Pathogens of Ips amitinus: new species and comparison with Ips typographus

Ips amitinus and I. typographus are two serious pests of spruce in Europe, have similar bionomics and are likely to occur and meet on the same host trees. We therefore hypothesized that the two species support similar levels of similar pathogens. To test this hypothesis, we collected mature beetles from three trap trees at each of eight study sites and determined beetle numbers and pathogen infection levels. In total, 938 mature I. amitinus beetles and 3435 of I. typographus were dissected; five pathogens, as well as intestinal nematodes and endoparasitoids, were detected. The neogregarine Mattesia schwenkei is reported here for the first time as a new pathogen in 9.4% of I. amitinus individuals at one site. Average infection levels of most pathogens (Chytridiopsis typographi, Gregarina typographi, Mattesia schwenkei and parasitoids) were significantly higher in I. typographus than in I. amitinus. Metschnikowia typographi was confirmed only in Ips amitinus, while the microsporidium of Nosema typographi occurred only in I. typographus. Within‐season increases in G. typographi infection levels were documented in Ips amitinus.     

Pathogens of the bark beetle Ips cembrae: microsporidia and gregarines also known from other Ips species

The objective of the current study was to identify pathogens of the large larch bark beetle, Ips cembrae, which is a secondary pest that has produced several local outbreaks across Europe in recent years. Beetles were collected from pheromone traps, trap trees and emergence traps (Larix decidua) during 2007 to 2011 at 10 study sites in central Europe. A total of 3379 mature and callow beetles were examined with a light microscope, and only two microsporidian pathogens [Chytridiopsis typographi and a diplokaryotic microsporidium (probably Nosema sp.)] and two gregarines (Gregarina typographi and Mattesia schwenkei) were found. Within the I. cembrae populations, the infection rate for C. typographi ranged from 2 to 58%. Nosema sp. occurred in only two beetles in 2007 (at two study sites). G. typographi was recorded only in Austria and Croatia and only in 1–2% of the beetles in those countries. Mattesia schwenkei was observed solely in Croatia in 0.6% of the beetles in that country. Only one fungal pathogen in the genus Fusarium was found and only in two mature beetles (0.7%) in 2010. The pathogen species found during our study of I. cembrae were very similar to the pathogens previously identified for Ips typographus. No species‐specific pathogen was detected.     

Budding: A new stage in the development of Chytridiopsis typographi (Zygomycetes: Microsporidia)

Chytridiopsis typographiWeiser, 1954, the microsporidian pathogen of the spruce bark beetle, Ips typographus L. (Coleoptera: Scolytidae), has an early developmental period with plurinucleate mother cells, each of which produces a single bud. The globular bud is connected with the mother cell by a collar and the cellular constituents are pushed to the distant end of the bud. Both the mother cell and the bud continue to develop; the bud then separates from the mother cell and grows to produce a cell of the same type. Both cells then continue sporogonial development and produce sporophorous vesicles with 16-32 spores. The process of a single mother cell producing a single bud that grows to an identical stage is new in the development of C. typographi and has no analogy in other Microsporidia.     

The comparative prevalence and demographic impact of two pathogens in swarming Ips typographus adults: a quantitative analysis of long term trapping data

• 1 Adult Ips typographus were collected using pheromone traps at a locality in the eastern part of Austria between 1995 and 2004. The occurrence of two pathogens, Gregarina typographi and Chytridiopsis typographi, was determined throughout the period of beetle swarming activities. Weekly and annual data sets were then analysed by smoothing statistical techniques and epidemiological models.
• 2 The pathogens spread differently within the beetle population with respect to their biological characteristics: infectious forms of C. typographi were immediately available after their development in the insect's gut, but not those of G. typographi.
• 3 Both of the pathogens had a low prevalence in swarming beetles, especially C. typographi, and there was no evidence of a between‐year or within‐year epidemiological process. Conversely, it was shown that G. typographi has a positive effect on the rate of increase of trapped beetles.
• 4 Fitting a nonlinear model to the data suggested that: (i) this was due to a higher catch ability of beetles infected with G. typographi than of healthy beetles; (ii) when this effect is taken into account, G. typographi induces a specific within‐year low mortality in beetle populations; and (iii) beetle populations increase naturally within a year, despite their infection by both pathogens. No clear effect of C. typographi was detected in the trapped data set when the prevalence of this species was high in beetle populations collected from trees.
• 5 It is hypothesized that both pathogens induce different behavioural effects on their host, resulting in: (i) favouring the trapping of G. typographi‐infected beetles and (ii) hindering the capture of C. typographi‐infected individuals. This could be the result of both of the pathogens having an opposite effect on the flight abilities of beetles and/or on the beetles' response to the aggregation pheromones used in the traps.