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.     

Contribution to the taxonomy of Pinus uncinata (Pinaceae) based on cone characters

Eight natural populations of Pinus uncinata Ramond from the Spanish Pyrenees, five of P. mugo Turra from the Carpathians, the Sudethians and Abruzzian Apennines, three of P. sylvestris L. from the Central-European Lowland and the Iberian Peninsula and one of P. uliginosa Neumann from SW Poland have been compared biometrically in 16 cone characters. P. uncinata appears the most distant from all other taxa, differing by having the largest and most asymmetric cones and the thickest apophyses with the most prominent umbo. In cone characters, P. mugo is more similar to P. sylvestris than to P. uncinata. Nevertheless, P. mugo and P. sylvestris remain distant from each other in a number of morphological characters examined. P. uliginosa has cones intermediate between the other three taxa and is moreover very variable. The locality of P. uliginosa in the Silesian Lowland may be of a relict character and conserve genes from P. uncinata.     

Influence of host species and age on host preference of Trissolcus semistriatus

Trissolcus semistriatus(Hymenoptera: Scelionidae) has a high potentialfor being an effective biological control agentfor the sunn pest, Eurygaster integriceps.This study was conducted to determine thepreferences of T. semistriatus in termsof host species and age of variousheteropteran host eggs (E. integriceps,Dolycoris baccarum, Graphosoma lineatum,Eurydema ornatum, Holcostethus vernalis). Theresults indicated that a small portion ofparasitism of E. ornatum eggs wasobtained (28.5%), although high parasitismrates were found in other host eggs(90.0–80.0%). A high percentage of adultemergence was also recorded. When tested for its ability toparasitize host eggs of different ages,T. semistriatus was found to preferyounger hosts: it showed parasitism ratesgreater than 50% with up to 3 daysold E. integriceps and D.baccarum, and with up to 4 days old G. lineatum and H. vernalis. However, theparasitism rates in all the ages of E. ornatum eggs were much lower than for other hostspecies. The developmental times in all hostspecies at different ages was extended withincreased host age. It was concluded that G. lineatum and D. baccarum could beused for mass production of egg parasitoids.     

Incidence of natural enemies of the Colorado potato beetle,Leptinotarsa decemlineata [Coleoptera: Chrysomelidae] on a native host in Mexico

In central Mexico, the center of origin forLeptinotarsa decemlineata (Say) and its principal host plants,L. decemlineata is associated with a diverse complex of natural enemies. Observations during 2 years in Morelos, Mexico revealed 4 species of asopine pentatomids, 7 species of foliar searching carabids, and 2 coccinellids among the predators ofL. decemlineata. Parasitoids include 3 species of tachinids andEdovum puttleri Grissel, an eulophid egg parasitoid. These natural enemies include several predators that have not previously been reported attackingL. decemlineata. University of Maine Agriculture Experiment Station No. 1553     

Measuring host finding capacity and arrestment of natural enemies of the cassava mealybug,Phenacoccus manihoti, in the field

In unifested fields, 80 cassava tips were artificially infested with 0, 1, 2, 4, 8, 16, 32, or 64 third instars, and 20 or 100 eggs of cassava mealybug,Phenacoccus manihoti Matile-Ferrero (Hom., Pseudococcidae). Another 80 uninfested tips served as a control. Tips were arranged in a circle of 28 m diameter, in the centre of which the following exotic natural enemies ofP. manihoti were released:Apoanagyrus (Epidinocarsis) lopezi De Santis andA. diversicornis (Howard) (Hym., Encyrtidae),Hyperaspis notata (Mulsant) andDiomus hennesseyi Fürsch (Col., Coccinellidae), and others. This experiment was repeated six times. During the 4–14 days following release, all experimental tips were inspected at two-hour intervals during each day and the presence of exotic as well as indigenous natural enemies, likeExochomus troberti Mulsant (Col., Coccinellidae), ants and spiders was noted. The experiment was repeated six times measured the aggregative response by the natural enemies to different host densities, achieved through host attractance and arrestment. All exotic natural enemies, except the males ofApoanagyrus spp., were fast attracted to the host colonies. As compared to the control tips, they concentrated on the infested tips about 50-fold for the twoApoanagyrus spp. and 10 to 20-fold for the exotic coccinellids. By contrast, non-coevolved indigenous coccinellids, as well as generalist predators like ants and spiders were attracted to the infested tips only 2 to 5-fold.A. lopezi responded best to different host densities, followed byA. diversicornis and the coccinellids, followed by ants and spiders. None of the parasitoids or predators was particularly attracted to egg masses. These results correspond closely to the known efficiencies of these natural enemies,A. lopezi standing out among all candidates. The results of such aggregation studies are compared with those of life-table studies.     

Parasitoids and other natural enemies of Maruca vitrata feeding on Sesbania cannabina in Taiwan

Natural enemies of Marucavitrata (Geyer) feeding concealed inside leafwhorls of a green manure legume, Sesbaniacannabina, were monitored during the 1996 and1997 summer seasons in Taiwan. A braconid,Apanteles taragamae, accounted for anaverage of 92% of all parasitoid specimensreared from M. vitrata larvae and pupae.Other parasitoid species included two otherbraconids: Bassus asper and Dolichogenidea sp., three ichneumonids: Trichomma sp., Triclistus sp., and Plectochorus sp., and two unidentifiedtachinids. All parasitoid species attacked M. vitrata larvae and emerged as mature larvaefrom host larvae (braconids) or as adults fromhost pupae (ichneumonids and tachinids). Oneunidentified predatory staphylinid and threeentomopathogenic fungi: Fusarium sp.Paecilomyces sp. and Beauveriabassiana were also observed attacking M.vitrata larvae. Apanteles taragamae is a solitary endoparasitoid. Its parasitismreached as high as 63% of M. vitratalarvae found attacking S. cannabina. Theparasitism was higher during June to August andreduced during September to November when otherparasitoid species were more active. Ninespecies of hyperparasitoids, all belonging toHymenoptera, were recovered, mainly from thebraconids.     

The effects of natural enemies,competition, and host plant water availability on an aphid population

Summary I used a factorial experiment repeated in two years to assess the relative effects of natural enemy attack, interspecific competition, and water availability to the host plant, and of interactions among these factors, on the population dynamics of the aphid Aphis varians feeding on fireweed (Epilobium angustifolium). The impact of a suite of coccinellid and syrphid predators emerged as the predominant factor affecting the success of aphid colonies: colonies protected from natural enemies grew in size at a rate of ten percent per day, were only one tenth as likely to go extinct, and produced over ten times more dispersing alates. In contrast, I found only minor effects of removing flea beetles, the most abundant herbivore with which A. varians colonies cohabit fireweed stems, and of supplementing water availability to fireweed host plants, in spite of a significant effect of watering frequency on aphid growth in the green-house. There was no evidence of significant two- or three-way interactions among factors. Hence, despite the potential complexity of the food web in which it is embedded, the dynamics of A. varians appears to be driven predominantly by a single factor, i.e. interactions with natural enemies.     

Direct effects of soybean varietal selection and Aphis glycines-resistant soybeans on natural enemies

The direct effects of three soybean parentages, each represented by an Aphis glycines-resistant and susceptible isoline, on the fitness and performance of two key predators (Orius insidiosus and Harmonia axyridis) were evaluated in the laboratory. Predators were reared from hatch through adulthood in Petri dishes with cut trifoliolate leaves of the designated soybean variety, using eggs of Ephestia kuehniella as surrogate prey to eliminate prey-mediated effects of the host plant. Preimaginal survival and development, sex ratio, adult longevity, fecundity, and size were compared among treatments and a no-plant control. An additional experiment compared life-history parameters of predators caged with soybean versus Ipomoea hederacea (ivyleaf morning glory). Aphid resistance reduced the adult longevity of H. axyridis, but O. insidiosus was unaffected by resistance traits. However, adult O. insidiosus lived longer on soybeans with Group C base genetics than the other soybean varieties. Other parameters were not affected by soybean base genetics or resistance, but both predators generally performed worse on soybean than on I. hederacea or no-plant controls. The results suggest that soybean varietal selection, particularly with respect to A. glycines-resistance, may directly affect biological control agents. Also, implications of the generally poor suitability of soybean for natural enemies are discussed within the context of current crop production practices. Handling editor: Michael Smith     

The influence of host plant species on parasitism of pollen beetles (Meligethes spp.) by Phradis morionellus

We investigated the response of the specialist insect predator Oligota kashmirica benefica (Coleoptera: Staphylinidae) to volatiles from lima bean leaves infested with the spider mite Tetranychus urticae (Acari: Tetranychidae), both in a Y-tube olfactometer and in a field in Kyoto, Japan. Adult male and female predators were significantly more attracted to T. urticae-infested leaves than to clean air. Adult male and female predators were not more attracted to uninfested leaves, artificially damaged leaves, or the spider mites and their visible products when compared to clean air. In a field trap experiment, 12 adult predators were caught in three traps containing T. urticae-infested lima bean plants over 13 days, whereas no adult predators were trapped in three traps containing uninfested lima bean plants during the same period. These results showed that O. kashmirica benefica adults responded to herbivore-induced plant volatiles from T. urticae-infested lima bean leaves under both laboratory and field conditions.     

Effects of manipulation of litter and humus layers on ectomycorrhizal colonization potential in Scots pine stands of different age

Effects of manipulation of litter and humus layers (removal, doubling and control treatments) on the colonization potential of ectomycorrhizal fungi were studied in two secondary stands of Pinus sylvestris (5 and 18 years old) in The Netherlands. Five-mont-hold, sterile-grown Scots pine seedlings, inoculated with Laccaria bicolor, Paxillus involutus or Rhizopogon luteolus and noninoculated seedlings were used as baits. The seedlings were harvested after one growing season. For comparison, sporocarps of ectomycorrhizal fungi were also investigated. Genus composition on the seedlings was independent of initial inoculum, but determined by both treatment and age of the stands. In both stands, removal of litter and humus layers increased, and addition of organic material decreased the number of ectomycorrhizal types on the seedlings. Not all indigenous genera were observed by either outplanting seedlings or sporocarp surveys.     

Influence of an endophytic fungus on host plant selection by a polyphagous moth via volatile spectrum changes

Recently the role of micro-organisms as mediators of plant-herbivore interactions has been increasingly acknowledged in ecological research. We investigated the interaction between an unspecialized root fungal endophyte (Acremonium strictum) and the polyphagous moth Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), in greenhouse and laboratory bioassays. Specifically we examined in tomato (Lycopersicon esculentum Mill.) the systemic effects of the endophytic fungus on the host selection behaviour of female moths for oviposition and the volatile profiles of host plants to understand the mechanisms acting in this multi-trophic model system. Both laboratory and field strains of H. armigera moths oviposited more on leaves of A. strictum inoculated plants as compared to endophyte-free plants, both in free flight cages and in tethered moth laboratory experiments; the moth’s preferences were significant between 10 and 18 days after inoculation. The analysis of volatile profiles showed strong quantitative differences between treatments. Endophyte inoculated plants emitted diverse terpenes and sesquiterpenes at significantly lower amounts as compared to endophyte free-plants, except for α-terpinene, which did not differ between the treatments, and trans-β-caryophyllene, which was emitted in significantly higher amounts on inoculated plants. β-Thujene and α-phellandrene accounted for 73.3 and 12.0% of total amounts of volatiles emitted from endophyte-free and inoculated plants, respectively. Our findings demonstrate that A. strictum is able to systemically influence the host selection of H. armigera moths for oviposition; conceivably mediated by the induced changes in volatile emissions (and probably additional biochemical parameters of the host plants, which have not been analysed so far). We argue for a more detailed assessment of micro-organisms invisibly colonizing plants, when studying plant-herbivore or multitrophic interactions.     

Effects of host age on parasitism and progeny allocation in Trichogrammatidae

The impact of host age on the number of hosts killed, survival of progeny, progeny allocation, and sex allocation was examined for several Trichogrammatidae (Hymenoptera) species in laboratory choice tests. Individual female parasitoids were provided with young, medium-aged and old eggs of one of three lepidopterous host species: Trichoplusia ni (Hübner) (Noctuidae), Pieris rapae (L.) (Pieridae), or Plutella xylostella (L.) (Plutellidae). Trichogrammatid species behaved as gregarious parasitoids with the first two host species, and as solitary parasitoids with eggs of the smaller latter one. They mostly preferred young eggs of T. ni, but did not discriminate among P. rapae eggs of different ages, and often preferred young or medium-aged P. xylostella eggs over old eggs. Survival of progeny did not vary constantly with host age, although it was often very low in P. rapae eggs of any age. Clutch size frequently decreased with host age in both T. ni and P. rapae. Offspring sex ratio did not change with age of T. ni and P. rapae eggs, and rarely did so in P. xylostella eggs. In regard to host age, the results with T. ni are the ones which are the most in agreement with optimal foraging theoretical predictions, as clutch size was the highest in preferred younger eggs.     

Influence of initial egg density and host size on the development of the gregarious parasitoid <Emphasis Type="Italic">Bracon hebetor</Emphasis> on three different host species

Bracon hebetor (Say) (Hymenoptera : Braconidae) is a gregarious parasitoid that attacks a variety of important lepidopterous pests of stored product and in the field. In this study the effect of host species, size and larval competition on parasitoid size, survival and development were investigated. In laboratory studies, wasp eggs at a range of densities, were placed on larvae of different weight of three Lepidoptera host species namely Adoxophyes orana (Fischer von Röslerstamm, Tortricidae), Plodia interpunctella (Hubner, Pyralidae) and, Lobesia botrana(Dennis & Schiffermueller, Tortricidae). On A. orana survival of immature parasitoids was very low at all densities and different host weights. On L. botrana survival progressively reduced as egg density increased at both host weights examined for this host. Survival on P. interpunctella was significantly affected by egg density but not by host weight. Initial egg density had a significant effect on the size of emerging adults from each rearing host. Smaller adult parasitoids emerged as egg density per larva increased. Larval host weight of P. interpunctella and A. orana had a significant effect on the size of emerging adult parasitoids mainly at the higher egg densities used in these experiments. The above results of host quality on fitness of parasitoid are discussed.     

Field evaluation of the combined deterrent and attractive effects of dimethyl disulfide on Delia radicum and its natural enemies

Delia radicum (L. 1758) is a major pest of cabbage crops in northern Europe. Due to more constraining laws relating to insecticide use, new strategies to control this pest are urgently needed. Manipulating insect behavior through infochemicals is a promising approach. The recent identification of dimethyl disulfide (DMDS) as a compound that both attracts the main predators of D. radicum and inhibits oviposition by the fly gives a challenging opportunity to develop such strategy. The aim of the present study was to confirm such potential of DMDS, in the field. Through the 8 weeks of the first egg laying peak of the fly we assessed, the potential of artificially increasing the levels of this molecule in the close vicinity of broccoli plants to 1/attract predators, 2/stimulate predatory activity and 3/limit damage done by the fly. Despite a lower number of D. radicum eggs as food resource, DMDS effectively increased predator catches in treated plots (119 Aleochara bilineata (Gyllenhal, 1810) caught in treated plot, while only 21 in control plots). However, damages done by the fly were of the same magnitude order in treated plots than in control ones. Number of D. radicum larvae and pupae recovered in plant roots were similar, despite the important decrease in eggs laid. This result, together with the observation that the numbers of eggs predated in artificial patches were lowered in the presence of the molecule, seems to indicate that increasing DMDS amounts disturbed the foraging activity of the fly predators. Consequences of these findings for the future of DMDS use in crop protection against D. radicum are discussed.     

马尾松不同部位挥发物对松墨天牛触角电位和行为反应的影响

【目的】探究马尾松健康植株不同位置挥发性物质,比较其不同部位的挥发物的种类差异及含量,分析松墨天牛成虫对健康马尾松各部位挥发物的触角电位反应和嗅觉反应,为开展松墨天牛的行为调控技术提供依据。【方法】采用静态顶空法和气相色谱-质谱联用技术分析了马尾松松针、1 cm枝条、5 cm枝条、20 cm树干、30 cm树干、松油脂6个部位的挥发物,并对松墨天牛雌雄成虫进行触角电位测定和行为反应观测。【结果】6个部位的挥发物提取物中,共检测出36种挥发性物质,各部位挥发物种类相近,相对含量差异较大,但其主要挥发物均为萜烯类物质,主要包括月桂烯、蒎烯、左旋-beta-蒎烯、莰烯等。在触角电位试验中,5 cm枝条挥发物提取物的触角电位值最高,松油脂挥发物提取物的触角电位值最高;在行为反应试验中,各部位挥发物对松墨天牛成虫均有一定的引诱作用,其中雌成虫对5 cm枝条挥发物提取物的趋向性最高,雄成虫对30 cm树干和松油脂挥发物提取物的趋向性最高。【结论】健康状态下,马尾松5 cm枝条及30 cm树干和松油脂中萜烯类挥发物制成的植物源引诱剂对松墨天牛两性成虫具有更好的引诱作用,这将为提高现有的诱捕剂效果提供参考,同时为松墨天牛虫害防治提供科学依据。     

Variation in selection pressures on the goldenrod gall fly and the competitive interactions of its natural enemies

Summary Larvae of the tephritid fly Eurosta solidaginis induce ball-shaped galls on the stem of tall goldenrod, Solidago altissima. Survival probability depends on gall size; in small galls the larva is vulnerable to parasitoid oviposition, whereas larvae in large galls are more frequently eaten by avian predators. Fly populations from 20 natural old fields in central Pennsylvania were monitored in 1983 and 1984 to examine the distribution of the selection intensity imposed by natural enemies, the parasitoids Eurytoma gigantea and E. obtusiventris, the inquiline Mordellistena unicolor, and the predatory birds Dendrocopus pubescens and Parus atricapillus. Mordellistena and E. obtusiventris are able to attack galls of all diameters while E. gigantea and the predatory birds preferentially assaulted small and large diameter galls, respectively. Eurosta in intermediate sized galls had the highest survivorship, hence selection had a stabilizing component. However, parasitoid attack was more frequent than bird attack, and the two did not exactly balance, thus there was also a directional component. The mean directional selection intensity on gall size was 0.21 standard deviations of the mean, indicating that larger gall size was favored. Interactions among the insect members of the Eurosta natural enemy guild are complex and frequent.     

Avoidance of natural enemies by adult whiteflies, Bemisia argentifolii, and effects on host plant choice

In this study we asked whether, in the context of a trap crop system, differential predation risks among plants influence host choice patterns of adult whiteflies, Bemisia argentifolii. We investigated whether adult whiteflies avoid natural enemies inhabiting poinsettia (a cash crop) and whether this behavior can be used to increase the movement of whiteflies to cucumber (a trap crop). The potential of cucumber as a trap crop was first evaluated and we found that significantly more whiteflies were attracted to cucumber when the whiteflies were released between the two plants. However, the accumulation of whiteflies on cucumber substantially diminished if the insects had first settled on poinsettia. Under such circumstances, we investigated whether movement of adult whiteflies to cucumber could be increased by creating conditions that would cause the whiteflies that had settled on poinsettia to leave the plant. A mechanical disturbance, consisting of shaking the plant, was first used to test this hypothesis. The shaking caused more whiteflies to leave poinsettia and move onto the trap crop, compared to undisturbed whiteflies. We then asked whether the presence of natural enemies on the cash crop could induce whiteflies to leave the cash crop and move onto the trap crop. Three natural enemies were tested: two predators, Amblyseius swirskii and Delphastus catalinae, and a parasitoid Encarsia formosa. The presence of D. catalinae on poinsettia induced significantly more whiteflies to disperse to cucumber compared to poinsettia with no natural enemies, whereas A. swirskii and E. formosa did not result in a significant increase. Predator avoidance behavior by adult whiteflies should be investigated further in the context of trap cropping and other crop-habitat alterations designed to help manage whitefly abundance.     

Effect of host age at parasitization by Aphidius sonchi on the development, survival, and reproduction of the sowthistle aphid, Hyperomyzus lactucae

Nineteen cohorts of Hyperomyzus lactucae (L.) (Homoptera: Aphididae) reared at various temperaturelight regimes were exposed to attack by Aphidius sonchi Marshall (Hymenoptera: Aphidiidae) at different ages of their development. Irrespective of host age at the initial parasitization and rearing conditions, the rates of reproduction, development and survival were unaffected until the parasites within had completed approximately 30, 40 and 55% of their total development respectively. As a result, aphids parasitized in the first or early second instar failed to reach maturity. Thereafter, the numbers of young produced by parasitized aphids increased rapidly as the age of the hosts at initial parasitization increased, and aphids parasitized 3 days after the final moult or later showed similar potential to contribute to future population increase.In comparison with the effects on host moulting, those on wing development of the potentially alatiform nymphs appeared much sooner after the beginning of parasitization, but again with the time lags being more or less independent of host age at the initial parasite attack. Consequently, the extent of wing development depended entirely on the age of the hosts when attacked.These results showed that the age of the host at the initial parasite attack is crucial in determining the consequences of parasitization on the host.

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Résumé Dix-neuf cohortes d'Hyperomyzus lactucae L. (Homoptera: Aphididae), élevées dans différentes conditions de lumière et de température, ont été exposées à différents moments de leur développement aux attaques d'Aphidius sonchi Marsh. (Hym. Aphidiidae). Quels que soient les conditions d'élevage et l'âge des pucerons quand ils ont été parasités, la survie, le développement et la reproduction n'ont été affectés que lorsque les parasites avaient accompli 30, 40 et 55% de leur développement total. Ainsi, les pucerons parasités au premier stade ou au début du second ne parvenaient pas à atteindre la maturité. Par la suite, le nombre de jeunes produits par les pucerons parasités a cru rapidement au fur et à mesure que l'âge de ces derniers au moment où ils avaient été parasités la dernière mue ou plus tard présentaient la même potentialité pour contribuer à l'accroissement ultérieur de la population.Comparés aux effets sur la mue, les effets sur le développement alaire des larves potentiellement ailées apparaissent beaucoup plus tôt après le moment où le puceron a été parasité, mais là aussi avec des délais plus ou moins indépendants de l'âge de l'hôte lors de la première attaque du parasite. Par conséquent, le degré de développement des ailes dépendait totalement de l'âge du puceron lorsqu'ila été parasité.Ces résultats ont montré que l'âge du puceron lors de la première attaque détermine les conséquences du parasitisme de l'hôte.

     

Species diversity, abundance, and phenology of aphid natural enemies on spring wheats resistant and susceptible to Russian wheat aphid

The species composition, relativeabundance, and seasonal dynamics of selectednatural enemies of cereal aphids were monitoredin spring wheat fields in Moscow, Idaho in 1997and 1998. Trials also examined the potentialimpact of resistance to Russian wheat aphid(RWA), Diuraphis noxia (Mordvilko)(Homoptera: Aphididae) in wheat, on aphidbiological control agents. Natural enemypopulations were monitored on two springwheats: D. noxia susceptible variety`Centennial' and resistant genotype `IDO488'. Field plots were artificially infested withadult D. noxia, and sampled for cerealaphids and parasitoids weekly. Coccinellidpredators were monitored once in 1997 and twicein 1998. The coccinellids Hippodamiaconvergens Guerin, Coccinellaseptempunctata L., C. transversoguttataBrown and C. trifasciata Mulsant weredetected. No significant differences in adultor immature coccinellid densities were observedbetween the D. noxia resistant andsusceptible genotypes. During both years, themost abundant primary hymenopteran parasitoidswere Diaeretiella rapae (M'Intosh), Aphidius ervi Haliday, A. avenaphis(Fitch), and Lysiphlebus testaceipes(Cresson), Aphelinus varipes (Foerster),Aphidius colemani Viereck, Aphidiuspicipes (Nees), Aphidius sp., Monoctonus washingtonensis Pike & Stary, Praon gallicum Stary, Praon occidentaleBaker, and Praon sp. were also detected. Numbers of both D. noxia and D.rapae were significantly greater on Centennialthan on IDO488 in both years. When all speciesof cereal aphids and parasitoids areconsidered, the total percentage parasitism wasnot significantly different between thegenotypes. There was no interaction betweenD. noxia resistance and the populationdensity of the predators or parasitoidsmonitored. These results suggest that the D. noxia resistant line had no adverse impacton natural enemies under the conditions ofthese field experiments.     

Influence of light and natural microbiota of the Butrón river on E. coli survival

The survival of an E. coli strain in water samples from the Butrón river has been studied. The input of E. coli cells in the aquatic system breaks down the established balance among the components of the natural microbiota: E. coli becomes the object of the active protozoal predation whereas the autochtonous heterotrophic community become alternative preys. As a result of this new situation, the natural microbiota increases but returns to the initial values once the E. coli cells have been removed from the system. The effect of the temperature of incubation on the survival is exerted through the effect of this parameter on the predatory activity of the protozoa. Light has a lethal and direct action on the E. coli cells, the effect of this parameter is even superior to that of predation.