Evaluation of Steinernema riobravis (Nematoda: Steinernematidae) Against the Mexican Rice Borer (Lepidoptera: Pyralidae)

The virulency of an endemic nematode, Steinernema riobravis Cabanillas, Poinar and Raulston (Nematoda: Steinernematidae), was tested against the Mexican rice borer, Eoreuma loftini(Dyar) (Lepidoptera: Pyralidae), in the laboratory and field. Steinernema riobravis caused 100% mortality in E. loftini larvae at all concentrations of 20 to 240 nematodes per larva 2 d post treatment. Numbers of juvenile progeny increased significantly with inoculum dosage of nematodes. Average juvenile progeny ranged from 2,000 per borer larva at 10 nematodes per larva to over 4,000 per larva at 120 per larva. A field experiment on sugarcane (Saccharum spp. cv ‘NCo 310’) was performed using three treatments: (1) control (no nematodes); (2) low application rate (1.24 × 10⁹ nematodes/ha); and, (3) high application rate (2.47 × 10⁹ nematodes/ha). At weekly intervals, the field was sampled for numbers of internodes per stalk, numbers of internodes damaged by borer larvae, and parasitoids reared from larvae collected. The field results showed the nematode treatments were ineffective in reducing borer incidence or damage. Percentage of bored internodes and numbers of borers collected significantly increased with time, but treatment effects were not significant. Numbers of parasitoids emerging from the larvae collected were too low to be analyzed statistically. Plant height, aboveground biomass, and juice quality were not significantly affected by treatment. The ineffectiveness of the nematode applications may be due to desiccation caused by exposure to sunlight and inadequate humidity, as well as poor contact with the target insect.