In planta bioassay on the effects of endophytic Beauveria strains against larvae of horse-chestnut leaf miner (Cameraria ohridella)

Cameraria ohridella is a major pest of horse-chestnut trees in Europe causing permanent outbreaks and severe defoliations. Entomopathogenic Beauveria species are common mortality factors of C. ohridella pupae over-wintering in leaf litters. In this study, laboratory bioassays were conducted to determine the ability of local Beauveria strains to colonize horse-chestnut leaves endophytically and test their antagonistic activity against C. ohridella in colonized leaves. Two strains of Beauveria bassiana (AM_EF0111 isolated from horse-chestnut leaf tissue and AM_EP0715 isolated from a C. ohridella pupa) and one strain of B. pseudobassiana (AM_SO1015 obtained from soil) were used for in planta bioassays. The selected strains demonstrated high virulence to leaf miner pupae after a topical treatment with spore suspensions. LC50 ranged between 2.49 and 3.19 x 10(6) conidia ml(-1). Horse-chestnut saplings were inoculated with the fungi through their direct immersion into spore suspensions (107 conidia ml(-1)). Endophytic colonization was successful and a mean colonization rate reached 57.44-66.08% 15 days post-inoculation. It decreased over time for all strains, but the fungi were still present in leaf tissue 54 days post-inoculation. AM_EF0111 strain was significantly more effective in colonizing saplings than AM_SO1015 and AM_EP0715. Fungus-colonized and untreated (control) saplings were infested with the leaf miner 31 days after inoculation and development of larval mines was monitored. In Beauveria-colonized saplings, damage by leaf miner was significantly lower compared to control plants. On day 23 post-infestation, the damaged leaf area of colonized leaves was 5-times smaller compared to control saplings. The negative effect on larval tunneling was significantly greater for strain AM_EP0715 than for the other two strains. Survival of leaf miners was considerably affected in colonized leaves, but mycosis was detected only in 5.41-9.23% of cadavers. No mycosed individuals were collected from control saplings. Pupae from colonized leaves were smaller in dimensions with lower weight than pupae form control plants. The same Beauveria strains recovered from leaves at the end of colonization bioassays showed no significant decrease in LC50 values.