Anatomy of an outbreak: the biology and population dynamics of a Cardiaspina psyllid species in an endangered woodland ecosystem

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Zebra chip disease incidence on potato is influenced by timing of potato psyllid infestation, but not by the host plants on which they were reared

Abstract  The Zebra chip (ZC) syndrome is an emerging disease of potato and a major threat to the potato industry. The potato psyllid, Bactericerca cockerelli (Sulc) is believed to be a vector of the ZC pathogen, which is now thought to be Candidatus Liberibacter, a bacterium. To further understand the relationship between potato psyllid infestation and ZC disease expression, healthy potato plants at different growth stages (4, 6 and 10 weeks after germination) were exposed separately to potato psyllids that were separately reared on four solanaceous hosts plants (potato, tomato, eggplant or bell pepper) for more than 1 year. ZC symptoms, leaf rates and total nonstructural carbohydrate accumulation in leaves and tubers of healthy and psyllid-infested plants were monitored and recorded. Typical ZC symptoms were observed in leaves and tubers of all plants exposed to potato psyllids regardless of the host plant on which they were reared. This was also accompanied by significant reductions in net photosynthetic rate. Caged potato plants without exposure to potato psyllids (uninfested controls) did not show any ZC symptom in both foliage and in harvested tubers. Foliage damage and ZC expression were most severe in the potato plants that were exposed to potato psyllids 4 weeks after germination compared to plants infested at later growth stages. Tubers from potato psyllid-infested plants had significantly higher levels of reducing sugars (glucose) and lower levels of starch than those in healthy plants, indicating that potato psyllid infestation interfered with carbohydrate metabolism in either leaves or tubers, resulting in ZC expression.     

Spatio-temporal variation in Lepidopteran larval assemblages associated with oak, Quercus crispula: the importance of leaf quality

Abstract.  1. Lepidoptera larval abundance and diversity in the canopies of oak ( Quercus crispula ) trees and saplings were surveyed in a cool-temperate, deciduous broadleaf forest in northern Japan.
2. In general, newly developed leaves were soft, rich in water and nitrogen, and low in tannin, whereas they became tough, poor in water and nitrogen, and high in tannin as the season proceeded. Leaf quality also varied among forest strata, such variations resulting in seasonal and among-strata differences in the structure of the Lepidoptera larval assemblage.
3. The greater Lepidoptera larval abundance and species richness may related to the higher leaf quality on spring foliage compared with summer foliage. On the other hand, diversity (Shannon's H' ) and evenness (Pielou's J' ) were greater on summer foliage than on spring foliage. Strengthened defences of the host plants against herbivory may cause these differences by filtering the larvae of Lepidoptera species and by constraining the super-dominance of a few species on summer foliage.
4. Canonical Correspondence Analysis (CCA) ordination also revealed a stratified structure of the Lepidoptera larval assemblage in the forest. In both spring and summer, the assemblage composition was more similar between sunlit and shaded canopies than between canopies and saplings. Such assemblage stratification was highly correlated with toughness and tannin content (in spring and summer) or water content (in summer).
5. This study emphasised the importance of spatio-temporal variations in leaf quality, even within the same host plant species, for promoting herbivore diversity in forests.     

Arthropod assemblages in a focal tree species (Eucalyptus microcarpa) depends on the species mix in restoration plantings

The composition of arthropods in trees has long been a topic of interest, with many studies exploring ways in which arthropod communities differ among tree species or entire forests. Few studies, however, have examined arthropods from trees in restoration plantings, and little is known about how different tree plantings might lead to different biodiversity outcomes. The aim of our study was to determine if a focal tree species hosted a different arthropod fauna depending on its context. We examined arthropod assemblages from the foliage of Eucalyptus microcarpa (Grey Box) trees present in two kinds of plantings: (i) simple plantings with only Eucalyptus trees, and (ii) mixed plantings with both Eucalyptus and Acacia trees. We examined the composition of the assemblages, and looked for associations between ant and psyllid (Hemiptera) diversity across each kind of planting. We found more species of psyllids, beetles and ants in E. microcarpa trees when they were grown in simple plantings compared with the mixed species plantings. We also found that psyllid richness and abundance was positively correlated with ant richness and abundance across all plantings. Our study shows that a more diverse context (i.e. a mixed planting) is not necessarily associated with a more diverse fauna within a focal tree species, and that simple Eucalyptus-only plantings may encourage higher numbers of insect herbivores. The apparent association between psyllids and ants suggests a complex interaction that may drive patterns in tree arthropod assemblages.     

Repellent effect of guava leaf volatiles on settlement of adults of citrus psylla, Diaphorina citri Kuwayama, on citrus

Abstract  The Asiatic citrus psyllid ( Diaphorina citri Kuwayama [Hemiptera: Sternorrhyncha: Psyllidae]) is a vector of huanglongbing (citrus greening), a devastating disease of citrus caused by phloem-limited bacteria. Growing guava ( Psidium guajava ) as an intercrop appears to be a successful means of reducing psyllid numbers within citrus orchards; however, the mechanism by which such a reduction is achieved is unknown. To determine the repellent effect of guava leaf and factors attributed to this activity, responses of adult psyllids to guava leaf and its odor were evaluated in cage tests and Y-tube olfactometer test. The results showed that guava leaf possessed a repellent effect against the adult citrus psyllids. Fewer psyllids were found on citrus leaves in the presence of guava foliage than in its absence. Young and old guava leaf showed equal repellent activity. By covering the guava shoots with net cloth, it was revealed that the repellent effect of guava leaf against adult psyllids on citrus was attributed to the volatile compounds, rather than physical factors. The olfactometer response of adult psyllids to guava leaf odor was dosage-dependent. Between guava odor and control, only 35.00%, 25.00% and 16.25% of the psyllids moved toward guava odor when presented with 5.0, 10.0 and 15.0 g of guava shoots, respectively. The olfactometer experiments also showed that both male and female psyllids responded similarly to the guava leaf odor.     

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.     

Cospeciation of psyllids and their primary prokaryotic endosymbionts

Psyllids are plant sap-feeding insects that harbor prokaryotic endosymbionts in specialized cells within the body cavity. Four-kilobase DNA fragments containing 16S and 23S ribosomal DNA (rDNA) were amplified from the primary (P) endosymbiont of 32 species of psyllids representing three psyllid families and eight subfamilies. In addition, 0.54-kb fragments of the psyllid nuclear gene wingless were also amplified from 26 species. Phylogenetic trees derived from 16S-23S rDNA and from the host wingless gene are very similar, and tests of compatibility of the data sets show no significant conflict between host and endosymbiont phylogenies. This result is consistent with a single infection of a shared psyllid ancestor and subsequent cospeciation of the host and the endosymbiont. In addition, the phylogenies based on DNA sequences generally agreed with psyllid taxonomy based on morphology. The 3' end of the 16S rDNA of the P endosymbionts differs from that of other members of the domain Bacteria in the lack of a sequence complementary to the mRNA ribosome binding site. The rate of sequence change in the 16S-23S rDNA of the psyllid P endosymbiont was considerably higher than that of other bacteria, including other fast-evolving insect endosymbionts. The lineage consisting of the P endosymbionts of psyllids was given the designation Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.).     

More is not necessarily better: the impact of limiting and excessive nutrients on herbivore population growth rates

Abstract 1. The body tissues of insect herbivores contain higher concentrations of nitrogen and phosphorus than do their host plants, leading to an elemental mismatch that can limit herbivore growth, fecundity and ultimately influence population dynamics. While low nutrient availability can limit herbivore growth and reproduction, nutrient levels that exceed an organism’s nutritional requirements, i.e. an organisms’ threshold elemental ratio, can also decrease performance. 2. We conducted a laboratory experiment to examine the impacts of nitrogen and phosphorus additions on population growth rates of a phloem‐feeding insect herbivore. 3. Herbivore per capita population growth rates were highest at intermediate foliar nitrogen concentrations, indicating a performance cost on the highest nitrogen foliage. While there was no direct effect of foliar phosphorus concentration on insect performance, there was a strong and unexpected indirect effect. High soil phosphorus availability increased both foliar nitrogen concentrations and aphid tissue nitrogen, resulting in low population growth rates when both soil nitrogen and phosphorus availabilities were high. 4. In this study, experimental increases in foliar nitrogen levels led to a decrease in herbivore performance suggesting that excessive nutrient levels can limit herbivore population growth rates.     

Observations on the entomopathogenic fungus Hirsutella citriformis attacking adult Diaphorina citri (Hemiptera: Psyllidae) in a managed citrus grove

A two-year field study was conducted in an orange grove in the United States (Florida) to characterize the phenology of the entomopathogen Hirsutella citriformis Speare infecting adults of the Asian citrus psyllid, Diaphorina citri Kuwayama. On the average, 23% of adults observed on mature leaves were killed by H. citriformis. These dead psyllids were characterized as being mummified and covered to various extents by synnemata produced by the fungus. Mummified cadavers were most abundant on citrus leaves during the fall and winter months, with the maximal percentage of mummified psyllids sometimes exceeding 75% of the total number of adults observed. Mummified cadavers were nearly absent each spring, presumably because relative humidity levels were suboptimal for the fungus at this time. Based on dispersion analyses, a monitoring plan for mummified cadavers would best include multiple samples in individual trees as well as multiple tree samples throughout a grove. Mummified cadavers with synnemata, which serve as point sources for new infections of the fungus in psyllids, were observed to remain on leaves for a mean of 68?days (one cadaver remained on a leaf for 168?days). Rainfall was positively correlated with the number of days mummies remained on leaves while mean daily air temperature was negatively correlated. Mummified cadavers were abundant in the summer during 2006 but not during 2007. This may have been a density-dependent consequence of low psyllid host populations in the grove in 2007. Alternatively, combination sprays of oil and copper applied during 2007 may have suppressed the fungus. This latter possibility prompted a laboratory investigation into the toxicity to H. citriformis of six chemicals commonly used in citrus. Copper hydroxide, petroleum oil, and elemental sulfur at maximum label rates each significantly reduced the infectivity of a laboratory culture of H. citriformis while copper sulfate pentahydrate, aluminum tris and alpha-keto/humic acids did not. This finding indicates that citrus growers interested in capitalizing on H. citriformis as a biological control agent of D. citri should avoid applying high rates of copper hydroxide, oil or sulfur.     

Cospeciation of Psyllids and Their Primary Prokaryotic Endosymbionts

Psyllids are plant sap-feeding insects that harbor prokaryotic endosymbionts in specialized cells within the body cavity. Four-kilobase DNA fragments containing 16S and 23S ribosomal DNA (rDNA) were amplified from the primary (P) endosymbiont of 32 species of psyllids representing three psyllid families and eight subfamilies. In addition, 0.54-kb fragments of the psyllid nuclear gene wingless were also amplified from 26 species. Phylogenetic trees derived from 16S-23S rDNA and from the host wingless gene are very similar, and tests of compatibility of the data sets show no significant conflict between host and endosymbiont phylogenies. This result is consistent with a single infection of a shared psyllid ancestor and subsequent cospeciation of the host and the endosymbiont. In addition, the phylogenies based on DNA sequences generally agreed with psyllid taxonomy based on morphology. The 3′ end of the 16S rDNA of the P endosymbionts differs from that of other members of the domain Bacteria in the lack of a sequence complementary to the mRNA ribosome binding site. The rate of sequence change in the 16S-23S rDNA of the psyllid P endosymbiont was considerably higher than that of other bacteria, including other fast-evolving insect endosymbionts. The lineage consisting of the P endosymbionts of psyllids was given the designation Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.).     

Discrimination among host plants (Leucaena species and accessions) by the psyllid pest Heteropsylla cubana and implications for understanding resistance

Abstract 1 The herbivorous bug Heteropsylla cubana Crawford (Homoptera: Psyllidae) is a pest of the cattle fodder crop Leucaena (Leguminosae: Mimosoideae). The interaction between the psyllid and three varieties of its Leucaena host plant was investigated in relation to the apparent resistance of some Leucaena varieties (Leucaena leucocephala, Leucaena pallida and their hybrids) to attack. 2 Field trials demonstrated that adult psyllids distinguished among the different varieties of Leucaena over a distance, and were attracted to L. leucocephala in significantly higher numbers than to L. pallida or to the hybrid. Pesticide treatment increased the attractiveness of Leucaena plants, even of those deemed to be psyllid resistant. Numbers of psyllid eggs and nymphs, sampled in the field, reflect the arrival rates of adults at the three plant varieties. 3 Wavelength reflectance data of the three Leucaena varieties were not significantly different from one another, suggesting that psyllids cannot discriminate among the three plants using brightness or wavelength cues. There was a differential release of caryophyllene among the three varieties. Release of caryophyllene in L. leucocephala and the hybrid appeared to be influenced by environmental conditions. 4 Experiments demonstrated that caryophyllene (at least on its own) did not influence the behaviour of leucaena psyllids in relation to leucaena plants. 5 The results suggest that host plant volatiles cannot be dismissed as significant in the interaction between the leucaena psyllid and its Leucaena host plants. Further avenues for investigation are recommended and these are related to novel ways of understanding resistance in insect plant inter‐relationships.     

The distribution, abundance and host plant relationships of Salix- feeding psyllids (Homoptera: Psylloidea) in arctic Alaska

Abstract. 1. Five species of psyllid occurred on seven species of Salix at Meade River, Alaska. Studies were made on the two common species Psylla pclmeni Löw and P.phlebophyllae Hodkinson. The former feeds on the phanerophy tes Salix pulchra, S.lanata, S.alaxensis and S.glauca , the latter on the chamaephytes S.phlebophylla and S.reticulata.
2. Both P.palmeni and P.phlebophyllae had a 1-year life cycle and nymphal development took place on the female Salix catkin. The life cycle was generally closely synchronized with the period of catkin development. However, only a few eggs were laid on S.glauca
3. Seasonal perturbation of the host plant by flooding, ice movement and blown sand prevented psyllids breeding in certain areas colonized by the host plant.
4. In P.palmeni densities and 'feeding pressure', measured as biomass of psyllids per gram of catkin, on the different host plants followed the sequence S.pulchra>S.lanata> S.alaxensis > S.glauca. In P.phlebophyllae densities and feeding intensities were similar on S.phlebophylla and S.reticulata and grazing intensity was comparable with P.palmeni on S.pulchra.
5. A highly significant negative correlation was found between psyllid density and catkin dry weight in S.pulchra, S.phlebophylla and S.reticulata , suggesting that psyllid feeding is affecting catkin growth.
6. Predation of psyllid nymphs by syrphid larvae was heavy but there was no evidence of parasitism.
7. The life history strategies of the five psyllid species are discussed within the context of the constraints imposed by the arctic environment.     

Effects of a particle film on biology and behavior of Diaphorina citri (Hemiptera: Psyllidae) and its infestations in citrus

Studies were conducted to investigate the effects of a kaolin-based hydrophilic particle film, Surround WP, on the biology and behavior of the psyllid Diaphorina citri Kuwayama (Hemiptera: Psyllidae) and to assess population densities of D. citri in citrus subjected to monthly applications of Surround WP. Laboratory investigations indicated a 3% (wt:vol) suspension of Surround WP in water applied directly was not acutely toxic to eggs, older nymphs or adults. Presence of the dried particle film on leaves interfered with the ability of adults to grasp and walk on citrus leaves. During a 30-s period, adults spent an average of 5 s moving on leaves with particle film compared with 16 s on leaves without particle film. When leaves were inverted, a significantly higher percentage of adults fell or flew from treated leaves (53%) than untreated leaves (16%). In a 12-mo study investigating infestations of D. citri on citrus treated monthly with Surround WP, cumulative reductions of 78% in adult numbers on mature leaves and of 60% in adult numbers on flush shoots (immature leaves) were observed in treated trees compared with untreated trees. Numbers of eggs and nymphs per flush shoot were reduced by 85 and 78%, respectively, in trees treated with particle film. Reductions in infestation levels of D. citri in treated trees were attributed to the negative effects of the particle film on the ability of adults to grasp, move, and oviposit. The suppressive effects of a Surround treatment against adult psyllids were degraded by rain.     

Oviposition preference of a Eucalyptus herbivore and the importance of leaf age on interspecific host choice

1. Paropsine chrysomelids are eucalypt folivores for which it has been hypothesized that the availability of suitable age class foliage (i.e. young foliage) is a more important determinant of host use than the physical and/or chemical characteristics of foliage from different hosts. This prediction was tested by assessing the oviposition preference of Chrysophtharta bimaculata (Olivier) on excised branches of Eucalyptus regnans F. Mueller and E. nitens (Deane & Maiden) Maiden.
2. In cage trials, and as observed in the wild, beetles oviposited preferentially on E. regnans when the branches of both eucalypt species displayed the full range of foliar developmental classes, from immature to fully expanded. However, when the branches were pruned so that both species displayed only immature and expanding leaves, no difference in oviposition between the species occurred. Further, when the branches were pruned so that E. regnans displayed only fully expanded leaves and E. nitens displayed only immature and expanding leaves, females oviposited entirely on the latter species.
3. These results support and extend the hypothesis that C. bimaculata 's ovipositional preference is dictated more by leaf age class than by the tree species from which the leaves derive. In the discussion it is proposed that a eucalypt that initiates and expands leaves rapidly will be less preferred for oviposition than one that develops and expands leaves slowly, because of the smaller area of its canopy with less sclerophyllous foliage.     

Carrot psyllid (Trioza apicalis) winter habitats – insights in shelter plant preference and migratory capacity

Abstract:  The carrot psyllid ( Trioza apicalis ) is a serious pest in carrot-growing areas in northern and parts of central Europe. The psyllids overwinter as adults on conifers and during summer feed and lay eggs on carrot plants ( Daucus carota ssp. sativus ), thereby destroying the crop. To investigate the migratory capabilities and preferences for different shelter plant species of the carrot psyllid, we made an inventory study of its winter habitats in three carrot-growing regions in southern Sweden. Norway spruce ( Picea abies ) was the preferred conifer over Scots pine ( Pinus sylvestris ) and juniper ( Juniperus communis ). We found psyllids on trees up to 1 km from the carrot fields, which was the largest distance sampled. The regression of catch numbers over distance was non-significant, however all samples containing more than seven psyllids were collected within 250m distance from the fields. There was no obvious pattern between catch directions and the prevailing wind directions of the preceding migratory period. Our study did not show any differences between males and females with respect to migration or shelter species preferences.     

Life history and phenology of the eucalyptus psyllid, Ctenarytaina eucalypti in Ireland

During the early 1990s, the eucalyptus psyllid, Ctenarytaina eucalypti Maskell, was accidentally introduced into Ireland and began to threaten economically a newly developing ornamental foliage production industry based in the south west. The life cycle of the psyllid was studied at three field sites in Co. Kerry. The psyllid exhibited a complex seasonal population dynamic suggesting that two, largely overlapping population cohorts over‐winter as eggs and nymphs, respectively, in a state of reduced development or temporary dormancy. The duration of this dormancy is short. Evidence of a resumption of activity was monitored at all sites in early February with increasing adult numbers and renewed egg laying being seen before any significant rise in temperatures. This strongly suggests that retarded development, and resumption of psyllid activity in early spring, are at least partially consequences of the changing nutritional quality of the food plant, probably mediated by changing day length. As a consequence of seasonally inhibited development, individual egg and nymphal over‐wintering cohorts probably alternate in the completion of two or three generations per growing season. Analysis of psyllid numbers on shoot samples showed that greater population densities were found on shoots at the top of the food plant compared with lower‐placed shoots. Evidence was also found that the distribution of nymphal instars changed seasonally, probably in response to population density and increased competition for preferred feeding sites. Overall psyllid abundance, and the finer details of seasonal population dynamics, varied between plantations. In the absence of effective predation or parasitism by indigenous natural enemies, these differences were also likely to be due to variation in nutritional quality of the host plant and may be influenced by crop husbandry factors such as the timing and severity of crop pruning.     

Leaf production of an overstorey rainforest tree and its effects on the temporal distribution of associated insect herbivores

Summary The host phenology and temporal distribution of insect herbivores associated with a rainforest canopy tree, Argyrodendron actinophyllum (Sterculiaceae), were monitored for 3 and 2 years respectively in an Australian subtropical rain forest near Brisbane. Leaf production of this evergreen species was synchronous among individuals, occurring from October to February, but differed markedly between years for some fast-growing trees. Most herbivore species fed selectively upon young leaves and, consequently, occurred only during windows of young foliage availability, with the exception of some mesophyll-feeders that preferred mature foliage. Fast-growing trees sustained high herbivore activity during periods of intense leaf production but some slow-growing trees, whose availability of young foliage appeared to be more constant and predictable, also supported high herbivore activity in the long term. Host suitability of particular trees during study years was dependent upon the intensity of leaf production and other unidentified factors, and was different for generalist and specialist herbiovores. Certain specialist species, such as psyllids, were able to overcome the changing availability of young foliage between years on vigorous A. actinophyllum trees by feeding upon both vegetative and reproductive meristems. Most of the data suggest that availability of young foliage is an important factor in this insect-plant system.     

Progressive restriction of host plant exploitation along a climatic gradient: the willow psyllid Cacopsylla groenlandica in Greenland

1. Host plant selection by the endemic willow psyllid Cacopsylla groenlandica was studied at eight sites in three locations along a N–S climatic gradient in west Greenland.
2. C. groenlandica oviposited and developed on four willow species: Salix glauca, S. arctophila, S. uva-ursi and S. herbacea. Development took place on female catkins of all species and on leaf shoots of S. glauca .
3. At sites in southern Greenland, C. groenlandica fully utilized all four willow species. However, there was progressive reduction of host plant species and tissues exploited as latitude increased, such that towards the northern limit of its range the psyllid became highly specialized, feeding only on female catkins of S. glauca , despite the presence of alternative hosts. Where plant growth was tightly constrained within a restricted growing season, the demands of synchronizing with more than one species may have become too great, resulting in specialization on a single host.
4. The probable mechanisms producing specialization were between-species differences in host plant phenology, linked to variation in the growing season, set against the partially non-overlapping geographical distribution patterns of the host willows.
5. A small degree of phenological asynchrony between host plants can be advantageous for the insect. Under favourable conditions the reproductive season can be extended by using a series of host plants with staggered phenologies. Some 'early' hosts also create predator-free space. However, as asynchrony increases, the advantages disappear as the insect becomes unable to exploit the full range of host options.
6. The importance of host plant synchrony in the life histories of arctic insects is discussed.