with the frass at the bottom of the container. By elevating the leaves in this manner, disease problems were avoided. When the larvae stopped feeding, they were allowed to pupate in the same plastic containers filled to a depth of 5 cm with vermiculite.
New adults (Fl generation) that emerged in the containers were sexed, and exposed to the same species of potted plant (silverleaf nightshade or turkey berry) on which they completed their development. In total, 12 cages of silverleaf nightshade and 12 of turkey berry, each containing 2 males and 1 female of L. texana, were maintained inside the quarantine room under the same environmental conditions. Survival of the F^sub 1^ females as well as the number of egg masses produced, eggs per mass, and percent larval eclosion on each test plant species were recorded.
A final single plant (no-choice) feeding and development test was conducted to determine the acceptability of potato tree as a host plant for L. texana. The experimental procedures and conditions were the same as those described above for the other single plant tests except the neonates used in this test were F^sub 2^ generation larvae of L. texana obtained from F^sub 1^ adults reared on turkey berry, the control plant in this experiment. The adult reproduction and potato tree risk assessment experiments were completed in late December 2001.
Data Analysis
The data on larval development time and leaf consumption were analyzed by ANOVA (SAS 1990). Leaf consumption means were compared with Tukey's Studentized Range (HSD) test. Nonparametric estimates of larval survival data were analyzed using the LIFETEST procedure (SAS 1990), and were compared with chi-square. The WEST procedure (SAS 1990) was used to compare the effect of plant species (silverleaf nightshade or turkey berry) on adult female reproductive performance, and plant species (turkey berry or potato tree) on larval feeding and development of L. texana. Data obtained on larval eclosion (%) were arcsine transformed prior to analysis.
RESULTS
Larval Feeding and Development
ACKNOWLEDGMENTS
We thank John Capinera and Howard Frank for reviewing an earlier version of the manuscript. We also thank Lucy Treadwell for technical assistance. This project was funded by grants from the Florida Department of Environmental Protection, Bureau of Invasive Plant Management Contract No. ERP039, and the Office of the Dean for Research, UF/IFAS. Florida Agricultural Experiment Station Journal Series No. R-07585.
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J. P. CUDA,1 P. E. PARKER,2 B. R. COON,1 F. E. VASQUEZ1 AND J. M. HARRISON3 1Entomology and Nematology Department, Institute of Food and Agricultural Sciences University of Florida, Gainesville, Florida 32611-0620
2USDA, APHIS Mission Plant Protection Center, Mission, Texas 78572 3formerly Department of Statistics, Institute of Food and Agricultural Sciences University Florida, Gainesville, Florida 32611-0339
Copyright Florida Entomological Society Dec 2002