Infection, course of disease and action of Canningia tomici in Tomicus piniperda and Tomicus minor (Coleoptera: Curculionidae)

Abstract

1. Background and purpose

The pine shoot beetles Tomicus piniperda and Tomicus minor are secondary tree pests attacking weakened Pinus sylvestris and Pinus nigra. Outbreaks occasionally occur, causing considerable economic damage. The microsporidian pathogen Canningia tomici infects T. piniperda as its principal host. Bioassays were used to study the infectivity of C. tomici to both beetle species, as well as vertical transmission, and effects on survival and fecundity.

Materials and methods

Field collected beetles from several locations in Austria (Lower Austria, Burgenland and Styria), Finland (Hyytiälä), Poland (Sękocin), the Czech Republic (Stará Boleslav), and Croatia (Korenica) (3410 T. piniperda, 413 T. minor) were dissected and evaluated for the presence of C. tomici. Uninfected beetles for infection experiments were only collected from the Austrian sites.Canningia tomici spores were extracted from the infected organs of T. piniperda by dissection and host tissue grinding. The number of spores suspended in water was counted in a hemocytometer. Infection experiments began with T. piniperda and T. minor as soon as parental beetles were available in the field in spring, or filial beetles were emerging from infested log sections in the laboratory. To test the effect of maturation feeding on infection success, filial beetles of both species were either inoculated immediately after emergence from a log section, or were allowed to feed on P. sylvestris twigs for several days first. Filial beetles were kept in Petri dishes containing P. sylvestris twigs at 8°C and long-day conditions (16L:8D) during the maturation feeding period. Afterwards, they were removed from the twigs, starved for 24 hours, and then inoculated with a 1-µl spore suspension or water. All beetles checked daily until death, dissected and inspected for the presence of C. tomici spores. All data were analyzed with the software program. Frequency data sets were compared using Chi-square analysis. Multiple comparisons were controlled for Type I errors using the Bonferroni method. The datasets of multiple dependent scale variables were analysed using the LM multivariate procedure, testing the effects of the following factors: year, temperature, inoculation, successful infection, maturation feeding, and sex

Results

The overall prevalence of C. tomici in T. piniperda was 1.9%, with significantly more female T. piniperda infected. No infections were observed in T. minor. After feeding spore suspensions to parental and filial T. piniperda and T. minor, between 0% and 67% of the beetles were successfully infected, regardless of the incubation temperature or the beetles’ sex. The beetles’ survival time was significantly influenced by the incubation temperature and the successful infection; in filial beetles the maturation feeding period was also an important factor. A lower incubation temperature and a successful infection caused both species and generations to survive longer.

Conclusion

We conclude that when C. tomici infects the reproductive organs of its hosts, this extends the host's lifespan, leading to better reproduction, transmission, and survival of the pathogen in host populations. This is also reflected by higher infection rates of T. piniperda in Finland and Poland, as compared to Austria.