Effects of temperature and hydrology on growth of shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820) in the lower Mississippi River

We evaluated the effects of thermal and hydrologic conditions on the growth of shovelnose sturgeon (Scaphirhynchus platorynchus) in the lower Mississippi River, USA. Duration of water temperatures 12–24°C had a positive influence and temperatures below 11°C had a negative influence on annual growth increment, but these two variables accounted for less than 15% of the variation in growth. Duration of water temperatures above 28°C, duration of floodplain inundation, duration of low water, and minimum and maximum river stage did not influence annual growth increment. Growth of shovelnose sturgeon in the lower Mississippi River appears to be positively influenced by duration of moderate water temperatures but minimally influenced by hydrologic conditions. The low variation accounted for by thermal and hydrologic variables suggests annual growth increment may be largely influenced by additional abiotic or biotic factors.     

Thermoregulatory trade-offs result from vegetation removal by a harvester ant

Abstract.  1. Analyses of ecological trade-offs help to explain how organisms balance competing demands. Harvester ants ( Pogonomyrmex occidentalis ), are conspicuous residents of shortgrass prairie in western North America; worker P. occidentalis actively clear all vegetation from the immediate vicinity of their large gravel mounds. This study is based on the prediction that vegetation clearing yields a thermal trade-off by increasing soil temperatures; during cool periods the resulting increase in soil temperature opens new time windows for activity, while during hot periods the soil temperature is more likely to exceed the maximum thermal tolerance for this species. To test the hypothesis that daily and seasonal trade-offs in ant activity result from vegetation removal, the effects of experimentally applied shade on activity patterns were measured.
2. Harvester ant activity correlated highly with ground temperature; experimental shading of ant mounds shifted daily activity patterns by lowering ground temperature. Shading in the morning significantly delayed the onset of ant activity by preventing solar warming of the mound. In contrast, mid-day experimental shading prevented elevation of ground temperatures to above 50 °C and allowed ants to remain active when lethally high temperatures would normally force them inside the mound.
3. A model derived from field data predicted surface ground temperature (and therefore ant activity) based on air temperature and solar radiation, under conditions of sun and shade. For each of six seasons modelled, shade removal yielded a net gain of activity time. These results indicate that vegetation removal by harvester ants produces an advantageous thermoregulatory effect by helping to maximise activity time.     

Effect of constant and fluctuating temperature on the circadian foraging rhythm of the red imported fire ant,Solenopsis invicta Buren (Hymenoptera: Formicidae)

Understanding circadian foraging rhythms activity of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae) foragers at different temperatures is an important step towards developing control measures in Integrated Pest Management (IPM) programs. In this study, the circadian foraging rhythm activities of S. invicta foragersat different temperature were investigated under laboratory and field conditions. Results indicated that the foraging activity increased after sunrise, and maximum foraging occurred at 14:00 (foraging rate was 69.22 ± 0.57 and 72.58 ± 1.15 foragers/min in the first and second year, respectively) in the tea fields of Guangzhou during autumn. Furthermore, foragers demonstrated circadian rhythms and exhibited a unimodal after 24 h. A significant correlation was found between foraging activity and temperature. S. invicta colonies were active at moderate soil temperatures (approximately 26.65 °C to 29.24 °C). The preferred temperature of the colonies was 26 °C, followed by 22 °C and 18 °C in the laboratory. The individual S. invicta activity was maximum at 17:00 (18.67 ± 1.66 times /10 min) and minimum at 5:00 (8.33 ± 2.51 times/10 min) at 26 °C. The fluctuating temperature had a significant impact on individual locomotor activity (r = 0.8979, P < 0.01) but did not alter the rhythm activity. Our results demonstrated that temperature might play an important role in circadian foraging rhythms activity of S. invicta. These results may have implications for the development of more effective fire ant management strategies.     

Effects of pasture and forest microclimatic conditions on the foraging activity of leaf-cutting ants

The fragmentation and transformation of land cover modify the microclimate of ecosystems. These changes have the potential to modify the foraging activity of animals, but few studies have examined this topic. In this study, we investigated whether and how the foraging activity of the leaf-cutter ant Atta cephalotes is modified by microclimatic variations due to land cover change from forest to pasture. We characterized the microclimate of each habitat and identified alterations in foraging behavior in response to relative humidity (RH), air temperature, and surface temperature along ant foraging trails by synchronously assessing foraging activity (number of ants per 5 min including incoming laden and unladen and outgoing ants) and microclimatic variables (air temperature, RH, and maximum and minimum surface temperature along the foraging trail). There were climatic differences between habitats during the day but not throughout the night, and A. cephalotes was found to have a high tolerance for foraging under severe microclimatic changes. This species can forage at surface temperatures between 17 and 45°C, air temperatures between 20 and 36°C, and an RH between 40% and 100%. We found a positive effect of temperature on the foraging activity of A. cephalotes in the pasture, where the species displayed thermophilic behavior and the ability to forage across a wide range of temperatures and RH. These results provide a mechanism to partially explain why A. cephalotes becomes highly prolific as anthropogenic disturbances increase and why it has turned into a key player of human-modified neotropical landscapes.     

Weight of food and time required to satiate brown trout, Salmo trutta L.

Brown trout of different weights (range 8-358 g) were fed to satiation at fifteen different water temperatures (range 3.8–21.6°C. Both the weight of the trout (Wg) and the water temperature (T°C) affected the maximum weight of food (Q mg) consumed in a meal, and the relationship between the three variables was well described by a multiple regression equation which can be used to estimate the value of Q (with 95% confidence limits) for trout of different weights at different temperatures between 3.8°C and 21.6°C. The satiation time (with 95% confidence limits) can also be estimated from a multiple regression equation for trout of different weights at temperatures between 6.8°C and 18.1°C. Estimates from the multiple regression equations were applicable to a wide range of food organisms with the exception of larvae ofTenebrio molitor (mealworms). Appetite (measured by voluntary food intake) varied with temperature and was greatest between 13.3°C and 18.4°C. From comparisons with the results of other workers, it was concluded that the maximum amount of food consumed in a meal may provide sufficient calories for both the daily metabolic requirements and the daily maintenance requirements of a trout at temperatures between 3.8°C and 18.4°C, but not at temperatures above 18.4°C.     

Host plant‐related variation in thermal tolerance of Eldana saccharina

Understanding tolerance of thermal extremes by pest insects is essential for developing integrated management strategies, as tolerance traits can provide insights into constraints on activity and survival. A major question in thermal biology is whether thermal limits vary systematically with microclimate variation, or whether other biotic or abiotic factors can influence these limits in a predictable manner. Here, we report the results of experiments determining thermal limits to activity and survival at extreme temperatures in the stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae), collected from either Saccharum spp. hybrids (sugarcane) (Poaceae) or Cyperus papyrus L. (Cyperaceae) and then reared under standard conditions in the laboratory for 1–2 generations. Chill‐coma temperature (CT_min), critical thermal maximum (CT_max), lower lethal temperatures (LLT), and freezing temperature between E. saccharina collected from the two host plants were compared. CT_min and CT_max of E. saccharina moths collected from sugarcane were significantly lower than those from C. papyrus (CT_min = 2.8 ± 0.4 vs. 3.9 ± 0.4 °C; CT_max = 44.6 ± 0.1 vs. 44.9 ± 0.2 °C). By contrast, LLT of moths and freezing temperatures of pupae did not vary with host plant [LLT for 50% (LT₅₀) of the moth population, when collected from sugarcane: ?3.2 ± 0.5 °C, from C. papyrus: ?3.9 ± 0.8 °C]. Freezing temperatures of pupae collected from C. papyrus were ?18.0 ± 1.0 °C and of those from sugarcane ?17.5 ± 1.8 °C. The E. saccharina which experienced the lowest minimum temperature (in C. papyrus) did not have the lowest CT_min, although the highest estimate of CT_max was found in E. saccharina collected from C. papyrus and this was also the microsite which reported the highest maximum temperatures. These results therefore suggest that host plant may strongly mediate lower critical thermal limits, but not necessarily LLT or freezing temperatures. These results have significant implications for ongoing pest management and thermal biology of these and other insects.     

Environmental factors affecting seed germination of gray birch (Betula populifolia) collected from abandoned anthracite coal mine spoils in northeast Pennsylvania

Germination of gray birch (Betula populifolia) seed collected from anthracite mine spoils in northeastern Pennsylvania was studied. Environmental conditions of the spoil banks are such that high mortality may occur at seed and germination stages because of low moisture availability and thermal stress. The mine spoil banks are harsh environments with respect to key seed germination factors: percent soil moisture as low as 1.8% and soil surface temperatures reaching 59°C. In the field, gray birch typically germinated in mid-April prior to severe environmental stress. Trends in germination success were inversely related to rising soil temperature and decreasing soil moisture availability. Although seeds were capable of survival and germination under laboratory conditions of constant temperatures in excess of 55°C, dramatic decline in germination was observed under fluctuating temperature regimes likely to be experienced in the field. No germinations occurred under fluctuating temperatures in excess of 30°C. Germinations in the field were seen to end after mid-June when substrate temperatures exceeded 30°C.     

Thermal sensitivity of scope for activity in Pagothenia borchgrevinki, a cryopelagic Antarctic nototheniid fish

The thermal sensitivity of scope for activity was studied in the Antarctic nototheniid fish Pagothenia borchgrevinki. The scope for activity of P. borchgrevinki at 0°C was 189 mg O₂ kg⁻¹ h⁻¹ (factorial scope 6.8) which is similar to that of temperate and tropical species at their environmental temperatures, providing no evidence for metabolic cold adaptation of maximum activity. The scope for activity increased to a maximum value of 266 mg O₂ kg⁻¹ h⁻¹ (factorial scope 8.3) at 3°C and then decreased from 3 to 6°C. The thermal sensitivity of critical swimming speed was also investigated and followed a similar pattern to aerobic scope for activity, suggesting oxygen limitation of aerobic performance. Oxygen consumption rates and ventilation frequencies were monitored for 24 h after the swimming challenge and the recovery of both parameters to resting levels was rapid and independent of temperature.     

Microclimatic Conditions Regulate Surface Activity in Harvester Ants <Emphasis Type="Italic">Messor barbarus</Emphasis>

This paper analyses the effect of microclimatic factors (internal soil temperature, surface temperature and surface relative humidity) on surface activity of Messor barbarus harvester ants. We selected 44 colonies in an area of Mediterranean grassland near Madrid (Central Spain), which were monitored for activity between March 1998 and September 1999. Results indicate that microclimatic factors are good predictors of colony activation and intensity of activity. Colonies became active above certain critical values of internal soil temperature and relative humidity. For active colonies, surface temperature was the main regulatory factor for worker departure rate, which peaked at around 25–30°C. Worker speed was positively correlated with surface temperature, although the relationship was weaker for large-sized workers. Microclimatic factors were not enough, however, to predict task allocation outside the nest. The explanation for this aspect of ant behavior probably requires the inclusion of biotic factors in the models.     

Radiated temperature from thermal imaging is related to feed consumption,growth rate and feed efficiency in grower pigs

Individual feed consumption and animal weight were continuously recorded in grower pigs using an automated feeding system. Infrared images were recorded each time a pig entered the feeding system and infrared thermography provided radiated thermal measurements of the dorsal surface of each animal. Feed was withdrawn and the animals fasted for a period of 24 h three times during the growth of the animals at body weights of approximately 35, 65 and 105 kg. There was a significant reduction of 0.28 °C in the maximum surface temperature (Tmax), and 0.48 °C in the average surface temperature (Tmean) during the periods of fasting. Maximum and average pig temperatures exhibited negative correlations to feed consumption and growth variables. There were negative correlations of residual feed intake (RFI) to Tmax and Tmean radiated temperatures. There were positive correlations of residual gain (RG) and residual intake and gain (RIG) with Tmax and Tmean. The Tmax and Tmean temperature responses to fasting were negatively associated with feed consumption and growth variables. Absolute temperature and temperature response variables were positively associated with RFI and negatively associated with residual intake and gain (RIG). These findings provide support for the concept of radiated heat losses as a measure of metabolic activity and a predictor of growth performance.     

Thermal biology,torpor and behaviour in sugar gliders: a laboratory-field comparison

Most studies on animal physiology and behaviour are conducted in captivity without verification that data are representative of free-ranging animals. We provide the first quantitative comparison of daily torpor, thermal biology and activity patterns, conducted on two groups of sugar gliders (Petaurus breviceps, Marsupialia) exposed to similar thermal conditions, one in captivity and the other in the field. Our study shows that activity in captive gliders in an outdoor aviary is restricted to the night and largely unaffected by weather, whereas free-ranging gliders omit foraging on cold/wet nights and may also forage in the afternoon. Torpor occurrence in gliders was significantly lower in captivity (8.4% after food deprivation; 1.1% for all observations) than in the field (25.9%), mean torpor bout duration was shorter in captivity (6.9 h) than in the field (13.1 h), and mean body temperatures during torpor were higher in captivity (25.3°C) than in the field (19.6°C). Moreover, normothermic body temperature as a function of air temperature differed between captive and free-ranging gliders, with a >3°C difference at low air temperatures. Our comparison shows that activity patterns, thermal physiology, use of torpor and patterns of torpor may differ substantially between the laboratory and field, and provides further evidence that functional and behavioural data on captive individuals may not necessarily be representative of those living in the wild.     

Thermal adaptation generates a diversity of thermal limits in a rainforest ant community

The Thermal Adaptation Hypothesis posits that the warmer, aseasonal tropics generates populations with higher and narrower thermal limits. It has largely been tested among populations across latitudes. However, considerable thermal heterogeneity exists within ecosystems: across 31 trees in a Panama rainforest, surfaces exposed to sun were 8 °C warmer and varied more in temperature than surfaces in the litter below. Tiny ectotherms are confined to surfaces and are variously submerged in these superheated boundary layer environments. We quantified the surface CT_min and CT_maxs (surface temperatures at which individuals grew torpid and lost motor control, respectively) of 88 ant species from this forest; they ranged in average mass from 0.01 to 57 mg. Larger ants had broader thermal tolerances. Then, for 26 of these species we again tested body CT_maxs using a thermal dry bath to eliminate boundary layer effects: body size correlations observed previously disappeared. In both experiments, consistent with Thermal Adaptation, CT_maxs of canopy ants averaged 3.5–5 °C higher than populations that nested in the shade of the understory. We impaled thermocouples in taxidermy mounts to further quantify the factors shaping operative temperatures for four ant species representing the top third (1–30 mg) of the size distribution. Extrapolations suggest the smallest 2/3rds of species reach thermal equilibrium in <10s. Moreover, the large ants that walk above the convective superheated surface air also showed more net heating by solar radiation, with operative temperatures up to 4 °C higher than surrounding air. The thermal environments of this Panama rainforest generate a range of CT_max subsuming 74% of those previously recorded for ant populations worldwide. The Thermal Adaptation Hypothesis can be a powerful tool in predicting diversity of thermal limits within communities. Boundary layer temperatures are likely key to predicting the future of Earth's tiny terrestrial ectotherm populations.     

Functional models for growth and food consumption of Atlantic salmon parr, Salmo salar, from a Norwegian river

1. The chief objectives were to analyse and model experimental data for maximum growth and food consumption of Atlantic salmon parr (Salmo salar) collected from a cold glacier fed river in western Norway. The growth and feeding models were also applied to groups of Atlantic salmon growing and feeding at rates below the maximum. The growth models were validated by comparing their predictions with observed growth in the river supplying the experimental fish.
2. Two different models were fitted, one originally developed for British salmon and the other based on a model for bacterial growth. Both gave estimates for optimum temperature for growth at 18–19 °C, somewhat higher than for Atlantic salmon from Britain. Higher optimal temperature for growth in salmon from a cold Norwegian river than from British rivers does not concur with predictions from the thermal adaptation hypothesis.
3. Model parameter estimates differed among growth groups in that the lower critical temperature for growth increased from fast to slow growing individuals. In contrast to findings for brown trout (Salmo trutta), the optimum temperature for growth did not decrease with decreasing levels of food consumption.
4. A new and simple model showed that food consumption (expressed in energy terms) peaked at 19.5–19.8 °C, which is similar to the optimal temperature for growth. Feeding began at a temperature 1.5 °C below the lower temperature for growth and ended about 1 °C above the maximum temperature for growth. Model parameter estimates for consumption differed among growth groups in a manner similar to the growth models. Maximum consumption was lower for Atlantic salmon than for brown trout, except at temperatures above 18 °C.
5. By combining the growth and food consumption models, growth efficiency was estimated and reached a maximum at about 14 °C for fast growing individuals, increasing to nearly 17 °C for slow growing ones, although it was lower overall for the latter group. Efficiency also declined with increasing fish size. Growth efficiency was generally higher for Atlantic salmon than for brown trout, particularly at high temperature.     

Thermoregulation and daily activity patterns in a black desert grasshopper,Taeniopoda eques

Behavioural thermoregulation was studied in the western horse lubber grasshopper Taeniopoda eques (Burmeister), a native of the Chihuahuan Desert of North America. The grasshoppers regulated their temperature through a series of daily cyclical vertical movements between vegetation and the soil, and by the adoption of four thermoregulatory postures: flanking, crouching, stilting and stem-shading. At dawn, the grasshoppers moved from their nocturnal roost-plants to the ground, returned to bushes during the middle of the day, moved back to the open ground in the afternoon, then reascended vegetation at dusk. The occurrences of the four thermoregulatory postures were synchronized with these microhabitat shifts. During the cooler mornings and afternoons, the insects maximized heat gain by flanking and crouching, achieving thoracic temperatures of up to 16°C above ambient. Throughout the hot middle of the day the insects stilted and shaded, minimizing heat gain. These behaviours effectively kept the grasshoppers' body temperatures near the preferred temperature (36·2°C), but lower than the maximum voluntarily tolerated temperature (41·9°C), critical thermal maximum (45·2°C) and instantaneous lethal maximum (46·5°C). The body size of flanking insects influenced heating and cooling rates, wind effects and temperature excess at equilibrium. Both infrared and visible radiation appeared to elicit flanking. The need and ability to thermoregulate are influenced by this insect's reliance on chemical deterrents for defence.     

Temperatures and thermal tolerances for cacti exposed to high temperatures near the soil surface

Abstract Soil surface temperatures in deserts can reach 70 °C, far exceeding the high-temperature tolerance of most vascular plants of about 55 °C. In this study a computer model indicated that the maximum temperatures of small spherical cacti would approach soil surface temperatures, in agreement with measurements on seedlings of Ferocactus acanthodes. Shortwave radiation was the most important environmental variable affecting maximum cactus temperatures: a 70% reduction in shortwave radiation by shading lowered both predicted and measured stem surface temperatures by 17 °C for plants 2 cm in diameter. High-temperature tolerance, measured as the temperature that halved the fraction of cells taking up a vital stain after a 1 h high-temperature treatment, could reach 60 °C for the detached stems of Opuntia bigelovii, which appears crucial for its vegetative reproduction, and 70 °C for O. ficus-indica, apparently the greatest high-temperature tolerance so far reported for higher vascular plants. Two-fold increases in shortwave absorptance from Epithelantha bokei to Mammillaria lasiacantha to Ariocarpus fissuratus led to a 5 °C predicted increase in maximum temperature. However, compensatory differences in high-temperature tolerances occurred for these dwarf cacti, helping to explain their occurrence in the same open habitat in the Chihuahuan Desert. All six species showed acclimation of their high-temperature tolerance as ambient temperatures were increased, including acclimation by the roots of the dwarf cacti, where the greater sensitivity to high temperatures of roots would exclude them from the upper 2 cm of the soil. Using the model, the observed high-temperature acclimation, and the temperatures needed to reduce stain uptake to zero, the three dwarf cacti were predicted to be able to survive soil surface temperatures of up to 74 °C.     

A thermal time model of post-initiation flower development in the shade plant, cineraria

Effects of temperature on flower development in cineraria cv. Cindy Blue were studied in controlled environment rooms and in glasshouses. The base, optimum and maximum temperatures respectively for progress to macroscopic flower appearance after flower initiation respectively were 1.6°C, 19.3°C and 39.8°C. From these cardinal temperatures, a thermal time requirement for flower appearance after flower initiation was calculated to be 130°Cd. The base, optimum and maximum temperatures for progress to anthesis after flower initiation were respectively 1.7°C, 22.3°C and 37.1°C and from these values, the thermal time required to reach anthesis after flower initiation was calculated to be 555°Cd. No significant difference was demonstrated between thermal times for flower development in plants grown in controlled environment growth rooms or under glasshouse conditions where irradiance and photoperiod varied markedly.     

Behavioral thermoregulation in a tropical gastropod: links to climate change scenarios

Tropical species are vulnerable to global warming because they live at, or near to, their upper thermal threshold limits. Therefore, the predicted increase in the frequency of warming events in the tropics is expected to be critical for the survival of local species. This study explored the major environmental variables which were thought to be correlated with body temperatures (BTs) of the tropical snail Littoraria scabra at the niche level. A correlation between BT and substrate temperature (ST) was detected from field observations which suggests a possible causal relationship between both substrate and BTs. In contrast, there was no correlation between BT and air temperature. Field observations suggest that 33.4 °C may be L. scabra upper limit of substrate surface temperature, although further experiments are needed to assess if the upper limit of physiological tolerance is actually different. As L. scabra individuals were free to choose their substrata, the observed distribution pattern at the niche level is related to L. scabra's behavior. Additionally, substrate surface temperatures were very heterogeneous at centimeter scale (i.e. from 22.5 to 53.1 °C) and L. scabra was shown to select specific STs (i.e. between 22.5 and 33.4 °C) rather than microhabitat type. Therefore, L. scabra did not seem to behaviorally thermoregulate through microhabitat selection nor aggregation. In contrast, behavioral experiments showed that L. scabra has the ability to actively select a thermally favorable site over short temporal scale (i.e. individual average speed of 1.26 cm min^?1) following exposure to high temperatures above 33.4 °C. Hence, this study supports the crucial need to integrate intertidal invertebrate behavioral responses to thermal constraints in climate change studies.     

Temporal environmental change, clonal physiology and the genetic structure of a Daphnia assemblage (D. galeata–hyalina hybrid species complex)

1. In a combined field and laboratory study, seasonal relationships between water temperature and oxygen content, genetic structure (composition of MultiLocus Genotypes, MLGs) of a Daphnia assemblage (D. galeata–hyalina hybrid species complex), and the physiological properties of clones of frequent MLGs were studied. In accordance with the oxygen‐limited thermal tolerance hypothesis, essential physiological variables of oxygen transport and supply were measured within the tolerable temperature range. 2. A few MLGs (types T1–T4) were frequent during early spring and late autumn at surface temperatures below 10 °C. Clones of T1–T4 showed a low tolerance towards higher temperatures (above 20 °C) and a high phenotypic plasticity under thermal acclimation in comparison to clones derived from frequent MLGs from later seasons, and stored high–medium quantities of carbohydrates at 12 and 18 °C. 3. Another MLG (T6) succeeded the MLGs T1–T4. T6 was frequent over most of the year at temperatures above 10 °C and below 20 °C. A clone derived from T6 exhibited a high tolerance towards warm temperatures and a more restricted phenotypic plasticity. It stored high–medium quantities of carbohydrates at 12, 18 and 24 °C and showed a high capacity for acclimatory adjustments based on haemoglobin expression. 4. During the summer period at temperatures ≥20 °C, the MLG T6 was found mainly near to the thermocline, where temperature and oxygen content were distinctly lower, and to a lesser extent in surface water. At the surface, another MLG (T19) was predominant during this period. A clone of this MLG showed a very high tolerance towards warm temperatures, minimal phenotypic plasticity, low carbohydrate stores and a high capacity for circulatory adjustments to improve oxygen transport at higher temperatures. 5. This study provides evidence for connections between the spatio‐temporal genetic heterogeneity of a Daphnia assemblage and the seasonal changes of water temperature and oxygen content. The data also suggest that not only the actual temperature but also the dynamics of temperature change may influence the genetic structure of Daphnia populations and assemblages.     

Factors Affecting the Production of Antifungal Compounds by Enterobacter aerogenes and Bacillus subtilis,Antagonists of Phytophthora cactorum1

The effects of temperature and pH on the production of antifungal compounds and growth in sterilized soil of Enterobacter aerogenes and Bacillus subtilis, antagonists of Phytophthora cactorum, the cause of crown rot of apple trees, were studied. With E. aerogenes maximum amounts of antifungal compounds were produced between 14 and 21°C and at pH levels between 3.5 and 5.0. Bacillus subtilis produced maximum amounts of antifungal compounds between 21 and 28°C andwith pH levels between 5.0 and 8.0. P. cactorum inoculum, produced in the presence of E. aerogenes or B. subtilis at optimum temperature and pH levels, was significantly less virulent compared with controls. The optimum temperature of maximum population growth in sterilized soil for E. aerogenes was 18°C and for B. subtilis 25°C. The population growth of B. subtilis was much slower than that of E. aerogenes. Fosetyl Al stimulated the multiplication of bacteria at lower temperatures while metalaxyl had the same effect at higher temperatures. These results indicate that populations of these antagonistic bacteria increase in sterile soil up to the 33rd day from inoculation and that the fungicides fosetyl, Al and metalaxyl did not limit their multiplication and production of antifungal compounds.