Generating Within-Plant Spatial Distributions of an Insect Herbivore Based on Aggregation Patterns and Per-Node Infestation Probabilities.

Most predator-prey models extrapolate functional responses from small-scale experiments assuming spatially uniform within-plant predator-prey interactions. However, some predators focus their search in certain plant regions, and herbivores tend to select leaves to balance their nutrient uptake and exposure to plant defenses. Individual-based models that account for heterogeneous within-plant predator-prey interactions can be used to scale-up functional responses, but they would require the generation of explicit prey spatial distributions within-plant architecture models. The silverleaf whitefly, Bemisia tabaci biotype B (Gennadius) (Hemiptera: Aleyrodidae), is a significant pest of tomato crops worldwide that exhibits highly aggregated populations at several spatial scales, including within the plant. As part of an analytical framework to understand predator-silverleaf whitefly interactions, the objective of this research was to develop an algorithm to generate explicit spatial counts of silverleaf whitefly nymphs within tomato plants. The algorithm requires the plant size and the number of silverleaf whitefly individuals to distribute as inputs, and includes models that describe infestation probabilities per leaf nodal position and the aggregation pattern of the silverleaf whitefly within tomato plants and leaves. The output is a simulated number of silverleaf whitefly individuals for each leaf and leaflet on one or more plants. Parameter estimation was performed using nymph counts per leaflet censused from 30 artificially infested tomato plants. Validation revealed a substantial agreement between algorithm outputs and independent data that included the distribution of counts of both eggs and nymphs. This algorithm can be used in simulation models that explore the effect of local heterogeneity on whitefly-predator dynamics.

Evidence for trade-offs in detoxification and chemosensation gene signatures in Plutella xylostella.

BACKGROUND: Detoxification genes have been associated with insecticide adaptation in the diamondback moth, Plutella xylostella. The link between chemosensation genes and adaptation, however, remains unexplored. To gain a better understanding of the involvement of these genes in insecticide adaptation, the authors exposed lines of P. xylostella to either high uniform (HU) or low heterogeneous (LH) concentrations of permethrin, expecting primarily physiological or behavioral selection respectively. Initially, 454 pyrosequencing was applied, followed by an examination of expression profiles of candidate genes that responded to selection [cytochrome P450 (CYP), glutathione S-transferase (GST), carboxylesterase (CarE), chemosensory protein (CSP) and odorant-binding protein (OBP)] by quantitative PCR in the larvae. Toxicity and behavioral assays were also conducted to document the effects of the two forms of exposure. RESULTS: Pyrosequencing of the P. xylostella transcriptome from adult heads and third instars produced 198,753 reads with 52,752,486 bases. Quantitative PCR revealed overexpression of CYP4M14, CYP305B1 and CSP8 in HU larvae. OBP13, however, was highest in LH. Larvae from LH and HU lines had up to five- and 752-fold resistance levels respectively, which could be due to overexpression of P450s. However, the behavioral responses of all lines to a series of permethrin concentrations did not vary significantly in any of the generations examined, in spite of the observed upregulation of CSP8 and OBP13. CONCLUSION: Expression patterns from the target genes provide insights into behavioral and physiological responses to permethrin and suggest a new avenue of research on the role of chemosensation genes in insect adaptation to toxins.

Permanent genetic resources added to Molecular Ecology Resources Database 1 August 2012 - 30 September 2012.

This article documents the addition of 83 microsatellite marker loci and 96 pairs of single-nucleotide polymorphism (SNP) sequencing primers to the Molecular Ecology Resources Database. Loci were developed for the following species: Bembidion lampros, Inimicus japonicus, Lymnaea stagnalis, Panopea abbreviata, Pentadesma butyracea, Sycoscapter hirticola and Thanatephorus cucumeris (anamorph: Rhizoctonia solani). These loci were cross-tested on the following species: Pentadesma grandifolia and Pentadesma reyndersii. This article also documents the addition of 96 sequencing primer pairs and 88 allele-specific primers or probes for Plutella xylostella.

Entomopathogenic nematodes as a model system for advancing the frontiers of ecology.

Entomopathogenic nematodes (EPNs) in the families Heterorhabditidae and Steinernematidae have a mutualistic-symbiotic association with enteric γ-Proteobacteria (Steinernema-Xenorhabdus and Heterorhabditis-Photorhabdus), which confer high virulence against insects. EPNs have been studied intensively because of their role as a natural mortality factor for soil-dwelling arthropods and their potential as biological control agents for belowground insect pests. For many decades, research on EPNs focused on the taxonomy, phylogeny, biogeography, genetics, physiology, biochemistry and ecology, as well as commercial production and application technologies. More recently, EPNs and their bacterial symbionts are being viewed as a model system for advancing research in other disciplines such as soil ecology, symbiosis and evolutionary biology. Integration of existing information, particularly the accumulating information on their biology, into increasingly detailed population models is critical to improving our ability to exploit and manage EPNs as a biological control agent and to understand ecological processes in a changing world. Here, we summarize some recent advances in phylogeny, systematics, biogeography, community ecology and population dynamics models of EPNs, and describe how this research is advancing frontiers in ecology.

Indirect selection for increased susceptibility to permethrin in diamondback moth (Lepidoptera: Plutellidae).

We have been exploring the behavioral response ot insect pests to heterogeneous distribution of toxins (low dose with refugia), and its genetic correlation with physiological tolerance to these toxins. A field-collected population of diamondback moth, Plutella xylostellu (L.) (Lepidoptera Plutellidae), from Celeryville, OH, was selected with permethrin to determine whether low heterogeneous doses could lead to increased susceptibility to permethrin by selecting indirectly on behavior. Two replicates of each of three selection regimes: uniform high concentration hypothesized to result in increased physiological tolerance, heterogeneous low concentration hypothesized to result in increased susceptibility through indirect selection on behavior, and a control with no exposure to permethrin, were maintained in 1-in3 cages in a greenhouse, for 33 generations. All life stages of the diamondback moth were exposed to the selection regimes, and new generations were started with a random selection of pupae from the previous generation. Lines selected with uniform high concentrations developed 76-fold levels of resistance to permethrin by the 17th generation, with little changes thereafter. For generations 1-20, lines selected with heterogeneous low concentrations remained slightly lower in LC50 but not significantly different from the unselected control lines. Based on confidence intervals from probit analyses, the LC50 of the lines selected with heterogeneous low concentration, however, were significantly lower than those of the control lines in generations 21-33. Our results demonstrate that selection on behavioral responses can result in greater susceptibility than no selection at all, despite exposure to the toxin and ample genetic variation and potential for increased physiological tolerance. The implications of our findings, which are based on selection scenarios that could take place in field situations, are that behavioral responses can prevent and even decrease the levels of resistance in insect populations, an important result with respect to resistance and resistance management.

Quantitative genetics of adult behavioral response and larval physiological tolerance to permethrin in diamondback moth (Lepidoptera: Plutellidae).

We investigated the genetic basis of adult behavioral response and larval physiological tolerance to permethrin within two diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), populations from Wooster and Celeryville, OH, with different average levels of larval tolerance. The adult behavioral response was measured as oviposition site preference and was investigated using full-sib design and parent-offspring regression. Additive genetic variance (0.134 +/- 0.02) and the heritability (h2 = 0.31 +/- 0.08) for the behavioral response was significant for the Celeryville population, suggesting that in this population, a high proportion of phenotypic variation for adult behavioral response to permethrin was heritable genetic variation. The larval physiological response was measured with a topical application bioassay and was investigated using a half-sib design. Significant additive genetic variances and heritabilities for physiological tolerance to permethrin were detected in both populations. The genetic correlation between adult behavioral response and larval physiological tolerance to permethrin were negative, but significant only in the Celeryville population; indicating that adults from this population that are more behaviorally responsive produced offspring that are more susceptible to permethrin. Our findings have implications for the evolution and management of insecticide resistance in the diamondback moth. The adult behavioral response can lower the exposure of larvae to the insecticide, lowering selection pressure for physiological resistance in larvae. Furthermore, to the extent that the adult behavioral response increases fitness, it can indirectly select for larval susceptibility because of the negative correlation between the two traits.

Relationship between carrot weevil infestation and parsley yield.

The relationship between numbers of carrot weevil, Listronotus oregonensis (LeConte), oviposition scars and parsley fresh weight and plant mortality was measured in research plots during 1999 and 2000. Fresh weight was measured in one to two cuttings of parsley planted on two planting dates. The average weight declined with increasing numbers of oviposition scars in the later planting in 1999. Compensatory growth in surviving plants may reduce this effect. Plant mortality increased as number of oviposition scars per plant increased in the second planting in both years and in the first cutting of the first planting in 2000. One oviposition scar per plant is sufficient to result in significant reduction in fresh weight per plant. In commercial parsley fields, the relationship between fresh weight of parsley per 30-cm row section of parsley was best described as a linear function of the proportion of plants with root feeding. Economic damage to parsley that is equivalent to the cost of controlling carrot weevil was estimated to result from approximately 1% of plants with root damage. Based upon this estimated economic injury level, we suggest an action threshold of 1% of plants containing carrot weevil oviposition scars earlier in the growing season when controls could be applied to prevent the damage.

Phenotypic variation in adult behavioral response and offspring fitness in Plutella xylostella (Lepidoptera: Plutellidae) in response to permethrin.

Phenotypic variation in behavioral response and physiological tolerance to permethrin was measured and compared between two populations of diamondback moth, Plutella xylostella (L.). A field population from Celeryville, OH, and a laboratory population originally collected from Wooster, OH, were compared. In laboratory choice bioassays, females from both populations were less likely to oviposit on cabbage leaf disks and seedlings treated with permethrin. The oviposition-deterrent effect was positively related to permethrin concentration. The laboratory population was significantly more behaviorally responsive to the insecticide and showed a significantly greater avoidance than the field population of the highest concentration of permethrin tested (1.50 g [AI] /liter). The physiological response of each population was measured by feeding bioassays, and the laboratory population was physiologically more susceptible to the permethrin. Larvae from the laboratory population that were fed permethrin-treated leaves had significantly lower growth rate, higher mortality, and lower adult fecundity compared with larvae from the field population. These data suggest that female moths that are more behaviorally responsive to permethrin produce offspring that tend to be more susceptible to the same insecticide, at least for the populations tested. The relationship between adult behavioral response and larval physiological tolerance to permethrin is discussed.

Molecular and symptom analyses of phytoplasma strains from lettuce reveal a diverse population.

ABSTRACT Epidemics of aster yellows in lettuce in Ohio are caused by at least seven distinct phytoplasma strains in the aster yellows (AY) group. Five of the strains are newly reported: AY-BW, AY-WB, AY-BD3, AY-SS, and AY-SG. All seven strains were characterized based on symptoms in aster and lettuce, and by polymerase chain reaction (PCR). Strain AY-BD2 (formerly 'Bolt') causes yellowing and leaf distortion in lettuce and bolting in aster, whereas strain AY-S (formerly 'Severe') causes stunting, leaf clustering, and phyllody. Strain AY-WB causes yellowing and wilting in lettuce and witches'-broom in aster. Strain AY-SG induces horizontal growth in lettuce and aster plants. Strain AY-BW causes chlorosis of emerging leaves and abnormally upright growth of leaf petioles. AY-SS causes symptoms similar to those caused by AY-S but has a different PCR-restriction fragment length polymorphism (RFLP) banding pattern. Strains AY-BD2 and AY-BD-3 cause mild leaf and stem distortion in lettuce but are differentiated by PCR-RFLP. All phytoplasma strains collected from lettuce in Ohio belong to the 16SrI group. AY-WB belongs to the 16SrI-A subgroup and the other six belong to the 16SrI-B subgroup. Five of the seven strains were distinguished from each other by primer typing. The results of phylogenetic analyses of sequences of the 16S rRNA genes were basically consistent with the classification based on PCR-RFLP, in which AY-WB clustered with phytoplasmas of the 16rIA subgroup and the other Ohio lettuce strains clustered with phytoplasmas in the 16SrI-B subgroup.

Cyromazine seed treatments to control onion maggot, Delia antiqua, on green onions.

Cyromazine seed treatments were evaluated for onion maggot control in green onion crops. The more tolerant to organophosphates of two populations of onion maggots was chosen for further research, based on the results of adult assays in a Potter spray tower. In the laboratory, first-instar mortality was compared between film-coated seed treatment and soil drench treatment. The LC50 for the film-coated seed treatment was approximately one fourth that of the soil drench treatment. In choice assays, no significant difference was observed between the number of eggs deposited on seedlings grown from film-coated seeds with cyromazine and film-coated seeds without cyromazine. Field studies demonstrated that all rates of cyromazine seed treatment resulted in protection of onion plants from onion maggot damage. Green onions may not require as high a rate of cyromazine for control of onion maggots as the rate established for use in dry onions for two reasons: the seeding rate is much higher for green onions resulting in more AI/ha for a given amount of AI/kg of seed, and a given percentage of stand loss is more difficult to detect in green onions than in dry onions.