BUENOS AIRES, April 24 (Reuters) - Argentine corn farmers are bracing for a warmer than usual Southern Hemisphere winter that will likely undermine efforts to reduce the population of leafhopper insects that damage the crop, the Rosario grains exchange (BCR) said on Wednesday.

Agricultural powerhouse Argentina is the world's No. 3 corn exporter, as well as a major soybean, wheat and beef producer.

Local corn yields, however, have been hit by explosive growth of leafhoppers, whose appetite for plant sap weakens corn plantings while triggering stunted growth and leaf yellowing. The insects also help spread plant disease.

Farmers were hoping that cooler weather in the weeks ahead could check the growth of leafhoppers, as they do not tolerate temperatures below about 39 degrees Fahrenheit (4 degrees Celsius).

But the exchange forecasts that upcoming winter weather conditions are unlikely to help control the pest with "minimum temperatures at average values or even slightly higher," according to a BCR report citing climate specialist Alfredo Elorriaga.

"A particularly cold year is not expected," Elorriaga said.

A separate report from the Buenos Aires grains exchange also released on Wednesday predicted cooler temperatures from this coming weekend, but they are not expected to fall below 41F (5C) in agricultural areas.

Earlier this month, BCR cut its corn harvest forecast by 6.5 million metric tons to settle at 50.5 million tons, citing damage attributed to the leafhopper outbreak for the downward revision. (Reporting by Maximilian Heath; Editing by Sonali Paul)