Deborah K. Letourneau

Vegetation management and biological control in agroecosystems

Abstract The application of diversity/stability considerations in theoretical ecology has led to the emergence of testable hypotheses with implications for biological control and agroecosystem management. The object of applied research in this area has been to achiece predictably stable pest populations (below an economic threshold level) in crop systems. The exacerbation of most insect-pest problems has been associated with increases in crop monocultures at the expense of the natural vegetation, thereby decreasing local habitat diversity. This can seriously affect the abundance and efficiency of natural enemies, which depend on habitat complexity for sources of alternate prey/hosts, pollen and nectar, shelter, nesting and overwintering sites. Plant diversification of agroecosystems can result in increased environmental opportunities for natural enemies and, consequently, improved biological pest control. Agronomically, there are several ways to design plsnt-diverse cropping systems. One way is by manipulating the vegetation of field margins, and managing the species composition and density of plants in ditchbanks, hedgerows, windbreaks and other types of shelter belts. Within-field plant diversity can be manipulated by designing polycultures of various temporal and spatial crop arrangements. The effects of some of these systems on the dynamics of insect populations are discussed, as well as the effects of cover-crop management on pest insects in orchards. Another way to enrich the vegetational structure of cropping systems is through weed management. Weed diversity in the form of weed borders, alternate rows, or by providing weeds in certain periods of the crop growth can have a major impact on insect dynamics.

Fundamental Differences Between Conventional and Organic Tomato Agroecosystems in California

In an integrated, multidisciplinary study we compared ecological characteristics and productivity of commercial farms categorized as either organic (ORG) or conventional (CNV) based on their use of synthetic fertilizers and pesticides or reliance on organic soil amendments and biological pest control. We measured belowground parameters: various soil chemical and biological properties and root disease severity; common agronomic indicators: biomass, fruit yield and insect pest damage; and community level indicators, including arthropod diversity and soil microbial activity and diversity. CNV and ORG production systems could not be distinguished based on agronomic criteria such as fruit yield and arthropod pest damage levels. However, differences were demonstrated in many soil, plant, disease, and diversity indicators suggesting that the ecological processes determining yields and pest levels in these two management systems are distinct. In particular, nitrogen mineralization potential and microbial and parasitoid abundance and diversity were higher in ORG farms. Differences between the agroecosystems were sufficiently robust to be distinguished from environmental variation and suggest that biological processes compensated for reductions in the use of synthetic fertilizers and pesticides.

Does plant diversity benefit agroecosystems? A synthetic review

Predictive theory on how plant diversity promotes herbivore suppression through movement patterns, host associations, and predation promises a potential alternative to pesticide-intensive monoculture crop production. We used meta-analysis on 552 experiments in 45 articles published over the last 10 years to test if plant diversification schemes reduce herbivores and/or increase the natural enemies of herbivores as predicted by associational resistance hypotheses, the enemies hypothesis, and attraction and repellency model applications in agriculture. We found extensive support for these models with intercropping schemes, inclusion of flowering plants, and use of plants that repel herbivores or attract them away from the crop. Overall, herbivore suppression, enemy enhancement, and crop damage suppression effects were significantly stronger on diversified crops than on crops with none or fewer associated plant species. However, a relatively small, but significantly negative, mean effect size for crop yield indicated that pest-suppressive diversification schemes interfered with production, in part because of reducing densities of the main crop by replacing it with intercrops or non-crop plants. This first use of meta-analysis to evaluate the effects of diversification schemes, a potentially more powerful tool than tallies of significant positive and negative outcomes (vote-counting), revealed stronger overall effects on all parameters measured compared to previous reviews. Our analysis of the same articles used in a recent review facilitates comparisons of vote-counting and meta-analysis, and shows that pronounced results of the meta-analysis are not well explained by a reduction in articles that met its stricter criteria. Rather, compared to outcome counts, effect sizes were rarely neutral (equal to zero), and a mean effect size value for mixed outcomes could be calculated. Problematic statistical properties of vote-counting were avoided with meta-analysis, thus providing a more precise test of the hypotheses. The unambiguous and encouraging results from this meta-analysis of previous research should motivate ecologists to conduct more mechanistic experiments to improve the odds of designing effective crop diversification schemes for improved pest regulation and enhanced crop yield.

Comparison of organic and conventional farms: challenging ecologists to make biodiversity functional

With the rise of organic farming in the United States and worldwide, ecologists are being presented with new opportunities to link basic and applied ecology through research on biodiversity and ecosystem services. We present evidence from our own research and a review of the literature to assess the evidence for enhanced insect pest control as a consequence of greater biodiversity on organic farms. Despite the frequency of claims in the literature that biodiversity is beneficial, we found that few studies have measured biodiversity effects on pest control and yield on organic farms compared to conventional farms. Relevant studies in agricultural or natural settings suggest that an increase in the diversity of insect predators and parasitoids can have positive or negative effects on prey consumption rates. We therefore call for a stronger scientific basis for evaluating pest suppression effects due to enhanced natural enemy diversity. We suggest several avenues of research to assess the relationship between biodiversity and effective biological control, to obtain the information needed to manage natural enemy diversity, and to estimate the value-added component of on-farm biodiversity in terms of pest control services.

ANTS, STEM-BORERS, AND FUNGAL PATHOGENS: EXPERIMENTAL TESTS OF A FITNESS ADVANTAGE IN PIPER ANT-PLANTS

This study tests experimentally the hypothesis that Pheidole bicornis ants increase the fitness of Piper ant-plants (Piperaceae) in Costa Rican forests. In two exper- iments with naturally occurring Piper ant-plants, -50 individuals were randomly selected and maintained either as controls (with ants) or as ant-exclusion plants (without ants) for 2 yr. Leaf replacement rates and seed production, as measures of relative plant vigor and reproductive potential, were significantly greater in control plants with intact ant colonies than in plants from which ant colonies were experimentally excluded by periodical treatment with dilute insecticide. A series of experiments was designed to assess the relative contribution of antiherbivore defense, nutrient procurement, and antipathogen defense as potential mutualistic functions of the ants. To assess the fitness effects of antiherbivore defense, leaf replacement rates and seed production were measured on plants to which artificial folivory was applied over a 2-yr period (33% of each leaf blade removed), and stem-boring weevil damage was monitored on plants with and without ants. Nutrient procurement by ants was estimated quantitatively, and net leaf production was monitored experimentally on control plants (with ants), on plants without ants, and on nutrient-enriched plants with ants (microquantities of fertilizer added to lower stem). Epiphyll loads, phylloplane spore densities, and disease incidence were compared on experimental plants with and without ant colonies. The results indicate that folivory, the classical parameter measured in ant-plant studies, was not related to differences in fitness in Piper ant-plants; instead, ant disruption of stem-borers and ant foraging on inflorescences to reduce fungal invasion were identified as probable mechanisms by which ants conferred an average fitness advantage of at least 60 and 10%, respectively, in the 2nd yr of the comparison. Although neither antifolivore defense nor nutrient pro- visioning by ants appeared to benefit Piper plants, they may contribute synergistically or may operate on temporal or spatial scales not included in the study. For example, nutrient provisioning may allow Piper ant-plant species to occupy and compete favorably in poor- soil habitats not evaluated in these experiments.

EXPERIMENTAL TEST IN LOWLAND TROPICAL FOREST SHOWS TOP‐DOWN EFFECTS THROUGH FOUR TROPHIC LEVELS

Approximately 50% of the variation in productivity in lakes is hypothesized to depend upon the cascading effects of top predators. A relative paucity of evidence for such trophic cascades in terrestrial systems has prompted proposals that resource availability (donor control) is more critical than are top-down forces in structuring terrestrial communities, and that trophic cascades with top-down dominance will be restricted to systems of low species diversity. To test the effects of a fourth trophic level on successive lower trophic levels under different levels of plant resource availability, we used Piper ant-plants and their associated arthropods as a model system in lowland, tropical rain forest. This interacting web of four trophic levels is composed of small trees, various herbivores, predaceous ants, and specialist clerid beetles as predators of ants. Three-hundred-sixty Piper cenocladum cuttings were randomly assigned to 36 plots with a factorial design: predator treatment (three levels), light treatment (two levels), and soil type (two levels). We monitored indicators of ant-colony size (percentage petioles occupied per tree), herbivore loads (leaf area loss), and tree biomass (total leaf area per tree) for 18 mo. When the top predator Tarsobaenus letourneauae was added experimentally to the three-trophic-level ant-plant system, the average abundance of Pheidole bicornis ants was reduced fivefold, average herbivory to Piper cenocladum leaves was increased nearly threefold, and tree leaf area was reduced by nearly half. Direct effects of predatory beetles on ants were more pronounced and more rapid than were indirect effects accumulating to the second and first trophic levels. Bottom-up effects of light and soil quality tended to be mitigated by these top-down cascades. Neither potential leaf-area accumulation by trees nor actual leaf area (left after herbivory) showed a direct response to relatively high or low availability of soil nutrients or light at different sites.

Synergistic Effects of Three Piper Amides on Generalist and Specialist Herbivores

The tropical rainforest shrub Piper cenocladum, which is normally defended against herbivores by a mutualistic ant, contains three amides that have various defensive functions. While the ants are effective primarily against specialist herbivores, we hypothesized that these secondary compounds would be effective against a wider range of insects, thus providing a broad array of defenses against herbivores. We also tested whether a mixture of amides would be more effective against herbivores than individual amides. Diets spiked with amides were offered to five herbivores: a naïve generalist caterpillar (Spodoptera frugiperda), two caterpillar species that are monophagous on P. cenocladum (Eois spp.), leaf-cutting ants (Atta cephalotes), and an omnivorous ant (Paraponera clavata). Amides had negative effects on all insects, whether they were naïve, experienced, generalized, or specialized feeders. For Spodoptera, amide mixtures caused decreased pupal weights and survivorship and increased development times. Eois pupal weights, larval mass gain, and development times were affected by additions of individual amides, but increased parasitism and lower survivorship were caused only by the amide mixture. Amide mixtures also deterred feeding by the two ant species, and crude plant extracts were strongly deterrent to P. clavata. The mixture of all three amides had the most dramatic deterrent and toxic effects across experiments, with the effects usually surpassing expected additive responses, indicating that these compounds can act synergistically against a wide array of herbivores.

Genetically engineered organisms : assessing environmental and human health effects

11.Preface, B.E. Burrows Variability and Uncertainty in Crop-to-wild Hybridization, T. Klinger Factors Affecting the Spread of Resistant Arabidopsis thaliana Populations, J. Bergelson and C.B. Purrington Bt Crops: Benefits, Risks, and Non-target Effects on Wild Relatives, D.K. Letourneau, J.A. Hagen, and G.S. Robinson Resisting Resistance to Bt Corn, D.A. Andow Ecological Risks of Transgenic Virus-resistant Crops, A.G. Power Impacts of Genetically-engineered Crops on Non-target Herbivores: Bt-corn and Monarch Butterflies as a Case Study, J. Losey, J. Obrycki, and R.A. Hufbauer Transgenic Host Plant Resistance and Non-target Effects, A. Hilbeck Release, Persistence, and Biological Activity in Soil of Insecticidal Proteins from Bacillus thuringiensis, G. Stotsky Survival, Persistence, Transfer: The Fate of Genetically Modified Microorganisms and Recombinant DNA in Different Environments, B. Tappeser, M. Jager, and C. Eckelkamp The Spread of Genetic Constructs in Natural Insect Populations, H.R. Braig and G. Yan Ecological and Community Considerations in Engineering Arthropods to Suppress Vector-borne Disease, A. Spielman, J.C. Beier, and A.E. Kiszewski Environmental Risks of Genetically Engineered Vaccines, T. Traavik Methods to Assess Ecological Risks of Transgenic Fish Releases, W,M. Muir and R.D Howard Controversies in Designing Useful Ecological Assessments of Genetically Engineered Organisms, A.R. Kapuscinski

Relative strengths of top-down and bottom-up forces in a tropical forest community

Abstract We tested integrative bottom-up and top-down trophic cascade hypotheses with manipulative experiments in a tropical wet forest, using the ant-plant Piper cenocladum and its associated arthropod community. We examined enhanced nutrients and light along with predator and herbivore exclusions as sources of variation in the relative biomass of plants, their herbivores (via rates of herbivory), and resident predaceous ants. The combined manipulations of secondary consumers, primary consumers, and plant resources allowed us to examine some of the direct and indirect effects on each trophic level and to determine the relative contributions of bottom-up and top-down cascades to the structure of the community. We found that enhanced plant resources (nutrients and light) had direct positive effects on plant biomass. However, we found no evidence of indirect (cascading through the herbivores) effects of plant biomass on predators or top predators. In contrast, ants had indirect effects on plant biomass by decreasing herbivory on the plants. This top-down cascade occurred whether or not plant resources were enriched, conditions which are expected to modify top-down forces.

Parasitism rates and sex ratios of a parasitoid wasp: effects of herbivore and plant quality

SummaryWe studied interactions among collards, Brassica oleracea var. acephala, the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Yponomeutidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae) by manipulating plant nitrogen (N) concentrations in field and laboratory experiments. Parasitoid abundance strongly reflected DBM abundance and was related to total leaf N. Parasitism rates were high (70.7%) and density-independent. Wasp sex ratios varied markedly (3–93% female) in response to the herbivores, the plants, or both. Higher proportions of female wasps emerged from DBM larvae on plants with high leaf N than on unfertilized plants. More female wasps also emerged from larvae parasitized as larger instars. We suggest that wasps have the potential to control DBM populations through long-term numerical responses mediated by variable sex ratios.