Seasonal and Scale Size Relationships between Citricola Scale (Homoptera: Coccidae) and Its Parasitoid Complex (Hymenoptera: Chalcidoidea) on San Joaquin Valley Citrus

The phenology of citricola scale, Coccus pseudomagnoliarum (Kuwana), and its associated parasitoid complex were studied on citrus in the San Joaquin Valley of central California over the period April 1995–March 1997. A total of 10,237 parasitoid specimens of 10 species were collected. Two of these species, Marietta mexicana (Howard) and Encyrtus lecaniorum (Mayr), each recovered from individually isolated scales, represent new parasitoid records for citricola scale. A third species, Encarsia citrinus citrinus (Craw), may represent a new parasitoid record, but this requires further confirmation because a single (male) specimen was recovered from individually isolated scales. The three most dominant parasitoid species, Coccophagus lycimnia (Walker), Metaphycus helvolus (Compere), and Metaphycus luteolus (Timberlake), accounted for the majority (>97%) of the specimens recovered. In contrast to the situation on citrus in southern California, where citricola scale is under effective biological control and is very rarely seen, citricola scale on citrus in the San Joaquin Valley is reemerging as a major pest, especially in groves employing integrated pest management with minimal use of broad-spectrum insecticides. Possible reasons uncovered in this study for the lack of effective biological control of citricola scale in the San Joaquin Valley include: (i) reduced presence of Metaphycus spp. because of hyperparasitism by the heteronomous hyperparasitoid C. lycimnia; (ii) absence of alternate hosts for those species of Metaphycus present; and (iii) absence of hosts of suitable size for Metaphycus at critical times of the year. Recommendations for improving the level of biological control in the San Joaquin Valley are discussed.     

Survey of brown soft scale Coccus hesperidum L. parasitoids in southern California citrus

The parasitoid complex of brown soft scale, Coccus hesperidum L., a multivoltine soft scale, was determined in southern California citrus over the period February 2004–March 2006. The survey was conducted by placing brown soft scale-infested yucca leaves in the canopy of citrus trees and subsequently rearing individually isolated parasitized scales in the laboratory. A total of 14 species parasitized brown soft scale in the field, the most abundant ones belonging to the genus Metaphycus Mercet (75%). The most abundant parasitoid species was Metaphycus angustifrons Compere (38% parasitism), and this is a new record of establishment for this species in California. Coccophagus species accounted for only 11% parasitism. There were important spatio-temporal differences across the parasitoid complex survey locations. We also found that the five most abundant encyrtid parasitoid species showed preferences for scales of different sizes. Our results have implications for biological control of citricola scale, Coccus pseudomagnoliarum (Kuwana), an important pest of citrus in the San Joaquin Valley of central California. Notably, this species is nearly absent in southern California. Brown soft scale is considered to be an alternate host for parasitoids of citricola scale, a univoltine soft scale, at times when the latter species is unavailable for parasitism.     

Classical biological control of Aphis gossypii (Homoptera: Aphididae) with Neozygites fresenii (Entomophthorales: Neozygitaceae) in California cotton

In August 1994 and 1995 classical biological control releases were made in cotton in the San Joaquin Valley, California, with an Arkansas strain of the entomopathogenic fungus, Neozygites fresenii, a pathogen of the cotton aphid, Aphis gossypii. Pre-release samples in both years indicated that N. fresenii was not naturally present in A. gossypii populations in the San Joaquin Valley. Two release methods were compared: dried N. fresenii-infected cotton aphid “cadavers” and chamber inoculation of A. gossypii. Both methods were successful in introducing N. fresenii to cotton aphids in California; however, higher prevalence of fungal infection resulted with the cadaver treatments. N. fresenii persisted and spread in the aphid population until early October 1994 and late September 1995. The highest mean percentage infection in the cadaver treatment in 1994 reached a level (14%) considered imminent for epizootics (12–15%). The use of predator exclusion cages resulted in higher N. fresenii prevalences.     

Identifying sources of dissolved organic carbon in agriculturally dominated rivers using radiocarbon age dating: Sacramento–San Joaquin River Basin, California

We used radiocarbon measurements of dissolved organic carbon (DOC) to resolve sources of riverine carbon within agriculturally dominated landscapes in California. During 2003 and 2004, average Δ¹⁴C for DOC was ?254‰ in agricultural drains in the Sacramento–San Joaquin Delta, ?218‰ in the San Joaquin River, ?175‰ in the California State Water Project and ?152‰ in the Sacramento River. The age of bulk DOC transiting the rivers of California’s Central Valley is the oldest reported for large rivers and suggests wide-spread loss of soil organic matter caused by agriculture and urbanization. Using DAX 8 adsorbent, we isolated and measured ¹⁴C concentrations in hydrophobic acid fractions (HPOA); river samples showed evidence of bomb-pulse carbon with average Δ¹⁴C of 91 and 76‰ for the San Joaquin and Sacramento Rivers, respectively, with older HPOA, ?204‰, observed in agricultural drains. An operationally defined non-HPOA fraction of DOC was observed in the San Joaquin River with seasonally computed Δ¹⁴C values of between ?275 and ?687‰; the source of this aged material was hypothesized to be physically protected organic-matter in high clay-content soils and agrochemicals (i.e., radiocarbon-dead material) applied to farmlands. Mixing models suggest that the Sacramento River contributes about 50% of the DOC load in the California State Water Project, and agricultural drains contribute approximately one-third of the load. In contrast to studies showing stabilization of soil carbon pools within one or two decades following land conversion, sustained loss of soil organic matter, occurring many decades after the initial agricultural-land conversion, was observed in California’s Central Valley.     

Characterization of microsatellite loci in chinook salmon (Oncorhynchus tshawytscha) and cross‐species amplification in other salmonids

Previous studies of population genetic structure of fall‐run chinook salmon (Oncorhynchus tshawytscha) in California’s Central Valley have either not focused on or have been unable to resolve intertributary differences within the San Joaquin River basin. The authors describe the isolation, the polymerase chain reaction conditions, and characterize the cross‐species amplification of 17 microsatellite loci in six species of salmonids. Fourteen of these loci are polymorphic in fall‐run chinook from the San Joaquin River drainage. These results indicate the potential utility of microsatellite markers developed for one species, for both congenerics and species within a closely related genus.     

Role of imidacloprid in integrated pest management of California citrus

Portions of three commercial citrus orchards were treated for 1 yr with foliar imidacloprid or for 2 yr with a systemic formulation in a replicated plot design to determine the impact of this neonicotinoid on the San Joaquin Valley California citrus integrated pest management (IPM) program. Foliar-applied imidacloprid had little effect on California red scale, Aonidiella aurantii (Maskell); cottony cushion scale, Icerya purchasi Maskell; or citrus red mite, Panonychus citri (McGregor), populations. Short-term suppression of the parasitoids Aphytis melinus DeBach and Comperiella bifasciata Howard; vedalia, Rodolia cardinalis (Mulsant); and the predacious mite Euseius tularensis (Congdon) were observed. Suppression of natural enemies allowed scales and mites to maintain higher populations in the treated areas compared with the nontreated areas. Thus, foliar imidacloprid did not exhibit control of these citrus pest species, and it disrupted biological control. Systemically applied imidacloprid suppressed California red scale and citricola scale populations 2-3 mo after treatment. Suppression of parasitoids of the California red scale also was observed. Thus, treatments of systemic imidacloprid applied in areawide management programs for invasive pests would provide a benefit of California red scale and citricola scale suppression. However, this treatment provided only single-season control of citricola scale, it was somewhat disruptive of biological control, and it did not suppress densities of either scale as low as a treatment of the organophosphate chlorpyrifos for citricola scale or the insect growth regulator pyriproxyfen for California red scale. Insecticides with longer periods of efficacy and greater IPM compatibility than imidacloprid should be used for a sustainable IPM approach in California citrus.     

Post-release dispersal of the introduced lygus bug parasitoid Peristenus relictus in California

Lygus spp. (Heteroptera: Miridae) are serious pests of numerous field and fruit crops in North America. In an effort to reduce crop damage, parasitoids known to attack these species in Europe were introduced into the USA beginning in the 1970s. Permanent populations of the nymphal endoparasitoid Peristenus relictus (Hymenoptera: Braconidae) were established at two locations in California during the 2000s. Both populations are associated with significant reductions in lygus bugs attacking alfalfa, commercially produced strawberries grown organically and in wild vegetation. Beginning in 2009, in an effort to determine the extent to which P. relictus has spread from the Sacramento Valley and Monterey Bay region, populations of lygus bug were sampled at increasing distances from their original release sites. P. relictus has dispersed at least 213 km in the central region of California and 150 km along the coastal mountains. These respective populations have averaged 16.6 km/year and 17.7 km/year since they were released. While not directly examined, the spread of P. relictus south into the central and major growing region of California, the San Joaquin Valley, where previous releases have failed, suggests this population may be evolving greater heat tolerance, relative to the populations originally introduced into California.     

Combining Forces - The Use of Landsat TM Satellite Imagery,Soil Parameter Information,and Multiplex PCR to Detect Coccidioides immitis Growth Sites in Kern County,California

Coccidioidomycosis is a fungal disease acquired through the inhalation of spores of Coccidioides spp., which afflicts primarily humans and other mammals. It is endemic to areas in the southwestern United States, including the San Joaquin Valley portion of Kern County, California, our region of interest (ROI). Recently, incidence of coccidioidomycosis, also known as valley fever, has increased significantly, and several factors including climate change have been suggested as possible drivers for this observation. Up to date details about the ecological niche of C. immitis have escaped full characterization. In our project, we chose a three-step approach to investigate this niche: 1) We examined Landsat-5-Thematic-Mapper multispectral images of our ROI by using training pixels at a 750 m×750 m section of Sharktooth Hill, a site confirmed to be a C. immitis growth site, to implement a Maximum Likelihood Classification scheme to map out the locations that could be suitable to support the growth of the pathogen; 2) We used the websoilsurvey database of the US Department of Agriculture to obtain soil parameter data; and 3) We investigated soil samples from 23 sites around Bakersfield, California using a multiplex Polymerase Chain Reaction (PCR) based method to detect the pathogen. Our results indicated that a combination of satellite imagery, soil type information, and multiplex PCR are powerful tools to predict and identify growth sites of C. immitis. This approach can be used as a basis for systematic sampling and investigation of soils to detect Coccidioides spp.     

Effects of grazing and invasive grasses on desert vertebrates in California

Much of California's San Joaquin Valley is a desert and, like portions of other North American deserts, is experiencing an ecological shift from being dominated by ephemeral native forbs, with widely spaced shrubs, to fire-prone non-native annual grasses. Small terrestrial vertebrates, many of which are adapted to open desert habitats, are declining. One hypothesis is that the invasive plants contribute to the decline by altering vegetative structure. Although cattle may have originally been a factor in the establishment of these non-native plants, their grazing may benefit the terrestrial vertebrates by maintaining an open structure, especially during average or wet winters when the exotic grasses grow especially dense. We experimentally tested the effect of cattle grazing on invasive plants and a community of small vertebrates at a site in the southwestern San Joaquin Desert. We established and monitored 4 treatment (grazed) and 4 control (ungrazed) plots from 1997 to 2006, and assessed the abundances of blunt-nosed leopard lizards (Gambelia sila), giant kangaroo rats (Dipodomys ingens), short-nosed kangaroo rats (Dipodomys nitratoides nitratoides), and San Joaquin antelope squirrels (Ammospermophilus nelsoni), all of which are listed as threatened or endangered by state or federal agencies. We also recorded abundances of the non-protected western whiptail lizards (Aspidoscelis tigris), side-blotched lizards (Uta stansburiana), San Joaquin pocket mice (Perognathus inornatus inornatus), and Heermann's kangaroo rats (Dipdomys heermanni). Based on repeated measures analysis of variance (ANOVA) and a 0.05 alpha level, only Heermann's kangaroo rats showed a treatment effect; they were more abundant on the control plots. However, this effect could be accounted for by the natural re-establishment of saltbush (Atriplex spp.) on part of the study site. Saltbush return also favored western whiptail lizards and San Joaquin antelope squirrels. A regression analysis indicated that populations of blunt-nosed leopard lizard and giant kangaroo rat increased significantly faster in grazed plots than the ungrazed controls, and abundances of 6 of 8 species were negatively correlated with increased residual dry matter. With relaxed alpha values to decrease Type II error, populations of blunt-nosed leopard lizards (500% greater), San Joaquin antelope squirrels (85% greater), and short-nosed kangaroo rats (73% greater) increased significantly on grazed plots over the course of the study compared to ungrazed plots. We did not find grazing to negatively affect abundance of any species we studied. When herbaceous cover is low during years of below average rainfall in deserts and other arid areas, grazing may not be necessary to maintain populations of small vertebrates. However, if cattle grazing is closely monitored in space and time to minimize adverse effects on the habitat, it could be an effective tool to control dense stands of non-native grasses and benefit native wildlife. © 2011 The Wildlife Society.     

Role of esterase enzymes in monitoring for resistance of California red scale, Aonidiella aurantii (Homoptera: Diaspididae), to organophosphate and carbamate insecticides

Eighty-seven populations of California red scale, Aonidiella aurantii (Maskell), from the San Joaquin Valley of California were tested for insecticide resistance by using chlorpyrifos, methidathion, and/or carbaryl in a standard fruit-dip bioassay as well as for general esterase activity by using alpha-naphthyl acetate as a substrate in a colorimetric test. The percentage of individuals that survived a discriminating concentration of methidathion, chlorpyrifos, or carbaryl was significantly correlated with the percentage of individuals showing > 0.4 nmol of esterase activity per minute per microgram of protein in the colorimetric test. Scale survival of the organophosphates showed a higher correlation with esterase activity than survival of carbaryl. These results suggest that the colorimetric test of esterase activity is useful as an indicator of the frequency of organophosphate-resistant and, to a lesser extent, carbamate-resistant individuals in California red scale populations. The results of tests for activity and inhibition of acetylcholinesterase activity suggest that California red scale is using increased amounts of esterase enzymes, including acetylcholinesterase, to sequester organophosphate and carbamate insecticides, rather than modified acetylcholinesterase. Third instars collected from twigs, leaves, and fruit showed similar levels of esterase activity. The colorimetric test of esterase activity is a useful tool to detect organophosphate and carbamate resistance in San Joaquin Valley California red scale because of its speed of testing over a wide range of months, allowing for within-season decision making by citrus growers.     

High genetic diversity among fossorial lizard populations (Anniella pulchra) in a rapidly developing landscape (Central California)

California is a biodiversity hotspot facing unbridled human population growth, especially in Central California. One of the poorly known, sensitive species in this area is the California legless lizard (Anniella pulchra), a fossorial worm-like reptile. We report mt and nuDNA sequences from 69 museum-vouchered samples of Anniella (A. pulchra and its sister species A. geronimensis) from 48 localities. Our genetic survey reveals substantially more genetic diversity within A. pulchra than previously reported. Our two independently evolving markers (mt and nuDNA) reveal five major lineages of A. pulchra. Two of the five major lineages of A. pulchra correspond to a north-south split found in other widespread California reptiles. These northern and southern clades also correspond to a previous study showing variation in chromosomal number. Unlike most other Californian reptiles, A. pulchra has major genetic lineages that are endemic to Central California including two that are endemic to the San Joaquin Valley and Carrizo Plain. Although A. pulchra is threatened throughout its range, the distinct San Joaquin lineages are seriously imperiled by urban sprawl. Some of the localities for the newly recognized genetic lineages have already been destroyed by development.     

Nuclear gene phylogeography reveals the historical legacy of an ancient inland sea on lineages of the western pond turtle,Emys marmorata in California

The historical biogeography of California’s taxa has been the focus of extensive research effort. The western pond turtle (Emys marmorata) is an example of a wide‐ranging taxon that spans several well‐known California diversity hotspots. Using a dataset comprised of one mitochondrial and five nuclear loci, we elucidate the major biogeographic patterns of the western pond turtle across the California landscape. By employing a combination of phylogenetic and network‐based approaches, we recovered a relatively ancient (c. 2–8 Ma) north/south split among populations of E. marmorata and find an area of intergradation centred in the Central Coast Ranges of California. In addition, discordant mitochondrial/nuclear genetic patterns suggest subsequent gene flow from northern populations and from San Joaquin Valley populations into the Central Coast Ranges after the Pliocene‐Pleistocene marine embayment of the Great Central Valley subsided. Our results emphasize the utility of nuclear DNA phylogeography for recovering the impact of relatively ancient biogeographic events, and suggest that the Central Coast Ranges of California have played a major role in the geographic structuring of the western pond turtle, and possibly other co‐distributed taxa.     

Seasonal population dynamics of Draeculacephala minerva (Hemiptera: Cicadellidae) and transmission of Xylella fastidiosa

The grass sharpshooter, Draeculacephala minerva Ball (Hemiptera: Cicadellidae), is a very common and often abundant grass-feeding leafhopper in California. Its population dynamics and ability to transmit Xylella fastidiosa were monitored over a 2-yr period in California's San Joaquin Valley. Collections of individuals from natural populations in irrigated pastures and alfalfa, Medicago savita L. fields adjacent to X. fastidiosa-infected almond (Prunus spp.) orchards indicated the occurrence of three discrete generations per year that peaked during the summer. Population densities varied significantly among experimental field survey sites. Insects captured on intercepting mesh traps, yellow sticky cards, and UV-light traps indicated local movement of these insects into and surrounding X. fastidiosa-infected, almond orchards. Local movement and seasonal transmission of X. fastidiosa from infected almonds to Catharanthus roseus (L.) G. Don indicated that this insect may be partly responsible for the slow spread of almond leaf scorch now recently observed in California's San Joaquin Valley.     

Biological and molecular characteristics of Beauveria bassiana isolates from California Lygus hesperus (Hemiptera: Miridae) populations

Lygus hesperus is an important pest of many crops grown in the Western US. In addition, other species of Lygus cause damage in other parts of the world. To date, no selective pesticide exists for the control of Lygus spp. and broad spectrum pesticides that also kill natural enemies may lead to secondary pests. Entomopathogenic fungi may offer an alternative to chemical pesticides. Isolates of Beauveria bassiana collected from San Joaquin Valley of California (SJV) L. hesperus populations were screened for their ability to grow at high temperatures and for their ability to infect and kill L. hesperus adults and nymphs under laboratory conditions. No isolate grew at 37 or 35 °C but most isolates were able to grow at 32 °C. In addition, one L. hesperus isolate was more efficacious at higher doses than the commercial isolate. Microsatellite markers were used to determine that selected isolates could be distinguished from other isolates. Preliminary information suggested 82 SJV isolates of B. bassiana were closely related to each other but distantly related to the commercial isolate.     

Distribution of algae in the San Joaquin River, California, in relation to nutrient supply, salinity and other environmental factors

1. The taxonomic composition and biomass of the phytoplankton and the taxonomic composition of the phytobenthos of the San Joaquin River and its major tributaries were examined in relation to water chemistry, habitat and flow regime. Agricultural drainage and subsurface flow contribute to a complex gradient of salinity and nutrients in this eutrophic, ‘lowland type’ river. 2. Because of light‐limiting conditions for growth, maintenance demands of the algae exceed production during summer and autumn in the San Joaquin River where there is no inflow from tributaries. In contrast to substantial gains in concentration of inorganic nitrogen and soluble reactive phosphorus during the summer of normal‐flow years, net losses of algal biomass (2–4 μg L^?1 day^?1 chlorophyll a) occurred in a mid‐river segment with no significant tributary inflow. However, downstream of a large tributary draining the Sierra Nevada, a substantial net gain in algal biomass (6–11 μg L^?1 day^?1) occurred in the summer, but not in the spring (loss of 1–6 μg L^?1 day^?1) or autumn (loss of 2–5 μg L^?1 day^?1). 3. The phytoplankton was dominated in summer by ‘r‐selected’ centric diatoms (Thalassiosirales), species both tolerant of variable salinity and widely distributed in the San Joaquin River. Pennate diatoms were proportionally more abundant (in biomass) in the winter, spring and autumn. Abundant taxa included the diatoms Cyclotella meneghiniana, Skeletonema cf. potamos, Cyclostephanos invisitatus, Thalassiosira weissflogii, Nitzschia acicularis, N. palea and N. reversa, and the chlorophytes Chlamydomonas sp. and Scenesdesmus quadricauda. Patterns in the abundance of species indicated that assembly of the phytoplankton is limited more by light and flow regime than by nutrient supply. 4. The phytobenthos was dominated by larger, more slowly reproducing pennate diatoms. Few of the abundant species are euryhaline. The diatoms Navicula recens and Nitzschia inconspicua and cyanophytes, Oscillatoria spp., were the principal late‐summer benthic species upstream in the mainstem and in drainages of the San Joaquin Valley. Many of the other abundant diatoms (Amphora veneta, Bacillaria paxillifer, Navicula symmetrica, Nitzschia amphibia, N. fonticola, N. palea, Pleurosigma salinarum) of late‐summer assemblages in these segments also are motile species. While many of these species also were abundant in segments downstream of confluences with rivers draining the Sierra Nevada, the relative abundance of prostrate (Cocconeis placentula var. euglypta, Navicula minima) and erect or stalked (Achnanthidium deflexum, Achnanthes lanceolata, Gomphonema kobayasii, G. parvulum var. lagenula) diatoms and Stigeoclonium sp. was greater in these lower San Joaquin River segments. 5. A weighted‐averaging regression model, based on salinity and benthic‐algal abundance in the San Joaquin River and segments of its major tributaries within the San Joaquin Valley, yielded a highly significant coefficient‐of‐determination (r²=0.84) and low prediction error between salinity inferred from the species and that observed, indicating that salinity tolerance is a primary constraint on growth and assembly of the phytobenthos. The same measures of predictability indicated poor performance of a model based on inorganic nitrogen. However, with a greater representation of tributaries (including segments within the Sierra Nevada foothills) in the sample set, an inorganic nitrogen model also yielded a highly significant coefficient‐of‐determination (r²=0.87) and low prediction error between the species‐inferred and the observed concentration. As with the salinity model (r²=0.94) for the enlarged data set, a systematic difference (increased deviation of residuals) existed at high inorganic nitrogen concentrations. These results indicate substantial interaction between salinity and inorganic nitrogen as constraints on the structure of benthic‐algal communities of the San Joaquin River basin.     

Genic diversity, genetic structure, and biogeography of Pinus sabiniana Dougl.

Pinus sabiniana Dougl. (grey pine) forms savanna forests in the foothills surrounding California's Great Central Valley. However, its fossil record, which dates from the late Miocene through the Pliocene and Pleistocene, is found exclusively in southern California, south of the species’ present range. A total of twenty-nine isozyme loci, representing eighteen enzyme systems, was assayed to analyse the genetic structure in eight populations of grey pine and attempt to track its migration history from southern to northern California. Expected heterozygosity in the two southernmost samples was 0.128 and 0.150, and heterozygosity tended to decrease with increasing latitude, suggesting the loss of diversity as grey pine dispersed northward. However, genetic distances between populations were very small, even on opposite sides of the treeless Great Central Valley; and estimated time since divergence was 900 to 9000 years at a maximum. Wright's F_ST, the proportion of total genetic diversity among populations, was only 0.057, which is similar to values found in many conifers with continuous distributions. Nm, the number of migrants among populations per generation, was 4.1 to 6.7, depending on estimator, and indicates that gene flow is extensive, or was so in the recent past. In every population, observed heterozygosity was less than expected heterozygosity, and the fixation index, F_IS, for the progeny was 0.128, which indicates a fairly high rate of inbreeding. The genetic similarity of disjunct populations, in combination with paleogeographic and paleoclimatic evidence, suggests that grey pine formed a continuous population throughout the Great Central Valley, perhaps between 12,000 and 8000 yrs BP . Its range became fragmented during the Xerothermic, when it ascended into the foothills. Gaps in its range correlate with late Pleistocene–early Holocene lakes in adjacent basins and with the Sacramento–San Joaquin Delta.     

How do riparian woody seedlings survive seasonal drought?

In semi-arid regions, a major population limitation for riparian trees is seedling desiccation during the dry season that follows annual spring floods. We investigated the stress response of first-year pioneer riparian seedlings to experimental water table declines (0, 1 and 3 cm day⁻¹), focusing on the three dominant cottonwood and willows (family Salicaceae) in California’s San Joaquin Basin. We analyzed growth and belowground allocation response to water stress, and used logistic regression to determine if these traits had an influence on individual survival. The models indicate that high root growth (>3 mm day⁻¹) and low shoot:root ratios (<1.5 g g⁻¹) strongly predicted survival, but there was no evidence that plants increased belowground allocation in response to drawdown. Leaf δ¹³C values shifted most for the best-surviving species (net change of +3.5 per mil from −30.0 ± 0.3 control values for Goodding’s willow, Salix gooddingii), implying an important role of increased water-use efficiency for surviving water stress. Both S. gooddingii and sandbar willow (S. exigua) reduced leaf size from controls, whereas Fremont cottonwood (Populus fremontii) sustained a 29% reduction in specific leaf area (from 13.4 to 9.6 m² kg⁻¹). The functional responses exhibited by Goodding’s willow, the more drought-tolerant species, may play a role in its greater relative abundance in dry regions such as the San Joaquin Basin. This study highlights the potential for a shift in riparian forest composition. Under a future drier climate regime or under reduced regulated river flows, our results suggest that willow establishment will be favored over cottonwood.     

Can the destruction of California’s oak woodlands be prevented? Potential for biological control of the goldspotted oak borer, <Emphasis Type="Italic">Agrilus auroguttatus</Emphasis>

The goldspotted oak borer (GSOB), Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is an introduced and aggressive phloem/wood borer infesting native oaks in southern California. Elevated levels of oak mortality have occurred continually for the last nine years on three oak species in San Diego Co., California, USA. Biological control is being assessed as an option for long-term and widespread management of the invasive population of GSOB. Foreign exploration in the native ranges of GSOB and a related sibling species (Agrilus coxalis Waterhouse) was conducted to determine life history information, to assess the natural enemy complex, and to collect specimens for molecular analyses that could help to identify the area of origin of California’s introduced population. Two species of parasitoids, Calosota elongata Gibson (Eupelmidae) and Atanycolus simplex Cresson (Braconidae), were discovered with GSOB populations in Arizona and California. No insect natural enemies were found with populations of A. coxalis in southern Mexico. However, Quercus conzatti Trel. and Quercus peduncularis Nee in Oaxaca and Chiapas, respectively, were recorded as the first known hosts of A. coxalis. A comparative analysis of our understanding of the natural enemy complexes for other pestiferous Agrilus with that of GSOB suggests that more effort should be directed at uncovering potential egg parasitoids and microbial pathogens of GSOB. Analyses of mitochondrial and nuclear ribosomal DNA (rDNA) revealed that the California population of GSOB was more similar to the Arizona population. Specimens of A. coxalis from southern Mexico were confirmed as a separate species. Additional surveys and sampling are needed across the complete native range of the GSOB species complex to develop a comprehensive inventory of parasitoid species that could be considered for use in a classical biological control program in California and to delineate the area of origin of California’s population.     

Book Reviews

Book reviewed in this article:
The Epidemic of 1830–1833 in California and Oregon . S. F. C ook
The Aboriginal Population of the San Joaquin Valley, California . S. F. C ook     

Gene flow among San Joaquin kit fox populations in a severely changed ecosystem

The San Joaquin kit fox (Vulpes macrotis mutica) was once ubiquitous throughout Californias San Joaquin Valley and its surrounds. However, most of its habitat has been lost to irrigated agriculture, urban development, and oil fields. The remaining foxes are concentrated in six areas, although there are several small pockets of foxes throughout the Valley. To help conserve kit foxes, we sought an ecological understanding of the level of genetic variation remaining in these locations and the extent of gene flow among them. We collected tissue from 317 kit foxes from 8 sites and estimated genetic variability in and gene flow among sites using data from 8 polymorphic, microsatellite markers. We found no differences in both observed and expected heterozygosity between locations using Bonferonni corrected paired t-tests. We found differences in mean number of alleles per locus, even after we used Monte Carlo simulations to adjust for sample size differences. Population subdivision was low among sites (F_st=0.043), yet a matrix of pairwise F_st values was correlated with a matrix of pairwise geographic distances. An assignment test classified only 45% of the individuals to the site where they were captured. Overall, these data suggest that kit fox dispersal between locations may still maintain genetic variation throughout most of the areas we sampled.