Bottom-up and top-down effects in a pre-dispersal seed predation system: are non-predated seeds damaged?

Seed predators that severely affect seed germination rates are well known for many plant species. Here, we hypothesised that due to differences in resource allocation within fruits, seed predation can negatively affect non-predated seeds in infested fruits when predation occurs during fruit maturation (a ‘top-down’ effect). We addressed this question using a system of bruchid beetles on Mimosa trees and we also investigated whether seed quality (nitrogen concentration) affects beetle body mass, which would have implications for adult fitness (‘bottom-up’ effect). To assess spatial variation, bottom-up and top-down effects were investigated in two plant populations. Nitrogen concentration was significantly higher in seeds from non-infested fruits than from infested fruits. This supports the hypothesis that resource allocation may differ between seeds from infested and non-infested fruits. Germination experiments showed that seeds from non-infested fruits germinated better than non-predated seeds from infested fruits. It was also confirmed that seed quality affected bruchid body mass. There was also evidence that more resources were taken from well-developed seeds. These results showed that seed predation can damage non-predated seeds.     

Effects of the seed predator Acanthoscelides schrankiae on viability of its host plant Mimosa bimucronata

Seeds of Mimosa bimucronata are heavily infested (pre-dispersal predation) by the bruchid beetle Acanthoscelides schrankiae in Brazil. In this study, firstly we set up experiments to assess seed germination under seven and six different light and temperature regimes, respectively, and then we evaluated the ability of seeds to germinate after predation. We tested the hypothesis that the non-predated seeds from infested fruits may respond differently when set for germination than those seeds of non-infested fruits. We also hypothesized that predation may increase the production of unviable seeds. Seeds under 18 hours of light presented the highest percentage of germination, and the alternating temperature 20-30 degrees C was considered as optimum for germination (abnormal seedlings were not considered as a successful germination). Germination of seeds from non-infested fruits was significantly higher than germination of non-predated seeds from infested fruits, and predation also caused a significant increase in the proportion of dead seeds. Our results also show a positive correlation between proportions of unviable seeds and predated seeds. These results demonstrated that seeds of M. bimucronata are strongly affected by predation because predated seeds did not germinate and non-predated seeds had their viability reduced when located in infested fruits, supporting our hypothesis.     

Benefits and costs to pollinating,seed-eating insects: the effect of flower size and fruit abortion on larval performance

Plant–pollinator interactions are well-known examples of mutualism, but are not free of antagonism. Antagonistic interactions and defenses or counter-defenses are expected particularly in nursery pollination. In these systems, adult insects, while pollinating, lay their eggs in flowers, and juveniles consume the seeds from one or several fruits, thereby substantially reducing plant fitness. The outcome of such interactions will depend, for the plant, on the balance between pollination versus seed predation and for the larvae on the balance between the food and shelter provided versus the costs imposed by plant defenses, e.g., through abortion of infested fruits. Here, we examine the costs and benefits to the larvae in the nursery-pollination system Silene latifolia/Hadena bicruris. Using selection lines that varied in flower size (large- vs. small-flowered plants), we investigated the effects of variation in flower and fruit size and of a potential defense, fruit abortion, on larval performance. In this system, infested fruits are significantly more likely to be aborted than non-infested fruits; however, it is unclear whether fruit abortion is effective as a defense. Larger flowers gave rise to larger fruits with more seeds, and larvae that were heavier at emergence. Fruit abortion was frequently observed (ca. 40% of the infested fruits). From aborted fruits, larvae emerged earlier and were substantially lighter than larvae emerging from non-aborted fruits. The lower mass at emergence of larvae from aborted fruits indicates that abortion is a resistance mechanism. Assuming that lower larval mass implies fewer resources invested in the frugivore, these results also suggest that abortion is likely to benefit the plant as a defense mechanism, by limiting both resources invested in attacked fruits, as well as the risk of secondary attack. This suggests that selective fruit abortion may contribute to the stability of mutualism also in this non-obligate system.     

Three–way interactions between Acacia, large mammalian herbivores and bruchid beetles - a review

Large mammalian herbivores are both predators and dispersers of Acacia seeds. While some of the seeds are destroyed during passage through the herbivore's digestive tract, others are defecated unharmed. Ingestion by large herbivores facilitates germination by scarification of the seed coat. The extent of the influence of herbivores on seed dispersal and germination depends on seed retention time and tooth size, which are both positively correlated with body size. Infestation by bruchid beetles (Bruchidae) reduces Acacia germination. Herbivores may reduce bruchid infestation in several ways. Larvae in recently infested seeds are killed by stomach acids penetrating the seed through the larval entry hole. Seeds that are partly excavated by burrowing larvae in more advanced stages may be crushed by the herbivore's teeth. Lastly, but probably most crucially, herbivores simply remove seeds from the natal tree prior to infestation or at least prior to reinfestation. The timing and magnitude of herbivory is crucial for both the reduction of bruchid infestation and Acacia seedling establishment. Although it is widely agreed that a three–way interaction exists between bruchid beetles, Acacia trees and large mammalian herbivores, it is also apparent that the relationship is highly complex and is not yet completely understood.     

Large African herbivores,bruchid beetles and their interactions with Acacia seeds

This study investigated the interactions of large African herbivores and bruchid seed beetles with Acacia seeds. The germination of bruchid-infested and uninfested seeds was compaed. The effects of pod consumption by large herbivores on bruchid infestation and seed germination was also assessed. Bruchid-infested seeds did germinate, and the germination of bruchid-infested and uninfested A. tortilis, A. nilotica and A. hebeclade seeds did not differ. Pod ingestion by large herbivores lowered the bruchid infestation of consumed and defaecated seeds compared to uningested seeds. Uninfested, ingested and voided A. tortilis seeds germinated seeds. Furthermore, infested A. tortilis seeds egested by giraffe, kudu and ostrich germinated better than infested, uningested seeds. Pod ingestion by large herbivores may reduce bruchid infestation, increase Acacia seed germination and therefore increase potential Acacia seedling recruitment.     

Effect of parasitoids,seed‐predators and ant‐mutualists on fruiting success and germination of Acacia drepanolobium in Kenya

Acacia drepanolobium is an obligate ant‐plant that bears dehiscent pods exploited by predispersal seed‐predators and parasitoids. Fruit set and seed germination in relation to ant‐association, bruchid and parasitoid infestation were investigated in a large‐scale multi‐year study in Kenya. Ant‐association had a significant impact on the overall numbers of fruiting trees. 94.8% of Crematogaster mimosae‐occupied trees, 25.6% of C. nigriceps‐occupied trees, and 82.2% of Tetraponera penzigi‐occupied trees set fruit. Within each tree, ant‐association had no significant impact on the amount of seed produced: C. mimosae‐occupied trees produced 47.5 seeds per branch, C. nigriceps‐occupied trees 44.9 seeds per branch and T. penzigi‐occupied trees 38.3 seeds per branch. Ant‐association did not limit seed‐predators or vary significantly by ant. Seeds infested by bruchids germinated in significantly lower proportions (6.2%) compared to uninfested seed (78.6%). Bruchid‐infested seed is also exploited by parasitoid wasps. Parasitoids appear to have a moderate but significant ‘rescue’ effect on bruchid‐infested seed with 18.4% of parasitoid‐infested seed germinating. Stable isotopes (δ15N) revealed the trophic structure of the seed‐associated insects, showing clearly that bruchids are seed predators and the parasitoid wasp Dinarmus magnus exploits the dominant Bruchidius sp.     

Seed size selection by olive baboons

Seed size is an important plant fitness trait that can influence several steps between fruiting and the establishment of a plant’s offspring. Seed size varies considerably within many plant species, yet the relevance of the trait for intra-specific fruit choice by primates has received little attention. Primates may select certain seed sizes within a species for a number of reasons, e.g. to decrease indigestible seed load or increase pulp intake per fruit. Olive baboons (Papio anubis, Cercopithecidae) are known to select seed size in unripe and mature pods of Parkia biglobosa (Mimosaceae) differentially, so that pods with small seeds, and an intermediate seed number, contribute most to dispersal by baboons. We tested whether olive baboons likewise select for smaller ripe seeds within each of nine additional fruit species whose fruit pulp baboons commonly consume, and for larger seeds in one species in which baboons feed on the seeds. Species differed in fruit type and seed number per fruit. For five of these species, baboons dispersed seeds that were significantly smaller than seeds extracted manually from randomly collected fresh fruits. In contrast, for three species, baboons swallowed seeds that were significantly longer and/or wider than seeds from fresh fruits. In two species, sizes of ingested seeds and seeds from fresh fruits did not differ significantly. Baboons frequently spat out seeds of Drypetes floribunda (Euphorbiaceae) but not those of other plant species having seeds of equal size. Oral processing of D. floribunda seeds depended on seed size: seeds that were spat out were significantly larger and swallowed seeds smaller, than seeds from randomly collected fresh fruits. We argue that seed size selection in baboons is influenced, among other traits, by the amount of pulp rewarded per fruit relative to seed load, which is likely to vary with fruit and seed shape.     

Seed arrival under different genera of trees in a neotropical pasture

Trees in pastures attract seed dispersers, leading to increased seed arrival under their canopies and more rapid regrowth around them. The characteristics that make some trees better `recruitment foci' than others, however, are poorly understood. In a neotropical pasture, we examined the arrival of seeds to open areas and underneath four genera of trees that varied in canopy architecture and type of fruit produced: Ficus trees had dense canopies and fleshy fruits, Pentaclethra trees had dense canopies and dry fruits, Cecropia trees had sparse canopies and fleshy fruits, and Cordia trees had sparse canopies and dry fruits. We found that all trees received more seeds than open pasture, probably because trees provided seed dispersers with better perches, protection from predators, nesting sites, etc. Among the tree genera, more seeds arrived under trees that produced fleshy fruits than trees that did not. This occured even during periods when trees were not fruiting (i.e., non-fruiting Ficus and Cecropia trees received more seeds than Cordia or Pentaclethra trees). Seed dispersers may periodically check Ficus and Cecropia trees for fruits, or they may become familiar with these trees while feeding and thereafter use them for other reasons. Height of trees had a slight positive effect on seed arrival, possibly because taller trees offered more protection from predators. Canopy architecture and distance to forest edge did not significantly affect seed arrival. This study demonstrates that trees in general are potentially important recruitment foci, but that different types of trees vary in the kind of recruitment that they foster in pastures.     

Seed predation,pathogen infection and life-history traits in Brassica rapa

Herbivory and disease can shape the evolution of plant populations, but their joint effects are rarely investigated. Families of plants of Brassica rapa (Brassicaceae) were grown from seeds collected in two naturalized populations in an experimental garden. We examined leaf infection by the fungus Alternaria, seed predation by a gall midge (Cecidomyiidae) and plant life-history traits. Plants from one population had heavier seeds, were more likely to flower, had less fungal infection, had more seed predation and were more fecund. Fungal infection score and seed predation rate increased with plant size, but large plants still had the greatest number of undamaged fruits. Spatial heterogeneity in the experimental garden was significant; seed predation rate and fecundity varied among blocks. An apparent tradeoff existed between susceptibility to disease and seed predation: plants with the highest fungal infection score had the lowest seed predation rate. Alternaria infection varied between populations, but the disease had no effect on fecundity. Seed predation did reduce fecundity. Damaged fruits had 31.4% fewer intact seeds. However, evidence for additive genetic variation in resistance to seed predation was weak. Therefore, neither disease nor seed predation was likely to be a strong agent of genetically based fecundity selection.     

Plant species variation in bottom‐up effects across three trophic levels: a test of traits and mechanisms

1. An increasing number of studies have addressed the mechanisms by which plant inter‐specific variation influence interactions at higher trophic levels, but little is known about the underlying plant traits driving these dynamics. 2. Here we investigated the effects of host plant species on herbivore‐parasitoid interactions and the underlying traits driving such effects. For this, we measured the abundance of seed‐eating bruchids and their parasitoids across seven sympatric populations of the bean species Phaseolus coccineus and Phaseolus vulgaris in Central Mexico. To investigate the mechanisms underlying differences between bean species in bruchid‐parasitoid interactions, we carried out two laboratory experiments to test whether bruchid and parasitoid performance differed between plant species. We also measured seed size and phenolic compounds to investigate if seed traits mediate bruchid‐parasitoid interactions by influencing herbivore susceptibility or resistance to parasitoids. 3. Field surveys revealed that the rate of parasitoid recruitment to bruchids was significantly higher on P. vulgaris than on P. coccineus. Subsequent laboratory bioassays indicated that bruchids developed more slowly and exhibited lower fitness on P. vulgaris seeds than on P. coccineus seeds. Accordingly, we found that bean species differed in seed size, with P. vulgaris having smaller (less nutritious) seeds, which explains why bruchid development was slower on this plant species. 4. These results provide a mechanism for why bruchids exhibited higher parasitism rates on seeds of P. vulgaris in the field which could be due to Slow‐Growth/High‐Mortality effects, a smaller physical refuge provided by the seed, or both factors. The roles of these mechanisms remain inconclusive without further study.     

壳斗科三种植物种子大小对昆虫寄生及种子存活率的影响

种子内的寄生昆虫可以严重影响种子的发育、损害种子活力。种子足余策略理论认为大种子有利于抵御和适应昆虫寄生取食,但动物最优觅食理论推测,大种子更易遭受昆虫寄生。为对这两种对立观点进行验证,本实验以青冈、苦槠和麻栎各2个种群的种子为材料,对昆虫寄生与完好种子间的体积和萌发率进行比较,并对寄生种子萌发率与种子体积的关系进行了分析。结果显示:(1)在6个种群的种子中,只有松阳麻栎和青冈种群的寄生种子体积大于完好种子,其余4个种群的寄生种子体积小于完好种子,但这种差异不显著;(2)所有寄生种子的整体萌发率(18%)显著低于完好种子(45.66%)(P<0.001),在不同种群内,寄生种子的萌发率也分别显著低于完好种子。(3)比较同种植物体积差异显著的寄生种子的萌发率发现,大种子总比小种子具有更高的萌发率,但差异不显著;在不同植物的寄生种子间比较时,体积最大的麻栎种子萌发率显著高于体积较小的青冈和苦槠种子。研究结果表明,象虫在种子上产卵时对大种子没有选择偏好,在昆虫寄生取食严重损害种子活力的压力下,大种子比小种子具有更强的耐受力。     

Seed dynamics of resprouting shrubs in grassy woodlands: Seed rain,predators and seed loss constrain recruitment potential

Abstract Measuring the fate of seeds between seed production and seedling establishment is critical in understanding mechanisms of recruitment limitation of plants. We examined seed fates to better understand the recruitment dynamics of four resprouting shrubs from two families (Fabaceae and Epacridaceae) in temperate grassy woodlands. We tested whether: (i) pre‐dispersal seed predation affected seed rain; (ii) post‐dispersal seed predation limited seed bank accumulation; (iii) the size of the seed bank was related to seed size; and (iv) viable seeds accumulated in the soil after seed rain. There was a distinct difference in seed production per plant between plant families with the legumes producing significantly more seeds per individual than the epacrids. Seed viability ranged from 43% to 81% and all viable had seed or fruit coat dormancy broken by heat or scarification. Pre‐dispersal predation by Lepidopteran larvae removed a large proportion of seed from the legume seed rain but not the epacrids. Four species of ants (Notoncus ectatomoides, Pheidole sp., Rhytidoponera tasmaniensis and Iridomyrmex purpureus) were major post‐dispersal seed removers. Overall, a greater percentage of Hardenbergia (38%) and Pultenaea (59%) seeds were removed than the fleshy fruits of Lissanthe (14%) or Melichrus (0%). Seed bank sizes were small (<15 seeds m^?2) relative to the seed rain and no significant accumulation of seed in the soil was detected. Lack of accumulation was attributed to seed predation as seed decay was considered unlikely and no seed germination was observed in our study sites. Our study suggests that seed predation is a key factor contributing to seed‐limited recruitment in grassy woodland shrubs by reducing the number of seeds stored in the soil.     

Seed Dispersal and Ingestion of Insect-Infested Seeds by Black Howler Monkeys in Flooded Forests of the Parana River, Argentina

All howler monkey species ( Alouatta spp.) have a folivorous–frugivorous diet. Howler monkeys are reported to be seed dispersers in several areas, including black howlers ( Alouatta caraya ), which are important seed dispersers in northern Argentinean forests. The goal of this work was to study the three-way interaction between insects, seeds, and black howlers, and assess the functional significance of this tri-trophic interaction for seed dispersal. I determined through direct observation that fruits of species with a high proportion of insect infestation were important components of howler monkey diet. Ocotea diospyrifolia seeds from fresh faeces of black howlers contained dead larvae, but seeds were still able to germinate. Seeds in which larvae had reached an advanced stage of development did not germinate. Larvae of infested Eugenia punicifolia fruits were killed by digestion when they occurred in the pulp early in the fruiting season, but were dispersed alive with seeds later in the season. Banara arguta fruits contained both healthy and infested seeds; infested seeds were destroyed during digestion, while healthy seeds were dispersed. Black howlers' ingestion of infested fruits could result in the: (1) killing of larvae and dispersion of healthy seeds; (2) spread of larvae; or (3) destruction of infested seeds. This will depend on the relationship between the time at which fruit is consumed by black howlers, the time at which insect infestation occurs, and also probably on the hardness of the seed coat and the seed–insect size ratio.     

Oviposition pattern and within-season spatial and temporal variation of pre-dispersal seed predation in a population of Mimosa bimucronata trees

Spatial and temporal variation of pre-dispersal seed predation was investigated in a population of Mimosa bimucronata trees located in the south-east region of Brazil. Three main hypotheses were addressed: (1) that the life stages of the seed predator Acanthoscelides schrankiae are synchronised with the reproductive stages of its host plant; (2) that seed predation levels vary spatially as a result of differences in fruiting phenology synchrony and fruit production among trees; and (3) that predation levels should be affected by the proximity of trees, showing a spatial structure. Also investigated was the oviposition pattern of A. schrankiae among seeds and fruits. Twenty spatially referenced trees were monitored throughout a year to examine tree phenology and egg laying and adult emergence. The bruchine’s life stages were synchronised with the reproductive stages of M. bimucronata trees. Egg distribution among seeds and fruits was aggregated. Infestation rates of adult bruchines were not spatially related to fruiting phenology and there is evidence that seed predation is a spatially density-independent process, because the relationship between infestation rates and fruit production was not significant. Finally, it was observed that the distribution of adult bruchines was spatially structured, because similar levels of infestation were found among nearby trees.     

Seed predation of Copaifera langsdorffii (Fabaceae): a tropical tree with supra‐annual fruiting

Seed predation is an important ecological and evolutionary force that directly affects the distribution of plant species. Copaifera langsdorffii is a tropical tree species with supra‐annual fruiting, which has its seeds predated by a specialist endogenous insect (Rynochenus brevicollis: Curculionidae) in the Brazilian savanna. Three hypotheses were addressed: (i) the predator satiation hypothesis, (ii) the resource concentration hypothesis and (iii) the larger seed predation hypothesis. A total of 112 individual C. langsdorffii were monitored monthly from January to August during four consecutive years (from 2008 to 2011) to determine the presence of fruits on each plant. All trees produced fruits in the year 2008, whereas none of them produced flowers or fruits in 2009 or 2010. Moreover, only 65 individuals (58%) marked in 2008 produced fruits in 2011. The number of fruits per plant was approximately 21% greater in 2008 than in 2011, while the percentage of seed predation was 76% greater in 2011, thereby supporting the predator satiation hypothesis. The percentage of seeds predated was not affected by the number of fruits per plant. Therefore, our data did not support the resource concentration hypothesis. Plants producing large seeds experienced more seed predation by R. brevicollis, supporting the larger seed predation hypothesis. In addition, we also observed a positive relationship between seed volume and adult R. brevicollis weight. This study demonstrates the importance of supra‐annual fruiting for increasing survivorship of C. langsdorffii seeds both at the individual and the population level, and suggests that seed predators select plants producing large seeds as a way of increasing the number of offspring.     

Seed Ecology of the Invasive Tropical Tree <Emphasis Type="Italic">Parkinsonia aculeata</Emphasis>

Parkinsonia aculeata is an invasive tree native to tropical America, but introduced to Australia. Propagation and stand regeneration is mainly by seed. To gain baseline knowledge for management decisions, seed bank dynamics were monitored for two months during the fruit dispersal period at a coastal wetland in Costa Rica (native habitat), and at a coastal wetland and two semi-arid rangeland sites in Northern Queensland, Australia (introduced habitats). Seed bank densities underneath dense, uniform Parkinsonia stands were found to be lowest in the Australian wetland but highest in the Costa Rican wetland. Post-dispersal seed losses were highest in the Australian wetland, primarily due to seed germination and/or death. At the other sites, seed losses were minor during the study period, and predation was the most important cause of losses. At the two rangeland sites bruchid beetles accounted for more than 95% of the seed losses by predation. Total predation was lowest in the Costa Rican wetland. In order to test for intrinsic differences of seed characteristics, germination trials were conducted using both canopy seeds and seeds from the soil seed bank. Dormancy release and germination rate were studied under four temperature treatments. In all populations, dormancy release increased with increasing temperature, but averaged responses were significantly different between Costa Rican and Australian seed populations, and between seeds collected from the soil and from trees. Germination rate of scarified seeds was fastest at 35 °C in all tested seed populations. While high seed germination levels seem to explain low seed bank densities in the Australian wetland, the large seed banks at the rangeland sites reflect the lower incidence of favourable conditions for germination. In the Australian wetland biocontrol with bruchids is unlikely to be successful, while control by conventional methods, such as killing stands by basal bark spraying, seems feasible, due to a lower long-term risk of re-infestation from the soil seed bank. At the rangeland sites conventional control will be difficult and costly. Parkinsonia stands may be better left to their own, while bruchid populations are monitored and management efforts are concentrated on preventing further invasion.     

Survival and Scatterhoarding of Frugivores-Dispersed Seeds as a Function of Forest Disturbance

The effect of forest disturbance on survival and secondary dispersal of an artificial seed shadow (N= 800) was studied at Brownsberg Natural Park, Suriname, South America. We scattered single seeds of the frugivore‐dispersed tree Virola kwatae (Myristicaceae), simulating loose dispersal by frugivores, in undisturbed and disturbed secondary forest habitats. Seed survival rate aboveground was high (69%) within 2 wk and was negatively correlated with scatterhoarding rate by rodents, the latter being significantly lower in the undisturbed forest (9%) than in the disturbed forest (20%). Postdispersal seed predation by vertebrates was low (3%) and infestation of seeds by invertebrates was almost zero in all instances. Therefore, secondary seed dispersal by rodents in forest is not as critical for recruitment as observed among other bruchid‐infested large‐seeded species. Secondary seed dispersal by rodents may, however, facilitate seedling recruitment whether cached seeds experience greater survival than seeds remaining above ground surface.     

Seed size variation in the palm Euterpe edulis and the effects of seed predators on germination and seedling survival

Intraspecific variation in seed size is common in wild plant populations and has important consequences for the reproductive success of individual plants. Multiple, often conflicting evolutionary forces mediated by biotic as well as abiotic agents may maintain such a variation. In this paper we assessed seed size variation in a population of the threatened, commercially important palm Euterpe edulis in southeast Brazil. We investigated (i) how this variation affects the probability of attack by vertebrate and invertebrate post-dispersal seed predators, and (ii) if seed size influences the outcome of seeds damaged by beetles in terms of seed germination and early survival of seedlings. Euterpe edulis seeds varied in diameter from 8.3 to 14.1 mm. Neither insects nor rodents selected the seeds they preyed upon based on seed size. Seed germination and total, shoot and root biomasses of one-year seedlings were significantly and positively affected by seed size. Root biomass and seedling survival were negatively affected by seed damage caused by a scolytid beetle (Coccotrypes palmarum) whose adults bore into seeds to consume part of the endosperm, but do not oviposit on them. Seed size had a marginally significant effect on seedling survival. Therefore, if any advantage is accrued by E. edulis individuals producing large seeds, this is because of greater seed germination success and seedling vigor. If this is so, even a relatively narrow range of variation in seed size as observed in the E. edulis population studied may translate into differential success of individual plants.     

Pre-dispersal seed predation in Central AmericanAcacia farnesiana: factors affecting the abundance of co-occurring bruchid beetles

Summary Pre-dispersal seed predation of the leguminousAcacia farnesiana byMimosestes nubigens andM. mimosae (Coleoptera: Bruchidae) was investigated in Santa Rosa National Park, northwestern Costa Rica. The purpose of the study was to determine the patterns of resource utilization by the seed predators and the mechanisms causing such patterns. Immature, mature, and fallen fruits were monitored during the dry seasons of 1987 and 1988 from different shrubs and areas. Parameters describing plant size, fecundity, and relative plant isolation were measured on each shrub. No evidence of spatial or temporal segregation was found between the two species and the intensity of seed predation was independent of the variables measured from each plant.M. mimosae was scarcer and always occurred withM. nubigens. Both were present in areas with low and high densities of the host plant, and the frequency distributions of their emergences from the fruits overlapped through the fruiting season.urosigalphus sp., a hymenopteran parasitoid, represented ca. 40% of all insect emergences in 1987 and ca. 30% in 1988. This wasp attacked a greater proportion of bruchid eggs on pods on the shrub than on pods beneath it, and more on green than on mature fruits. Parasitism thus appears to select against bruchid females that oviposit at an early stage of pod maturation. The harsh conditions of the dry season, namely heat and desiccation, also accounted for a high level of bruchid pre-emergence mortality, especially in fallen fruits, where survival from egg to adult was only about 18%. Beneath the shrubs, bruchid females exhibited selectivity, ovipositing more on pods in the shade than on those exposed to direct sunlight. In contrast to parasitoids, abiotic factors probably impose a selective force against those bruchid females that oviposit at a late stage of maturation or on pods already dropped. Both bruchid species can have more than one generation during the fruiting period. The intensity of seed predation did not, however, change during the season. The data obtained in this study suggest that factors like natural enemies and severe weather are more likely to limit the bruchid population densities than intra- or interspecific competition.     

Parasitism by Pteromalus cerealellae (Hymenoptera: Pteromalidae) on the Cowpea weevil, Callosbruchus maculatus (Coleoptera: Bruchidae): Host density, temperature effects, and host finding ability

Studies on interactions between a larval parasitoid, Pteromalus cerealellae (Boucek) and one of its hosts, Callosobruchus maculatus (F.) were carried out in the laboratory. The number of host larvae parasitized by P. cerealellae increased with host larvae at low densities and tended to a plateau at a density of 25 larvae per female parasitoid. Each parasitoid was able to parasitize more hosts and produced more offspring at 20 and 25 °C than at 30 °C. The number of non-infested seeds mixed with seeds infested with the last instar of C. maculatus did not preclude P. cerealellae from identifying infested seeds and attacking larvae inside them. When infested seeds were tightly packed, several host larvae escaped parasitism. P. cerealellae may be a useful biological control agent in newly harvested cowpea with low C. maculatus infestation, and lowering the temperature of the storage system may enhance the effectiveness of this parasitoid.