Economics versus alleles: balancing integrated pest management and insect resistance management for rotation-resistant western corn rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte, has overcome crop rotation in several areas of the central United States. We expanded a simple model of adult behavior and population genetics to explain how rotation resistance may have developed and to study ways to manage the western corn rootworm in a landscape of corn, soybean, and winter wheat where evolution of resistance may occur. We modeled six alternative management strategies over a 15-yr time horizon, as well as a strategy involving a 2-yr rotation of corn and soybean in 85% of the landscape, to investigate their effectiveness from both a biological and economic perspective. Generally, resistance to crop rotation evolves in fewer than 15 yr, and the rate of evolution increases as the level of rotated landscape (selection pressure) increases. When resistance is recessive, all six alternative strategies were effective at preventing evolution of rotation resistance. The two most successful strategies were the use of transgenic rotated corn in a 2-yr rotation and a 3-yr rotation of corn, soybean, and wheat with unattractive wheat (for oviposition) preceding corn. Results were most sensitive to increases in the initial allele frequency and modifications of the density-dependent survival function. Economically, three alternative strategies were robust solutions to the problem, if technology fees were not too high. Repellant soybean, attractive rotated corn, and transgenic rotated corn, all in 2-yr rotations, were economically valuable approaches. However, even the currently common 2-yr rotation was economical when resistance was recessive and the actual costs of resistance would not be paid until far in the future.     

Analysis of the dynamics of adaptation to transgenic corn and crop rotation by western corn rootworm (Coleoptera: Chrysomelidae) using a daily time-step model

Western corn rootworm, Diabrotica virgifera virgifera LeConte, has overcome crop rotation in several areas of the north central United States. The effectiveness of crop rotation for management of corn rootworm has begun to fail in many areas of the midwestern United States, thus new management strategies need to be developed to control rotation-resistant populations. Transgenic corn, Zea mays L., effective against western corn rootworm, may be the most effective new technology for control of this pest in areas with or without populations adapted to crop rotation. We expanded a simulation model of the population dynamics and genetics of the western corn rootworm for a landscape of corn; soybean, Glycine max (L.); and other crops to study the simultaneous development of resistance to both crop rotation and transgenic corn. Results indicate that planting transgenic corn to first-year cornfields is a robust strategy to prevent resistance to both crop rotation and transgenic corn in areas where rotation-resistant populations are currently a problem or may be a problem in the future. In these areas, planting transgenic corn only in continuous cornfields is not an effective strategy to prevent resistance to either trait. In areas without rotation-resistant populations, gene expression of the allele for resistance to transgenic corn, R, is the most important factor affecting the evolution of resistance. If R is recessive, resistance can be delayed longer than 15 yr. If R is dominant, resistance may be difficult to prevent. In a sensitivity analysis, results indicate that density dependence, rotational level in the landscape, and initial allele frequency are the three most important factors affecting the results.     

Recombinant and classically selected factors of potato plant resistance to the Colorado potato beetle, Leptinotarsa decemlineata, variously affect the potato aphid parasitoid Aphidius nigripes

Different forms of crop resistance developed against majorpotato pests such as the Colorado potato beetle (CPB), Leptinotarsadecemlineata, may be variously compatible with biological controlof secondary pests such as aphids. We compared the performance of theparasitoid Aphidius nigripes developing in the aphid hostMacrosiphum euphorbiae, on five potato lines, including atransgenic `Superior-BT' line expressing the CryIIIA toxin ofBacillus thuringiensis, which is specific to Coleoptera; andthe `NYL 235-4' line derived from Solanum berthaultii,characterized by relatively unspecific resistance to herbivores based onmoderately-high density of glandular trichomes. The other lines testedwere a `Kennebec-OCI' transgenic line expressing the protease inhibitorrice cystatin I (OCI), a potential resistance factor against coleopteranpests; and the commercial cultivars `Superior' and `Kennebec' used ascontrols. Parasitoid immature survival and adult size were reducedcompared to controls when the wasps developed on aphids fed the`Superior-BT' potato. In contrast, parasitoid size and fecundityincreased above control when the wasps developed on aphids fed the`Kennebec-OCI' potato. Parasitoids reached the adult stage faster andwere more fecund on `NYL 235-4' than control lines. The results indicatethat the different forms of potato resistance currently developed mainlyagainst the CPB had various unexpected effects on aphid parasitoidfitness. These effects on the parasitoid were complex but were generallyinterpretable in terms of host aphid quality variation among potatolines used as food by the aphids during parasitoiddevelopment.     

Effects of contemporary agricultural land cover on Colorado potato beetle genetic differentiation in the Columbia Basin and Central Sands

Landscape structure, which can be manipulated in agricultural landscapes through crop rotation and modification of field edge habitats, can have important effects on connectivity among local populations of insects. Though crop rotation is known to influence the abundance of Colorado potato beetle (CPB; Leptinotarsa decemlineata Say) in potato (Solanum tuberosum L.) fields each year, whether crop rotation and intervening edge habitat also affect genetic variation among populations is unknown. We investigated the role of landscape configuration and composition in shaping patterns of genetic variation in CPB populations in the Columbia Basin of Oregon and Washington, and the Central Sands of Wisconsin, USA. We compared landscape structure and its potential suitability for dispersal, tested for effects of specific land cover types on genetic differentiation among CPB populations, and examined the relationship between crop rotation distances and genetic diversity. We found higher genetic differentiation between populations separated by low potato land cover, and lower genetic diversity in populations occupying areas with greater crop rotation distances. Importantly, these relationships were only observed in the Columbia Basin, and no other land cover types influenced CPB genetic variation. The lack of signal in Wisconsin may arise as a consequence of greater effective population size and less pronounced genetic drift. Our results suggest that the degree to which host plant land cover connectivity affects CPB genetic variation depends on population size and that power to detect landscape effects on genetic differentiation might be reduced in agricultural insect pest systems.     

大肠杆菌的耐酸机制及其改造研究进展

微生物细胞在自然环境或工业应用中经常受到酸胁迫,严重制约细胞生长性能和产物合成效率。为了在各种酸性环境中生存,耐酸细菌发展出多种保护机制来维持细胞内pH稳态,如氢离子消耗、细胞膜保护、代谢修饰等。因此,深入研究耐酸机制、改进菌株耐酸能力对于利用微生物发酵合成高附加值产品具有重要意义。作为模式微生物,大肠杆菌耐酸机制的研究较为透彻,近年来其耐酸性改造也取得了重大进展。本文主要总结了大肠杆菌的氧化或葡萄糖抑制系统(acid resistance system 1, AR1)、谷氨酸依赖型耐酸系统(acid resistance system 2, AR2)、精氨酸依赖型耐酸系统(acid resistance system 3, AR3)、赖氨酸依赖型耐酸系统(acid resistance system 4, AR4)和鸟氨酸依赖型耐酸系统(acid resistance system 5, AR5)、细胞膜保护以及生物大分子修复等方面的耐酸机制,并概述了利用传统代谢工程、全局转录工程和适应性实验室进化等方法构建大肠杆菌耐酸菌株的研究进展,同时展望了大肠杆菌耐酸机制及其改造的后续研究方向...     

Resistance and Susceptibility of Plants to Fungal Pathogens

Plants are under continuous threat of infection by pathogens endowed with diverse strategies to colonize their host. Comprehensive biochemical and genetic approaches are now starting to reveal the complex signaling pathways that mediate plant disease resistance. Initiation of defense signaling often involves specific recognition of invading pathogens by the products of specialized host resistance (R) genes. Potential resistance signaling components have been identified by mutational analyses to be required for specific resistance in the model Arabidopsis and some crop species. Strikingly, many of the components share similarity to that of innate immune systems in animals. Evidence is also accumulating that plant pathogens have a number of ways to evade host defenses during the early stages of infection, similar to animal pathogens. These strategies are becoming much better understood in a number of plant–pathogen interactions. In this review, we focus on the current knowledge of host factors that control plant resistance and susceptibility to fungal pathogens. The knowledge accumulated in these studies will serve a fundamental basis for combating diseases in strategic molecular agriculture.     

Occurrence of an herbicide‐resistant plant trait in agricultural field margins

Agricultural environments allow study of evolutionary change in plants. An example of evolution within agroecological systems is the selection for resistance to the herbicide glyphosate within the weed, Conyza canadensis. Changes in survivorship and reproduction associated with the development of glyphosate resistance (GR) may impact fitness and influence the frequency of occurrence of the GR trait. We hypothesized that site characteristics and history would affect the occurrence of GR C. canadensis in field margins. We surveyed GR occurrence in field margins and asked whether there were correlations between GR occurrence and location, crop rotation, GR crop trait rotation, crop type, use of tillage, and the diversity of herbicides used. In a field experiment, we hypothesized that there would be no difference in fitness between GR and glyphosate‐susceptible (GS) plants. We asked whether there were differences in survivorship, phenology, reproduction, and herbivory between 2 GR and 2 GS populations of C. canadensis in agrestal and ruderal habitats. We found that geographic location was an important factor in the occurrence of GR C. canadensis in field margins. Although not consistently associated with either glyphosate resistance or glyphosate susceptibility, there were differences in phenology, survivorship, and herbivory among biotypes of C. canadensis. We found equal or greater fitness in GR biotypes, compared to GS biotypes, and GR plants were present in field margins. Field margins or ruderal habitats may provide refugia for GR C. canadensis, allowing reproduction and further selection to occur as seeds recolonize the agrestal habitat. Agricultural practices may select for ecological changes that feed back into the evolution of plants in ruderal habitats.     

Aggregating fields of annual crops to form larger-scale monocultures can suppress dispersal-limited herbivores

An important part of landscape ecology is determining how the arrangement (aggregation or fragmentation) of patches in space influences the population dynamics of foraging organisms. One hypothesis in agricultural ecology is that fine-grain spatial heterogeneity in cropping (many small agricultural fields) should provide better pest control than coarse-grain heterogeneity (few large agricultural fields); this hypothesis has been proposed as an explanation for the increased pest abundance associated with agricultural intensification. However, empirical studies have found mixed support for this hypothesis, and some, surprisingly, demonstrate a strong decrease in pest abundance with increased crop aggregation. We developed a spatially explicit simulation model of pest movement across an agricultural landscape to uncover basic processes that could reduce pest abundance in landscapes with fewer, larger fields. This model focuses on herbivore movement and does not include predation effects or other biological interactions. We found that field aggregation in the model led to severely reduced pest densities and further discovered that this relationship was due to an increased distance between fields and a decreased “target area” in more aggregated landscapes. The features that create a negative relationship between aggregation and pest densities rely on crop rotation and limited dispersal capabilities of the pests. These findings help to explain seemingly counter-intuitive empirical studies and provide an expectation for when field aggregation may reduce pest populations in agro-ecosystems.     

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     

Diversifying mechanisms in the on‐farm evolution of crop mixtures

While modern agriculture relies on genetic homogeneity, diversifying practices associated with seed exchange and seed recycling may allow crops to adapt to their environment. This socio‐genetic model is an original experimental evolution design referred to as on‐farm dynamic management of crop diversity. Investigating such model can help in understanding how evolutionary mechanisms shape crop diversity submitted to diverse agro‐environments. We studied a French farmer‐led initiative where a mixture of four wheat landraces called ‘Mélange de Touselles’ (MDT) was created and circulated within a farmers' network. The 15 sampled MDT subpopulations were simultaneously submitted to diverse environments (e.g. altitude, rainfall) and diverse farmers' practices (e.g. field size, sowing and harvesting date). Twenty‐one space‐time samples of 80 individuals each were genotyped using 17 microsatellite markers and characterized for their heading date in a ‘common‐garden’ experiment. Gene polymorphism was studied using four markers located in earliness genes. An original network‐based approach was developed to depict the particular and complex genetic structure of the landraces composing the mixture. Rapid differentiation among populations within the mixture was detected, larger at the phenotypic and gene levels than at the neutral genetic level, indicating potential divergent selection. We identified two interacting selection processes: variation in the mixture component frequencies, and evolution of within‐variety diversity, that shaped the standing variability available within the mixture. These results confirmed that diversifying practices and environments maintain genetic diversity and allow for crop evolution in the context of global change. Including concrete measurements of farmers' practices is critical to disentangle crop evolution processes.     

Using a generational time-step model to simulate dynamics of adaptation to transgenic corn and crop rotation by western corn rootworm (Coleoptera: Chrysomelidae)

We expanded a simulation model of the population dynamics and genetics of the western corn rootworm for a landscape of corn, soybean, and other crops to study the simultaneous development of resistance to both crop rotation and transgenic corn. Transgenic corn effective against corn rootworm was recently approved in 2003 and may be a very effective new technology for control of western corn rootworm in areas with or without the rotation-resistant variant. In simulations of areas with rotation-resistant populations, planting transgenic corn to only rotated cornfields was a robust strategy to prevent resistance to both traits. In these areas, planting transgenic corn to only continuous fields was not an effective strategy for preventing adaptation to crop rotation or transgenic corn. In areas without rotation-resistant phenotypes, gene expression of the allele for resistance to transgenic corn was the most important factor affecting the development of resistance to transgenic corn. If the allele for resistance to transgenic corn is recessive, resistance can be delayed longer than 15 yr, but if the resistant allele is dominant then resistance usually developed within 15 yr. In a sensitivity analysis, among the parameters investigated, initial allele frequency and density dependence were the two most important factors affecting the evolution of resistance. We compared the results of this simulation model with a more complicated model and results between the two were similar. This indicates that results from a simpler model with a generational time-step can compare favorably with a more complex model with a daily time-step.     

Fitness landscapes emerging from pharmacodynamic functions in the evolution of multidrug resistance

Adaptive evolution often involves beneficial mutations at more than one locus. In this case, the trajectory and rate of adaptation is determined by the underlying fitness landscape, that is, the fitness values and mutational connectivity of all genotypes under consideration. Drug resistance, especially resistance to multiple drugs simultaneously, is also often conferred by mutations at several loci so that the concept of fitness landscapes becomes important. However, fitness landscapes underlying drug resistance are not static but dependent on drug concentrations, which means they are influenced by the pharmacodynamics of the drugs administered. Here, I present a mathematical framework for fitness landscapes of multidrug resistance based on Hill functions describing how drug concentrations affect fitness. I demonstrate that these ‘pharmacodynamic fitness landscapes’ are characterized by pervasive epistasis that arises through (i) fitness costs of resistance (even when these costs are additive), (ii) nonspecificity of resistance mutations to drugs, in particular cross‐resistance, and (iii) drug interactions (both synergistic and antagonistic). In the latter case, reciprocal drug suppression may even lead to reciprocal sign epistasis, so that the doubly resistant genotype occupies a local fitness peak that may be difficult to access by evolution. Simulations exploring the evolutionary dynamics on some pharmacodynamic fitness landscapes with both constant and changing drug concentrations confirm the crucial role of epistasis in determining the rate of multidrug resistance evolution.     

Genetic research as the valid base of strategies for breeding rust resistant wheats

It is known that few wheat cultivars maintain their resistance to rust diseases for a long period of time, particularly when crop populations become genetically more uniform. A number of genetically diverse, so far unexploited, sources of rust resistance in the natural as well as mutagenized population of wheat cultivars were identified. Several of these genes were placed in agronomically superior well-adapted backgrounds so that they could be used as pre-breeding stocks for introducing genetic diversity for resistance in a crop population. Some of these stocks when employed as parents in several cross combinations in a breeding programme have generated a number of promising cultivars with diversity for resistance.Many presently grown wheats in India, near-isogenic lines each with Lr14b, Lr14ab, Lr30 and certain international cultivars were identified as possessing diverse sources of adult plant resistance (APR) to leaf rust. Prolonged leaf rust resistance in some of the Indian cultivars was attributed to the likely presence of Lr34 either alone or in combination with other APR components. Tests of allelism carried out in certain cultivars that continue to show adequate levels of field resistance confirm the presence of Lr34, which explains the role that this gene has played in imparting durability for resistance to leaf rust. Also, Lr34 in combination with other APR components increases the levels of resistance, which suggests that combination of certain APR components should be another important strategy for breeding cultivars conferring durable and adequate levels of resistance. A new adult plant leaf rust resistance source that seems to be associated with durability in Arjun has been postulated. Likewise, cultivars possessing Sr2 in combination with certain other specific genes have maintained resistance to stem rust.Further, non-specific resistances that were transferred across widely different genotypes into two of the popular Indian wheats provided easily usable materials to the national breeding programmes for imparting durable resistance to stripe rust.     

Weed Management in Rice: Wheat Rotation by Allelopathy

Rice is major crop in India and its cultivation in northwest India started 25 to 30 years ago in assured irrigation areas during the summer rainy season. In this region, rice-wheat rotation became most popular owing to its high yields; however, these crops are highly infested by the weeds, thus farmers use herbicides for their control. Hence, this rotation consumes a maximum quantity of herbicides in this region, which has resulted in several problems (environmental pollution, human health hazards, development of herbicide resistance in weeds). Thus, serious ecological questions about the reliance on herbicides for weed control in this rotation have been raised. One of the alternatives to overcome these problems is with the use of allelopathic strategies, including the use of weed-smothering crops for weed management and for the sustainability of agriculture. The field, pot culture, and laboratory studies have shown that inclusion of weed-smothering crops in rotation considerably reduced the weed population in the current and succeeding crops. Early summer (April-June) fodder crops of sorghum, pearlmillet and maize drastically smothered the weed population and biomass. The residual suppression effect of peralmillet also persisted in the next crop up to 45 days. Thus, it is conceptualized that the inclusion of such summer fodder crops before the rice crop in the rice-wheat rotation may provide satisfactory weed control in the succeeding rice crop and may minimize the use of herbicides. Likewise, the replacement of wheat by winter fodder crops of oat and berseem (Trifolium alexandrinum) may also help in the control of winter weeds. Hence, further studies in this direction may provide satisfactory weed management in rice-wheat rotation and may minimize the use of herbicides and thereby help indeveloping sustainable agricultural practices.     

Evolution of resistance to transgenic crops: interactions between insect movement and field distribution

The refuge strategy is designed to delay evolution of pest resistance to transgenic crops producing Bacillus thuringiensis Berliner (Bt) toxins. Movement of insects between Bt crops and refuges of non-Bt crops is essential for the refuge strategy because it increases chances that resistant adults mate with susceptible adults from refuges. Conclusions about optimal levels of movement for delaying resistance are not consistent among previous modeling studies. To clarify the effects of movement on resistance evolution, we analyzed simulations of a spatially explicit model based partly on the interaction of pink bollworm, Pectinophora gossypiella (Saunders), with Bt cotton. We examined resistance evolution as a function of insect movement under 12 sets of assumptions about the relative abundance of Bt cotton (50 and 75%), temporal distribution of Bt cotton and refuge fields (fixed, partial rotation, and full rotation), and spatial distribution of fields (random and uniform). The results show that interactions among the relative abundance and distribution of refuges and Bt cotton fields can alter the effects of movement on resistance evolution. The results also suggest that differences in conclusions among previous studies can be explained by differences in assumptions about the relative abundance and distribution of refuges and Bt crop fields. With fixed field locations and all Bt cotton fields adjacent to at least one refuge, resistance evolved slowest with low movement. However, low movement and fixed field locations favored rapid resistance evolution when some Bt crop fields were isolated from refuges. When refuges and Bt cotton fields were rotated to the opposite crop type each year, resistance evolved fastest with low movement. Nonrecessive inheritance of resistance caused rapid resistanceevolution regardless of movement rate. Confirming previous reports, results described here show that resistance can be delayed effectively by fixing field locations and distributing refuges uniformly to ensure that Bt crop fields are not isolated from refuges. However, rotating fields provided better insect control and reduced the need for insecticide sprays.     

Molecular markers for wheat leaf rust resistance gene Lr41

Leaf rust, caused by Puccinia triticina Eriks., is an important foliar disease of common wheat (Triticum aestivum L.) worldwide. Pyramiding several major rust-resistance genes into one adapted cultivar is one strategy for obtaining more durable resistance. Molecular markers linked to these genes are essential tools for gene pyramiding. The rust-resistance gene Lr41 from T. tauschii has been introgressed into chromosome 2D of several wheat cultivars that are currently under commercial production. To discover molecular markers closely linked to Lr41, a set of near-isogenic lines (NILs) of the hard winter wheat cultivar Century were developed through backcrossing. A population of 95 BC₃F_2:6 NILs were evaluated for leaf rust resistance at both seedling and adult plant stages and analyzed with simple sequence repeat (SSR) markers using bulked segregant analysis. Four markers closely linked to Lr41 were identified on chromosome 2DS; the closest marker, Xbarc124, was about 1 cM from Lr41. Physical mapping using Chinese Spring nullitetrasomic and ditelosomic genetic stocks confirmed that markers linked to Lr41 were on chromosome arm 2DS. Marker analysis in a diverse set of wheat germplasm indicated that primers BARC124, GWM210, and GDM35 amplified polymorphic bands between most resistant and susceptible accessions and can be used for marker-assisted selection in breeding programs.     

Bombus terrestris in a mass‐flowering pollinator‐dependent crop: A mutualistic relationship?

Bumblebees (Bombus spp.) rely on an abundant and diverse selection of floral resources to meet their nutritional requirements. In farmed landscapes, mass‐flowering crops can provide an important forage resource for bumblebees, with increased visitation from bumblebees into mass‐flowering crops having an additional benefit to growers who require pollination services. This study explores the mutualistic relationship between Bombus terrestris L. (buff‐tailed bumblebee), a common species in European farmland, and the mass‐flowering crop courgette (Cucurbita pepo L.) to see how effective B. terrestris is at pollinating courgette and in return how courgette may affect B. terrestris colony dynamics. By combining empirical data on nectar and pollen availability with model simulations using the novel bumblebee model Bumble‐BEEHAVE, we were able to quantify and simulate for the first time, the importance of courgette as a mass‐flowering forage resource for bumblebees. Courgette provides vast quantities of nectar to ensure a high visitation rate, which combined with abundant pollen grains, enables B. terrestris to have a high pollination potential. While B. terrestris showed a strong fidelity to courgette flowers for nectar, courgette pollen was not found in any pollen loads from returning foragers. Nonetheless, model simulations showed that early season courgette (nectar) increased the number of hibernating queens, colonies, and adult workers in the modeled landscapes. Synthesis and applications. Courgette has the potential to improve bumblebee population dynamics; however, the lack of evidence of the bees collecting courgette pollen in this study suggests that bees can only benefit from this transient nectar source if alternative floral resources, particularly pollen, are also available to fulfill bees’ nutritional requirements in space and time. Therefore, providing additional forage resources could simultaneously improve pollination services and bumblebee populations.     

Effect of a turnip rape <Emphasis Type="Italic">(Brassica rapa)</Emphasis> trap crop on stem-mining pests and their parasitoids in winter oilseed rape <Emphasis Type="Italic">(Brassica napus)</Emphasis>

Concerns about the negative effects of chemical control of oilseed rape (Brassica napus L.) pests on non-target species, human safety, and development of insecticide resistance, require alternative control strategies such as the use of trap crops and biocontrol to be developed. Psylliodes chrysocephala(L.) (Coleoptera: Chrysomelidae) (cabbage stem flea beetle) and Ceutorhynchus pallidactylus (Marsh.) (Coleoptera: Curculionidae) (cabbage stem weevil) are two major stem-mining pests of oilseed rape. This study investigated the phenology of these pests and their main parasitoids in the UK, the potential use of turnip rape (Brassica rapa L.) as a trap crop to reduce oilseed rape infestation, and the effects of insecticide treatment on pest incidence and larval parasitism. Water trap samples, plant dissections and pest larval dissections were done to determine: the incidence of adult pests and their parasitoids, the level of plant infestation by the pests and percentage larval parasitism, respectively. The turnip rape trap crop borders reduced P. chrysocephalabut not C. pallidactylus infestation of oilseed rape plots. Treatment of the trap crop with insecticide had little effect on either pest or parasitoid incidence in the oilseed rape. TersilochusmicrogasterSzép. andT. obscurator Aub. (Hymenoptera: Ichneumonidae) were the main larval parasitoids of P. chrysocephalaand C. pallidactylus, respectively. Tersilochus microgasteris reported for the first time in the UK. The implications for integrated pest management are discussed.     

Effects of forest canopy on habitat selection in treefrogs and aquatic insects: implications for communities and metacommunities

The specific dispersal/colonization strategies used by species to locate and colonize habitat patches can strongly influence both community and metacommunity structure. Habitat selection theory predicts nonrandom dispersal to and colonization of habitat patches based on their quality. We tested whether habitat selection was capable of generating patterns of diversity and abundance across a transition of canopy coverage (open and closed canopy) and nutrient addition by investigating oviposition site choice in two treefrog species (Hyla) and an aquatic beetle (Tropisternus lateralis), and the colonization dynamics of a diverse assemblage of aquatic insects (primarily beetles). Canopy cover produced dramatic patterns of presence/absence, abundance, and species richness, as open canopy ponds received 99.5% of propagules and 94.6% of adult insect colonists. Nutrient addition affected only Tropisternus oviposition, as females oviposited more egg cases at higher nutrient levels, but only in open canopy ponds. The behavioral partitioning of aquatic landscapes into suitable and unsuitable habitats via habitat selection behavior fundamentally alters how communities within larger ecological landscapes (metacommunities) are linked by dispersal and colonization.     

Contribution of soil-borne bacteria to the rotation effect in corn

Few efforts have been directed at understanding how the rhizosphere microbiology of continuous corn may effect crop yields. This relationship may explain, in part, the decreases in yield associated with continuous corn as compared to the corn in rotation with a second crop. This study was conducted to determine the importance of soil-borne microorganisms to yield declines in long term continuous corn. Continuous corn (Zea mays L.) or rotated corn-soybean (Glycine max L.) field plots, established in 1975, under either fall plowing or no-till tillage treatments were used. Treatments consisted of methyl bromide applied at 48.8 g m⁻² 3 days prior to planting in all four combinations. Total plant samples from both the fumigated and non-fumigated areas were collected 14 days after planting. Rhizosphere bacteria were recovered and tested for their ability to impact plant growth. Bacterial assessments were made in a test tube bioassay where germinated corn was transported in to agar containing a bacterial isolate. In the first year of the study a highly significant interaction of fumigation and rotation was indicated. With fumigation continuous corn yields were similar to that of rotated corn-bean. Rotated corn yields were less affected by fumigation. In the second year, the effects were similar but less significant. Over 130 bacterial isolates were tested for their effect on plant growth. Approximately 22% were able to inhibit plant growth. Of these, 72% were from the continuous corn system. Clearly, the interaction of rotation and yield is at a microbiological level. The suggestion that microorganisms similar to those isolated are responsible for controlling early plant growth in the continuous corn system is indicated.