Beauveria brongniartii Sacc. (Petch) against Melolontha spp. white grubs in forest nurseries with different soil pH

Przemysław Szmit

In 2011, the General Directorate of State Forests in Poland managed 669 forest nurseries on the total area of 2411 ha that produced forest tree seedlings bare root systems, hence vulnerable to Melolontha spp. white grubs. Up to date, no chemical or biological plant protection product for control of cockchafer grubs in forests has been registered in Poland. The study was carried out with the aim to evaluate the efficacy of a biocontrol product Bovecol with BP strain of Beauveria brongniartii on sterilized wheat grain in control of Melolontha spp. white grubs in forest nurseries, established on acidic soils (with pH analogous to that of forest soils) as well as on those alkaline. The active substance of Bovecol was 108 fungal conidia per 1g of the product. Study plots were established in 3 bareroot nurseries, situated in the areas with different soil reaction values and abundant populations of Melolontha spp. white grubs (assessed before Bovecol treatments). The product was applied against L1 larvae (rates in the Czerniawka and Bałtów nurseries: 120 kg/ha and 240 kg/ha) and against L2 larvae (the Iłki nursery: 240 kg/ha). Grub population numbers were assessed 2, 3 and 4 months after the treatments. Application of Bovecol against L1 into the soil with neutral reaction (pH 6.8) caused the reduction of white grub numbers in 2 months, down to the threshold recommended by the Instruction of the protection of forests (mandatory guidelines for the protection of Poland’s State Forests), i.e. less than 1 grub/sampling pit. Bovecol treatment against L1 cockchafer larvae into acidic soil (pH 4.8) had no statistically significant effect on the reduction of grub population numbers, even 4 months after product application. Bovecol treatment against L2 larvae applied into the soil with medium pH value (5.3) gave poorer results when compared to the soil with pH 6.8, but considerably better – when compared to the soil with pH 4.8. The selection of a fungal strain with insecticidal properties against soil insect pests should include the evaluation of strain requirements with regard to soil pH range – optimal for germination, development and growth of a given strain.