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Abstract. Invasive pathogen-insect symbioses have been extensively studied in many different ecological niches. Whether the damage of symbioses in different introduced regions might be influenced by other microorganisms has, however, received little attention. Eight years of field data showed that the varied levels of the nematode and beetle populations and infested trees of the invasive Bursaphelenchus xylophilus-Monochamus alternatus symbiosis were correlated with patterns in the isolation frequencies of ophiostomatoid fungi at six sites, while the laboratory experiments showed that the nematode produced greater numbers of offspring with a female-biased sex ratio and developed faster in the presence of one native symbiotic ophiostomatoid fungus, Sporothrix sp. 1. Diacetone alcohol (DAA) from xylem inoculated with Sporothrix sp. 1 induced B. xylophilus to produce greater numbers of offspring. Its presence also significantly increased the growth and survival rate of M. alternatus, and possibly explains the prevalence of the nematode-vector symbiosis when Sporothrix sp. 1 was dominant in the fungal communities. Studying the means by which multispecies interactions contributed to biogeographical dynamics allowed us to better understand the varied levels of damage caused by biological invasion across the invaded range.

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The pinewood nematode (PWN) Bursaphelenchus xylophilus, vectored primarily by the sawyer beetle, Monochamus alternatus, is an important invasive pest and causal agent of pine wilt disease of Chinese Masson pine, Pinus massoniana. Previous work demonstrated that the ratios and concentrations of α-pinene∶β-pinene differed between healthy trees and those trees containing blue-stain fungus (and M. alternatus pupae). However, the potential influence of the altered monoterpene ratios and concentrations on PWN and associated fungi remained unknown. Our current results show that low concentrations of the monoterpenes within petri dishes reduced PWN propagation, whereas the highest concentration of the monoterpenes increased PWN propagation. The propagation rate of PWN treated with the monoterpene ratio representative of blue-stain infected pine (α-pinene∶β-pinene = 1∶0.8, 137.6 mg/ml) was significantly higher than that (α-pinene∶β-pinene = 1∶0.1, 137.6 mg/ml) representative of healthy pines or those damaged by M. alternatus feeding, but without blue stain. Furthermore, inhibition of mycelial growth of associated fungi increased with the concentration of the monoterpenes α-pinene and β-pinene. Additionally, higher levels of β-pinene (α-pinene∶β-pinene = 1∶0.8) resulted in greater inhibition of the growth of the associated fungi Sporothrix sp.2 and Ophiostoma ips strains, but had no significant effects on the growth of Sporothrix sp.1, which is the best food resource for PWN. These results suggest that host monoterpenes generally reduce the reproduction of PWN. However, PWN utilizes high monoterpene concentrations and native blue-stain fungus Sporothrix sp.1 to improve its own propagation and overcome host resistance, which may provide clues to understanding the ecological mechanisms of PWN's successful invasion.

Insects are often exposed to high temperature stress in natural environments, but the mechanisms involved in thermotolerance in many insect groups like Hemiptera are not well known. To explore possible mechanisms of thermotolerance in the hemipteran pest Laodelphax striatellus, which damages rice through direct feeding and viral transmission, small heat shock proteins (sHsps) implicated in thermotolerance in other insect groups were identified. The seven sHsps identified have a conserved alpha crystallin domain, a variable N-terminal region, and shared relative low identities to each other. Three of the sHsp genes (LsHsp20.5, LsHsp21.5, and LsHsp21.6) exhibited higher basal expression than the other four genes but showed weak or no heat-induced expression. The other four genes (LsHsp20.1, LsHsp21.2, LsHsp21.4, and LsHsp22.0) were induced up to 3306-fold by heat. Injection of dsRNA indicated that expression of these sHsps was associated with thermotolerance, and Escherichia coli transformed with LsHsp21.2 and LsHsp20.1 showed relatively higher thermotolerance. These results point to an important functional role of these sHsps for thermotolerance in L. striatellus.

Using microtitration method, the relationship between Phenoloxidase activity and the resistance of the diamondback moth Plutella xylostella (Linnaeus) to the novel insecticide butane-fipronil was determined in vitro. After selection of the tenth-generation by butane-fipronil, the resistance of the fourth instar larvae was increased 83.80-fold as compared to the susceptible strain. Phenoloxidase activity of the resistant strain (POr) was 1.29-fold higher than the susceptible one (POs). However, the Km and optimum pH values were similar in resistant and susceptible strains, which were 1.11 mM and 6.5, respectively. Both POr and POs have maximum stability at pH values less than 7.0, although POs was less stable at lower pH values than POr. In addition, the thermal stabilities of the two phenoloxidase were very similar. It is suggested that PO may play an important role in the increasing resistance of pests to pesticides.

Maternally-inherited facultative symbionts are widespread in most insect species, and it is common that several symbionts coexist in the same host individual. Hence, the symbionts may compete or share for the limited resources and space in the host. The whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodoidea), harbors a diverse array of facultative symbionts, among which Hamiltonella sp. and Cardinium sp. are abundant species. Hamiltonella alone increases host fitness, while Cardinium alone confers lower fitness. Locking those different partners together creates ideal situations for the evolution of interactions between symbionts. In this study, we compared the fitness effects of whiteflies infected with only Hamiltonella to Hamiltonella-Cardnium co-infected whiteflies and measured the density of Hamiltonella and Cardinium during host aging, aiming to explore Hamiltonella-Cardinium interactions in B. tabaci. Our results illustrated that Hamiltonella-Cardinium coinfection induced lower fecundity, egg hatchability and number of female offspring, leading to a male-biased sex ratio in offspring, while there is no evidence for reproductive incompatibility between the infections. We also found an antagonistic interaction between Hamiltonella and Cardinium given that the density of the latter increased across time and led to a decrease of Hamiltonella density, which may be the underlying causes of the fitness cost in double-infected B. tabaci. Exploring the ecological consequences of co-infections of these different symbionts helps us to understand the nature of host-symbiont interactions in this species and potential for evolutionary conflict.

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