William O. Lamp

Arthropod pest resurgence: an overview of potential mechanisms

Abstract The phenomenon of insecticide-induced resurgence of arthropod pests has long been known to occur in response to a reduction in natural enemy populations, releasing the pest population from regulation. However, studies of resurgent populations infrequently examine other mechanisms, although numerous alternative mechanisms such as physiological enhancement of pest fecundity, reduction in herbivore-herbivore competition, changes in pest behaviour, altered host-plant nutrition, or increased attractiveness may also cause, or enhance the probability of, resurgence. Additionally, many studies that have identified natural enemy mortality as the primary cause of resurgence do not document a priori regulation by natural enemies and, therefore, are correlative in nature. In this paper, a universal definition of resurgence is proposed and criteria for determining the occurrence of resurgence are listed. Both of these are essential to rigorous evaluation of this important phenomenon. Alternative mechanisms for resurgence and potential areas of future research are identified. It is argued that insecticide resistance is not a mechanism underlying resurgence; rather, it simply enhances the probability that resurgence may occur. The similarity of resurgence to some insect outbreaks is discussed. In some cases natural outbreaks differ only in the initiating factor. The importance of resurgence management to plant protection is that resurgence is totally contradictory to the intended outcome of insecticidal application. This conflict strengthens the need to identify specifically the causal factors for each case of resurgence in order to manage this detrimental phenomenon effectively.

Evidence for divergent selection between the molecular forms of Anopheles gambiae: role of predation

BackgroundThe molecular forms of Anopheles gambiae are undergoing speciation. They are characterized by a strong assortative mating and they display partial habitat segregation. The M form is mostly found in flooded/irrigated areas whereas the S form dominates in the surrounding areas, but the ecological factors that shape this habitat segregation are not known. Resource competition has been demonstrated between species undergoing divergent selection, but resource competition is not the only factor that can lead to divergence.ResultsIn a field experiment using transplantation of first instar larvae, we evaluated the role of larval predators in mediating habitat segregation between the forms. We found a significant difference in the ability of the molecular forms to exploit the different larval sites conditioned on the presence of predators. In absence of predation, the molecular forms outcompeted each other in their respective natural habitats however, the developmental success of the M form was significantly higher than that of the S form in both habitats under predator pressure.ConclusionOur results provide the first empirical evidence for specific adaptive differences between the molecular forms and stress the role of larval predation as one of the mechanisms contributing to their divergence.

Dispersal by terrestrial stages of stream insects in urban watersheds: a synthesis of current knowledge

Abstract Adult dispersal and completion of life cycles by aquatic insects are essential for the persistence of populations, colonization of new habitats, and maintenance of genetic diversity. However, life-cycle stages and processes associated with the terrestrial environment often are overlooked when the effect of watershed urbanization on the persistence of insects associated with streams is examined. We reviewed and synthesized current literature on the known effects of watershed urbanization on the terrestrial stage of stream insects. Some research has directly examined the effects of watershed urbanization on dispersal, but much of the evidence we present is indirect and from related studies on aquatic insect life-history traits and dispersal abilities in nonurban watersheds. Our goal is to provide examples of potential impacts that warrant further study, rather than to provide a comprehensive review of all life-history studies. We discuss how watershed land use, riparian condition, and habitat quality affect: 1) adult fitness, 2) adult dispersal, and 3) habitat fragmentation, and 4) how these factors interact with species traits. In general, we found that the local- and landscape-scale changes to stream, riparian, and upland habitats that typically result from anthropogenic activities have the potential to prevent the completion of aquatic insect life cycles and to limit adult dispersal, and therefore, can affect population persistence. When considered within the spatial context of dendritic stream networks, these effects, particularly those on adult dispersal, might have important implications for design and assessment of restoration projects. We discuss a framework for how to determine the relative importance of effects on specific life-cycle stages and processes for the absence of larval populations from urban streams. Overall, more research on terrestrial life-cycle stages and processes and on adult dispersal is required to understand how urbanization might affect population persistence of insects in urban streams.

Exposure and Nontarget Effects of Transgenic Bt Corn Debris in Streams

ABSTRACT Corn (Zea mays L.) transformed with a gene from the bacterium Bacillus thuringiensis (Bt) comprises 49% of all corn in the United States. The input of senesced corn tissue expressing the Bt gene may impact stream-inhabiting invertebrates that process plant debris, especially trichopteran species related to the target group of lepidopteran pests. Our goal was to assess risk associated with transgenic corn debris entering streams. First, we show the input of corn tissue after harvest was extended over months in a stream. Second, using laboratory bioassays based on European corn borer [Ostrinia nubilalis (Hübner)], we found no bioactivity of Cry1Ab protein in senesced corn tissue after 2 wk of exposure to terrestrial or aquatic environments. Third, we show that Bt near-isolines modify growth and survivorship of some species of invertebrates. Of the four nontarget invertebrate species fed Bt near-isolines, growth of two closely related trichopterans was not negatively affected, whereas a tipulid crane fly exhibited reduced growth rates, and an isopod exhibited reduced growth and survivorship on the Cry1Ab near-isoline but not on the stacked Cry1Ab + Cry3Bb1 near-isoline. Because of lack of evidence of bioactivity of Bt after 2 wk and because of lack of nontarget effects on the stacked near-isoline, we suggest that tissue-mediated differences, and not the presence of the Cry1Ab protein, caused the different responses among the species. Overall, our results provide evidence that adverse effects to aquatic nontarget shredders involve complex interactions arising from plant genetics and environment that cannot be ascribed to the presence of Cry1Ab proteins.

Genetically modified crops and aquatic ecosystems: considerations for environmental risk assessment and non-target organism testing

Environmental risk assessments (ERA) support regulatory decisions for the commercial cultivation of genetically modified (GM) crops. The ERA for terrestrial agroecosystems is well-developed, whereas guidance for ERA of GM crops in aquatic ecosystems is not as well-defined. The purpose of this document is to demonstrate how comprehensive problem formulation can be used to develop a conceptual model and to identify potential exposure pathways, using Bacillus thuringiensis (Bt) maize as a case study. Within problem formulation, the insecticidal trait, the crop, the receiving environment, and protection goals were characterized, and a conceptual model was developed to identify routes through which aquatic organisms may be exposed to insecticidal proteins in maize tissue. Following a tiered approach for exposure assessment, worst-case exposures were estimated using standardized models, and factors mitigating exposure were described. Based on exposure estimates, shredders were identified as the functional group most likely to be exposed to insecticidal proteins. However, even using worst-case assumptions, the exposure of shredders to Bt maize was low and studies supporting the current risk assessments were deemed adequate. Determining if early tier toxicity studies are necessary to inform the risk assessment for a specific GM crop should be done on a case by case basis, and should be guided by thorough problem formulation and exposure assessment. The processes used to develop the Bt maize case study are intended to serve as a model for performing risk assessments on future traits and crops.

Comparison of insect communities between adjacent headwater and main-stem streams in urban and rural watersheds

Abstract Watershed urbanization decreases diversity and taxonomic richness of aquatic insect communities, and headwater streams are particularly susceptible to degradation from urbanization. Patterns of taxon loss between urban headwater communities and communities in adjacent downstream, higher-order reaches might indicate which processes are controlling taxon loss and the extent to which unique headwater taxa are lost after urbanization. We compared insect communities in urban and rural watersheds and investigated if community similarity between headwater streams and adjacent higher-order main-stem reaches was greater in urban than in rural watersheds. We sampled insect communities in 3 urban and 3 rural watersheds in Marylands Piedmont region during 3 seasons. Mean taxonomic richness was 4.3× greater and the Shannon diversity index was 1.8× greater in rural than in urban headwater streams. Simpsons index was 1.9× greater in urban than in rural headwater streams. The Jaccard similarity index calculated between headwater and main-stem communities was 1.6× greater for urban sites than rural sites during autumn, and the proportion of headwater taxa shared with the main-stem community was 1.8× greater for urban than rural sites. Redundancy analysis also indicated significantly greater similarity between urban headwater and main-stem communities than between rural headwater and main-stem communities. As expected, urbanization decreased diversity, and the communities remaining in urban headwaters were mostly subsets of the communities in the main-stem streams. This result suggested that taxa unique to headwaters were at the greatest risk of local extirpation after watershed urbanization. A significant interactive effect of landuse type and the longitudinal position of a reach along the headwater on taxonomic richness and the Jaccard index suggested that patterns of taxon loss partially depended on the proximity of a headwater reach to the main-stem stream. Overall, the results suggested that water- and habitat-quality degradation were not the only effects of watershed urbanization that determined the composition of insect communities in urban headwaters.

Adjacent habitat influence on stink bug (Hemiptera: Pentatomidae) densities and the associated damage at field corn and soybean edges.

The local dispersal of polyphagous, mobile insects within agricultural systems impacts pest management. In the mid-Atlantic region of the United States, stink bugs, especially the invasive Halyomorpha halys (Stål 1855), contribute to economic losses across a range of cropping systems. Here, we characterized the density of stink bugs along the field edges of field corn and soybean at different study sites. Specifically, we examined the influence of adjacent managed and natural habitats on the density of stink bugs in corn and soybean fields at different distances along transects from the field edge. We also quantified damage to corn grain, and to soybean pods and seeds, and measured yield in relation to the observed stink bug densities at different distances from field edge. Highest density of stink bugs was limited to the edge of both corn and soybean fields. Fields adjacent to wooded, crop and building habitats harbored higher densities of stink bugs than those adjacent to open habitats. Damage to corn kernels and to soybean pods and seeds increased with stink bug density in plots and was highest at the field edges. Stink bug density was also negatively associated with yield per plant in soybean. The spatial pattern of stink bugs in both corn and soybeans, with significant edge effects, suggests the use of pest management strategies for crop placement in the landscape, as well as spatially targeted pest suppression within fields.

Transcriptome analysis of the salivary glands of potato leafhopper, Empoasca fabae

The potato leafhopper, Empoasca fabae, is a pest of economic crops in the United States and Canada, where it causes damage known as hopperburn. Saliva, along with mechanical injury, leads to decreases in gas exchange rates, stunting and chlorosis. Although E. fabae saliva is known to induce plant responses, little knowledge exists of saliva composition at the molecular level. We subjected the salivary glands of E. fabae to Roche 454-pyrosequencing which resulted significant number (30,893) of expressed sequence tags including 2805 contigs and 28,088 singletons. A high number of sequences (78%) showed similarity to other insect species in GenBank, including Triboliumcastaneum, Drosophilamelanogaster and Acrythosiphonpisum. KEGG analysis predicted the presence of pathways for purine and thiamine metabolic, biosynthesis of secondary metabolites, drug metabolism, and lysine degradation. Pfam analysis showed a high number of cellulase and carboxylesterase protein domains. Expression analysis of candidate genes (alpha amylase, lipase, pectin lyase, etc.) among different tissues revealed tissue-specific expression of digestive enzymes in E. fabae. This is the first study to characterize the sialotranscriptome of E. fabae and the first for any species in the family of Cicadellidae. Due to the status of these insects as economic pests, knowledge of which genes are active in the salivary glands is important for understanding their impact on host plants.

Mitochondrial lineages and DNA barcoding of closely related species in the mayfly genus Ephemerella (Ephemeroptera:Ephemerellidae)

Abstract We compared genetic lineages in the mayfly genus Ephemerella (Ephemeroptera:Ephemerellidae) identified from mitochondrial DNA (mtDNA) to current taxonomy in 9 morphological taxa, including 2 geographically widespread species, Ephemerella invaria ( = E. inconstans, E. rotunda, E. floripara) and Ephemerella dorothea ( = E. infrequens). Maximum likelihood and parsimony analyses of the mtDNA sequences placed E. inconstans and E. invaria in a well-supported clade; however, mean Kimura 2-parameter genetic distance between the lineages was high (5.2%) relative to distance within lineages (1.3%). The phylogenetic relationships of synonyms E. rotunda and E. floripara were not resolved, but estimates of mean genetic distance to E. invaria were high for both (8.5% and 11.6%, respectively). Populations of E. dorothea were grouped in 2 well-supported clades (12.9% mean divergence) that did not include the synonym E. infrequens (20.9% mean divergence, based on a single sample). A large genetic distance (18.6%) also was found between eastern and western populations of Ephemerella excrucians. Western samples of Ephemerella aurivillii were so genetically distant from all other lineages (32.2%) that doubt about its congeneric status is raised. mtDNA data have been useful for identifying genetic lineages in Ephemerella, but our results do not support use of cytochrome oxidase I (COI) as a DNA barcode to identify species in this genus because we also found evidence of incomplete mtDNA lineage sorting in this gene. Use of the barcoding gene rediscovered some old taxonomic problems in Ephemerella, a result that emphasizes the importance of completing empirical systematic description of species before using single-character systems for identification.

Spatiotemporal Dynamics of the Invasive Halyomorpha halys (Hemiptera: Pentatomidae) in and Between Adjacent Corn and Soybean Fields

ABSTRACT Knowledge on movement and spatial patterns of insect pest populations among preferred hosts aids in the development of effective pest management strategies. In this study, we quantified the spatiotemporal dynamics of the invasive brown marmorated stink bug, Halyomorpha halys (Stål 1855), in relation to field corn, Zea mays L., and soybean, Glycine max (L.), crop phenology.We also examined the potential role of corn as a source of stink bugs in adjacent soybean. The highest density of stink bugs in each crop coincided with blister to milk-dough stages in corn (R2–R3/R4), and beginning seed to full seed (R5–R6) stages in soybean. In entire fields of adjacent corn and soybean, H. halys was found in very low density (<0.5/m2) or absent beyond 25m from the field edge. Inverse distance weighted interpolations of H. halys densities suggest potential dispersal of H. halys, particularly adults and large nymphs, from corn into soybean, coinciding with the end of dough stage in corn and beginning of soybean seed development stage. These findings have important implications for managing H. halys through location and timing of scouting efforts, consideration of crop arrangement, and decisions on management interventions. Repeated scouting of field corn to assess H. halys densities, particularly from blister stage onwards, could inform decisions on management interventions for preventing or mitigating H. halys colonization into soybean. Where H. halys is an economic problem, reducing the extent of boundary shared between corn and soybean could reduce dispersal into soybean.