Efficacy of entomopathogenic nematode and Bacillus thuringiensis combinations against Holotrichia parallela (Coleoptera: Scarabaeidae) larvae

Abstract

The larvae of Holotrichia parallela Motschulsky (Coleoptera: Scarabaeidae) are notorious soil insect pests that cause severe damage to crop production in China. However, the long-term use of insecticides seriously pollutes the environment and threatens human health. Therefore, an environmentally friendly, alternative control method to insecticides is crucial to effectively control these pests. In this study, we assessed the efficacies of possible combinations of entomopathogenic nematodes (EPNs), the Heterorhabditis beicherriana LF strain and Bacillus thuringiensis (Bt) strain HBF-18 to control H. parallela larvae under laboratory and field conditions. In the laboratory, the newly identified H. beicherriana LF strain showed higher pathogenicity than Heterorhabditis bacteriophora H06, Steinernema carpocapsae ALL and Steinernema carpocapsae SF-SN against H. parallela second instar larvae. The H. beicherriana LF strain exhibited a strong potential to control H. parallela larvae at an optimum temperature of 25 °C and soil moisture of 18%. The H. beicherriana LF strain was compatible with Bt (HBF-18). All EPN-Bt combinations caused significantly higher corrected mortality of H. parallela larvae than nematodes were applied alone after 7 and 11 days; in particular, a stronger synergistic effect was detected when Bt (HBF-18) at LC50 concentrations (0.79 × 107 CFU/g of soil) were combined with EPNs. In the peanut field experiment, synergistic interactions were observed for the combinations of the H. beicherriana LF strain (1 × 103 IJs/plant) and Bt (HBF-18) (1.14 × 1010 CFU/plant) against H. parallela third instar larvae, resulting in a sizable white grub reduction of 83.9 ± 0.82%. Our findings suggested that combining the H. beicherriana LF strain and Bt (HBF-18) is a recommended and practical strategy to manage pests in crop production systems.