DED TRANSMISSION

As they enter elms to breed, bark beetles may be contaminated with O. ulmi  s.l. spores resulting in fungal colonization of dying elm bark in the vicinity of the developing broods (Photo 47). Often an expanding lesion produced by O. ulmi  s.l. centred on the maternal hall can be observed. This lesion lies behind the line of the feeding larvae. Since colonization can occur in summer or in winter, reunion of pathogen and vector is ensured by the time the beetle emerges in early summer. Although at the initial stage of infection there is no physical contact between the fungus and the larvae, O. ulmi  s.l. may eventually grow through the whole gallery system. Contamination of the new adult beetles occurs in the pupal chambers {436}.

Photo 47:A) Setae on the head of Scolytus scolytus.

Fransen {[381]} observed that immature larvae of S. scolytus and S. multistriatus contain faecal lumps in their gut system that can be contaminated with fungal spores (e.g. O. ulmi, Fusarium sp., Phomopsis sp.). Mature larvae in the pupal chamber have completely lost their gut content, therefore transmission of O.ulmi  s.l. from larvae via pupae to the new adult beetle does not occur via the gut system. However, the faecal lumps can contaminate the pupal chamber {[381]}.

B) Spores of O. ulmi s.l. stick to the hairs of the large European elm bark beetle (Photo 47A and B are scanning electron micrographs).