Examining the Potential Role of Foliar Chemistry in Imparting Potato Germplasm Tolerance to Potato Psyllid, Green Peach Aphid, and Zebra Chip Disease

Long-term, sustainable management of zebra chip disease of potato, caused by 'Candidatus Liberibacter solanacearum' (Lso) and vectored by potato psyllids (Bactericera cockerelli Sulc [Hemiptera: Triozidae]), requires development of cultivars resistant or tolerant to infection or capable of reducing spread or both. We examined the influence that five experimental breeding clones of potato had on potato psyllids and their ability to vector Lso. The ability of these potato clones to resist aphids (green peach aphids, Myzus persicae Sulzer [Hemiptera: Aphididae]) also was examined. Due to the importance of host chemistry on plant-insect interactions, levels of primary metabolites of amino acids and sugars, as well as secondary metabolites including polyphenolics, terpenoids, and alkaloids were compared between breeding clones and a commercial cultivar. Findings for compound levels then were associated with observed changes in host susceptibility to psyllids or aphids. Psyllids oviposited less on three breeding clones than Atlantic, but no significant effects of breeding clones on psyllid feeding or choice were observed. Aphid reproduction was reduced on two clones relative to Atlantic. A05379-211 had greater sugar levels and postpsyllid amino acid levels than Atlantic. Total alkaloid and phenolic levels were greater in all breeding clones than Atlantic. Total terpenoid levels were greater in PALB03016-3 and PALB03016-6 than Atlantic, which might explain, in part, the observed resistance to psyllid oviposition and aphid reproduction. Overall, these results suggest that increased levels of certain metabolites in breeding clones could affect psyllid and aphid reproduction.

De Novo Genome Sequence of "Candidatus Liberibacter solanacearum" from a Single Potato Psyllid in California

The draft genome sequence of "Candidatus Liberibacter solanacearum" strain RSTM from a potato psyllid (Bactericera cockerelli) in California is reported here. The RSTM strain has a genome size of 1,286,787 bp, a G+C content of 35.1%, 1,211 predicted open reading frames (ORFs), and 43 RNA genes.

Ozone-induced changes in host-plant suitability: Interactions ofKeiferia lycopersicella andLycopersicon esculentum

Tomato pinworms,Keiferia lycopersicella (Walsingham), survived better and developed faster on tomato plants,Lycopersicon esculentum Mill., damaged by ozone than on plants not subjected to ozone fumigation. Other measures of fitness, including survival during pupation, sex ratio of adults, female longevity, and fecundity, were not affected. Analyses of ozonated foliage at zero, two, and seven days following fumigation demonstrated a transient but significant increase (18-24%) in soluble protein concentration. Although the concentration of the total free amino acids in ozonated foliage did not increase significantly, significant changes were observed in at least 10 specific amino acids, some of which are critical for either insect development or the production of plant defensive chemicals. A reduction in total nitrogen in ozonated foliage at seven days postfumigation indicated that nitrogen was being translocated to other portions of the plant. The implications of increases in assimilable forms of nitrogen in ozonated foliage, which lead to improved host-plant suitability for insect herbivores, are discussed both in relation to some current ecological theories and in regard to pest-management strategies.

Individual and joint actions of selenate and methylmercury on the development and survival of insect detritivore Megaselia scalaris (Diptera: Phoridae)

Despite the important roles played by insects in most ecosystems, surprisingly little is known about how anthropogenic pollutants or their mixtures interact to affect insect populations. The independent and joint actions of selenate and methylmercury on a ubiquitous insect detritivore, Megaselia scalaris (Loew), were determined in this study. Ovipositing females did not distinguish between untreated food sources and those contaminated with toxic concentrations of selenate, methylmercury, or both chemicals in combination. Even at the highest concentrations of pollutants, no negative effects were observed for the egg stage. However, larval survival was significantly decreased and development significantly prolonged by selenate and methylmercury individually at low or intermediate ecologically relevant treatment levels. Potentiation was strongly evident because mixtures containing concentrations as little as only 1% of the respective individual median lethal tolerances (LC(50)s) caused significantly more mortality and delayed larval development than would be expected from the responses selenate and methylmercury elicit individually. However, survival and pupal development was not affected at any rate tested. Female fecundity was significantly decreased by methylmercury but not by selenate or mixture treatments. The relative toxicity to M. scalaris of each of the individual and joint treatments was selenate (LC(50) = 260 microg/g) < methylmercury (LC(50) = 22 microg/g) < the mixture at approximately 5% of the LC(50) concentration of each of the components (12 microg/g selenate plus 1.0 microg/g methylmercury). The increased mortality and delayed larval development within sites contaminated by selenate, methylmercury, or combination of the two have substantial implications for the ecology, population dynamics, and sustainability of M. scalaris populations. If these results can be extrapolated to other arthropod detritivores, ecosystem food-web function may be substantially affected.

Ovipositional preferences, damage thresholds, and detection of the tomato-potato psyllid Bactericera cockerelli (Homoptera: Psyllidae) on selected tomato accessions

The tomato-potato psyllid Bactericera [Paratrioza] cockerelli (Sulc) has recently caused losses exceeding 50% on fresh market tomatoes in California and Baja, Mexico by injecting a toxin that results in a condition known as 'psyllid yellows'. The objectives of this study were to: (i) document oviposition preferences on a range of tomato cultivars; (ii) determine threshold levels for psyllid densities that would cause psyllid yellows on tomatoes within the first three weeks following transplanting; and (iii) identify the most important 'psyllid yellows' symptoms that might be used in surveying and monitoring for this pest. Plant lines tested included the commonly-planted commercial cultivars 'Shady Lady' and 'QualiT 21', an older, previously commercial cultivar '7718 VFN', a common cultivar planted by consumers 'Yellow Pear', and a wild type plant accession, PI 134417. When given a choice, psyllids significantly preferred 'Yellow Pear' and avoided PI 134417 for oviposition. Under no-choice conditions psyllids laid significantly fewer eggs on PI 134417, but all the other plant lines were equally good substrates for laying eggs. Thus, oviposition preference is not likely to provide a functional management strategy in large plantings. On 'Shady Lady', psyllids preferred to oviposit on plants already infested with adults. On both 'Shady Lady' and '7718 VFN' oviposition was significantly greater on plants previously infested by nymphs as compared to uninfested control plants. This suggests that, at least for some cultivars, there is a physiological change in plant attractiveness following psyllid feeding. 'Yellow Pear' and 'QualiT 21' were relatively tolerant of psyllids, requiring 18 nymphs per plant to produce the disease symptoms. Only eight nymphs per plant were needed on 'Shady Lady' and '7718 VFN'. For all cultivars, the pest density showed strong correlations with measurements such as the number of yellowing leaves and leaflets and distorted leaves, which were as good as or better than the first factor extracted from principal component analysis. Therefore, such measurements have the potential to simplify field surveys.

Evaluation of Atriplex lines for selenium accumulation, salt tolerance and suitability for a key agricultural insect pest

Thirty Atriplex lines were examined for potential habitat improvement and phytoremediation of selenium (Se) contaminated sites. Studies were conducted to determine the biomass production, Se accumulation, and resistance of each line to the beet armyworm, Spodoptera exigua, an agriculturally important insect. Plants were tested using three salinity treatments: (1) control, no Se; (2) NaCl and CaCl2 salts and 1 mg l(-1) Se (12.7 microM) added as sodium selenate; and (3) iso-osmotic to treatment 2 containing high concentrations of sulfate and I mg l(-1) Se added as sodium selenate. Insect bioassays measured survival, growth, and development. Atriplex patula. A. spongiosa 415862, A. hortensis, A. hortensis 379088 and A. hortensis 379092 were among the top biomass producers and Se accumulators, yet they exhibited significantly reduced insect growth, development, and survival. High background sulfate strongly reduced Se accumulation, suggesting that phytoremediation potential is greatest in saline areas having low to moderate sulfate levels. However, these lines grew well in high salinity soils, indicating possible use as a self-seeding cover crop to improve habitat. All plant lines grown in control and high sulfate salinity treatments are acceptable oviposition sites for S. exigua, indicating that these plants would help reduce populations of this key agricultural pest.

Avocadofurans and their tetrahydrofuran analogues: comparison of growth inhibitory and insecticidal activity

The importance of the double bonds in the furan ring of avocadofurans with relation to their insecticidal activity was examined. The insecticidal activity of two naturally occurring avocadofurans, 2-(pentadecyl)furan and 2-(heptadecyl)furan, was compared to the toxicity of five tetrahydrofurans with alkyl chains at position 2 and varying side chains from 14 to 18 carbons. We found that eliminating the sites of unsaturation in the furan ring of avocadofurans significantly reduced the detrimental effects on the mortality and growth of the generalist insect herbivore Spodoptera exigua. In 7-day bioassays, S. exigua larvae were significantly more affected when fed a diet containing avocadofurans as compared to a larvae fed diet treated with the analogous tetrahydrofurans. Although larvae fed with the tetrahydrofurans showed reduced growth as compared to controls, larval mortality was not significantly increased. We conclude that the double bonds in the furan ring of avocadofurans play an important role in their insecticidal effects.

The resolution of Californian populations of Liriomyza huidobrensis and Liriomyza trifolii (Diptera: Agromyzidae) using PCR

Liriomyza trifolii (Burgess) and Liriomyza huidobrensis (Blanchard) are important vegetable pests in California. Populations of each species differ in their impact in central and southern regions. This difference may be explained by geographical or host plant differences in each of the regions. We used random amplified polymorphic DNA polymerase chain reactions to assess genetic differences between two laboratory populations of each species collected from central and southern California. Individual L. trifolii from the two regions could be discriminated by the presence/absence of PCR products. No such qualitative differences were apparent in PCR products amplified from L. huidobrensis individuals, but the origins of individuals could be differentiated using a bootstrap analysis of marker frequencies. Marker primers were used to compare field and laboratory individuals. No evidence was found for the existence of further populations or of hybrid populations in central and southern California. The distribution of populations of L. huidobrensis was explained completely by geographical differences. As a consequence of the absence of leafminer infestations on the same host plant varieties in both regions, factors governing L. trifolii population distribution differences were less apparent. The presence of the same host plant varieties at both sites suggests that the two L. trifolii populations differ in host plant preference.

Feeding preferences of spodoptera exigua in response to form and concentration of selenium

Minimal information is available on the impact of various organic and inorganic forms of the ecologically and agriculturally important pollutant, selenium (Se), on insect herbivores. We conducted bioassays with artificial diet to examine the feeding responses of a generalist herbivore, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae), to various forms and concentrations of Se. Two different-aged cohorts of larvae were examined in choice tests with control diets vs. test diets incorporating lethal concentrations (LC(10), LC(30), LC(50), and LC(70)) of sodium selenate, sodium selenite, seleno-DL-cystine, and seleno-DL-methionine. Tests initiated with neonates showed larvae significantly preferred control diet over diet with sodium selenate, sodium selenite, or selenocystine, but at most concentrations showed no preference between selenomethionine and control diet. Choice tests initiated with third instars demonstrated a preference for control diet over sodium selenate treatments, and sodium selenite treatments. In contrast, no significant responses were found in tests initiated with third instars offered the choice between selenocystine or selenomethionine and untreated controls. Additionally, comparisons of consumption demonstrated that inorganic selenium compounds were antifeedants whereas the organic selenium compounds tested have little antifeedant activity. The toxicity of all of the tested forms of selenium, in combination with the lack of antifeedant activity of some compounds, has the potential to affect both the distribution and diversity of terrestrial herbivores in both agricultural and natural systems. Arch. Copyright 1999 Wiley-Liss, Inc.

Alkylfurans: effects of alkyl side-chain length on insecticidal activity

The insecticidal activity of five alkylfurans against the generalist insect herbivore beet armyworm, Spodoptera exigua, was examined. Two naturally occurring compounds, the avocadofurans 2-(pentadecyl)furan (1) and 2-(heptadecyl)furan (2), previously isolated from specialized avocado idioblast oil cells, and three homologues, 2-(tetradecyl)furan (3), 2-(hexadecyl)furan (4), and 2-(octadecyl)furan (5), were synthesized. Bioassays of alkylfurans 1-5 using a 9-day diet-incorporation initiated with neonates showed that all alkylfurans tested significantly increased S. exigua larval mortality and reduced larval weights, with maximal biological activity detected among the naturally occurring alkylfurans 1 and 2.

Isolation, identification, and biological activity of isopersin, a new compound from avocado idioblast oil cells

A new compound, (12Z,15Z)-1-hydroxy-4-oxo-heneicosa-12,15-dien-2-yl acetate, isopersin (2), has been isolated from avocado idioblast oil cells. In artificial diet bioassays, 2 showed no effects on either larval survivorship or growth of early-instar beet armyworm Spodoptera exigua. In contrast, the isomeric persin (1), (12Z, 15Z)-1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene, reduces larval growth at equivalent concentrations (500 &mgr;g g-1). Compound 2 is not very stable and isomerizes readily to 1. Both compounds are acid-labile, rearranging rapidly to alkylfuran 3 in the presence of traces of acid.

Influence of form and quantity of selenium on the development and survival of an insect herbivore

Even plants classified as 'nonaccumulators' can sequester concentrations of sodium selenate, sodium selenite, selenocystine and selenomethionine that can strongly influence insect development and survival. These forms of selenium (Se), tested in diet-incorporation bioassays, proved toxic to larvae of a generalist insect herbivore at relatively low levels. Sodium selenite was the most toxic form tested against Spodoptera exigua (Hübner), with an LC(50) of 9.14 microg g(-1) wet wt (21.11 microg g(-1) dry wt). Selenocystine was intermediate with an LC(50) of 15.2 microg g(-1) wet wt. The least toxic forms, sodium selenate and selenomethionine, had LC(50)s below 50 microg g(-1) dry wt, the upper level for tissues of plants classified as nonaccumulators. Ingestion of some forms of Se also affected growth and development. Increasing concentrations of sodium selenate and sodium selenite decreased pupal weight and added significantly to the time needed for development to the pupal and adult stages. The time required to complete the larval stage increased by over 25% and the time from egg to adult emergence was extended by 22% to nearly 30%. Selenocystine and selenomethionine did not significantly increase developmental times, even at concentrations that killed 90% or more of the test populations. Analyses of relative growth rate, relative growth index, and an analysis of covariance technique for measuring growth indicated that the form of Se affected growth rates, growth inhibition responses of the larvae, and toxicological effects. Thus, quantity and the form of Se accumulating in plants grown on Se-contaminated sites are likely to influence the population dynamics of insect herbivores. The implications of these results for the ecology of contaminated sites are discussed.

Interactions between the Encelia leaf beetle and its host plant, Encelia farinosa: The influence of acidic fog on insect growth and plant chemistry

The impact of acidic deposition on interactions between the plant Encelia farinosa and the herbivorous beetle Trirhabda geminata (Chrysomelidae) was determined under greenhouse conditions. Acidic fogs (pH 2.75) did not significantly affect the overall foliar concentrations of water or soluble protein as compared with control fogs (pH 5.6). Nonetheless, E. farinosa foliage was altered by exposure to three 3-h acidic fogs such that growth and biomass gain by T. geminata increased by more than 30% as compared to beetles feeding on control-fogged plants. Thus, previous indications that changes in soluble proteins or water content were responsible for increased biomass gain and growth of T. geminata cannot be substantiated by this study. Additionally, changes in the plant defensive chemistry were not responsible for increased herbivore growth, as farinosin, encecalin, and euparin foliar concentrations did not vary significantly between fog treatments. Significant increases in CO2 assimilation rates of E. farinosa exposed to acidic fogs were documented at 3, 7, and 21 days following treatment, suggesting that carbohydrate-based products of increased plant metabolism may have played a role (e.g. soluble carbohydrates). However, the key factors responsible for increasing herbivore performance on acidic-fogged E. farinosa remain largely unknown.

Effect of Insecticides on Celery Insectsc, 1995

Celery was transplanted in a sandy loam soil on 29-30 Aug at the University of California’s South Coast Field Station. The plants were sprinkler irrigated for three weeks and drip irrigated (water pH 7.2-7.5) thereafter. Experimental plots were four beds wide (two rows per bed on 40-inch centers) by 40 ft and separated by a 5 ft buffer with four replicates of each treatment in a RCB design. Applications were weekly from 5 Oct through 30 Nov (total of 9 applications for all chemicals except Avid and Avid + Lannate). Actual dates of applications were: 5, 12, 19, 26 Oct, and 3, 9, 16, 21, 30 Nov. Avid was applied on 5, 12 Oct and again on 3, 9, 30 Nov. The Avid + Lannate treatment was the same except that Lannate was applied on 19, 26 Oct and 16, 21 Nov. All chemicals were applied by a tractor-mounted boom sprayer operated at 125 psi. Nozzles were 4 per bed and carrier (H2O) was at 100 gpa. Disc-type cone nozzles incorporated D3 orifice discs, #25 cores and 50-mesh screens. All treatments received spreader-sticker. Antifoam (No-Foam®) was used with all treatments. Leafminer populations were evaluated by weekly counts of leafminer prepupae and pupae in four 5 1/2 X 11 inch trays/replicate when the plants reached a suitable height. Actual sampling dates were 16, 25, 31 Oct and 8, 16, 20, 30 Nov. The number of dead adult parasites of Liriomyza species were recorded in the same manner and on the same dates listed for pupal counts. Evaluation of BAW populations was based on number of damaged plants found in 25 plants per replicate (100/treatment) from the center two rows of each replicate at harvest (6-7 Dec).

Effect of Insecticides on Tomato Insects 1995

Seedlings were transplanted in a sandy loam type soil on 21 June at the University of California’s South Coast Field Station in Santa Ana, California. Experimental plots were 4 rows wide (5-ft centers) by 40 ft long and separated by a 5-ft buffer. The tomato transplants were drip irrigated (water pH 7.2-7.5). Treatments were replicated 4 times in a RCB design. Weekly applications were initiated at first fruiting. Application dates were 27 Jul, 3, 10, 16, 24, 30 Aug, and 7, 14 Sep. (except Agrimek which was applied on 27 Jul 3, 24, 30 Aug, and Agrimek + Lannate in which Lannate was applied every wk). A tractor-mounted boom sprayer with 6 nozzles per row incorporated D-3 orifice disks, #25 cores, and 50 mesh screens. Operating pressure was 100 psi delivering 100 gal/acre. Spreader sticker (Leaf Act 80) at 0.04% was included for all treatments. Leafminer populations were evaluated by weekly counts of leafminer prepupae and pupae in four 9 by 11 inch trays/replicate from 1 Aug through 12 Sep., inclusive. On 18-21 Sep., 200 mature-green to ripe fruit were harvested from the center 2 rows of each replicate (800 per treatment) and examined for internal damage (primarily H. zea) and external damage (primarily S. exigua and K. lycopersicella). Leafminer populations were low early, then moderate late in this test. Some treatments increased leafminer density as compared to the control. Beet armyworm and tomato pinworm populations were moderate as were tomato fruitworm populations. No phytotoxicity was observed in any treatment.

Trirhabda geminata (Coleoptera: Chrysomelidae) resistance to the direct impact of simulated acidic fog on larval growth and mortality

The direct effects of acidic fog (pH 2.75) upon the mortality and growth of Trirhabda geminata Horn (Coleoptera: Chrysomelidae), the dominant herbivore of the California coastal sage scrub, Encelia farinosa (Compositae: Asteraceae), were evaluated. Although there was a consistent pattern among and between experiments, suggesting that acidic fogs could reduce insect growth and survivorship within the first few days following application of treatments, an exposure to three consecutive, 3-h fogs over a five day period did not significantly affect mortality, biomass gain, or larval growth rate. There are two important implications from this study. First, even the highly acidic fogs found in southern California will have minimal direct effects on T. geminata performance. Second, the impacts on T. geminata biology observed in previous studies were likely mediated by host-plant responses to acidic-fog episodes.

Insecticidal activity of the CryIIA protein from the NRD-12 isolate of Bacillus thuringiensis subsp. kurstaki expressed in Escherichia coli and Bacillus thuringiensis and in a leaf-colonizing strain of Bacillus cereus

A 4.0-kb BamHI-HindIII fragment encoding the cryIIA operon from the NRD-12 isolate of Bacillus thuringiensis subsp. kurstaki was cloned into Escherichia coli. The nucleotide sequence of the 2.2-kb AccI-HindIII fragment containing the NRD-12 cryIIA gene was identical to the HD-1 and HD-263 cryIIA gene sequences. Expression of cryIIA and subsequent purification of CryIIA inclusion bodies resulted in a protein with insecticidal activity against Heliothis virescens, Trichoplusia ni, and Culex quinquefasciatus but not Spodoptera exigua. The 4.0-kb BamII-HindIII fragment encoding the cryIIA operon was inserted into the B. thuringiensis-E. coli shuttle vector pHT3101 (pMAU1). pMAU1 was used to transform an acrystalliferous HD-1 strain of B. thuringiensis subsp. kurstaki and a leaf-colonizing strain of B. cereus (BT-8) by using electroporation. Spore-crystal mixtures from both transformed strains were toxic to H. virescens and T. ni but not Helicoverpa zea or S. exigua.

Host plant resistance and linear furanocoumarin content of Apium accessions

Linear furanocoumarin contents and antibiotic resistance to Liriomyza trifolii (Burgess) were documented for Apium species being investigated in a celery breeding program. In no-choice tests, L. trifolii fed more, produced more offspring, and had the highest pupal and adult productivity on the widely planted cultivar 'Tall Utah' 52-70R (Apium graveolens L.). Antibiotic effects of the commercial cultivar 'Tall Utah' 52-70 HK and University of California families 87A-147 and 87A-338, derived from A. chilense Hook and Arn., were intermediate. Only A. nodiflorum (L.) Lag (accession 87A-236) did not allow survival beyond the larval stage. Concentrations of the carcinogenic and mutagenic linear furanocoumarins varied by location within plants (leaves usually greater than petioles), by specific compound (trend: psoralen less than xanthotoxin less than bergapten or isopimpinellin), and between accessions. A. nodiflorum had the lowest foliar levels of phototoxic furanocoumarins (11.8 micrograms/g fresh weight) and the best potential for use in the breeding program. Foliar levels of phototoxic furanocoumarins (psoralen, bergapten, and xanthotoxin) in plants 87A-147-3 (406 micrograms/g), 87A-147-2 (292.9 micrograms/g), and the family 87A-338 (265.9 micrograms/g) were 22.6, 16.3, and 14.8 times higher, respectively, than the concentration known to produce contact dermatitis (18 micrograms/g). Even with such variability in concentration, the foliar content of linear furanocoumarins (individually or total) and L. trifolii adult production were not correlated.