Potential use of entomopathogenic nematodes against the soil dwelling stages of onion thrips, Thrips tabaci Lindeman: Laboratory, greenhouse and field trials

Onion thrips Thrips tabaci Lindeman (Thysanoptera: Thripidae) is one of the most damaging insect pests of onion Allium cepa L., which is an economically important agriculture crop cultivated worldwide. This is first comprehensive study in which we have tested a broad range of entomopathogenic nematodes (EPN) species against different developmental stages of T. tabaci in laboratory, greenhouse and field trials. We performed several bioassays to assess the efficacy of different species of EPNs against different soil inhabiting stages of T. tabaci under various conditions. In screening bioassays, at 100 infective juveniles (IJs) cm-2 mortality ranged from 8.5 to 74.2% and 5.0-62.7% among all the tested EPNs (Steinernema carpocapsae (ALL strain), S. glaseri (NC), S. riobrave (355), S. feltiae (SN strain), S. rarum (17 C&E), Heterorhabditis bacteriophora (VS), H. georgiana (Kesha), H. indica (HOM1), H. floridensis (K22) and H. megidis (UK211 strain)) in prepupae and pupae, respectively. Five EPNs species (H. bacteriophora, S. feltiae, S. carpocapsae, S. riobrave and H. indica) selected from the screening bioassay were further evaluated against prepupae, pupae and late second instar larvae at four different concentrations i.e. 50, 100, 150 and 200 IJs cm(-2). A clear dose response relationship was observed among all the tested species against all the developmental stages. The prepupal stage was the most susceptible (87.9% mortality) followed by pupae (78.1% mortality) and late second instar larvae (59.4% mortality) when exposed to H. bacteriophora at highest dose tested (200 IJs cm-2). Among the different temperatures tested, maximum efficacy was observed by all the tested EPNs at 25 degrees C and 30 degrees C followed by 35 degrees C and 20 degrees C with H. bacteriophora and S. feltiae being most virulent among all species at all the tested temperatures. In a potted soil bioassay, the lowest adult emergences were observed in the H. bacteriophora treatment for both prepupae and pupae. In potted plant bioassay under the greenhouse condition, significant lower number of adult emergence was observed among H. bacteriophora as compare to control group. Under field conditions, the lowest number of larvae (4.9 per plant in 2017-18 and 9.6 per plant in 2018-19) and adult (0.1 per plant in 2017-18 and 5.2 per plant in 2018-19) were observed in H. bacteriophora treated plots compared to control larvae (29.4 per plant in 2017-18 and 44.1 per plant in 2018-19) and adults (20.0 per plant in 2017-18 and 28.1 in 2018-19). Our results revealed that EPNs could be included in integrated pest management programs for T. tabaci in onion production systems.