Use of the entomopathogenic nematode, Steinernema carpocapsae in controlling common cutworm and diamond back moth and its mechanisms governing the infection process

Present experiments were conducted to demonstrate how the infectivity of Steinernema carpocapsae against the common cutworm and diamond back moth depending on the body size. The time required to kill insect varied, small larvae tend to be earlier than larger ones. For Spodoptera litura, the average body weight of larvae dying before 24 hr, and at 48 hr was 176.75 and 262.77 mg, respectively. For Plutella xylostella, the average body weight of larvae dying before 24, 48 and at 72 hr were 4.0, 4.3 and 5.8 mg, respectively. The total number of nematodes recovered from the body of Spodoptera litura and Plutella xylostella larvae fluctuated due to body weight of insect. Nematodes recovered 24 h after inoculation were range from 3 to 82 in dead larvae of Spodoptera litura weighting about 155.07 mg and living larvae weighting about 211.66 mg had range from 2 to 14 nematodes. The same tendency was observed the number of invading for Plutella xylostella. Nematodes recovered were range from 1 to 8 in dead larvae weighting about 5.89 mg and living larvae weighting about 4.17 mg had range from 1 to 6 nematodes. The mechanisms that govern the infectivity process was also observed. In these experiments, nematodes were separated into two groups; those attempt to establish infection and those do not, then difference between the groups was tested. The both groups were collected by exposing hosts to nematodes in 60 mm petri dishes lined with moist filter paper. Infectious nematodes were collected from the insect culticle and tracheae by vortexing in water after 2 hours exposure. Sample of non-infectious nematodes were collected from that remain on the filter paper after 24 hrs. The proportion of the population that is in an infectious stage was 2.33. Infectivity was assessed in 60 mm petri dishes with moistened filter paper. One hundred infective juveniles (IJs) of each treatment group were added to the dishes, then one Chilo tumidicostalis larvae was added. In this insect tested, infectious nematodes showed a higher infectivity than non-infectious and non-separated (intact) nematodes resulted in 70, 40 and 15 percent insect mortality in 24 hr, respectively. However, most of the insects died within 48 hr after inoculation. This data will enable us to manipulate infectivity ie., the effective field dose rate, or increase the portion of infectious IJs in an application.