Quantity of the inoculum

Experiments with artificial feeding grooves that were performed by Webber et al. {[436]} and Sutherland et al. {[8]} showed that at least 500-1000 O. ulmi spores are necessary to successfully infect U. minor var. vulgaris (U. procera). However, the precise threshold spore inoculum required for elm xylem infection appears to depend on environmental factors and the level of host tree resistance to DED {[8]}. Therefore, even an inoculum of 50.000 spores does not always guarantee infection. The number of O. ulmi s.l. spores carried by individual elm bark beetles varies both within and between species. Frequently the beetles show additional contamination with spores derived from fungi like Penicillium {[367]}. The body size of the beetle appears not to be the sole determinant of spore load and the subsequent success as a DED vector. Sex-morphological features (e.g. the presence or absence of setae on the front head of the beetle) and distinct breeding preferences between species also influence the number of spores carried by the beetle{[354],[366],[367]}. The spore load of an individual beetle is determined by the dilution method described by Webber et al. {[371]}. However, since this method does not distinguish between single spores and clumps of spores the percentage recovery rate of the pathogen may be underestimated. S. scolytus is reported to carry up to 350.000 O. ulmi sporeson its body {[354],[362]}. On an average large elm bark beetle 250-2500 spores can be detected. Roughly  50% of the S. scolytus beetles has a O. ulmi spore load of 1000 or more spores. The number of spores carried by S. scolytus emerging in summer is similar to that spring{[371]}. The individual spore load of S. multistriatus can vary between 1 and 57.000 spores {[354]}. Faccoli et al.{[27]} recently reported an average load of 130.1 ± 14.7 spores per beetle for S. multistriatus and S. pygmaeus.

Many authors have estimated the percentage of beetles of a particular species that is actually contaminated with O. ulmi spores during emergence. Since the experiments were performed in different months of the year and at different locations/conditions the comparison may be questionable. Gibbs et al. {[426]} reported that 18-55% adult S. scolytus beetles carry O. ulmi at on emerging from a diseased elm. However, according to Webber et al. {[354]} the large elm bark beetle is seldom completely free of spores. For the small elm bark beetle S. multistriatus, the spore-carrying percentage at emergence varies considerably: 5.7-7.7% {[316]}, 3-20% {[426]}, 1.3-38% {[427]}, 35-64% {[354]}, 57.6% {[367]} and 76% {[329]}. At emergence 57.5% of the S. pygmaeus  beetles were found to carry spores {[366]}. Recently Favaro et al. {[27]} reported a reduction in the percentage of S. multistriatus and S. pygmaeus spore carriesduring summer emergence (8% on average) compared with spring emergence (58% on average).

The extent to which the fungus colonizes the pupal chambers determines contamination of the new adult beetles. Since the outer bark (or thin bark) tends to dry out quickly, sporulation of O. ulmi  s.l. occurs less frequently in pupal chambers located in these regions compared with those in the moist inner bark. The location of the pupal chambers, which differs among species, strongly influences the spore load of the emerging beetles {[436]}. In addition the timing of beetle brood attack affects transfer. Pteleobius vittatus lives in the dry bark of the elm tree where O. ulmi  s.l. fails to form coremia. Since the adult P. vittatus attacks the elm in the non-DED susceptible period (early spring and autumn) this beetle is considered to play a minor role in DED transmission {[381]}. So far no data about individual spore load and the proportion of beetles that are contaminated with spores have been reported for S. laevis and H. rufipes.