J. R. Fisher

GRAPES, GALLS, AND GEOGRAPHY: THE DISTRIBUTION OF NUCLEAR AND MITOCHONDRIAL DNA VARIATION ACROSS HOST‐PLANT SPECIES AND REGIONS IN A SPECIALIST HERBIVORE

Abstract.— Studies of patterns of molecular variation in natural populations can provide important insights into a number of evolutionary problems. Among these, the question of whether geographic factors are more important than ecological factors in promoting population differentiation and ultimately speciation has been an important and contentious area in evolutionary biology. Systems involving herbivorous insects have played a leading role in this discussion. This study examined the distribution of molecular variation in a highly specialized gall‐forming insect, grape phylloxera (Daktulosphaira vitifoliae Fitch), that is found on both sympatric and allopatric host‐plant species of the genus Vitis. In addition, the relationship of insects in the introduced range in the United States to ancestral populations in the native range was examined. Evidence for differentiation along host‐plant lines from both nuclear (RAPD) and mitochondrial (COI) DNA was confounded with the effect of geography. Differentiation was found where hosts were allopatric or parapatric, but no evidence was found for such differentiation on two hosts, V. vulpina and V. aestivalis, that are broadly sympatric. The question of population differentiation onto these sympatric hosts can be considered to be resolved—it has not occurred in spite of a long history of association. Evidence was equivocal, but suggestive of a period of divergence in allopatry prior to reestablishment of contact, for insects associated with another host plant species, V. cinerea, found in both sympatric and parapatric populations. A low level of diversity and placement of samples collected from the grape species V. riparia at the tip of a phylogenetic tree supports the hypothesis that this host has been recently colonized from populations from the Mississippi Valley. A polyphyletic origin for biotype B grape phylloxera was supported: Although most samples collected from vineyards in the introduced range in California had similar haplotypes, they were closely related to natives on V. vulpina from the Atlantic Coast‐Piedmont region. All samples collected from vineyards in Oregon and Washington were closely related to natives on V. riparia in the northern United States.

Fecundity, longevity and establishment of Otiorhynchus sulcatus (Fabricius) and Otiorhynchus ovatus (Linnaeus) (Coleoptera: Curculionidae) from the Pacific North‐west of the United States of America on selected host plants

1 The fecundity, longevity and establishment of Otiorhynchus sulcatus and Otiorhynchus ovatus from the Pacific North‐west U.S.A. was studied on five selected host plants: Picea abies‘Nidiformis’, Picea glauca‘Conica’, Taxus baccata, Rhododendron catawbiense‘Boursault’ and Fragaria×ananassa‘Totem’.

Influence of corn planting date on the life stage development and phenology of Diabrotica virgifera virgifera.

Under field conditions in eastern South Dakota, USA three different planting dates of corn and three times of egg infestation were used to imposed synchronous and asynchronous timing of corn growth and Diabrotica virgifera virgifera LeConte egg hatch and subsequent development. Median occurrence of each of the three larval stages and of the adult stage for each of the infestation‐planting treatments was determined by relating occurrence to thermal units and to days after infestation. No significant differences were found among the treatments for median occurrence of each life stage when the thermal unit approach was used. However, significant differences were found among treatments for median occurrence when the day after infestation approach was used. Consistent parameters are needed for prediction of the occurrence of life stages of this insect. Despite the various imposed conditions, this study indicates that the least variable method of predicting life stage occurrence and adult emergence of D. v. virgifera was the use of thermal unit accumulations (base 11 °C).

Influence of Below-Ground Feeding by Garden Symphylans (Cephalostigmata: Scutigerellidae) on Plant Health

Abstract The influence of garden symphylan (Scutigerella immaculata Newport) root feeding on crop health was measured in the laboratory and in the field. In the laboratory, the relative susceptibility of sweet corn (Zea mays L.), potato (Solanum tuberosum L.), spinach (Spinacia oleracea L.), and tomato (Lycopersicon esculentum L.) to S. immaculata feeding was investigated by subjecting each crop to three levels of pest pressure (0, 15, and 45 S. immaculata). As S. immaculata densities increased from 0 to 45, root length and dry weight of above-ground plant parts of 1-wk-old spinach and tomato seedlings were reduced from 85 to 98%, whereas corn root length was reduced by 34%. Potato and corn dry weight and potato root length were not reduced. The pest-host association was investigated in the field by examining the relationships between estimated S. immaculata densities and two vegetational parameters: (1) crop health measured by the normalized difference vegetation index (NDVI) and (2) post-tillage spring weed densities. S. immaculata densities were estimated using a baiting method. NDVI in corn and potato was only slightly reduced with increasing within-field S. immaculata densities after adjusting for spatial autocorrelation. Squash and broccoli showed sharp decreases in NDVI at densities from 1 to 10 S. immaculata. The nonlinear asymptotic form of the Bleasdale-Nelder curve was selected as the best curve to describe the damage/pest relationship for all crops using Akaike information criteria. The relative susceptibilities of direct seeded crop varieties to S. immaculata in western Oregon reported in 1937 by Morrison were reanalyzed to make comparisons with our data. In further analysis, seed size seemed to be linearly related with susceptibility of direct-seeded crops to S. immaculata. Stand count was predicted to increase by 0.24% for each 1-mg increase in seed size. These findings will be used to help develop action thresholds and sample size requirements and to help reduce S. immaculata damage by avoiding the planting of highly susceptible crops at sites with high S. immaculata populations.