Effect of mineral dusts alone and in combination with the entomopathogenic fungus Beauveria bassiana (Bals.) Vuill. against the bark beetle Ips typographus L. (Col., Scolytidae) in the laboratory and under field conditions

The objective of our study was to determine the effect of mineral dusts alone or in combination with the entomopathogenic fungus Beauveria bassiana to control the spruce bark beetle Ips typographus. In laboratory experiments with silicon dioxide (SiO₂), diatomaceous earth (DE), Otavi-Perlit® (OP) and SilicoSec® (Ssec), 100% of all beetles died within 7 days on spruce bark at a temperature of 25 °C and a relative humidity (RH) of 40%, while the control mortality was 21%. The median survival time (MST) was between 2.0 and 4.1 days compared to the control with 9.8 days. After treatment on artificial diet, only in the SIO₂ and DE treatment the mortality and MST were significantly different from the control. At 40% RH all beetles treated with siloxide (SiO) were dead after 3 days, while at 70% and 100% RH no effect of SiO on the beetles was observed, which shows that the effect of SiO on I. typographus is dependent on RH. An aqueous solution of SiO₂ (1% and 10%) was as effective against I. typographus as the powder formulation, when the bark was dipped into the solution and dried afterwards. In the combination test, conidia of B. bassiana (1x10⁷ spores/ml) were mixed with a solution of SiO₂ at a final concentration of 0.5% and 5%. The highest mortality and lowest MST at 40%, 70% and 100% RH and 25 °C were observed in the 5% SiO₂/B. bassiana mixture and in the powder formulation of both. In the field, I. typographus beetles were treated in bore holes or nuptial chambers with the B. bassiana product Boverol® or SiO₂ alone and in combination on spruce logs. After 5 days, the highest number of galleries and pupal chambers was recorded in the untreated control. The mortality in all treatments was between 83 and 93% (control 54%). The rate of mycosis was 75–93% in all Boverol® treatments. In a final field experiment, beetles were trapped with pheromones, treated with B. bassiana or Ssec alone and in combination, and then were released on spruce bark in cages. After 7 days, the mortality and MST were significantly higher in all treatments compared to the control. In the B. bassiana treatments the mycosis rate was more than 90%. We suggest testing further mineral dusts alone or in combination with an entomopathogenic fungus (e. g. B. bassiana) against I. typographus in the field and to study the practicability of treatments within forest management strategies.