Randa Jabbour

Microbial community profiling to investigate transmission of bacteria between life stages of the wood-boring beetle, Anoplophora glabripennis.

Many insects harbor specific bacteria in their digestive tract, and these gut microbiota often play important roles in digestion and nutrient provisioning. While it is common for a given insect species to harbor a representative gut microbial community as a population, how this community is acquired and maintained from generation to generation is not known for most xylophagous insects, except termites. In this study, we examined acquisition of gut microbiota by the wood-feeding beetle, Anoplophora glabripennis, by identifying and comparing microbial community members among different life stages of the insect and with microbes it encounters in the environment. Automated ribosomal intergenic spacer analysis was employed to compare bacterial communities present in the egg and larval stages of A. glabripennis as well as with microbes found in the oviposition site and the surrounding woody tissue. Multivariate analyses were used to identify relationships between sample type and specific bacterial types (operational taxonomic units). From this analysis, bacteria that were derived from the environment, the oviposition site, and/or the egg were identified and compared with taxa found in larvae. Results showed that while some larval microbes were derived from environmental sources, other members of the larval microbial community appear to be vertically transmitted. These findings could lead to a better understanding of which microbial species are critical for the survival of this insect and to development of techniques that could be used to alter this community to disrupt the digestive physiology of the host insect as a biological control measure.

Labile Carbon and Other Soil Quality Indicators in Two Tillage Systems during Transition to Organic Agriculture

Weed management is one of the primary challenges for producers transitioning from conventional to organic agriculture. Tillage and the use of cover crops are two weed control tactics available to farmers transitioning to organic management, but little is known about their interactive effects on soil quality during the transition period. We investigated the response of soils to tillage and initial cover crop during the 3-year transition to organic in a cover crop–soybean ( Glycine max )–maize ( Zea mays ) rotation in the Mid-Atlantic region of the USA. The tillage treatment contrasted full, inversion tillage with moldboard plowing (FT) versus reduced tillage with chisel plowing (RT). The cover crop treatment contrasted annual versus mostly perennial species during the first year of the rotation. The experiment was initiated twice (Start 1 and Start 2), in consecutive years in adjacent fields. By the end of the experiment, labile carbon, electrical conductivity, pH and soil moisture were all greater under RT than under FT in both starts. Soil organic matter and several other soil attributes were greater under RT than under FT in Start 1, but not in Start 2, perhaps owing to differences between starts in initial field conditions and realized weather. Soil attributes did not differ between the two cover crop treatments. Combining our soils results with agronomic and economic analyses on these plots suggests that using RT during the organic transition can increase soil quality without compromising yield and profitability.

Natural enemy functional identity, trait-mediated interactions and biological control

Recent yearshave seen great interest in the importance of species richness for the functioning and stability of ecological communities (Ives and Carpenter 2007). Empirical examinations of richness effects typically vary the number of species in experimental treatments and measure resulting ecosystem functions such as biomass accumulation or resource uptake (Naeem et al. 2009). Across trophic levels and communities of many types, a clear pattern has emerged from these experiments: community processes (biomass accumulation, resource uptake, etc.) generally become more efficient when more species are present (Hooper et al. 2005; Cardinale et al. 2006). This pattern is generally attributed to resource partitioning among species, where species differ in ecologically significant ways such that they complement one another (Hooper et al. 2005). For example, in English meadow communities multiple plant species coexist, because different plant species exploit different hydrological conditions (Silvertown et al. 1999). The plants that dominate drought-prone areas are different from those that thrive in flood-prone areas and, presumably, total plant biomass is greatest when both plant groups (drought tolerant and flood tolerant) are present.

Effects of Initial Seed-Bank Density on Weed Seedling Emergence during the Transition to an Organic Feed-Grain Crop Rotation

Abstract The transition period to certified organic production can present a significant weed management challenge for growers. Organic certification requires that prohibited fertilizers and pesticides must not have been used for 36 mo before harvest of the first organic crop. Understanding how organic management practices and initial weed seed-bank densities affect weed population dynamics during the transition period may improve weed management efficacy and adoption of organic practices. We examined how tillage systems (full or reduced) and cover crop species planted during the first transition year (rye or a mixture of timothy and red clover) affect the seedling densities of three common annual weed species, common lambsquarters, velvetleaf, and foxtail spp., during the 3-yr transition period. Weed seeds were applied in a one-time pulse at the beginning of the study at three densities, low, medium, and high (60, 460, and 2,100 seeds m−2, respectively), and cumulative seedling densities of each species were assessed annually. Treatment factors had variable and species-specific effects on weed seedling densities. In general, the full-tillage system, with an initial cover crop of timothy and red clover, resulted in the lowest density of weed seedlings following seed-bank augmentation. There was little consistent association between the initial densities of applied weed seeds in the weed seed bank at the start of the transition and weed seedling densities at the end of the transition period. This suggests that when multiple crop and weed cultural management practices are employed during the organic transition period, initial failures in weed management may not necessarily lead to persistent and intractable annual weed species management problems following organic certification. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medik. ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. SETLU; red clover, Trifolium pretense L.; rye, Secale cereal L.; timothy, Phleum pratense L.

Organic Farmer Knowledge and Perceptions are Associated with On-Farm Weed Seedbank Densities in Northern New England

Abstract Weed management remains a high priority for organic farmers, whose fields generally have higher weed density and species diversity than those of their conventional counterparts. We explored whether variability in farmer knowledge and perceptions of weeds and weed management practices were predictive of variability in on-farm weed seedbanks on 23 organic farms in northern New England. We interviewed farmers and transcribed and coded interviews to quantify their emphasis on concepts regarding knowledge of ecological weed management, the perceived risks and benefits of weeds, and the perceived risks and benefits of weed management practices. To characterize on-farm weed seedbanks, we collected soil samples from five fields at each farm (115 fields total) and measured germinable weed seed density. Mean weed seed density per farm ranged from 2,775 seeds m−2 to 24,678 seeds m−2 to a soil depth of 10 cm. Farmers most often reported hairy galinsoga and crabgrass species (Digitaria spp.) as their most problematic weeds. The proportion of the sum of these two most problematic weeds in each farms seedbank ranged from 1 to 73% of total weed seed density. Farmer knowledge and perceptions were predictive of total seed density, species richness, and proportion of hairy galinsoga and crabgrass species. Low seed densities were associated with farmers who most often discussed risks of weeds, benefits of critical weed-free management practices, and learning from their own experience. These farmers also exhibited greater knowledge of managing the weed seedbank and greater understanding of the importance of a long-term strategy. Targeted education focusing on this set of knowledge and beliefs could potentially lead to improved application and success of ecological weed management in the future, thus decreasing labor costs and time necessary for farmers to manage weeds. Nomenclature: Large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; smooth crabgrass, Digitaria ischaemum (Schreb.) Schreb. ex Muhl. DIGIS; hairy galinsoga, Galinsoga quadriradiata Cav. GAQU.

Cover crop and tillage intensities alter ground-dwelling arthropod communities during the transition to organic production

We conducted a cropping systems experiment in central Pennsylvania, USA, to determine the effects of initial cover crop species and soil management on the abundance and composition of the ground-dwelling arthropod community. We hypothesized that we would detect legacy effects of the cover crops planted in year 1 of a 3-yr crop sequence on the arthropod community in the subsequent 2 yrs, and that these effects would be influenced by the intensity of tillage. We compared four systems in a factorial combination of perennial sod and legumes or annual cereal grain and legume as initial cover crops and moldboard or chisel plow tillage implemented in soybeans followed by maize in the subsequent 2 yrs. The entire experiment was initiated twice in adjacent locations, starting in 2003 (Start 1) and 2004 (Start 2). We quantified soil arthropod activity-density and community composition and identified all arthropods to order or family, and the ground and tiger beetles (Coleoptera: Carabidae) to species. In Start 1, but not Start 2, arthropod activity-density increased with each year following implementation of organic management. We observed few legacy effects of cover crop or tillage intensity on arthropod activity-density. The composition of the soil arthropod community was primarily defined by the initial cover crop in the first year, and by the interaction between cover crop and tillage intensity in the second and third year. A legacy effect associated with a yr-1 cover crop of cereal rye was observed for Scarabaeidae beetles and Formicidae (ants) in yr 2 and Carabidae beetles in yr 3 of Start 1, but not Start 2. Weed indicators contributed significantly to the variation in the soil arthropod community that was explained by the environment in yr 2 in Start 1, and in yr 3 in both Starts. Our observations support the concept that both immediate and legacy effects of management shape arthropod communities during the organic transition period, suggesting that transitioning systems could be managed in ways that conserve or enhance natural enemy populations.

A unifying gravity framework for dispersal

Most organisms disperse at some life-history stage, but different research traditions to study dispersal have evolved in botany, zoology, and epidemiology. In this paper, we synthesize concepts, principles, patterns, and processes in dispersal across organisms. We suggest a consistent conceptual framework for dispersal, which utilizes generalized gravity models. This framework will facilitate communication among research traditions, guide the development of dispersal models for theoretical and applied ecology, and enable common representation across taxonomic groups, encapsulating processes at the source and destination of movement, as well as during the intervening relocation process, while allowing each of these stages in the dispersal process to be addressed separately and in relevant detail. For different research traditions, certain parts of the dispersal process are less studied than others (e.g., seed release processes in plants and termination of dispersal in terrestrial and aquatic animals). The generalized gravity model can serve as a unifying framework for such processes, because it captures the general conceptual and formal components of any dispersal process, no matter what the relevant biological timescale involved. We illustrate the use of the framework with examples of passive (a plant), active (an animal), and vectored (a fungus) dispersal, and point out promising applications, including studies of dispersal mechanisms, total dispersal kernels, and spatial population dynamics.

Commercial Crop Yields Reveal Strengths and Weaknesses for Organic Agriculture in the United States

Land area devoted to organic agriculture has increased steadily over the last 20 years in the United States, and elsewhere around the world. A primary criticism of organic agriculture is lower yield compared to non-organic systems. Previous analyses documenting the yield deficiency in organic production have relied mostly on data generated under experimental conditions, but these studies do not necessarily reflect the full range of innovation or practical limitations that are part of commercial agriculture. The analysis we present here offers a new perspective, based on organic yield data collected from over 10,000 organic farmers representing nearly 800,000 hectares of organic farmland. We used publicly available data from the United States Department of Agriculture to estimate yield differences between organic and conventional production methods for the 2014 production year. Similar to previous work, organic crop yields in our analysis were lower than conventional crop yields for most crops. Averaged across all crops, organic yield averaged 80% of conventional yield. However, several crops had no significant difference in yields between organic and conventional production, and organic yields surpassed conventional yields for some hay crops. The organic to conventional yield ratio varied widely among crops, and in some cases, among locations within a crop. For soybean (Glycine max) and potato (Solanum tuberosum), organic yield was more similar to conventional yield in states where conventional yield was greatest. The opposite trend was observed for barley (Hordeum vulgare), wheat (Triticum aestevum), and hay crops, however, suggesting the geographical yield potential has an inconsistent effect on the organic yield gap.

Habitat and Time Are More Important Predictors of Weed Seed Predation than Space on a Diversified Vegetable Farm in Maine, USA

Abstract Postdispersal weed seed predation is a significant source of weed mortality in agroecosystems. The magnitude of seed predation, however, is variable. Understanding the relative importance of factors driving variability in seed predation rates will increase the potential utility of seed predation to farmers. We conducted landscape-scale field experiments to quantify and compare the effects of space, time of sampling, and habitat on weed seed predation. Seed predation assays, with and without vertebrate exclosures, measured seed predation rates at spatially explicit sample sites across 8.5 ha of crop and noncrop habitats on a diversified organic vegetable farm in Maine. Total and invertebrate seed predation averaged 8% and 3% d−1, respectively. Vertebrate seed predators detected by motion-sensing cameras included small mammals and birds. A ground beetle, Harpalus rufipes, was highly dominant in pitfall traps, comprising 66% of invertebrate seed predators captured within crop fields. Seed predation was randomly distributed in space. However, time of sampling and habitat were highly significant predictors of seed predation. Variance partitioning indicated that habitat factors explained more variation than did time of sampling. Total seed predation was greater in crop and riparian forest habitats than in mowed grass, meadow, or softwood forest. Generally, invertebrate seed predation was greatest at sites with an intermediate degree of vegetative cover, whereas habitat type was the chief biotic determinant of vertebrate seed predation rates. These results suggest cover cropping and wetland conservation as practices that may bolster seed predation rates. Nomenclature: Harpalus rufipes DeGeer.

Spud Web: Species Interactions and Biodiversity in Potatoes

Agroecologists often see greater biodiversity as the key to reducing pest problems on farms. Others have suggested, however, that increasing species number only increases the risk of negative interactions among species, such as predation of one predator by another, which could disrupt biological control. Such multi-species interactions have long been a topic of interest among entomologists working in potato crops, and here we review the basic ecological knowledge that has come from work in this important model cropping system. We examine the effects of increasing biodiversity on potato-insect species interactions at multiple trophic levels: among plants, herbivores, and natural enemies. Increasing plant diversity at both local and landscape scales can help build predator populations and increase suppression of potato pests. However, planting flowering plants near potato crops sometimes provides supplemental food for pests, making pest problems worse. Effects of herbivore diversity are equally complex. Feeding by early-season herbivores sometimes makes plants resistant to late-arriving herbivores, thus harming those pests most active later in the growing season. On the other hand, multiple herbivore species can “distract” predators from feeding on particular target pests, providing less-preferred herbivore species with protection from predation. Predator diversity likewise exerts varying effects. In most cases predator species appear to complement one another by foraging in different locations in the crop or by attacking different pest stages, thus improving biological control. In some cases, however, predator species feed on one another, disrupting biological control. In summary, although increasing biodiversity within potato crops sometimes worsens pest problems, it most often makes pest outbreaks less likely. Thus, work in potatoes largely supports the view of early agroecologists that increasing biodiversity restores natural balance among plants, herbivores, and natural enemies.