Laboratory and field evaluation of Beauveria bassiana against sugarcane stalkborers (Lepidoptera: Pyralidae) in the lower Rio Grande Valley of Texas

Of approximately 18,200 ha planted to sugarcane in south Texas, only approximately 80 ha (<0.5%) are treated with insecticides because this type of control is widely regarded as ineffective against stalkboring pyralids, the key pests of sugarcane. Therefore, nonchemical control measures, such as resistant varieties and biological controls, must be evaluated to mitigate the losses caused by stalkborers. We performed laboratory and field evaluations on the use of Beauveria bassiana (Balsamo) Vuillemin (Deuteromycotina: Hyphomycetes) (strain GHA) against the Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Pyralidae), which causes damage in south Texas estimated at between 10 and 20 million dollars annually. We also performed bioassays against the sugarcane borer, Diatraea saccharalis (F.) (Pyralidae), the key pest in other sugarcane growing areas. In the bioassays, E. loftini was substantially more susceptible to B. bassiana than D. saccharalis, based on both 5-d LD50 values and survival times. A commercial oil-based formulation of B. bassiana was evaluated in the field using the following treatments: oil alone (control), B. bassiana + oil, and B. bassiana + Silwet L-77 carrier at an application rate of 5 x 10(13) spores per hectare. Neither numbers of E. loftini per stalk, nor stalk damage (approximately 20% bored internodes) were significantly affected by treatment. The application of B. bassiana + Silwet significantly affected the numbers of internodes showing high damage, but not those with low or medium damage. Analysis of yield data and juice quality showed no significant treatment effects. We conclude that the application of Beauveria + Silwet offers the best chances for reducing damage caused by E. loftini of those treatments tested. However, reductions in insect incidence or damage did not result in measurable increases in yield or sugar quality, probably because of insufficient coverage. Effective control of stalkboring pyralids in sugarcane using B. bassiana will likely require improvements in delivery technology.