In Vitro Compatibility and Combined Efficacy of Entomopathogenic Nematodes with Abamectin and Imidacloprid Against the White Grub, Pentodon bispinosus Kust.

One of the challenges in mixing entomopathogenic nematodes (EPNs) with chemical control is low predictable compatible information; hence, pesticides are misused by farmers.  Therefore, the study aimed to clarify the result of mixing abamectin and imidacloprid on the infectivity of EPNs against larvae of hard-black beetle Pentodon bispinosus Kust., besides, investigating the fluctuations of the joint action of EPN species and chemical pesticides during the study period. Recommended dose (RD) of imidacloprid (LT₅₀= 5.20 and 14.20 d) was more efficient comparing with abamectin (LT₅₀= 18.14 and 24.22 d) on the first third and instar larvae of white grubs. Heterorhabditis bacteriophora (Ar-4) showed the highest tolerance (LT₅₀=10.11 and 6.22 d) while Steinernema feltiae (Filipjev) was the most sensitive (LT₅₀=5.53 and 3.55 d) after exposure to abamectin and imidacloprid at RD, respectively. EPNs plus chemical insecticides combinations on first instar larvae recorded the higher median lethal concentration (LC₅₀) except S. feltiae (Filipjev) plus abamectin and all combinations of EPNs with imidacloprid. All combinations with EPNs showed lower LC₅₀ values than EPN alone except abamectin with some tested H. bacteriophora with III instar larvae. Based on LC₅₀ values, all EPNs combinations with imidacloprid surpassed EPN alone or abamectin combinations. Moreover, mortality reached 100% in the 1^st instar of white grubsat concentration 250 IJs/larva H. bacteriophora (HP88 strain) compared with 92% in the 3^rd instar larvae at the same concentration after 4 weeks of incubation. The potentiation effect was observed after one week in the 3^rd instar larvae only when abamectin combined with H. bacteriophora (Ar-4 strain) and S. carpocapsae (All strain) while after 3 weeks with 1^st instar larvae in combination between S. feltiae (Filipjev) and the RD of abamectin and imidacloprid at concentration 150 IJs/larva. No synergistic interaction was observed in combinations of EPNs with neither abamectin nor imidacloprid. During mixing chemical pesticides and bioagents, the chemical pesticide is the independent variable effect on EPNs viability and infectivity which is affected by combining period and species. The final interaction mainly depends on the chemical pesticide selectivity and toxicity to target insects (stage and instar).