The development of the truffle beetle Leiodes cinnamomeus at low temperature, a determining factor for the susceptibility of adults and larvae to entomopathogenic nematodes

Abstract

The European truffle beetle Leiodes cinnamomeus (Panzer) (Coleoptera: Leiodidae) is the most important pest in black truffle (Tuber melanosporum) plantations. Adults and larvae feed on truffles during the cold months of autumn and winter, while during spring and summer larvae are in diapause. This study aims to test the susceptibility of L. cinnamomeus adults and larvae to different entomopathogenic nematode (EPN) species at various temperatures under laboratory conditions. Different populations of Steinernema carpocapsae and Steinernema feltiae were applied against adults and mycophagous larvae at 20 °C, 10 °C and 10–15 °C (10 °C during 18 h and 15 °C during 6 h a day), while Heterorhabditis bacteriophora was only applied against diapause larvae at 25 °C. S. carpocapsae caused 100% mortality of adults three days after application at 20 °C. At 10 °C, adults were not susceptible to any EPN species seven days after treatment, while at 10–15 °C S. carpocapsae was the most virulent species (76.6–96.6% mortality). In the case of larvae, all EPN species were infective at 20 °C (43.3–83.3% mortality), despite differences among some populations. At 10 °C, only two populations of S. feltiae caused higher mortality of larvae (50–53.3%) than control seven days after treatment. H. bacteriophora caused 100% mortality against diapause larvae five days after application at 25 °C. Soil temperature was measured in a truffle plantation for each hour every day from September 2021 to April 2022 at 20 cm depth. From September to mid-October it was registered a temperature above 15 °C for more than 20 h a day. Temperatures were generally below 10 °C from November to March. An appropriate timing of field applications should be considered due to the short periods of time when temperature is optimal for each EPN species tested.