Northern Tamaulipas Research Articles

Effect of Ear Wounding and Cultural Practices on Abundance of Carpophilus freemani (Coleoptera: Nitidulidae) and Other Microcoleopterans in Maize in Northeastern Mexico

Field experiments were conducted from 1993to 1997in northern Tamaulipas, Mexico, to identify the sap beetles and other microcoleopterans attracted to maize ears, and to evaluate their abundance in relationship to growing season (spring or fall), ear wounding (caterpillars, birds, and artificial), crop phenology, cultivar, and aflatoxin contanlination. During the 5-yr study, 14 species of microcoleopterans in 7 families were identified. However, only 3 of these species comprised 97% of the total captures: Carpophilus freemani Dobson (Nitidulidae), Cathartus quadricollis (Guerin- Meneville) (Cucujidae), and Sitophilus zeamais Motschulsky (Curculionidae). G.freemani was by far the predominant species, comprising nearly 90% of all collections regardless of growing season, crop management, or type of ear damage. Sap beetles occurred commonly during both the spring and fall growing seasons. Compared with undamaged ears, microcoleopterans were 2- to 5-fold more abundant in caterpillar-damaged ears, and 5- to 28-fold more abundant in bird-damaged and artificially damaged ears. G.freemani seemed to respond more to ear wounding (5- to 10-fold) than C. quadricollis and S. zeamais (2- to 4-fold). A maximum average density of 57 sap beetles per ear was observed in artificially damaged ears during the spring of 1993. Maximum abundance of microcoleopterans occurred from dough-hard to 25% kernel moisture stages. C. freemani was abundant during all maize reproductive stages, whereas C. quadricolis and S. zeamais were common only when kernels were drying down (15-20% moisture). Number of sap beetles varied significantly among cutivars in both undamaged and damaged ears. Infection by Aspergillus flavus Link:Fr and aflatoxin contamination of maize were enhanced by ear wounding and incidence of sap beetles.

A Sixteen-Year Study on the Bivoltinism of Anomala flavipennis (Coleoptera: Scarabaeidae) in Mexico

Seasonality and abundance of the scarab beetle Anomala flavipennis Burmeister were studied using a black light trap in Rio Bravo, in northern Tamaulipas, Mexico, from 1979 to 1994. A bivoltine life cycle in A. flavipennis was evident throughout the period of study as shown by 2 distinctive flight periods: one from mid-April to mid-May (spring or overwintering generation) and the other from late August to late September (fall generation). Bivoltinism of A. flavipennis in northeastern Mexico, as compared with univoltinism of this species in the United States (e.g., Georgia, Kansas, Nebraska), is attributed to the warmer, subtropical climate of the former, where annual heat units nearly double those accumulated in northern parts of the range of this beetle. An average of 1,174 and 3,509 accumulated heat units (>10°C) starting 1 January was required for 50% adult emergence in the spring and fall generations, respectively. However, heat units resulted in a more variable parameter than did Julian dates for predicting emergence of A. flavipennis . Although rainfall appeared to trigger A. flavipennis adult emergence during the fall, the influence of rainfall was not as clear during the spring, probably because of the irrigation practices in this region during the spring growing season. Captures were generally greater during the fall than the spling, with a maximum of 21,444 adults having been captured on 7 September 1994. Abundance (total captures during spring or fall) of A. flavipennis was not associated with temperature and precipitation. Abundance of A. flavipennis during the spring season depended on the adults captured during the previous fall. However, fall generation did not depend on abundance of adults emerging during the previous spring.

Seasonal Feeding by Phyllophaga crinita and Anomala spp. (Coleoptera: Scarabaeidae) Larvae in Northeastern Mexico

Larval feeding by Phyllophaga crinita (Burmeister) and Anomala spp. (mixed populations of A. flavipennis Burmeister and A. foraminosa Bates) was studied under simulated field conditions in northern Tamaulipas, Mexico. Each month from July 1994 to February 1995, 50 field-collected larvae of P. crinita and Anomala spp. were placed individually in plastic pots with soil and corn seeds, and damage was evaluated 10 d after seedling emergence. Secondinstar P. crinita caused 41% root loss in July, whereas third instars caused most damage (66–88% root loss) during July–September. Feeding by P. crinita decreased gradually from October to December, and ceased in January–February. In contrast, feeding by Anomala spp. peaked in July and again in November (73 and 53% root loss, respectively), a result of the bivoltine life cycle of these species in this region. The relationship of these findings to similar studies is discussed.

Susceptibility of Bermudagrass Cultivars to Eoreuma loftini (Lepidoptera: Pyralidae) in Subtropical Mexico

Susceptibility of eight bermudagrasses (Tifton 68, Tifton 78, Tifton 85, Gigante, Brazos, Cruza 1, Callie, and NK 37) to the Mexican rice borer, Eoreuma loftini (Dyar) was evaluated monthly during 1994 in northern Tamaulipas, northeastern Mexico. Damage by E. loftini occurred year around, peaking in April and December. Borer attack was reduced substantially after mowing the pastures to 3-5 cm height. Susceptibility of the cultivars to borer damage was influenced by stolon width, an important characteristic for E. loftini tunneling. This relationship between stolon width and borer damage was explained closely (R 2 = 0.86) by the curvilinear model y = (1.2 + 0.1757 x 3 ) 2, where y = borer damage and x = stolon diameter. The widest stemmed bermudagrass, and hence the most susceptible to E. loftini, was Tifton 68. Tunneling behavior of E. loftini, as affected by stem width of host plants, is discussed.

Winter Mortality and Spring Emergence of Corn Stalkborers (Lepidoptera: Pyralidae) in Subtropical Mexico

Winter survival and spring emergence of the crambine stalkborers Diatraea lineolata (Walker), D. saccharalis (F.), and Eoreuma loftini (Dyar) were studied in the corn agroecosystem of northern Tamaulipas, Mexico, from 1985 to 1992. Laboratory experiments to evaluate the effect of cold on survival of stalkborer larvae complemented the field studies. Numbers of stalkborers throughout the winter decreased linearly ,and resulting regression slopes differed among species. Overall, 50% of larvae remained 42, 64, and 66 d after corn physiological maturation for E. loftini, D. saccharalis, and D. lineolata, respectively. Larval disappearance was a result of mortality and development to pupae. Low temperatures caused significant mortality (45-100%) only during the winter of 1989-1990 when temperatures decreased to -8 degrees C, very unusual for northern Tamaulipas. Laboratory studies showed stalkborer populations were significantly reduced only if exposed for several hours at temperatures below -5 degrees C, which has only occurred during two winters in this region during the fast 28 yr. Borer mortality attributed to low temperatures was lower in stacked corn than in field corn. Percentage of survivorship from full grown larvae to adulthood in emergence cages averaged 4.1, 11.0, and 21.8% for D. saccharalis, D. linealata, and E. loftini, respectively. Although cumulative moth emergence of the Diaeraea species was sigmoid, that of E. loftini was linear. Fifty percent of cumulative emergence occurred during late February, mid-March, and early April for E. loftini, D. saccharalis, and D. lineolata, respectively.

Seasonal Abundance of Diabrotica balteata and Other Diabroticina Beetles (Coleoptera: Chrysomelidae) in Northeastern Mexico

Abundance and seasonality of Diabroticina species were investigated using black light traps in Rio Bravo and San Fernando, in northern Tamaulipas, Mexico, from 1980 to 1993. Diabrotica balteata LeConte was the most abundant species with average captures of 11,666 beetles per year. Other species captured included D. undecimpunctata howardi Barber , D. longicomis (Say), D. tibialis Jacoby, Paranapiacaba connexa (LeConte), and Acalymma trivittatum (Mannerheim). Multivoltinism was evident for D. balteata , D. undecimpunctata howardi , and A. trivittatum . Beetles were captured more abundantly from midspring to early fall but occurred practically year around. More than 90% of the total captures of D. balteata occurred from May through July. Flight activity of D. balteata decreased as wind speed increased. A multiple regression model consisting of climatic factors as independent variables ( x n ) explained 86% of the variability of total yearly catches of D. balteata ( y ). The model indicated abundance was adversely affected by preceding cold winter and early rainfall (1 January to 18 May) and was favored by rainfall from 19 May to 30 August. Periodic high captures of D. balteata during May to July might be caused by dispersal behavior, probably associated to changes in habitat quality, i.e., corn flowering and maturation. The results found herein are discussed in relation to those reported for the same or related species elsewhere.

Crop/Weed gene flow:Cucurbita argyrosperma Huber andC. fraterna L. H. Bailey (Cucurbitaceae)

A mixed population of Cucurbita at Vado El Mow in northern Tamaulipas, Mexico showed an anomalous pattern of fruit bitterness. Some domesticated plants (C. argyrosperma andC. moschata) expressed cucurbitacin bitterness whereas some sympatric free-living plants produced non-bitter fruits. This reversal of typical cucurbitacin expression suggested gene flow between crop and weed at the site. Isozyme analysis provided little insight as to taxa involved in gene exchange, although progeny from a single free-living plant carried IDH allozymes that are associated with Mexican landraces ofC. pepo. Synthetic hybridization revealed that fertile F, hybrids are produced from crosses involvingC. fraterna as the pistillate parent andC. argyrosperma as the staminate parent. Interspecific crop/weed hybrids can produce viable progeny upon self-pollination or backcrossingto either parent, andF2 families display normal allozyme segregation. Hybrid fertility, as indicated by pollen stainability, increases in progeny produced by backcrossingfrom theC. argyrosperma parent. Interspecific hybridfertility represents a potential for crop/weed gene flow that would be realized under natural conditions if pollen flow occurs betweenC. fraterna andC. argyrosperma in the fields of Tamaulipas. Oligolectic “squash bees” (Teponapis), efficientCucurbita pollen vectors, are present at the site. Thus, it is likely that natural interspecific crop/weed hybridization has occurred at Vado El Moro and this might at least partially explain the anomalous distribution of fruit bitterness among extant populations at the site.

Postemergence Shattercane (Sorghum bicolor) Control in Corn (Zea mays) in Northern Tamaulipas, Mexico

Field studies were conducted in northern Tamaulipas, Mexico in 1991 and 1992 to determine the effectiveness of POST applications of primisulfuron and nicosulfuron on shattercane control in corn. Primisulfuron at 15 g/ha and nicosulfuron at 30 g/ha were the minimal doses required for shattercane control. Both herbicides applied on 5- to 9-leaf shattercane provided excellent (> 90%) control 35 d after application. No corn injury was evident with any herbicide and rates evaluated. Untreated check plots sustained 30 to 56% of yield reduction as compared to herbicide treatment plots.

Prepupal and Pupal Parasitism of Helicoverpa zea and Spodoptera frugiperda (Lepidoptera: Noctuidae) by Steinernema sp. in Cornfields in the Lower Rio Grande Valley

An indigenous Steinernema sp. (Rhabditida: Steinernematidae) nematode parasitized prepupae and pupae of corn earworm, Helicoverpa zea (Boddie), and fall armyworm, Spodoptera frugiperda (J. E. Smith), collected from fruiting cornfields in the Lower Rio Grande Valley of Texas and northern Tamaulipas, Mexico. Over a 5-yr period, 34% of all fields sampled contained parasitized corn earworm and 24.2% contained parasitized fall annyworm. Of 3,036 corn earwonn prepupae and pupae collected during this study, 11.6% were parasitized; of 1,802 fall armyworm collected, 9.3% were parasitized. Parasitism by Steinernema sp. accounted for 49.4 and 46.1% of the mortality of corn earworm and fall armyworm prepupae and pupae, respectively.

Seasonality of Cornstalk Borers (Lepidoptera: Pyralidae) in Northeastern Mexico

The seasonal abundance and phenology of the borers Diatraea lineolata (Walker), D. saccharalis (F.), and Eoreuma loftini (Dyar) were investigated on corn in northern Tamaulipas, Mexico, from 1985 to 1988. D. lineolata was the dominant species throughout the study. The presence of E. loftini was practically limited to the fall seasons, and D. saccharalis wasmostcommon during the fall of 1985. Positive interspecific associations between borer species were detected, particularly during the fall, when densities were highest. Two borer generations were detected within each corn growing season (spring or fall) for all borer species; the first generation was initially detected at middle to late whorl stage, whereas the second generation normally occurred after tasseling. All species overwintered as mature larvae in dry cornstalks, with D. lineolata also expressing summer diapause. Plant maturation was the major factor responsible for diapause induction of D. lineolata regardless of time of year. A minimal accumulation of heat units and contact moisture was required for D. linealata larvae to terminate diapause. The relationship of these studies to those reported for the same species in the tropics or other species elsewhere is discussed.

Seasonal Parasitism of Cornstalk Borers (Lepidoptera: Pyralidae) by Indigenous and Introduced Parasites in Northeastern Mexico

Parasitism of cornstalk borers was evaluated near Rio Bravo, Tamaulipas, Mexico, from 1984 to 1988. Additional surveyswere made on grain sorghum and johnsongrass. A total of 12,897 larvae and pupae of Diatraea lineolata (Walker), D. saccharalis (F.), and Eoreuma loftini (Dyar) was collected from 10,820 host plants. Total parasitism throughout this study was 6.8%, with no small larval or pupal parasites detected in corn. Six indigenous parasites were reared from all borer species and host plants. Chelonus sonorensis Cameron attacked only E. loftini and was the most common native parasite with 18% overall parasitism. Percentage of parasitism by Chelonus sonorensis was highest (34%) during the fall seasons, particularly at the vegetative corn stage. The remaining indigenous parasites appeared sporadically at very low levels of parasitism and included Digonogastra sp., Orgilus gelechiaevarus (Cushman), Glyptapanteles sp., Spilochalcis sp., and Eucelatoria sp. The exotic parasites Cotesia fiavipes (Cameron) and Allorhogas pyralophagus Marsh were field released during the fall of 1985. A. pyralophagus was not recovered from field collections. Cotesia fiavipes was recovered from D. saccharalis and D. lineolata immediately after releasesbegan and became the most abundant parasite of those borer species thereafter. Cotesia fiaoipes was recovered most frequently during the fall during the reproductive and senescent corn stages. Average parasitism by Cotesia flavipes during the fall was 13% on D. saccharalis and 5% on D. lineolata with the highest parasitism (53% on D. saccharalis ) during the fall of 1987. The consistent recoveries of Cotesia fiavipes from 1985 to 1988 suggest that this parasite is permanently established in northern Tamaulipas, Mexico

Parasitization of Diatraea lineolata Pupae and Diapausing Larvae by Several Exotic Parasites

The neotropical cornstalk borer (NCB), Diatraea lineolata (Walker), is a widely distributed species of the economically important genus Diatraea (Lepidoptera: Pyralidae) (Rodriguez-del-Bosque et al. 1988). A recent study on the parasitization of NCB in the corn agroecosystem of northern Tamaulipas, Mexico, revealed a minimal impact of native parasites. No pupal parasites were detected, and only three larval parasites appeared sporadically causing negligible levels of parasitization. The braconid larval parasite Cotesia flavipes (Cameron) was introduced into the corn agroecosystem of northern Tamaulipas in 1985, and rapidly became the most important parasite of both NCB and the sugarcane borer, D. saccharalis (F.) (Rodriguez-del-Bosque 1988). However, the impact of this exotic parasite was greatest on D. saccharalis, dictating the need for introduction of additional natural enemies against NCB. The successful establishment of potential parasites in this area could be limited by their ability to develop on diapausing NCB larvae, which are present during both summer and winter. Diapause of NCB is characterized by a change from a spotted to an immaculate morph, triggered by host plant maturation (Rodriguez-del-Bosque 1988). We investigated the suitability of NCB pupae and diapausing larvae to several exotic parasites. This report serves as a precursor for further detailed investigations and as a guide for selecting the best parasites for trial colonization. Small-scale laboratory tests were conducted from January to May 1988 to determine the host suitability of NCB to several exotic larval and pupal parasites, cultured at the Biological Control Facility, Department of Entomology, Texas A&M University, as part of a biological control program aimed at graminaceous stalkborers in Texas. Diapausing NCB larvae used in the tests were field collected from maturing cornstalks in Rio Bravo, Tamaulipas, Mexico, placed in 20-ml cups with artificial diet (Martinez et al. 1988), and transported to College Station, Texas. Pupae (1-2 days old) were obtained from a NCB colony maintained in College Station (Rodriguez-del-Bosque 1988). Larval parasites tested included the braconids Digonogastra kimballi (Kirkland) and Allorhogas pyralophagus Marsh from Mexico, C. flavipes and Rhacanotus roslinensis Lal from Pakistan, Apanteles minator Muesebeck from Bolivia, and the bethylid Goniozus natalensis Gordh from South Africa. The braconids all developed successfully on nondiapausing (spotted) NCB larvae in preliminary trials. Goniozus natalensis was not tested against nondiapausing NCB larvae. Pupal parasites were the eulophids Trichospilus diatraeae Cherian & Margabandhu from India (founder culture obtained from Florida, U.S.A.) and Pediobiusfurvus Gahan from Kenya, and the ichneumonid Xanthopimpla stemmator Thunberg from Asia (founder culture obtained from Mauritius). Although T. diatraeae has previously been reared from NCB pupae (Bennett & PschornWalker 1970), information on parasitization, emergence, progeny, and developmental time was not reported. Twenty hosts (pupae or diapausing larvae) were used to determine NCB suitability and developmental time (host exposure to adult parasite emergence) for each parasite, except X. stemmator, in which 81 pupae were used. Host larvae and pupae were held at constant 26-+?0.5?C and 14:10 (L:D) photoperiod during and after exposure to parasites. Host larvae were exposed for 24 h to >20 parasite females held in Plexiglas cages

Damage by Stalkborers (Lepidoptera: Pyralidae) to Corn in Northeastern Mexico

Commercial corn fields were surveyed in northern Tamaulipas, Mexico, during the spring and fall growing seasons of 1985 and 1986 to assess the damage and yield losses caused by the stalkborers, Diatraea lineolata (Walker), D. saccharalis (F.), and Eoreuma loftini (Dyar). Tunneling by E. loftini was characteristically different from that of Diatraea spp. Diatraea spp. occurred more frequently in the lower internodes during the fall than during the spring, which is probably related to the overwintering habits of the species. D. lineolata was more abundant than D. saccharalis during all seasons. E. loftini was the most common species during the fall and it was scarce during the spring. The simultaneous occurrence of both genera within the same plant was a random process. Significant correlations were found between the number of borer larvae, percentage of internodes damaged, and percentage of plants damaged. Damage and yield losses were greater in the fall when the borer populations were denser. Yield reductions per field averaged 1.3, 17.0, and 2.0%during spring of 1985, fall of 1985, and spring of 1986, respectively. A maximum of 39.3% of yield loss was detected in one field in the fall of 1985.

History of the White Pelican Colonies in South Texas and Northern Tamaulipas

History of the White Pelican Colonies in South Texas and Northern Tamaulipas

Fall Armyworm Distribution and Population Dynamics in the Texas-Mexico Gulf Coast Area

Fall armyworm, Spodoptera frugiperda J. E. Smith, population trends were studied along the Mexican Gulf Coast, the Isthmus of Tehuantepec, and the Yucatan Peninsula using Hartstack pheromone traps. Trap capture generally peaked in November and December, while the lowest capture period occurred during mid year. A temporal progression in trap capture was noted during the early part of the year from Veracruz, Mexico, to the Lower Rio Grande Valley of Texas. A similar temporal progression in occurrence of larval populations from southern areas of the State of Tamaulipas to the Rio Grande Valley was also observed. Studies in an irrigated corn-growing region encompassing the Lower Rio Grande Valley in both southern Texas and northern Tamaulipas, Mexico (with ca. 200,000 ha of corn) showed the major fall armyworm emergence from the area occurred in June, resulting in an adult population of from 6.11X10^8 to 1.72X10^9 moths.

Identification of Eoreuma loftini (Dyar) (Lepidoptera: Pyralidae) in Texas, 1980: Forerunner for Other Sugarcane Boring Pest Immigrants from Mexico?

Eoreuma loftini (Oyar) was intercepted from Mexico regularly at ports of entry in the Lower Rio Grande Valley of Texas (LRGV) about 10 years before its identification in LRGV plant hosts. E. loftini appears to have migrated from western Mexico to eastern Mexico before entering Texas. Interceptions were primarily from sugarcane during late fall and winter months when sugarcane is highest in sugar content. Corn, sorghum, and wild grasses were other intercepted hosts. Two other Pyralid sugarcane pests in Mexico, Diatraea considerata Heinrich and D. magnifactella Oyar, appear to be following a similar pattern. Both have been intercepted at LRGV ports of entry since 1980. Sites of origin of intercepted hosts suggest that both species have had the opportunity to establish in northern Tamaulipas, Mexico, bordering the LRGV. Intercepted host plants and seasonal interception patterns are similar to those of E loftini . Research on these species in Mexico is suggested.

EPIDEMIC VENEZUELAN EQUINE ENCEPHALITIS IN NORTH AMERICA IN 1971: VECTOR STUDIES1

A major epidemic of Venezuelan equine encephalitis occurred in south Texas in the summer of 1971. More than 1500 equines died of VEE in Texas, and 110 human cases with no deaths were reported. Vector studies in south Texas and northern Tamaulipas revealed that the overall mosquito infection rates during the peak of the epidemic were about 1:100, one of the highest rates observed for a major epidemic. Mosquito infection rates of this magnitude could easily explain the intensity of VEE outbreaks in both equines and man. A total of 943 VEE virus isolations were made from mosquitoes. Eight of the 12 mosquito species found infected were implicated in the epidemic cycle of VEE for the first time. Sufficient laboratory and field evidence is available to prove that Psorophora confinnis was one of the primary vectors of VEE. The lack of laboratory evidence necessitates the use of the term "probable" primary vectors for other species apparently equally as involved on the basis of field infections; these include Aedes sollicitans, Aedes thelcter and Psorophora discolor. Eight other species from which less than 10 VEE virus isolations were made were considered auxiliary vectors. Mosquitoes of some species were tested individually; such tests showed 2-4% of the probable primary vectors to be infected. The first isolation of VEE virus of the epidemic was made from P. confinnis on June 28, 1971. Highest mosquito infection rates occurred during the week of July 5. Mosquito infection rates declined precipitously in the last 3 weeks of July 1971, signaling the end of the epidemic in the study area. One explanation for the decline was that equines, the principal epidemic hosts, were eliminated as a source of virus by death or by acquisition of natural or induced immunity. Mosquito control appeared to be effective in reducing the infected mosquito population while the immunization of equines with TC 83 VEE vaccine was accomplished. Quarantines appeared to be effective in restricting the VEE virus activity to south Texas. Undoubtedly all of the control measures contributed to stopping the epidemic. Continued VEE surveillance by various government and other agencies failed to reveal any further epidemic VEE activity in the US in 1972. Other arboviruses isolated during the VEE studies in south Texas included St. Louis encephalitis virus, and San Angelo subtype of the California Group. A virus of the Bunyamwera Group was also isolated from Palo Blanco, Tamaulipas.

The Colubrid Snake Genus Thamnophis: A Revision of the Sauritus Group

The Sauritus group of the gartersnakes is composed of two polytypic species, Thamnophis sauritus (Linnaeus) and Thamnophis proximus (Say), commonly called the ribbonsnakes. Both species vary geographically in number of ventrals, number of subcaudals, relative tail length, and several features of the color pattern. Supralabial number is subject to geographic variation in T. sauritus but not in T. proximus. Most of the geographic variation is discordant, transition zones in one character seldom corresponding with those of other characters. As color pattern fluctuates less locally than do the meristic characters, it is more reliable in defining geographic races. Four subspecies of T. sauritus are recognized, two of them previously undescribed ( T. s. septentrionalis of the Great Lakes region, northern New England, and Nova Scotia; T. s. nitae of northwestern peninsular Florida). Of six races of T. proximus four are new (T. p. orarius of coastal Louisiana, Texas, and northern Tamaulipas; T. p. rubrilineatus of the Edwards Plateau region of central Texas; T. p. diabolicus of New Mexico, trans-Pecos Texas, Coahuila, and Nuevo Leon; T. p. alpinus of the Chiapas Highlands in southern Mexico). The evolutionary history of this species group and the trends within each species are discussed. Members of the group are highly specialized and occupy an advanced position within the genus.

Morphological Variation in Mesquite (Prosopis, Leguminosae) in the Lowlands of Northeastern Mexico

A detailed study of morphological variation in mesquite (Prosopis) from 17 sites in northeastern Mexico shows a definite correlation of leaf length with secondary-leaflet size, number of pairs of primary leaflets per leaf, and number of spines per node. These morphological characters are also correlated to some extent with geographic location, with some exceptions; intermediates occur which indicate the occurrence of hybridization between the long-leaf mesquite, P. glandulosa Torr., and the short-leaf form, for which Neltuma palmeri Britt. & Rose is the only avail- able name. The contact zone between the two kinds remains fairly static at present because of environmental pressures and/or partial isolation, the mechanisms for which are at present unknown. Mesquites (Prosopis Sect. Algarobia DC. emend. Burk.) of the southwestern United States and Mexico are polymorphic and variable (Standley, 1922; Burkhart, 1940; Benson, 1941). During the course of work in northeastern Mexico, opportunity was provided for a de- tailed study of some of the morphological variation. Preliminary reconnaissance over a wide area in the Gulf lowlands of northeastern Mexico showed that, in general, the mesquite growing in southern Tamaulipas, southern Nuevo Leon, extreme eastern San Luis Potosi, and extreme northern Veracruz had, among other dif- ferences, shorter leaves and smaller secondary leaflets than those growing in northern Tamaulipas and northern Nuevo Leon, and that the contact zone between the short-leaf and long-leaf mesquites is in the vicinity of the Rio San Fernando. The map (figure 1) shows this contact zone.

A Plainview Point from Northern Tamaulipas

Immediately after the exploration of the Santa Isabel Iztapan mammoth site in the Valley of Mexico had been completed (see pp. 332-40) the recently created Direccion de Prehistoria of the Instituto Nacional de Antropologia e Historia of Mexico undertook further reconnaissance in the reservoir basin of Falcon Dam on the Rio Grande. Work was concentrated on the Mexican side, where Aveleyra (1951) had located nine sites with abundant lithic materials.