Georgia Research Articles

Polarimetric scanning radiometer C- and X-band microwave observations during SMEX03

Soil Moisture Experiment 2003 (SMEX03) was the second in a series of field campaigns using the National Oceanic and Atmospheric Administration Polarimetric Scanning Radiometer (PSR/CX) designed to validate brightness temperature (T/sub B/) data and soil moisture retrieval algorithms for the Advanced Microwave Scanning Radiometer (AMSR-E) for the Earth Observing System on the Aqua satellite. Objectives related to the PSR/CX during SMEX03 included: calibration and validation of AMSR-E T/sub B/ observations over different climate/vegetation regions of the U.S. [Alabama (AL), Georgia (GA), Oklahoma (OK)], identification of possible areas of radio-frequency interference (RFI), comparison of X-band observations from Tropical Rainfall Measurement Mission Microwave Imager (TMI), AMSR-E, and PSR/CX, and exploring the potential of soil moisture retrieval algorithms using C- and X-band imagery in diverse landscapes. In the current investigation, more than 100 flightlines of PSR/CX data were extensively processed to produce gridded T/sub B/ products for the four study regions. Due to the lack of significant rainfall in OK, generally dry soil moisture conditions were observed. Observations obtained over AL include a wide range of soil moisture and vegetation conditions. Results from the AL site clearly showed a lack of sensitivity to rainfall/soil moisture under forest canopy cover. Quantitative comparisons made with the TMI validated that both the PSR/CX and AMSR-E X-band channels were well calibrated. Spectral analyses indicated that the PSR/CX observations at C-band also are reasonable. As expected, there were varying degrees of RFI in the AMSR-E C-band data for the study sites that will prevent further soil moisture analysis using these data. X-band comparisons of the PSR/CX high-resolution and AMSR-E and TMI low-resolution data indicated a linear scaling for the range of conditions studied in SMEX03. These results will form the basis for further soil moisture investigations.

Molecular Epidemiology of Foodborne Hepatitis A Outbreaks in the United States, 2003

Molecular epidemiologic investigations can link geographically separate foodborne hepatitis A outbreaks but have not been used while field investigations are in progress. In 2003, outbreaks of foodborne hepatitis A were reported in multiple states. Case-control studies were conducted in 3 states. Hepatitis A virus was sequenced from serologic specimens from individuals associated with outbreaks and from individuals concurrently ill with hepatitis A in non-outbreak settings in the United States and Mexico. Case-control studies in Tennessee (TN), North Carolina (NC), and Georgia (GA) found green onions to be associated with illness among restaurant patrons (TN: odds ratio [OR], 65.5 [95% confidence interval {CI}, 8.9-482.5; NC: OR, 2.4 [95% CI, 0.3-21.9]; GA: OR, 20.9 [95% CI, 3.9-110.3]). Viral sequences from TN case patients differed by 2 nt, compared with those from case patients in NC and GA. A third sequence, differing from the TN and GA/NC sequences by 1 nt, was identified among case patients in a subsequent outbreak in Pennsylvania. Each outbreak sequence was identical to > or =1 sequence isolated from northern Mexican resident(s) with hepatitis A. The sources of green onions served in restaurants in TN and GA were 3 farms in northern Mexico. Ongoing viral strain surveillance facilitated the rapid implementation of control measures. Incorporation of molecular epidemiologic methods into routine hepatitis A surveillance would improve the detection of hepatitis A outbreaks and increase our understanding of hepatitis A epidemiology in the United States.

Genetic Parameters for Days Open and Pregnancy Rates in US Holsteins Using Different Editing Criteria

The influence of various editing criteria for days open (DO) records on genetic parameter estimates of DO and pregnancy rates (PR) in US Holsteins was investigated. Data included first parity 305-d milk yield and DO records from 8 states: Georgia (GA), Florida (FL), North Carolina (NC), Texas (TX), Arizona (AZ), California (CA), New York (NY), and Wisconsin (WI). The pregnancy rate was computed as 1/[(DO−VWP)/HI + 1)], where VWP was the approximate voluntary waiting period and HI was the heat interval set as 21 d. The upper limit for PR was set to 1.0. A bivariate animal model for DO (or PR) and 305-d milk yield was fit separately for each state. The model included fixed effects of herd-year, month of calving, and age of cow, as well as random animal and residual effects. In separate analyses, maximum DO records were limited to 150, 200, 250, 300, and 365 d. Analyses for PR used values of 50, 80, and 120 d for the VWP. Genetic and residual variances for DO were strongly dependent on the upper limit; both variances were 8 times larger as the upper bound increased from 150 to 365 d. Estimates of heritability for DO varied between 0.03 and 0.06. There was a 30% increase in the heritability estimate as the upper limit increased from 150 to 250 d for FL and NC, and small or no increases for the other states. The increase of the upper limit from 250 to 365 d resulted in little change. The genetic correlation between milk and DO was the highest for FL (0.6) and the lowest for GA (0.12 to 0.23). For PR with VWP = 50, the heritability was higher than the corresponding estimate for DO in GA, equal to that in AZ, and lower in the remaining states. Heritabilities of PR also varied by the length of VWP; highest heritabilities were obtained at VWP = 50 d for GA and AZ; at VWP = 80 d for NY and WI; at VWP = 120 d for FL, NC, and CA. Increase of genetic variation for records of DO<250 d was small. Days open and PR are strongly influenced by differences in management protocols among states.

First Report of the Cucurbit Powdery Mildew Fungus (Podosphaera xanthii) Resistant to Strobilurin Fungicides in the United States.

Resistance to strobilurin fungicides was documented in isolates collected from three fungicide efficacy experiments conducted in research fields in Georgia (GA), North Carolina (NC), and New York (NY). In these fields in 2002, strobilurins (fungicide group 11, quinone outside inhibitors [QoI]) when used alone on a 7-day schedule (use pattern not labeled) did not effectively control cucurbit powdery mildew. Strobilurin efficacy declined dramatically after the second application in New York (3). Efficacy also was reduced in commercial fields in Kentucky and research fields in Arizona, California, Kentucky, Illinois, Michigan, and Virginia in 2002 where strobilurins were used predominantly or exclusively. Isolates were collected on 22 July and 8 and 17 October after the last of four, five, and five applications of strobilurin (trifloxystrobin formulated as Flint or azoxystrobin formulated as Quadris) in experiments conducted by J. D. Moore in Chula, GA, M. McGrath in Riverhead, NY, and G. J. Holmes in Clayton, NC, respectively. A leaf-disk bioassay was used to determine fungicide sensitivity (2). Strobilurin sensitivity was determined using trifloxystrobin at 0, 0.5, 5, 50, and 100 μg/ml. Four of nine NY isolates, 19 of 21 GA isolates, and 13 of 15 NC isolates were resistant to strobilurins (grew well on disks treated with trifloxystrobin at 100 μg/ml). The geometric mean of the azoxystrobin baseline was 0.258 μg/ml for Podosphaera xanthii isolates collected in 1998 and 1999 in North America (4). Poor control with strobilurins under field conditions was associated with reduced sensitivity in vitro. Strobilurin sensitivity appeared to be qualitative as reported elsewhere (1). Two sensitive and three resistant isolates responded similarly when tested in another laboratory using kresoxim-methyl and pyraclostrobin (H. Ypema, personal communication). These findings and experiences elsewhere with QoI-resistant P. xanthii indicate that cross-resistance probably extends among multiple QoI's (1). Strobilurins have been available for commercial use in the United States since 1998, when azoxystrobin received Section 18 registration in some states. Federal registration was granted in March 1999. Strobilurin resistance was detected after 2 years of commercial use elsewhere in the world (1). All isolates tested in the current study were from research fields where selection pressure for resistance could have been higher than in commercial fields where strobilurins are used with demethylation inhibitors (DMIs; fungicide group 3) and contact fungicides in alternation or tank mixtures to prevent or delay resistance development. Resistance in commercial fields will reduce the utility of strobilurins, including those not yet registered, and eliminate an important tool for managing DMI resistance. Strobilurins and DMIs are the only systemic fungicides registered for cucurbit powdery mildew in the United States. Managing DMI resistance may be challenged by multiresistant strains. Strobilurin-resistant isolates also exhibited reduced sensitivity to DMIs, tolerating triadimefon at 50 to 100 μg/ml (2). One suggestion to improve resistance management is to apply a contact fungicide with strobilurins as well as DMIs.

An Evaluation of Dazomet Incorporation Methods on Soilborne Organisms and Pine Seedling Production in Southern Nurseries

Abstract The use of dazomet as a fall and spring fumigant for pine seedling production and control of soilborne pests was evaluated at two southern nurseries. Dazomet was applied at low (280–325 kg/ha) and high (493–560 kg/ha) rates and incorporated with a rototiller or spading machine. Comparisons were made with methyl bromide/chloropicrin (MBC) fumigation and nonfumigated control treatments. Dazomet incorporation method had no effect on seedling density at either nursery, and often did not affect seedling morphological characteristics. At the Georgia (GA) nursery, seedling density and morphological characteristics did not differ among fumigant treatments except in the spring study area where shoot weight was greater in the MBC treatment than the dazomet or nonfumigated control treatments. In the fall study area at the North Carolina (NC) nursery, seedling density was greater in the high-rate dazomet treatment than the nonfumigated control. Seedlings were generally larger in MBC and dazomet treatments than the control. Seedling density and morphological characteristics did not differ among fumigation treatments in the spring study area. Fumigation with MBC or dazomet generally reduced the percentage of roots withPythium andFusarium spp. compared to controls at the GA nursery and the fall fumigation area in the NC nursery. Plant parasitic nematodes were found infrequently at both nurseries and did not differ among treatments. Nutsedge (Cyperus spp.) was the major problem at the GA nursery and was effectively controlled only with MBC. Compared to the MBC treatment, the abundance of soilborne fungi and the association of certain types ofTrichoderma with roots was often lower in the dazomet treatments. The importance of these differences for long term seedling production and management of soilborne diseases is not known at this time. South. J. Appl. For. 27(1):41–51.

Mapping the threat of seawater intrusion in a regional coastal aquifer-aquitard system in the southeastern United States

The Upper Floridan aquifer (UFA) beneath the southeastern US Atlantic Coastal Plain and inner continental shelf is a major source of groundwater for coastal Georgia (GA) and South Carolina (SC), where it supplies over 50% of water requirements. Since pumping from the UFA first began at Savannah, GA in the late 1800s, a large (~50 km radius) cone of depression has developed on the aquifer's potentiometric surface. Where a regional Miocene-age aquitard overlying the UFA is thin or absent within this cone, the opportunity exists for seawater to leak downward through the seabed and into the aquifer. Recently completed marine reflection seismic surveys on the Georgia–South Carolina coast identified 11 areas of concern (AOCs) where the UFA is susceptible to seawater intrusion. Results from this project, and from an AOC test-drilling program recently completed, are contributing to a large database of information and models being developed for the UFA by several agencies and academic institutions. The results will provide input necessary for managing groundwater resources and mitigating potential seawater intrusion on the Georgia–South Carolina coast.

Macrogeographic Allozyme Variation in Passiflora incarnata

the southern United States and common in old fields and roadsides. This herbaceous, perennial vine has hermaphroditic flowers yet is self-incompatible. Large bees pollinate these flowers that produce fruits that may be eaten by mammals. Although the mode of sexual reproduction suggests the potential for long distance dispersal of genes, this species also displays extensive vegetative reproduction from rhizomes. Vegetative reproduction may reduce the amount of genetic variation within a population and increase differentiation among populations. We report on the macrogeographic allozymic variation within P. incarnata. Plants were collected from three regions in South Carolina (SC) and Georgia (GA). Genetic data were collected from nine polymorphic loci with starch gel electrophoresis of allozymes. In our sample of 168 plants from SC and GA, we observed 82 different multilocus genotypes. Plants with the same multilocus genotype were collected within and among regions. Genetic data indicated that there was little genetic differentiation among regions. These data suggest that there is extensive long distance transport of genes and that the species may rely heavily on sexual reproduction to establish new habitat.

Genetic Variation of RAPD Markers for North American White Clover Collections and Cultivars

New white clover (Trifolium repens L.) cultivars with improved winter hardiness and persistence are needed for pasture‐based cropping systems in temperate regions of North America. This study evaluated, quantified, and compared genetic variation of populations developed from five recently collected germplasms from Georgia (GA), Iowa (IA), Pennsylvania (PA), West Virginia (WV), and Prince Edward Island (PEI), Canada, and three improved cultivars (‘Regal’, ‘Sacramento’, and ‘Will’). ‘Aurora’ alsike clover (T. hybridum L.) was included for comparison. Random amplified polymorphic DNA (RAPD) profiles of these populations were compared. Euclidean metric distance matrices were calculated for all possible pairwise combinations and were evaluated by analysis of molecular variance (AMOVA). An interpopulation distance matrix [Φst, an analog of F as described by Excoffier et al. (1992)] for the nine populations was used to calculate a dendrogram based on the unweighted paired group method of arithmetic averages. As anticipated, alsike clover was separated from white clover. At a higher level of similarity, the white clovers were separated into only two groups. One group consisted of GA, IA, PA, and PEI germplasms collected from long‐established pastures. The second group included Regal, Sacramento, and Will cultivars and the WV collection. Although Will was originally derived from pasture collections, it has a larger leaf size than any of the five white clover collections. The surprising genetic similarities of eight populations derived from different climates and geographic regions of the continent may indicate a common European origin for much of the naturalized white clover in North American pastures.

Genetic Variation of RAPD Markers for North American White Clover Collections and Cultivars

New white clover (Trifolium repens L.) cultivars with improved winter hardiness and persistence are needed for pasture-based cropping systems in temperate regions of North America. This study evaluated, quantified, and compared genetic variation of populations developed from five recently collected germplasms from Georgia (GA), Iowa (IA), Pennsylvania (PA), West Virginia (WV), and Prince Edward Island (PEI), Canada, and three improved cultivars (‘Regal’, ‘Sacramento’, and ‘Will’). ‘Aurora’ alsike clover (T. hybridum L.) was included for comparison. Random amplified polymorphic DNA (RAPD) profiles of these populations were compared. Euclidean metric distance matrices were calculated for all possible pairwise combinations and were evaluated by analysis of molecular variance (AMOVA). An interpopulation distance matrix [Φst, an analog of F as described by Excoffier et al. (1992)] for the nine populations was used to calculate a dendrogram based on the unweighted paired group method of arithmetic averages. As anticipated, alsike clover was separated from white clover. At a higher level of similarity, the white clovers were separated into only two groups. One group consisted of GA, IA, PA, and PEI germplasms collected from long-established pastures. The second group included Regal, Sacramento, and Will cultivars and the WV collection. Although Will was originally derived from pasture collections, it has a larger leaf size than any of the five white clover collections. The surprising genetic similarities of eight populations derived from different climates and geographic regions of the continent may indicate a common European origin for much of the naturalized white clover in North American pastures.

Development of a Multi-state Fruit Weed Extension Position

Tree fruit researchers and extension specialists from North Carolina (NC), South Carolina (SC), and Georgia (GA) have been collaborating informally for many years. There has been a desire to formalize some of these arrangements, and in late 1998 planning was intiated to develop an extension specialist position to cover orchard and vineyard floor management in NC, SC, and GA. Wayne Mitchem, who had this responsibility for NC as well as serving as the coordinator for the regional IR-4 field research center at NCSU, presented us with the opportunity to create a three-quarters time extension specialist position dealing solely with the management of weeds in tree fruits and vineyards on a regional basis. The proposal was presented to Extension Directors from NC, SC, and GA in Oct. 1998, and over the following 6 months a memorandum of understanding was developed among the three states to establish the position. The position is located in NC at the Mountain Horticultural Crops Research and Extension Center, Fletcher, and each of the states agreed to share equally in funding the salary. None of the three states had this expertise in their faculty and if the position were not created, we would have had a void in this important aspect of orchard and vineyard maintenance. In addition to weed manangement, the position will have responsibilty for conducting residue trials for the IR-4 program as it pertains to the labeling of minor use pesticides for tree fruits and vineyards.

Near Real-Time Agricultural Simulations on the Web

Easy access to the Internet allows farmers to search for site-specific information and applications. The agricultural scientific community has developed computer simulation models and decision support systems during the last 20 years in order to transfer scientific knowledge to farmers. We describe an at tempt to create an Interrzet-based decision support system for delivering weather data and executing near real-time weather applications and crop simu lation models on the Web. A user accesses an easy- to-use interface to select management scenarios and run crop simulation models remotely. The weather data inputs are linked to data collected by the Geor gia Automated Environmental Monitoring Net work. A user defines a scenario by selecting a crop, cultivar, planting date, plant density and spacing, irrigation schedules and fertilizer inputs. After de fining all inputs associated with the scenario, the simulation model runs remotely on the server. The proposed distributed processes scheme can be imple mented on different platforms. Client/Server, CGI, Perl and other Internet technologies are providing a cost-effective, robust and simplified access to crop simulation models for the user community.

Analysis of the intergenic region of tomato spotted wilt Tospovirus medium RNA segment

The intergenic region (IGR) of the medium (M) RNA of tomato spotted wilt Tospovirus (TSWV) isolates naturally infecting peanut (groundnut), pepper, potato, stokesia, tobacco and watermelon in Georgia (GA) and a peanut isolate from Florida (FL) was cloned and sequenced. The IGR sequences were compared with one another and with respective M RNA IGRs of TSWV isolates from Brazil and Japan and other tospoviruses. The length of M IGR of GA and FL isolates varied from 271 to 277 nucleotides. The M IGRs of TSWV from potato and stokesia, and tobacco and watermelon were identical with each other in their length and sequence. IGR sequences were more conserved (95–100%) among the populations of TSWV from GA and FL, than when compared with those of TSWV isolates from other countries (83–94%). The conserved motif (CAAACTTTGG) present in the IGRs of both M and small (S) RNAs of a Brazilian isolate of TSWV was also conserved in the isolates studied. Cluster analysis of the IGR sequences showed that all GA and FL isolates are closely clustered and are distinct from the TSWV isolates from other countries as well as from other tospoviruses.

Chemical and sensory quality of processed carrot puree as influenced by stress-induced phenolic compounds.

Physicochemical analysis of processed strained product was performed on 10 carrot genotypes grown in Texas (TX) and Georgia (GA). Carrots from GA experienced hail damage during growth, resulting in damage to their tops. Measurements included pH, moisture, soluble phenolics, total carotenoids, sugars, organic acids, and isocoumarin (6-MM). Sensory analysis was conducted using a trained panel to evaluate relationships between chemical and sensory attributes of the genotypes and in carrots spiked with increasing levels of 6-MM. Preharvest stress conditions in GA carrots seemed to elicit a phytoalexic response, producing compounds that impacted the perception of bitter and sour flavors. Spiking 6-MM into strained carrots demonstrated the role bitter compounds have in lowering sweetness scores while increasing the perception of sour flavor. Screening fresh carrots for the phytoalexin 6-MM has the potential to significantly improve the sensory quality of processed products.

The effectiveness of a single treatment with doramectin or ivermectin in the control of gastrointestinal nematodes in grazing yearling stocker cattle

Four studies were conducted to a similar experimental design in the U.S. to evaluate the effectiveness of doramectin injectable administered to yearling stocker cattle in the control of gastrointestinal nematodiasis over the subsequent grazing period. Studies were conducted in Wisconsin (WI) and Arkansas (AR) during the summer season. The other two studies were conducted in Georgia (GA) and Mississippi (MS) during the winter/spring season. Doramectin was compared with both ivermectin injectable and ivermectin pour-on in the WI study, with ivermectin injectable alone in the GA study and with ivermectin pour-on alone in the other two studies. At each study site, an area of permanent pasture previously grazed by parasitized animals was subdivided by fencing into equal pasture units each with its own water supply. A treatment designation (non-medicated control, doramectin injectable, ivermectin injectable or ivermectin pour-on) was randomly assigned to each pasture unit. Weaned beef calves with confirmed gastrointestinal nematode infections were randomly allotted to a pasture unit and corresponding treatment group. Each treatment group consisted of three replicates of seven animals per pasture unit (total 21 animals) in the WI study, three replicates of four or six animals per pasture unit (total 16 animals) in the AR study, five replicates of six animals per pasture unit (total 30 animals) in the GA study and three replicates of 12 animals per pasture unit (total 36 animals) in the MS study. Treatments were 1% doramectin injectable solution, 1% ivermectin injectable solution, 0.5% ivermectin pour-on solution or non-medicated controls. The injectables were administered at a dose of 1 ml/50 kg body weight (200 μg doramectin or ivermectin/kg) by subcutaneous injection in the neck. Ivermectin pour-on solution was administered topically at a dose of 1 ml/10 kg body weight (500 μg ivermectin/kg). After receiving their prescribed treatment, animals were placed on their designated pasture unit where they remained for the entire grazing period (84–140 days). Fecal nematode egg counts and body weights were monitored at predetermined intervals throughout each study. Doramectin treatment reduced pretreatment egg counts by between 95 and 100% by 21 days post-treatment. Subsequent rises in egg output from exposure to infective pastures were delayed by two to four weeks resulting in substantial reductions in total egg deposition over the grazing period and, therefore, potential pasture recontamination. Doramectin treatment resulted in substantial average daily weight gain advantages (0.152–0.272 kg) over the grazing season compared to non-medicated controls. Advantages were statistically significant ( P < 0.05) in three of the four studies. There were no significant differences ( P > 0.05) in average daily gain between the doramectin and ivermectin injectable or ivermectin pour-on treated groups.

Provence and georgia: Two poets of the courtly passion

Provence and georgia: Two poets of the courtly passion

Melatonin receptors and signal transduction in melatonin-sensitive and melatonin-insensitive populations of white-footed mice ( Peromyscus leucopus)

The pineal hormone melatonin times seasonal alterations in reproductive function in photoperiodic mammals. In white-footed mice, there is variation in responsiveness to the reproductive effects of melatonin between populations originating in different locations; mice from Connecticut (CT) respond normally to melatonin, while mice from Georgia (GA) appear insensitive to melatonin. In the present paper, we compare melatonin receptor distribution and a second messenger response to melatonin in white-footed mice from CT and GA. Specific binding of 125I-labeled melatonin (I-MEL) was observed in a variety of brain regions in each population, but there were no consistent differences in the distribution or intensity of I-MEL binding between the populations. Furthermore, melatonin inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis explants from both populations. These results suggest that insensitivity to melatonin in GA mice is not due to a gross defect in melatonin receptors or receptor-effector coupling.

Geographic Differences for Delay of Sexual Maturation in Peromyscus Leucopus: Effects of Photoperiod, Pinealectomy, and Melatonin1

Effects of short-day photoperiod, pinealectomy, and melatonin on sexual maturation were tested in Peromyscus leucopus from either Connecticut (CT) or Georgia (GA). Laboratory reared-stocks from CT and GA were exposed to short daylength (photoperiod) from birth or 25 days of age. At 12 wk of age, delay in sexual maturation was indicated in most CT mice by decreased testis length, combined testes weight, and seminal vesicle weight. Conversely, GA animals did not delay sexual maturation when exposed to short-day photoperiod from either birth or 25 days of age. These results indicate that responses to short daylengths differ for juvenile CT and GA populations. In a second experiment, pinealectomized or sham-operated CT males were exposed to short-day (9L:15D) or long-day (16L:8D) photoperiod from birth. Pinealectomy blocked the effect of short daylength on reproduction. Therefore, the pineal must be involved in the delay of sexual maturation observed for short-day CT mice. The effects of melatonin, a pineal gland hormone, were tested with chronic s.c. implants or daily injections. In CT mice given either melatonin implants or afternoon injections, sexual maturation was delayed. GA mice were insensitive to all melatonin treatments. Further, no differences in circadian organization (phase angle, duration of activity, period under constant dark) between GA and CT animals were apparent. Collectively, these studies indicate that melatonin is involved in the mechanism responsible for delay of sexual maturation in CT mice. Short-day insensitivity of GA Peromyscus leucopus probably results from a deficiency in the melatonin effector pathway and is not due to a disruption of circadian organization.

Effect of population density on agronomic performance and chemical yields in pokeweed, Phytolacca americana

Pokeweed ( Phytolacca americana L.) has been identified as a promising species for development as a whole-plant source of polyphenols and oil and as a high protein feed. Plants were established in the field in the spring of 1982 at Beltsville, Maryland (MD) and Experiment, Georgia (GA) and harvested in the fall of 1982 and 1983. In 1982, maximum dry matter (DM) yields (leaves, stems, and berries) in GA and MD were 7·5 and 3·1 Mg ha −1 respectively, maximum acetone extract (EX) yield in GA was 0·53 Mg ha −1 (leaves and stems only). In 1983 in GA and MD, maximum DM yields were 13·6 and 4·1 Mg ha −1 respectively, amd maximum EX yields were 0·59 and 0·32 Mg ha −1, respectively. Population densities of 12 000, 18 000 and 36 000 plants ha −1 had no significant effect on yields in GA, but in MD, DM yields were highest at 36 000 plants ha −1 in 1982 and at 18 000 plants ha −1 in 1983. Genetic variation for DM and EX yield was noted.

Intraspecific Differences in the Use of Photoperiod and Temperature as Environmental Cues in White-Footed Mice Peromyscus leucopus

This study compared the effects of photoperiod and temperature on populations of Peromyscus leucopus from two different latitudes. Laboratory-reared mice which were descendants of Connecticut (CT) or Georgia (GA) wild-caught animals were exposed to either a long (16L:8D) or short (9L:15D) photoperiod for 12 wk. All mice were kept at 23 C for the first 6 wk, then half of each group was exposed to 13 C for the remaining 6 wk. Nesting was assessed during week 10 and spontaneous torpor during weeks 11-12. At the end of week 12, all mice were killed and examined for the presence of winter pelage. Testicular index, testes weight, seminal vesicle weight, spermatogenic index in males, and reproductive tract weight and follicular index in females were determined. Short days caused gonadal involution, molts to winter pelage, and torpor at both temperatures in CT mice. In contrast, the majority of GA mice remained reproductively competent, and thermoregulatory adjustments were less pronounced. At 23 C, short-day exp...

Growth and development of geographic and crossbred strains of colonized Simulium decorum (Diptera: Simuliidae)

The developmental times from larval hatch to adult emergence were computed for strains of Simulium decorum from Warm Springs, Georgia (GA), and Dryden Lake, New York (DL). Rearings were conducted at several constant temperatures. Differences between geographic strains and between sexes within strains were significant in nearly every instance (P &lt; 0.01). At cool temperatures (13 to 17 °C), the GA strain developed more quickly; however, this trend was reversed at intermediate temperatures (18 to 20 °C), and at 21 °C only GA males developed faster than DL males. With few exceptions, both sexes of the GA strain required more cumulative degree-days for development than their northern counterparts.The number of instars was determined for each strain by measuring the length of the postgena of head capsules collected during warm (21 °C) and cool (13 °C) rearings. Although both strains pass through seven stadia, differences between strains were noted in the size of middle instars. In all rearings sexual dimorphism was evident in the morphology of last-instar head capsules; in rearings conducted at cool temperatures (13 °C), sexes could also be separated by the size of last-instar head capsules.Strains were crossed and their progeny reared. High values for net reproductive rate (R0), fecundity, and adult survival suggest that heterosis resulted from crossbreeding the two geographic strains.