Life-history trade-off in two predator species sharing the same prey: a study on cassava-inhabiting mites

In cassava fields, two species of predatory mites, Typhlodromalus aripo and T. manihoti, co-occur at the plant level and feed on Mononychellus tanajoa , a herbivorous mite. The two predator species are spatially segregated within the plant: T. manihoti dwells on the middle leaves, while T. aripo occurs in the apices of the plant during the day and moves to the first leaves below the apex at night.
To monitor the prey densities experienced by the two predator species in their micro-environment, we assessed prey and predator populations in apices and on the leaves of cassava plants in the field. Prey densities peaked from November to January and reached the lowest levels in July. They were higher on leaves than in the apices. To test whether the life histories of the two predator species are tuned to the prey density they experience, we measured age-specific fecundity and survival of the two predators under three prey density regimes (1 prey female/72 h, 1 prey female/24 h and above the predators level of satiation). T. manihoti had a higher growth rate than T. aripo at high prey densities, mainly due to its higher fecundity. T. aripo had a higher growth rate at low prey density regimes, due to its late fecundity and survival. Thus, each of the two species perform better under the prey density that characterizes their micro-habitat within the plant.     

Living at the threshold: Where does the neotropical phytoseiid mite Typhlodromalus aripo survive the dry season?

The establishment of the neotropical predatory mite Typhlodromalus aripo in sub-Saharan Africa has resulted in broadly successful biological control of the cassava green mite Mononychellus tanajoa throughout the cassava belt of Africa. In some mid-altitude areas and drier lowland savannahs of sub-Saharan Africa, which are characterized by cool or hot long (≥5 months) dry seasons, the predator disappears from its habitat in the cassava apex during the dry season and reappears after the onset of rains. It is not known, however, where the predator remains during this time period. In this study, we conducted a field enclosure experiment of cassava plants with the objectives to determine if (a) T. aripo survives at very low densities in the apex, if (b) it survives in the soil or leaf litter below the cassava plant, and if (c) it recolonizes the cassava plant from the surrounding vegetation. Towards the end of the dry season, when the predators had disappeared from all cassava plants included in the experiment, five treatments were applied: (1) plants without enclosure; (2) plants with enclosure; (3) plants with enclosure, apices removed; (4) plants with enclosure, glue barrier around stem; and (5) plants kept free of T. aripo, without enclosure. Predator (re)appearance on cassava apices was monitored non-destructively at weekly intervals and was expressed as the proportion of plants with at least one apex with T. aripo per total number of plants of the treatment. The predators reappeared first on the plants of the treatments (1), (2), and (4). With a time lag of 7–8 weeks, the predators appeared also on the plants of the treatments (3) and (5). The time pattern of the predator’s (re)appearance in the cassava apex of the different treatments suggests that (a) T. aripo survives the dry season in very low densities in the cassava apex; this result is supported by an assessment of the efficiency of non-destructive visual in-field apex inspections which proved that about 10% of the cassava apices that had T. aripo were not recognized as such; (b) T. aripo does not survive in the soil or leaf litter, but we did document cases in a screenhouse experiment, where few individuals migrated down to the ground and walked over exposed soil until they reached the apex bouquet traps; additionally, microclimate measurements in various cassava plant strata proved that the cassava apex and the cassava stem base are the locations with the highest relative humidity during the dry season—which makes the stem base a potentially interesting refuge; (c) T. aripo does not survive in the surrounding vegetation, which is supported by a vegetation survey, where T. aripo was not found on any other plant species than cassava.     

Interactions in an acarine predator guild: impact on Typhlodromalus aripo abundance and biological control of cassava green mite in Benin, West Africa

To determine the impact of an acarine predator guild on the abundance of a shared herbivorous prey and its principal exotic predator, a series of surveys were conducted in ca. 200 cassava fields in swamp and non-swamp areas in southwestern Benin, West Africa. For each field, the surveys provided data on the density of a pest arthropod, the cassava green mite Mononychellus tanajoa (Bondar), of an introduced and successfully established natural enemy, the apex-inhabiting predatory mite Typhlodromalus aripo DeLeon, and on occurrence of other predator species that inhabit the leaves and share the same prey. These other predators included one exotic species, Typhlodromalus manihoti Moraes, that is successfully established mainly in swamp areas, and two indigenous species, Euseius fustis (Pritchard and Baker) and Typhlodromalus saltus (Denmark and Matthysse), that are commonly found on cassava in Africa. Our aim was to assess the association between the density of M. tanajoa and that of T. aripo, the most successful predator in terms of establishment and abundance, and subsequently determine how this association was affected by the presence or absence of the other predator species. No obvious density-dependent relationship was found by inspecting the scattergrams of T. aripo versus M. tanajoa densities, but high T. aripo densities did not occur when M. tanajoa densities were low and--during the dry season in February--the densities of M. tanajoa steeply increased when T. aripo numbers were low. Given the establishment of T. aripo in all fields, the presence of other species of predatory mites (T. manihoti, both in swamp and non-swamp areas; T. saltus in absence--as well as presence--of T. manihoti in swamp areas; E. fustis in absence of T. manihoti in non-swamp areas) reduced the density of M. tanajoa by a factor 2-3. Thus in all these cases, the presence of an exotic or indigenous species of predatory mite in addition to T. aripo was associated with lower M. tanajoa density. The density of T. aripo was usually positively affected by the presence of other predator species except for T. saltus in presence of T. manihoti that negatively affected the density of T. aripo in swamp areas, an effect likely mediated by either intraguild predation or competition for food. Path analysis showed that indigenous phytoseiid species were more important in suppression of M. tanajoa populations in cassava fields than previously thought. We suggest that the lack of negative effects of the predator species complex is likely due to differential niche use by the various species which reduces interference among the predators. Manipulative experiments are, however, needed to provide details on the relative importance in M. tanajoa suppression by each species within this acarine predator guild.     

Infochemical-mediated intraguild interactions among three predatory mites on cassava plants

Carnivorous arthropods exhibit complex intraspecific and interspecific behaviour among themselves when they share the same niche or habitat and food resources. They should simultaneously search for adequate food for themselves and their offspring and in the meantime avoid becoming food for other organisms. This behaviour is of great ecological interest in conditions of low prey availability. We examined by means of an olfactometer, how volatile chemicals from prey patches with conspecific or heterospecific predators might contribute to shaping the structure of predator guilds. To test this, we used the exotic predatory mites Typhlodromalus manihoti and T. aripo, and the native predatory mite Euseius fustis, with Mononychellus tanajoa as the common prey species for the three predatory mite species. We used as odour sources M. tanajoa-infested cassava leaves or apices with or without predators. T. manihoti avoided patches inhabited by the heterospecifics T. aripo and E. fustis or by conspecifics when tested against a patch without predators. Similarly, both T. aripo and E. fustis females avoided patches with con- or heterospecifics when tested against a patch without predators. When one patch contained T. aripo and the other T. manihoti, females of the latter preferred the patch with T. aripo. Thus, T. manihoti is able to discriminate between odours from patches with con- and heterospecifics. Our results show that the three predatory mite species are able to assess prey patch profitability using volatiles. Under natural conditions, particularly when their food sources are scarce, the three predatory mite species might be involved in interspecific and/or intraspecific interactions that can substantially affect population dynamics of the predators and their prey.     

Parasitism by the mite Trombidium breei on four U.K. butterfly species

Abstract 1.  The incidence of parasitism by larvae of the mite species Trombidium breei was reported in one population of the lycaenid butterfly Polyommatus icarus , four populations of the satyrine butterfly Maniola jurtina , one population of the satyrine butterfly Aphantopus hyperanthus , and two populations of the satyrine butterfly Pyronia tithonus , as well as on one specimen of the dipteran Alophorus hemiptera . A considerable proportion of butterflies (11-50%) was infested in all study populations.
2. The pattern of infestation was examined in detail in M. jurtina . Males had a significantly higher incidence of infestation than females, and middle-aged butterflies had a higher incidence of infestation than old or young butterflies. The incidence of infestation peaked in the middle of the flight season, and this seasonal effect was independent of the effect of butterfly age.
3. Using a model based on capture-recapture data, it was estimated that a hypothetical ideal male M. jurtina that lives exactly the mean expected lifespan of 9-10 days has an approximately 75% chance of becoming infested with mites at least once during its lifetime, a mean time to first infestation of 3-4 days, and an average infestation persistence time of 2-3 days.
4. Capture-recapture data failed to show any effect of mite infestation on the lifespan or within-habitat movement rate of M. jurtina .
5. In experiments in which individual butterflies were taken out of their normal habitat and released, M. jurtina and P. tithonus that were infested with mite larvae did not differ from uninfested individuals in the efficiency with which they returned to suitable habitat. Thus, parasitism by T. breei larvae had no detectable effects on flight performance or orientation ability.
6. The results suggest that trombidiid mite larvae have limited potential in the biological control of insect pests.     

Bottom-up cascading effects in a tritrophic system: interactions between plant quality and host-parasitoid immune responses

Abstract.  1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that host-plant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer – lepidopteran [ Plutella xylostella (L.)] – parasitoid [ Cotesia plutellae (Kurdjumov)], system.
2. The effects of host-plant suitability on herbivore performance and parasitism were examined. The bottom-up effect of plant suitability on host-parasitoid immune responses was then evaluated using measures of cellular and humoral effectors.
3. Host-plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence.
4. Poor host-plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized.
5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid.
6. These results demonstrated that plant-mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella , nor to host-plant effects on superparasitism.
7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host-plant-mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts.     

Genetic diversity of the cotton aphid Aphis gossypii in the unstable environment of a cotton growing area

1 Spatial and temporal habitat heterogeneity represented by annual crops is a major factor influencing population dynamics of phytophagous insect pests such as the cotton aphid Aphis gossypii Glover. We studied the effects of instability of the cotton agroecosystem resulting from the temporary availability of the plant resource and the repeated use of insecticides on the genetic variability of the cotton aphids.
2 Samples of A. gossypii were collected in cotton plots, treated or not with insecticides and from vegetable crops (Malvaceae, Cucurbitaceae and Solanaceae) within the cotton growing area of northern Cameroon. The genetic structure of the samples was assessed using eight microsatellite markers. Insecticide resistance was estimated through the detection of two mutations in the ace -1 gene that are associated with insensitivity of acetylcholinesterase to carbamate and organophosphate insecticides.
3 The results obtained show that both host plants and insecticides act in genetic structuring of A. gossypii . Ninety-three percent of aphids collected on cotton were characterized by the same microsatellite multilocus genotype, Burk1 , which also displays the insecticide resistant alleles.
4 During the dry season, the cotton crop season after, the genotype Burk1 was principally found on two other malvaceous cultivated plants, rosella and okra, acting as suitable reservoir plants. The ability of the cotton aphid to move among asynchronous suitable habitats in response to changes in resource availability enables the pest to exploit unstable cropping systems. An understanding of the cotton aphid life system may aid to improve strategies for integrated resistance management.     

Whitefly Bemisia tabaci (Homoptera: Aleyrodidae) infestation on cassava genotypes grown at different ecozones in Nigeria

Large-scale screening of cassava, Manihot esculenta Crantz, genotypes for resistance to infestation by whitefly Bemisia tabaci Gennadius, the vector of cassava mosaic geminiviruses, is limited. A range of new cassava elite clones were therefore assessed for the whitefly infestation in the 1999/2000 and 2000/2001 cropping seasons in experimental fields of International Institute of Tropical Agriculture, Ibadan, Nigeria. On each scoring day, between 0600 and 0800 hours when the whiteflies were relatively immobile, adult whitefly populations on the five topmost expanded leaves of cassava cultivars were counted. All through the 6-mo scoring period, there was a highly significant difference in whitefly infestation among the new cassava elite clones. Vector population buildup was observed in Ibadan (forest-savanna transition zone) and Onne (humid forest), 2 mo after planting (MAP). Mean infestation across cassava genotypes was significantly highest (16.6 whiteflies per plant) in Ibadan and lowest in Zaria (0.2). Generally, whitefly infestation was very low in all locations at 5 and 6 MAP. During this period, cassava genotypes 96/1439 and 91/02324 significantly supported higher infestations than other genotypes. Plants of 96/1089A and TMS 30572 supported the lowest whitefly infestation across cassava genotypes in all locations. The preferential whitefly visitation, the differences between locations in relation to whitefly population, cassava mosaic disease, and the fresh root yield of cassava genotypes are discussed.     

The effect of relative humidity on the behaviour and development of Triatoma brasiliensis

Abstract The preference for relative humidity (RH) and suitability of different levels of this environmental parameter were investigated in the haematophagous bug Triatoma brasiliensis Neiva, 1911 (Hemiptera, Reduviidae). The hygropreference of T. brasiliensis was studied using a RH gradient and the effect of different RHs on the egg hatching, nymph mortality and moulting success was also analysed. The results show that egg hatching in first-instar nymphs of T. brasiliensis was lower at extreme RHs and, particularly, it was lowest at 9.3% RH. The survival of starved nymphs was not affected by RH, but the percentage of engorged nymphs and the ecdysis success of these nymphs once fed was diminished strongly by high humidity. Fourth-instar nymphs preferred to stay at the lowest RH during the first 5 days after feeding and during ecdysis. This preference changed markedly during starvation. Fifteen days after ecdysis, the bugs moved towards intermediate humidities, and 30 days after ecdysis they even preferred the most humid sectors of the gradient. Females preferred to lay eggs in dry environments, suggesting that they may not have a particular hygropreference for oviposition, but that they simply lay their eggs at the RHs where they prefer to stay.     

Effects of prey mite species on life history of the phytoseiid predators Typhlodromalus manihoti and Typhlodromalus aripo

The effects of prey mite suitability on several demographic characteristics of phytoseiid predators and the relationship of these effects to the potential of phytoseiid predators to control herbivorous mite populations are well documented. Evidence has also accumulated in the last 20 years demonstrating that phytoseiid predators utilize herbivorous prey mite-induced plant volatiles as olfactory cues in locating their herbivorous mite prey. but less well established is the predictability of reproductive success from the ability of the predators to utilize olfactory cues to locate their prey, and how these processes are related to the success of the predators as biological control agents of the herbivorous mite. In this study, we determined in laboratory no choice experiments, the development, survivorship and fecundity of the two neotropical phytoseiid predators Typhlodromalus manihoti Moraes and T. aripo DeLeon when feeding on three herbivorous mites, including the key prey species Mononychellus tanajoa (Bondar), and the two alternative prey species Oligonychus gossypii (Zacher) and Tetranychus urticae (Koch). Intrinsic rate of increase (rm) of T. aripo was 2.1 fold higher on M. tanajoa as prey compared with T. urticae as prey, while it was almost nil on O. gossypii. For T. manihoti, rm was 2.3 fold higher on M. tanajoa as prey compared with O. gossypii as prey, while reproduction was nil on T. urticae. An independent experiment on odor-related prey preference of the two predator species (Gnanvossou et al. 2002) showed that T. manihoti and T. aripo preferred odors from M. tanajoa-infested leaves to odors from O. gossypii-infested leaves. Moreover, both predator species preferred odors from M. tanajoa-infested leaves over those from T. urticae-infested leaves. As reported here, life history of the two predatory mites matches odor-related prey preference if the key prey species is compared to the two inferior prey species. The implications of our findings for the persistence of T. manihoti and T. aripo and biological control of M. tanajoa in the cassava agroecosystem in Africa are discussed.     

Hydrological connectivity drives patterns of macroinvertebrate biodiversity in floodplain rivers of the Australian wet /dry tropics

1. Floodplain rivers in Australia's wet/dry tropics are regarded as being among the most ecologically intact and bio-diverse lotic ecosystems in the world, yet there have been relatively few community-based studies of their aquatic fauna.
2. To investigate relationships between hydrological connectivity and biodiversity in the region, macroinvertebrates were collected from sites within two contrasting floodplain rivers, the 'tropical' Gregory River and 'dryland' Flinders River systems, during the dry season and analysed at various spatial scales. A subset of sites was re-sampled in the following dry season to explore temporal variation. The fauna consisted of 124 morphotaxa, dominated by gatherers and the Insecta.
3. As predicted, hydrological connectivity (the lotic or lentic status of waterbodies) had a major influence on macroinvertebrate assemblage composition and diversity, both in space and time. Assemblages from waterbodies with similar connection histories were most alike, and beta-diversity between assemblages was greatest between lotic and lentic waterbodies, tending to increase with increasing spatial separation.
4. At smaller spatial scales, a number of within-waterbody, habitat and water quality characteristics were important for explaining variation (61%) in the taxonomic organization of assemblages, and characteristics associated with primary productivity and habitat diversity were important for explaining variation (45%) in the functional organization of assemblages. However, much of the small-scale environmental variation across the study region appeared to be related to broad-scale variation in hydrological connectivity, which had both direct and indirect effects on macroinvertebrate assemblages.
5. Conservation of the biodiversity in Australia's wet/dry tropics may depend on conserving the natural variation in hydrological connectivity and the unregulated flow of floodplain rivers.     

Importance of rare habitats and riparian zones in a tropical forest fragment: preferential use by Tayassu pecari, a wide-ranging frugivore

In forest fragments, rare habitats contribute to heterogeneity and may provide unique resources for frugivorous species like peccaries with spatially and temporally complex patterns of range use. This study examined seasonal habitat use by two sympatric peccary species ( Tayassu pecari and Tayassu tajacu ) in an Atlantic forest fragment on the plateau region of São Paulo state, Brazil. Previous studies showed that range use by T. pecari was highly nonrandom, and that both species persisted at population densities typical of larger forest fragments. To explain this, we quantified the use and availability of habitats and riparian zones with compositional analysis. Use by T. pecari was nonrandom with respect to availability. Habitat preferences were different during dry and wet seasons and corresponded closely with seasonal movements, core range and fruit use. Although aquatic habitats made up a small proportion of the herd home range, they were most preferred by T. pecari during both seasons. Headwater palmito Euterpe edulis swamps were most favored in the dry season, whereas swamps and marshes near larger streams were preferred in the wet season. Tayassu pecari preferred riparian zones <50 m from streams over drier zones. These habitats were important sources of fruits, travel routes and corridors between forest patches in the agricultural matrix. The least-preferred habitat of T. pecari during both seasons was disturbed forest edge dominated by bamboo. Habitat and riparian zone use were herd-specific for T. tajacu and related to habitat quality and composition where stable home ranges had been established. The persistence of viable peccary populations after 75 years of fragmentation-associated pressures is related to preservation of rare habitats and overall habitat diversity. Thus, T. pecari is an indicator of high habitat diversity in forest fragments and will function as an umbrella species when targeted for conservation.     

Mites associated with soybean crop in Rio Grande do Sul State, Brazil

During the last growing seasons, high infestations of phytophagous mites were observed in the State of Rio Grande do Sul, Brazil, becoming necessary to apply pesticides for their control. The objective of this study was to identify phytophagous and predatory mite species associated with soybean in ten counties of that state, during the 2002/03 and 2003/04 growing seasons, in five soybean cultivars (A 6001 RG, A 7001 RG, A 8000 RG, A 8100 RG, Anta 82), all genetically modified. In samples of soybean leaves four phytophagous mite species (Mononychellus planki (McGregor), Polyphagotarsonemus latus (Banks), Tetranychus desertorum Banks and Tetranychus gigas Pritchard & Baker) and two predatory mite species (Phytoseiulus fragariae Denmark & Schicha and Typhlodromalus aripo De Leon) were found. T. desertorum was found for the first time associated with soybean in the country. Phytoseiulus fragariae and T. aripo are reported for the first time on soybean. The potential of phytoseid mites as biological control agents in soybean crop was discussed. Among the hypotheses to explain the increasing infestation of soybean fields with phytophagous mites area are the progressively larger cultivated area, the dry spells observed in the last few years in the growing season, changes in soybean cropping system that led to increased use of pesticides and utilization of new soybean cultivars with morphological or biochemicals characteristics that favour the development of these mite populations.     

The predatory mite Typhlodromalus aripo prefers green-mite induced plant odours from pubescent cassava varieties

It is well known that plant-inhabiting predators use herbivore-induced plant volatiles to locate herbivores being their prey. Much less known, however, is the phenomenon that genotypes of the same host plant species vary in the attractiveness of these induced chemical signals, whereas they also differ in characteristics that affect the predator’s foraging success, such as leaf pubescence. In a series of two-choice experiments (using a Y-tube olfactometer) we determined the preference of Typhlodromalus aripo for pubescent versus glabrous cassava cultivars infested with the cassava green mite Mononychellus tanajoa and also the preference for cultivars within each of the two groups. We found that when offered a choice between pubescent and glabrous cassava cultivars (either apex or leaves), T. aripo was significantly more attracted to pubescent cultivars. For each cultivar, M. tanajoa infested leaves and apices were equally attractive to T. aripo. There was however some variation in the response of T. aripo to M. tanajoa-infested plant parts within the group of pubescent cultivars, as well as within the group of glabrous cultivars. Our study confirms not only that T. aripo uses herbivore-induced plant volatiles to search for prey in cassava fields, but it also shows that it can discriminate between glabrous and pubescent cultivars and prefers the latter. This knowledge can be useful in selecting cultivars that are attractive and suitable to T. aripo, which, in turn, may promote biological control of the cassava green mite.     

Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.     

Influence of host plant vs. natural enemies on the spatial distribution of a pine sawfly, Neodiprion autumnalis

Abstract .1. The pine sawfly, Neodiprion autumnalis , infests ponderosa pine, Pinus ponderosa , growing at low densities near the bottom of an altitudinal gradient in Arizona, U.S.A. The relative importance of host-plant quality vs. natural-enemy effects in determining the spatial distribution of this sawfly was examined over a 3-year period.
2. Field and laboratory bioassays were conducted on all life stages of N. autumnalis at two forest stand densities (high ≥ 23 m² ha^–1, low ≤ 7 m² ha^–1) and at two elevations (bottom slope = 2390 m, top slope = 2540 m). These experiments were used for constructing life tables of N. autumnalis that compared the effects of host-plant quality on oviposition preference and progeny performance with the effects of natural enemies at different tree densities and elevations.
3. Life-table analyses determined that mortality attributed to host-plant effects during the egg and larval stages had the largest impact on fitness between tree densities and elevations.
4. Natural enemies caused a significant reduction in progeny survival, but their effects were similar across all tree densities and elevations during egg and larval life stages. However, cocoon-stage survival did vary between tree densities and elevations due to natural-enemy effects.
5. It was concluded that the observed oviposition preference for, and higher progeny performance on, trees at low densities and bottom slope elevations were caused primarily by host-plant effects.
6. These results further the argument that heterogeneity at the resource level (i.e. bottom-up forces) determines potential outcomes of multitrophic level interactions.     

Bottom-up and top-down effects in a tritrophic system: the population dynamics of Plutella xylostella (L.)–Cotesia plutellae (Kurdjumov) on different host plants

Abstract.  1. The effects of host-plant resistance on the population dynamics of the Diamondback moth, Plutella xylostella L., and its solitary parasitoid, Cotesia plutellae (Kurdjumov), were studied in replicated time-series experiments.
2. Host-plant resistance did not affect the equilibrial abundance of the Diamondback moth, but it affected the dynamics of Diamondback moth populations.
3. The mean population size of Diamondback moth showed no significant difference between Brassica rapa (a susceptible host plant) and Brassica napus (a partially resistant host plant) either in the presence or absence of the parasitoid.
4. Time-series analysis suggests that the dynamics of Diamondback moth on B. rapa were underpinned by delayed density-dependent processes. In contrast, the dynamics of the moth on B. napus were influenced by a direct density-dependent process.
5. Although measures of short-term parasitism showed a significantly higher rate of parasitism by C. plutellae on Diamondback moth feeding on B. napus compared with B. rapa , this individual performance does not translate into differences in the population dynamics. Analysis shows no significant difference in the persistence time of the population-level interaction between the host and parasitoid on the two different host plants.     

Infochemical-Mediated Niche Use by the Predatory Mites Typhlodromalus manihoti and T. aripo (Acari: Phytoseiidae)

In Africa, Typhlodromalus manihoti and T. aripo, two introduced predators of the cassava green mite Mononychellus tanajoa, occupy different parts of cassava foliage. In the present study, niche use by these two predators, as mediated by prey-induced infochemicals, was investigated. In response to prey feeding damage, cassava plant parts emit volatile blends, that attract phytoseiidae predators. When given a choice between old cassava leaves infested with M. tanajoa and either apices or young cassava leaves infested with M. tanajoa, T. aripo displayed a marked preference for odors emitted from either infested apices or infested young leaves over infested old leaves but showed no preference for odors from apices versus young leaves, all infested with M. tanajoa. Typhlodromalus manihoti did not discriminate between volatiles from the three infested cassava plant parts. Our data show that T. aripo uses differences in volatile blends released by infested cassava plant parts and restricts its fundamental niche to a realized niche, which enables coexistence with its competitor T. manihoti.     

Multi-trophic level interactions in a cassava-maize mixed cropping system in the humid tropics of West Africa

Multi-trophic level interactions in a mixed crop, involving cassava and maize, were studied in derived-savanna in Benin, West Africa. Two trials were planted, one during the short rainy season two months before onset of the dry season and one during the long rainy season in spring. Key pests under study on maize were the noctuid Sesamia calamistis Hampson and the pyralids Eldana saccharina Walker and Mussidia nigrivenella Ragonot, and on cassava, the exotic mealybug, Phenacoccus manihoti Matile-Ferrero and its encyrtid parasitoid Apoanagyrus lopezi De Santis. Both crops received insecticide treatments to assess the crop loss by a pest species. On maize, intercropping with cassava reduced egg and immature numbers of S. calamistis by 67 and 83%, respectively, as a result of reduced host finding by the ovipositing adult moth and of higher egg parasitism by Telenomus spp. Both trials showed similar effects on maize yields: on insecticide-treated maize, intercropping with cassava reduced maize yields by 9-16%, while on untreated maize the net effect of reduced pest density and increased plant competition resulted in zero yield differences; yield losses were lower in inter- compared to monocropped maize. For cassava, cropping system had no effect on parasitism by A. lopezi. Yield differences between mono- and intercropped cassava depended on time of harvest: they were large at the beginning and zero at final harvest. Land equivalent ratios were mostly > 1.5 indicating that a maize/cassava mixed crop, protected or unprotected, considerably increased the productivity per unit area of land.     

Interdemic variation of cannibalism in a wolf spider (Pardosa monticola) inhabiting different habitat types

Abstract.  1. Cannibalism was investigated in the wolf spider Pardosa monticola (Clerck) using spiders collected from four populations with varying densities, inhabiting two different coastal dune habitat types. Sampled individuals were paired randomly and tested immediately for their cannibalism propensity.
2. The occurrence of cannibalism was found to be influenced by the size (cephalothorax width) of both the smaller and the larger spider of a pair. Larger size differences enhanced cannibalism.
3. Cannibalism rates were not significantly different in spiders from high-density compared with low-density populations. Cannibalism rates showed, however, large variability between habitat types, with higher rates in spiders from dune grasslands than from dune slacks. This is suggested to result from differences in prey availability throughout the growing season between both habitat types.
4. Different size classes of spiders did not use different microhabitats, indicating that microhabitat segregation as a cannibalism-avoidance behaviour is absent in this species.