Scolytid bark beetles that colonize living conifers are frequently associated with specific fungi that are carried in specialized structures or on the body surface. These fungi are introduced into the tree during the attack process. The continuing association suggests that there is mutual benefit to the fitness of both beetles and fungi. The fungal species may benefit from the association with the beetles by transport to new host trees. Beetle species may benefit from the association with fungi by feeding on the fungi, or by the fungi contributing to the death of the host trees through mycelial penetration of host tissue, toxin release, interactions with preformed and induced conifer defenses, or the combined action of both beetles and fungi during colonization. Extensive research has been directed towards characterizing the interactions of beetle-fungal complexes with live host conifers and determining the ecological advantages for maintaining the associations. However, differences among systems and how species interact under different population and environmental conditions make it difficult to generalize about the importance of the separate biological components in successful host colonization.

A new huanglongbing (HLB) "Candidatus Liberibacter" species is genetically characterized, and the bacterium is designated "Candidatus Liberibacter psyllaurous." This bacterium infects the psyllid Bactericera cockerelli and its solanaceous host plants potato and tomato, potentially resulting in "psyllid yellowing." Host plant-dependent HLB transmission and variation in psyllid infection frequencies are found.

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Eucalyptus species, native to Australia, Indonesia, the Philippines, and New Guinea, are the most widely planted hardwood timber species in the world. The trees, moved around the globe as seeds, escaped the diverse community of herbivores found in their native range. However, a number of herbivore species from the native range of eucalypts have invaded many Eucalyptus-growing regions in North America, Europe, Africa, Asia, and South America in the last 30 years. In addition, there have been shifts of native species, particularly in Africa, Asia, and South America, onto Eucalyptus. There are risks that these species as well as generalist herbivores from other parts of the world will invade Australia and threaten the trees in their native range. The risk to Eucalyptus plantations in Australia is further compounded by planting commercially important species outside their endemic range and shifting of local herbivore populations onto new host trees. Understanding the mechanisms underlying host specificity of Australian insects can provide insight into patterns of host range expansion of both native and exotic insects.

Two endosymbionts, an obligate primary symbiont and a facultative secondary symbiont, are harbored within the invasive red gum (eucalyptus) lerp psyllid, Glycaspis brimblecombei, in California. An extensive survey of diversity and frequency of G. brimblecombei's secondary symbiont in multiple populations throughout the state of California was conducted using PCR detection, restriction enzymes, cloning, and sequencing. A total of 380 G. brimblecombei individuals in 19 populations were screened for secondary symbionts. Based on molecular screening results, only one type of secondary symbiont was present in G. brimblecombei populations in California. Overall, 40% of the 380 psyllids screened were infected with the secondary symbiont. Interestingly, secondary symbiont infection frequencies in G. brimblecombei populations varied dramatically from 0 to 75% and were significantly related to parasitism pressure by Psyllaphaegus bliteus, a solitary endoparasitoid of the psyllid.Insect bacterial endosymbionts are harbored in a wide diversity of insects (2,7,14,32). Despite the prevalence of endosymbionts throughout many insect orders, little is known about the role and maintenance of endosymbionts within insect hosts. In general, an endosymbiont-host association can be obligate for survival and/or reproduction of the host (termed a primary symbiont here) or facultative and therefore not necessary for host survival and/or reproduction (termed a secondary symbiont [SS] here) (7, 26). The role of primary symbionts to their host is thought to be nutritional enrichment (4,5,14,15,19,20,27). Alternatively, the roles of facultative SSs are not well understood (2, 3). Nevertheless, SSs have recently been found to exert a variety of effects on the phenotype of Acyrthosiphon pisum Harris (Hemiptera: Aphidae) (8,17,18,25,29,35).Little is known about the roles SSs play within their insect hosts other than A. pisum. Although SSs are known to occur in 33 psyllid species (26,30,34), no information has yet been published on the frequency of SSs within and among psyllid populations or their effect on psyllid phenotypes. It is important to understand insect-endosymbiont interactions because endosymbionts may play major evolutionary roles in eukaryotes by influencing interspecific interactions, distributions, and fitness during ecological time scales in populations.In the state of California, the invasive red gum lerp psyllid Glycaspis brimblecombei Moore (Hemiptera: Psylloidea), its preferred and prevalent host plant Eucalyptus camaldulensis Dehnhardt (Myrtales: Myrtaceae) (6), and its parasitoid Psyllaphaegus bliteus Riek (Hymenoptera: Encyrtidae) provide an ideal system for the study of endosymbiont-host interactions on a geographic scale. Eucalyptus spp. were introduced from Australia to California as seeds around 1850 (13); E. camaldulensis is the most abundant host plant species of G. brimblecombei and has been planted throughout the state (13). In 1998, G. brimblecombei invaded California from Australia (24) and quickly became estab...

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Standing loblolly pines in southeastern North America are colonized by four sympatric species of bark beetles: Dendroctonus frontalis (Zimm.), Ips calligraphus (Germ.), I. grandicollis (Eichh.) and I. avulsus (Eichh.). The beetles compete for the limited amount of phloem tissue used as a site for reproduction. Using indices of niche breadth and niche overlap determined from the surface areas attacked, the interaction of colonizing beetle species in partitioning resources in entire trees and within each sample level was examined. The broadest niche breadth was exhibited by I. avulsus, while I. grandicollis had the narrowest. D. frontalis dominated the lower bole and overlapped primarily with I. calligraphus. The upper bole was similarly dominated by I. avulsus, which overlapped only slightly with D. frontalis, but overlapped extensively with I. calligraphus. Within tree species diversity was highest in the mid-bole sections and declined progressively toward the stump and top. Increasing species diversity showed a strong positive correlation with increasing mean niche overlap.