Foliar application of the entomopathogenic nematode Steinernema carpocapsae for biological control of diamondback moth larvae (Plutella xylostella)

For the use of the entomopathogenic nematode Steinernema carpocapsae on cabbage foliage to control larvae of the diamondback moth (DBM), Plutella xylostella, a formulation containing 0.3% of the surfactant Rimulgan and 0.3% of the polymer xanthan was tested in leaf bioassays and compared to nematodes applied in water. Compared to water, the surfactant-polymer formulation (SPF) significantly improved efficacy. Using 75 dauer juveniles (DJs) cm−2 in SPF, 80% mortality was recorded, whereas at 60% RH insect mortality reached almost 60%. The survival time (LT50) for S. carpocapsae applied in water was 36 h at 80% RH and only 3 h at 60% RH. With SPF the LT50 was prolonged to 58 h at 80% RH and >20 h at 60% RH. The addition of fumed silica, cross-linked sodium polyacrylate or alginate gel did not significantly improve DJ survival compared to SPF alone. Nematode caused mortality decreased when DBM larvae were added 9 h after DJ application. As 98% of the nematodes were still alive after 9 h, the nematodes must have lost efficacy. No significant increase in DBM morality was recorded when insects were exposed to DJs for 1, 4 or 20 h. The results indicate that host penetration on the leaf occurs within the first hour after application. Thus, the major advantage using the formulation is not to enhance nematode survival but rather to provide optimal environmental conditions that support nematode invasion of the host on foliage.