Fungal entomopathogens found in natural populations of 7 lepidopteran forest pests were isolated and
identified. Beauveria bassiana was established as a dominating species followed by Fusarium sp. and
Aspergillus sp. The pine processionary moth Thaumetopoea pityocampa was the most affected by B.
bassiana – 66.7% of the analysed dead larvae of the pest. The occurrence of B. bassiana in Malacosoma
neustria, Tortrix viridana and Melitaea didyma is the first record in Bulgaria.
Relative susceptibility of first-second instar Lymantria dispar larvae was estimated to three B. bassiana
and one Metarhizium anisopliae isolate and susceptibility of third instars – to seven B. bassiana and one
M. anisopliae isolate. The bioassays showed that L. dispar larvae were tolerant to mycoses caused by the
tested B. bassiana isolates and M. anisopliae. M. anisopliae isolate caused mycosis to first-second instar
larvae with the highest lethal effect 56.67% ± 4.84 and B. bassiana isolates 561Bb and 575Bb to third
instar larvae – 47.78% ± 4.06 and 34.44% ± 3.28, respectively, in 17 days period.