Volume 61 – Issue 3 (Sep 1996)

The Lower Rio Grande Valley includes the Texas counties of Cameron, Hidalgo, Starr, and Willacy. It is a relatively level plain with a semi-arid and megathermal climate, a maximum elevation of 47 m, and an area of ca. 1,200,000 hectares. This book is a useful identification guide to the 183 species and 63 genera of grasses in the valley’s dwindling “natural” habitats (river floodplains, salt flats, marshes, clay dunes, barrier islands, and brush-grasslands) and in its expanding anthropogenic habitats (croplands and other “developed” sites), which dominate much of the region

ABSTRACT
The integrity of many natural environments in the southeastern United States is being threatened by invasive plants. Biological control offers a selective method for the control of these weeds because the insects and diseases it employs effect few if any nontarget species. This contribution evaluates the suitability of the biological control method for 19 invasive weeds. Criteria that can help predict the number of natural enemies associated with a weed are: (1) size of the plant’s native range, (2) commonness of the plant in its native range, (3) number of congeneric species in the world, (4) and the natural enemies already known. Criteria that help predict conflicts with valued plants, which can limit the biological control approach, are the number of congeneric species and important economic relatives the United States. The relative seriousness of the weed (not examined here), and the degree to which practical controls exist, determine the need for biological control.

ABSTRACT
Many plant species that have been moved beyond their historical ranges by humans survive only under cultivation. However, some introduced species do naturalize and become a threat to the biodiversity of natural areas and/or production capacity of developed or agricultural ecosystems. Such invaders may be termed biological pollutants. One example of a new arrival that has become a biological pollutant in the southeastern United States is tropical soda apple (Solanum viarum Dunal) (TSA). TSA is a native of South America that has recently been reported in Florida, Georgia, Alabama, Mississippi, and South Carolina, and poses a threat to all of the South. One objective of APHIS is to prevent the entry and establishment of foreign weeds such as TSA in the United States under authority of the Federal Noxious Weed Act (FNWA). Regulatory strategies that are utilized to protect the United States from foreign weeds include prevention (production of weed free commodities overseas); preclearance of high risk commodities at foreign ports of export; port of entry inspections; treatment or other mitigation systems; and finally early detection, containment, and eradication of incipient infestations that become established in the United States. Over the past several years, efforts have been made to eradicate 12 Federal Noxious Weeds at localized sites in the United States through cooperative projects with affected states.

ABSTRACT
The submersed macrophyte hydrilla [Hydrilla verticillata (L.F.) Royle], which is native to the warmer areas of Asia, was first discovered in the United States in 1960. A highly specialized growth habit, physiological characteristics, and reproduction make this plant well adapted to life in submersed freshwater environments. Consequently, hydrilla has spread rapidly through portions of the United States and become a serious weed. Where the plant occurs, it causes substantial economic hardships, interferes with various water uses, displaces native aquatic plant communities, and adversely impacts freshwater habitats. Management techniques have been developed, but sufficient funding is not available to stop the spread of the plant or implement optimum management programs. Educational efforts to increase public and political awareness of problems associated with this weed and the need for adequate funding to manage it are necessary.

ABSTRACT
Parasitic weeds are of limited economic importance in the Southern United States although almost 30 species are native and several others have been introduced. The most important is Striga asiatica, a pathogen on corn and other grains. Broomrapes, especially Orobanche cernua, 0. crenata, and 0. ramosa have the greatest potential as possible introductions. They attack a broad range of hosts including tomatoes, tobacco, and kole crops. The most serious native parasite is Seymeria cassioides (Scrophulariaceae) that parasitizes young pines. Cuscuta pentagona, although native, is a more serious problem in other parts of the country and has been spread around the world. Each parasite family occurring in the South is discussed as well as those that could be introduced. Prediction of parasitic weed problems is based on an understanding of the host range and biology of the parasite in its native setting.

ABSTRACT
The role of the United States Department of Agriculture, Agricultural Research Service (ARS) in the research and control of introduced weeds is discussed. Introduced weeds are a major economic threat to the United States. Currently, about 60 % of all weeds in the United States are of foreign origin. These alien weed species reduce the quality of food, feed, and fiber and increase the cost of crop production by several billion dollars annually. In addition, introduced weeds threaten native plant species in natural areas and reduce environmental quality on public and private lands.

ABSTRACT
Tropical soda apple (Solanum viarum Dunal), an exotic noxious weed, displaces pasture grasses and native plants resulting in significant economic and ecological losses. This perennial broadleaf weed, native to Argentina and Brazil, is 1-2 m tall at maturity, has yellowish prickles up to 20 mm long on the leaves, and the fruits are globose (2.5 cm in diameter) with the immature fruit green with white mottling (like a watermelon). Weed distribution includes Alabama, Florida, Georgia, Mississippi, and South Carolina. In Florida, tropical soda apple (TSA) has been observed in pastures, citrus (Citrus spp.), sugar cane (Saccharum offinarum L.), vegetable fields, sod fields, natural areas (state parks, nature preserves, hammocks), and roadsides. Recent (1993) surveys indicate there are nearly 161,000 ha of TSA occupying improved pastures in peninsular Florida, which is an increase of 95 % from 1990. In 1995, estimated TSA infested pasture was 303,000 ha. Tropical soda apple is listed on the Federal Noxious Weed List which prohibits the possession, movement, or release of TSA. Seed production averages 50,000 per plant, seed germination is high (75%), and weed dispersion occurs from wildlife and livestock that feed on the fruit and spread seed through the feces. Weed management strategies are centered around prevention, detection and control.

The Florida Exotic Pest Plant Council (EPPC, known affectionally as “Epp-See”) was formed in 1984 to help cope with the increasingly severe problems of invasive exotic plants that threaten the survival of ecosystems such as the Everglades. Since then, three additional councils have formed in California, Tennessee, and the Pacific Northwest. Groups are in the formation stages in Texas and the Northeast.

ABSTRACT
Polygonum perfoliatum L. (Polygonaceae) or mile-a-minute weed, from eastern Asia, has been spreading through wild and disturbed areas of the mid-Atlantic United States. It has a wide ecological amplitude and is found on stream banks, moist thickets, roadsides, nurseries, wood-piles, clearings, and ditches. Polygonum perfoliatum presents serious problems for reforestation because it thrives where forests are clear-cut. A southward direction of spread indicates that the species will probably proliferate in at least some southern states. Mechanical control is not likely to be completely effective because seeds are often left behind. No appropriate biocontrols are known. The plant poses a threat to natural and restored ecosystems because of its capacity to grow rapidly and overgrow other species. It is expected to cover large areas unless it is controlled.

ABSTRACT
Alligatorweed, Alternanthera philoxeroides (Mart.) Griseb., a South American immigrant, is an aquatic weed in the southern United States where it roots in shallow water or on shore. Floating stems grow across the surface of the waterway forming a dense interwoven mat. This mat clogs the waterway and outcompetes native plants along the shore. Three species of South American insects were released from 1964 to 1971 for biological control of this aggressive invader. The flea beetle Agasicles hygophila Selman & Vogt strips the leaves from the stems and the moth Vogtia malloi Pastrana bores inside the stems. Heavy damage by either species kills the stems thereby causing the mat to break up, clearing the waterway. The thrips Amynothrips andersoni O’Neill feeds on the young apical leaves. Heavily damaged plants are often stunted. Alligatorweed has been controlled or the populations reduced throughout much of its range, especially in Florida and along the Gulf Coast. Colder climates, though, from Arkansas to North Carolina along the margin of the range preclude establishment of the insects. In some colder areas, for example the Lower Mississippi River Valley, the moths and flea beetles immigrate from the warm coastal areas and provide local control.