Sex-specific physiological, allocation and growth responses to water availability in the subdioecious plant Honckenya peploides

The gender of dimorphic plant species is often affected by ecophysiological variables. Differences have been interpreted as a response of the sexes to meet specific resource demands associated with reproduction. This study investigated whether sex‐specific variations in ecophysiological traits in response to water availability determine the performance of each sex in different habitats, and therefore promote extreme spatial segregation of the sexes in the subdioecious plant, Honckenya peploides. Twenty‐seven plants of each sex were individually potted in dune sand and assigned randomly to one of three water treatments. Well‐watered plants were watered daily to field capacity, whereas plants in the moderate and high‐water stress treatments received 40% and 20%, respectively, of the water given to well‐watered plants. Photochemical efficiency, leaf spectral properties and components of relative growth rate (leaf area ratio and net assimilation rate) were measured. Photochemical efficiencies integrated over time were higher in male than in female plants. Water deficit decreased maximum quantum yield in female plants more rapidly than in male plants, but female plants (unlike male plants) had recovered to initial values by the end of the experiment. Maximum quantum yield in male plants was more affected by water stress than in female plants, indicating that male plants were more susceptible to photoinhibition. The two sexes did not differ in growth rate, but male plants invested a higher proportion of their biomass in leaves, had a higher leaf area per unit biomass and lower net assimilation rate relative to female plants. Female plants had a higher water content and succulence than male plants. Differences in stomatal density between the sexes depended on water availability. The results suggest that the two sexes of H. peploides have different strategies for coping with water stress. The study also provides evidence of sex differences in allocation traits. We conclude that between‐sex differences in ecophysiological and allocation traits may contribute to explain habitat‐related between‐sex differences in performance and, therefore, the spatial segregation of the sexes.     

Females make tough neighbors: sex-specific competitive effects in seedlings of a dioecious grass

If males and females of a species differ in their effect on intraspecific competition then this can have significant ecological and evolutionary consequences because it can lead to size and mortality disparities between the sexes, and thus cause biased population sex ratios. If the degree of sexual dimorphism of competitive effect varies across environments then this variation can generate sex ratio variation within and between populations. In a California population of Distichlis spicata, a dioecious grass species exhibiting extreme within-population sex ratio variation (spatial segregation of the sexes), I evaluated the intraspecific competitive effects of male and female D. spicata seedlings in three soil types. The sex of seedlings was determined using a RAPD-PCR marker co-segregating with female phenotype. Distichlis spicata seedlings, regardless of sex, were six times larger when grown with male versus female conspecific seedlings in soil from microsites where the majority of D. spicata plants are female, and this sexual dimorphism of competitive effect was weaker or did not occur in other soil types. This study suggests that it is not just the higher costs of female versus male reproduction itself that cause spatial segregation of the sexes in D. spicata, but that differences in competitive abilities between the sexes—which occur as early as the seedling stage—can generate sex ratio variation.     

Patterns of water use and the tissue water relations in the dioecious shrub,Salix arctica: the physiological basis for habitat partitioning between the sexes

Summary Within the high arctic of Canada, Salix arctica, a dioecious, dwarf willow exhibits significant spatial segregation of the sexes. The overall sex ratio is female-biased and female plants are especially common in wet, higher nutrient, but lower soil temperature habitats. In contrast, male plants predominate in more xeric and lower nutrient habitats with higher soil temperatures that can be drought prone. Associated with the sex-specific habitat differences were differences in the seasonal and diurnal patterns of water use as measured by stomatal conductance to water vapor and the bulk tissue water relations of each gender. Within the wet habitats, female plants maintained higher rates of stomatal conductance (g) than males when soil and root temperatures were low (<4° C). In contrast, within the xeric habitats, male plants maintained higher g and had lower leaf water potentials _leaf at low soil water potentials and a high leaf-to-air vapor pressure gradient (w) when compared to females. Female plants had more positive carbon isotope ratios than males indicating a lower internal leaf carbon dioxide concentration and possibly higher water use efficiency relative to males. Tissue osmotic and elastic properties also differed between the sexes. Male plants demonstrated lower tissue osmotic potentials near full tissue hydration and at the turgor loss point and a lower bulk tissue elastic modulus (higher tissue elasticity) than female plants. Males also demonstrated a greater ability to osmotically adjust on a diurnal basis than females. These properties allowed male plants to maintain higher tissue turgor pressures at lower tissue water contents and _soil over the course of the day. The sex-specific distributional and ecophysiological characteristics were also correlated with greater total plant growth and higher fecundity of females in wet habitats, and males in xeric habitats respectively. The intersexual differences in physiology persisted in all habitats. These results and those obtained from growth chamber studies suggest that sex-specific differences have an underlying genetic basis. From these data we conjecture that selection maintaining the intersexual differences may be related to different costs associated with reproduction that can be most easily met through physiological specialization and spatial segregation of the sexes among habitats of differing conditions.     

Effects of nutrients and arbuscular mycorrhizal colonization on the growth of Salix gracilistyla seedlings in a nutrient-poor fluvial bar

A field survey and a pot culture experiment were conducted to examine the effects of nutrients (N and P) and arbuscular mycorrhizal (AM) fungi on the growth of Salix gracilistyla, a pioneer plant in riparian habitats. The plants growing in the field were colonized by AM and/or ectomycorrhizal fungi. However, the direct effect of AM colonization on seedling growth was not detected in the pot culture experiment. In contrast, N application significantly promoted plant growth, suggesting that the growth of S. gracilistyla seedlings is largely limited by the availability of N in the field.     

Patterns of growth,reproduction, defense,and herbivory in the dioecious shrub Baccharis halimifolia (Compositae)

Summary Patterns of growth, reproduction, defense (leaf resin) and herbivory were compared between the sexes of the dioecious shrub Baccharis halimifolia (Compositae). Male plants possessed longer shoots and more tender leaves, grew faster, and flowered and senesced earlier than female plants. Levels of leaf nitrogen, water content, and acetone-soluble resin (shown to deter feeding by polyphagous insect herbivores) did not differ between male and female plants. When offered a choice between leaves from male and female plants, adults of two leaf beetles (Chrysomelidae), the monophagous Trirhabda bacharidis and the polyphagous Paria thoracica, both preferred to feed on male leaves. Similarly, the daily fecundity of older females of T. bacharidis was higher when they were fed leaves from male compared to female plants. However, adult survivorship and total fecundity of T. baccharidis did not differ between male and female leaf treatments. We attribute the feeding preference for and slight increase in fecundity on male plants to the tenderness of male leaves. Larvae of the fly Tephritis subpura (Tephritidae) fed exclusively in the sterile receptacle of male flower heads (85% infested), but the phenology was such that pollen production was not adversely affected. Larvae of two other flies Dasineura sp. and Contarinia sp. (Cecidomyiidae) occupied >95% of only female flower heads where they fed among and on the developing seeds. We conclude that foliage-feeding herbivores are unlikely candidates to explain the female-biased sex ratio (59% female) of B. halimifolia plants in the field, and that their preference for male plants is a result of plant characteristics (e.g. rapid growth) that have been selected by some other factor. However, our data on selective floral herbivory in B. halimifolia are in accord with the argument that dioecy reduces the inadvertent loss of flower parts of one sex when herbivores feed on flower parts of the opposite sex.     

Invasive sweetclover (<Emphasis Type="Italic">Melilotus alba</Emphasis>) impacts native seedling recruitment along floodplains of interior Alaska

Sweetclover (Melilotus alba) is a non-native legume that has formed dense and extensive patches along several rivers in Alaska. Our research objective was to determine if sweetclover impacts recruitment of native seedlings in floodplain habitats. To determine if sweetclover impacted recruitment, we conducted a removal experiment along two rivers in interior Alaska. When compared to areas where sweetclover was removed, areas with sweetclover had approximately 50% greater mortality of native seedlings, 25% less recruiting species, and a significant reduction in the quantity of light available to seedlings on floodplain surfaces. To determine if sweetclover shading was a mechanism that limited seedling recruitment, we grew eight common early-successional floodplain plant species in a greenhouse under a range of lighting conditions that were representative of shading under sweetclover. We observed that species restricted to the earliest seral stages of floodplain succession experienced greater reductions in biomass than species that persist into later stages of floodplain succession. Shading seedlings in a greenhouse did not lead to mortality during the growing season. However, when seedlings were over-wintered, we observed that greater shading during the growing season can result in higher seedling mortality. Our study indicates sweetclover invasions have created a novel shade environment in early seral floodplain plant communities, which has the potential to alter community composition. To preserve biodiversity and structure of plant communities, sweetclover should be actively managed to prevent its spread onto additional glacial rivers in Alaska.     

Response of the sexes of the subdioecious plant Honckenya peploides to nutrients under different salt spray conditions

Sex dimorphic plants often show sex-specific differences in growth and biomass allocation. These differences have been explained as a consequence of the different reproductive functions performed by the sexes. Such differences may determine the performance of each sex in different habitats and therefore might explain the spatial segregation of the sexes described in many dimorphic plants. We examined the growth, reproductive and physiological responses of the sexes of the subdioecious plant Honckenya peploides to two levels each of salt spray and nutrients, which are assumed to be important selective forces in coastal environments. We found sex-related differences in H. peploides. In particular, females allocated proportionally more dry mass to reproduction and grew less and more slowly than males regardless of salt spray and nutrient conditions, which is interpreted as a trade-off between reproductive and vegetative growth. Regarding physiological response, nutrients significantly increased values of photochemical reflectance index (PRI) in females but not in males, suggesting that photochemical efficiency is more limited by nutrients in females than in males. PRI values also suggest intersexual differences in protection requirements against photoinhibition. The study did not find sex-differential responses to salt spray, which caused a decrease in reproductive effort in both sexes. The sex-related differences in relative growth rates, reproductive allocation and photosynthetic features found here may contribute to explaining habitat-related between-sex differences in performance and, therefore, the spatial segregation of the sexes observed in H. peploides.     

Survivorship and growth of seedlings of four dipterocarp species in a tropical rainforest of Sarawak,East Malaysia

Survivorship and growth of seedlings of four dipterocarp species (Dipterocarpus actangulus, D. globosus, Dryobalanops aromatica, Dryobalanops lanceolata) were studied for 2.5 years in a mixed dipterocarp forest in Sarawak, East Malaysia. Predispersal seed predation rates were larger forD. globosus (75%) thanD. lanceolata (27–34%) andD. aromatica (18–26%). Less than 20% of the twoDryobalanops seeds were damaged by vertebrates after seed dispersal. During the period from seed dispersal to the time when the seedlings had shed cotyledons, more dispersed seeds died in the twoDipterocarpus (ca 90%) than the twoDryobalanops (ca 60–70%). The major mortality factors during this period were uprooted and seed/seedling predation by insects or vertebrates. After the seedlings shed cotyledons, all species showed constant mortality rates of 34, 15–16, 17 and 6%/year forD. actangulus, D. lanceolata, D. aromatica andD. globosus, respectively, in the forest understorey. Mortality was lower in less shaded conditions than in more shaded ones forD. aromatica andD. actangulus, but not significantly different forD. lanceolata andD. globosus. A majority of dead seedlings were killed by fallen branches or were found standing with wilted leaves, probably due to water stress. No significant correlation was found between seed/seedling mortality and distance from mother trees or the initial density of seeds/seedings for all species. The mean leaf production was positively correlated with the estimated diffuse light factor of their habitats for each species.     

Effects of plant sex on insect abundance across three trophic levels in the perennial shrub Buddleja cordata

Although there is a growing interest in the effects of intra‐specific plant genetic variation on species interactions, the effects of plant sex, an important axis of genetic variation, have been less studied. In addition, previous work investigating plant sex effects on species interactions has frequently focused on bitrophic interactions (e.g., herbivory), usually ignoring plant sex effects on higher trophic levels (i.e., natural enemies). Here, we investigated the effects of plant sex on herbivore abundance and that of their natural enemies associated with the dioecious shrub Buddleja cordata Kunth (Scrophulariaceae). Furthermore, we measured a subset of plant traits frequently involved in herbivore resistance and the potentially underlying plant sex effects. To this end, we recorded the abundances of a specialist leaf‐chewing caterpillar [Acronyctodes mexicanaria Walker (Lepidoptera: Geometridae)] throughout an entire growing season. We also recorded information about the caterpillar’s parasitoids, as well as leaf water content, phenolic compounds, phosphorus, and nitrogen for male and female plants of B. cordata. Plant sex did not significantly influence caterpillar abundance but did have an effect on natural enemies, with parasitoid abundance being 2.4‐fold greater on female than on male plants. The effect of plant sex on parasitoids remained significant after accounting for caterpillar abundance, suggesting that it was underlain by a trait‐mediated (rather than density‐mediated) mechanism. Finally, we found that male plants had a higher concentration of phenolic compounds (other traits did not differ between plant sexes). These results provide valuable evidence for the extended effects of plant sex on the third trophic level and point at plant traits potentially mediating such effects.     

Sexual differences in reproductive phenology and their consequences for the demography of Baccharis dracunculifolia (Asteraceae), a dioecious tropical shrub

Patterns of phenological variation and reproductive investment were studied in the dioecious shrub Baccharis dracunculifolia DC (Asteraceae), and possible consequences on survivorship were evaluated. The sex ratio was determined in a natural field population (n = 921) of B. dracunculifolia in Belo Horizonte, Brazil. Fifty-two males and 56 females were sampled at random from this population. During the reproductive season of 1999, inflorescence production, shoot growth and mortality, and xylem water potential were recorded for each individual. The population sex ratio was male-biased (1.27 : 1, P < 0.05), and was associated with a higher mortality of female shoots (38.4 vs. 23.1 %, P < 0.05), and individuals (17.8 vs. 11.5 %, P < 0.1), despite lower water stress in female plants. Flowering phenology also differed between the sexes, with males producing more inflorescences, and earlier, than females. Owing to fruit maturation, the number of inflorescences supported by females was higher than that supported by males later in the reproductive season. This occurred during the dry season, and drought stress may have been responsible for the greater female mortality. Thus, the male-biased sex ratio in this population of B. dracunculifolia is probably due to different reproductive functions of males and females. Intersexual differences in reproductive phenology had consequences for plant demography.     

Sex ratio from germination through maturity and its reproductive consequences in the liverwort Sphaerocarpos texanus

Summary The dioecious winter ephemeral liverwort, Sphaerocarpos texanus disperses spore tetrads consisting of two males and two females. I examined the subsequent sex ratio of S. texanus at different stages in its life cycle to detect possible mechanisms affecting deviations from a 1:1 sex ratio and the effect of sex ratio on reproductive success. As S. texanus occurs in pure male, pure female and mixed sex clumps, I examined the proportions and sizes of these, the reproductive success of pure female and mixed sex clumps in the field and the sex ratio of germinating plants in a growth chamber. In both the field and the growth chamber the most abundant clump type was pure female followed by mixed sex and pure male clumps. These abundance patterns suggest that males have a lower survival rate than females before emergence and this lower survival rate continues through the gametophytic stage. This disadvantage may be due to the higher susceptibility of males to environmental conditions, to their competitive inferiority to females, and/or to differential resource allocation to the sexes within the spore tetrad. The female biased sex ratio at germination is consistent with predictions from sex ratio theory. Further my field data indicate that males may gain a survival benefit from growing in a mixed sex clump and both males and females benefit reproductively when they occur in mixed sex clumps.     

Gender variation inSilene acaulis (Caryophyllaceae)

Silene acaulis (Caryophyllaceae) is an alpine-arctic plant with a gynodioecious breeding system, but significant variation in sex expression has been reported. In addition, population sex ratio may be modified by the anther-smutMicrobotryum violaceum (Pers.)Deml & Oberwinkler, which sterilizes individuals of both sexes. A survey was undertaken at several sites on Baffin Island, Canada, to determine sex ratio and assess variation in female function among morphologically hermaphroditic individuals. The degree of anthersmut infection was also measured. Six sites had high female frequencies ranging from 72–80% and < 2% smut infection. High female frequencies may indicate cytoplasmic control of male-sterility. A seventh site from a mesic habitat had only 50% females and a higher rate of smut infection (22%). Of the three sites studied in detail, 84% of females set at least one capsule compared to only 25% of the hermaphrodites, indicating reduced female function. Hermaphrodites displayed significant variability in female function. Flowers with short styles (< 4.0mm) had degenerated ovules, and field estimates confirmed that only 5% of these individuals set capsules, but comprised the majority (> 55%) of hermaphrodites. Although hermaphrodites with short-styled flowers functioned solely as males, there was no increase in pollen production compared to long-styled hermaphrodites. Long-styled hermaphrodites produced the same number of ovules as females, and all set at least one capsule but these plants were uncommon (< 11%) at all sites.     

Sex allocation in Epidinocarsis lopezi: local mate competition

Epidinocarsis lopezi is used as a biological control agent against the cassava mealybug, Phenacoccus manihoti, a serious pest of cassava in Africa. The efficiency of parasitoid mass-rearing is maximized when maximum numbers of healthy female wasps are obtained, since only female parasitoids attack the mealybugs.Highly variable sex ratios are often found in parasitic Hymenoptera. Local mate competition (LMC) is one of the evolutionary models which provide predictions about sex allocation. In this paper we show that E. lopezi does not respond to parasitoid density with a change in sex ratio. We also show that in the field, no local mating structure exists, and that mating is random. Therefore, a shift in sex ratio in response to parasitoid density as predicted by LMC theory would not be adaptive. E. lopezi also does not change its sex allocation when ovipositing in already parasitized hosts. Hence host-size distribution and differential mortality are the only factors that can influence sex ratio in mass-rearings.

---

Résumé E. lopezi est utilisé dans la lutte biologique contre Phenacoccus manihoti, important ravageur du manioc en Afrique. Puisque seules les femelles du parasitoïde attaquent la cochenille, l'efficacité de l'élevage de masse de l'entomophage sera optimale quand le maximum de femelles saines sera obtenu.Les rapports des sexes des hyménoptères parasites varient très souvent. La compétition sexuelle locale (LMC) constitue l'un des modèles qui fournissent des prédictions de la distribution des sexes. Cette note montre que la proportion des sexes de E. lopezi n'est pas modifiée par la densité du parasitoïde. Par ailleurs, les accouplements s'effectuent au hasard dans la nature et il n'y a pas de structure locale d'accouplement. Par conséquent, le biais, prévu par la théorie du LMC, et introduit par la densité du parasitoïde dans la distribution des sexes, n'a pas de valeur adaptative. E. lopezi ne modifie pas non plus la distribution du sexe de ses descendants quant il pond dans de hôtes déjà parasités. Ainsi, la répartition en taille des hôtes et la mortalité différentielle sont les seuls facteurs qui influent sur la proportion des sexes dans les élevages de masse.

     

Dispersion pattern and mortality of seeds and seedlings ofFagus crenata Blume in a cool temperate forest in western Japan

The seed and seedling mortality ofFagus crenata Blume after a mast year (1993) was examined in relation to density and distance from the nearest conspecific adult tree in a mixed conifer-hardwood forest in Ohdaigahara, western Japan. The mortality of fallen seeds during winter amounted to 93.7%, and 79.2% of the current-year seedlings died in the first growing season. The most important factor of death for both seeds and seedlings was predation by vertebrates. The mortality of seeds during winter was positively correlated with sound seed density. The mortality of seedlings was positively correlated with density but not significantly related to the distance from the nearest crown edge of a conspecific adult tree. Mortality patterns varied with stages and spatial scales due to the behavior of predators; it is thus important to investigate the spatial pattern of seeds and seedling mortality at various temporal and spatial scales. After the first growing season, the difference in seedling density between distance classes was not significant at <4m from the nearest adult trees due to density-dependent mortality. However, seedling density was significantly lower in the ≥4 m class than in the <4 m classes.     

Inter-sexual competition in a dioecious grass

Spatial segregation of the sexes (SSS) occurs in many dioecious angiosperms, but little data are available on the fitness advantages, if any, for males and females. We examined whether reciprocally transplanted male and female seedlings of Distichlis spicata, a dioecious grass species that exhibits extreme SSS, differed in their responses to microhabitats and competition treatments. Plants grown without conspecific competitors grew equally well in both male- or female-majority habitats, suggesting that male and female plants do not have differential resource needs at the juvenile life-history stage. However, plants subject to intra-sexual competition were significantly larger than plants subject to inter-sexual competition, suggesting that niche partitioning may occur in D. spicata.     

Woody plant regeneration in three South Carolina Taxodium/Nyssa stands following Hurricane Hugo

Long-term monitoring began in 1990 to follow community changes resulting from hurricane disturbance. In addition, one-year-old Taxodium distichum seedlings were planted to determine if planting was feasible in saltwater-flooded areas. The canopy of the least impacted swamp recovered rapidly, but there were few seedlings growing in the understory. Planted seedlings survived well, but they grew very little. Both lack of seedlings and poor growth of planted seedlings were probably due to intense shading and flooding. Two impacted areas contained a greater number of seedlings, most of which were found growing on raised microsites like Taxodium knees. The majority of the seedlings in all areas were shrub species. Planted seedlings grew cell (30 cm/yr) where open canopy conditions allowed sunlight to reach the forest floor and no new saltwater has been introduced since the hurricane.     

Biased sex ratios in the dioecious annual Croton texensis (Euphorbiaceae) are not due to environmental sex determination

At Arapaho Prairie, in the sandhills of western Nebraska, the dioecious annual Croton texensis (Euphorbiaceae) exhibits biased sex ratios. Moreover, the direction of bias changes from year to year: in 1994 the study population was significantly female biased, in 1995 and 1996 it was significantly male biased, and in 1997 and 1998 the sex ratio did not differ from 1 : 1. Such variation in the observed sex ratio in plants is frequently attributed to environmental sex determination (ESD), which is favored by natural selection if the rate of fitness gain across an environmental gradient is greater for one sex than the other. We performed experiments to determine: (1) whether variation in the sex ratio is correlated with environmental conditions, as would be expected if ESD is operating, and (2) whether ESD, if present, would be favored by natural selection. In a common garden experiment in which water and fertilizer were manipulated the sex ratio was marginally male biased in treatments in which water was added, but not different from 1 : 1 in other treatments. In field plots into which seeds were planted none of several soil characteristics, nor overall plot quality for C. texensis (measured as average plant biomass) were correlated with plot sex ratio. However, plots in which a large number of planted seeds emerged tended to be female biased. These results provide very weak evidence for sex ratio bias across an environmental gradient, and thus provide little evidence for ESD. Moreover, sex-by-environment interactions for fitness, which are required for the evolution of ESD, were absent for all measured variables. Thus, ESD does not appear to be favored by natural selection in this population. Instead, these biases may have been caused by differences between the sexes in germination and/or early mortality.     

Variations of flower size and reproductive traits in self-incompatible <Emphasis Type="Italic">Trollius ranunculoides</Emphasis> (Ranunculaceae) among local habitats at Alpine Meadow

For plants that rely on animals for pollination, the ability to attract the animals to their flowers can be a crucial component of fitness. A large number of studies have documented pollinators to be important selective agents driving the evolution of flower size and correlated traits on a large scale. In this paper, we studied variations of reproductive traits in self-incompatible Trollius ranunculoides (Ranunculaceae) among local habitats at Alpine Meadow. The results showed significant variations of floral size, seed mass per fruit and sex allocation (male/female mass ratio) between different habitats, where floral size and seed mass was not explained fully by variation of plant size among habitats. It suggested that other factors unrelated to plant size might also influence floral variation. However, in our manipulated experiment, it showed no effects of manipulated floral size not only on visit rate of effective pollinators (bees and flies) but also on female success (seed set, seed mass per fruit), irrespective of flower density. Consequently, we could not conclude that the variation of floral size in T. ranunculoides was due to phenotypic plasticity, or natural selection. But if selection occurred, it should not be mediated by pollinators. It was likely that variation of sex allocation between habitats lead to changes of flower or corolla size, because plant invested much less to male function (female-biased sex allocation and larger single seed mass) in shade habitat (bottom of bush) than other exposed habitats, to gain higher fitness. In addition, high-floral density in T. ranunculoides had a negative effect on service of main pollinator (bees) and female success. This situation would influence the strength of selection on floral size.     

The effects of light on foliar chemistry,growth and susceptibility of seedlings of a canopy tree to an attine ant

Summary Seedlings of Inga oerstediana Benth. (Mimosaceae) growing in three different light environments (the understory, tree-fall gaps and full sun) were tested for differences in chemistry (nutrients and tannins), wound-induced increases in tannins, growth, and susceptibility to leaf-cutter ants, Atta cephalotes (L.) (Formicidae: Attini). I hypothesized that seedlings of I. oerstediana would contain higher concentrations of tannins when growing in high light conditions and, therefore, would be less susceptible to leaf-cutter ants.Foliar concentrations of condensed tannins were much higher in plants growing in full sun compared to those growing in the understory. The concentrations of condensed tannins did not increase following damage. Despite higher concentrations of condensed tannins in sun foliage, leaf-cutter ants found these leaves more acceptable. The preference for sun leaves was consistent with higher concentrations of foliar nutrients. I suggest that the magnitude of the increase in condensed tannins was not great enough to override the benefits of increased concentrations of foliar nutrients. Finally, based on these results and those of others, I suggest that foraging by leaf-cutter ants may be an important factor determining patterns of succession in early successional habitats.     

The progeny production of a hymenopterous parasitoid,Apanteles sp. groupultor,as affected by temperature

Adults of the parasiteApanteles sp. groupultor were held unter constant temperatures (15, 20, 26, 30°C) each within ±1°C. Total progeny production was significantly greater at 26°C, averaging 85 per female and decreased at 15°C and 30°C. Mating behavior showed that both sexes were a polygamous. The range of constant temperature did not greatly alter the progeny sex ratio when female parent had been mated. At 15°C, male activity was adversely affected when both sexes were exposed to it without previous mating, resulting in a higher number of male progeny. A temperature of 26°C was the most appropriate for normal sex ratio.