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December 24, 2013

Scientists Advise Brazilian Farmers not to Over Apply Insecticides

Scientists in the state of Parana are advising farmers to only apply insecticides and fungicides as needed on their soybeans and not to panic and over apply chemicals in an effort to control the corn earworm (Helicoverpa armigera) or soybean rust. News reports of various states in Brazil declaring phytosanitary emergencies in regard to the corn earworm has led some farmers to needlessly over apply chemicals. The emergencies were declared in order to bypass the normal regulatory process to register new insecticides in Brazil and they were not declared because of excessive damage being done by the insect.

In order to demonstrate that farmers should be more cautious in their use of chemicals, Embrapa has established 200 demonstration plots in the state of Parana where they are closely monitoring insect populations and disease pressure. Some of the plots have been monitored for up to 85 days and at this point; only one application of insecticide has been needed to keep the corn earworm under control.

In contrast, some farmer's fields near the demonstration plots have already received five insecticide applications and many farmers are using chemicals that are stronger than necessary. Scientists are worried that the multiply applications of strong chemicals may actually do more harm than good by killing off the natural enemies of the corn earworm. In the demonstration plots the natural ermines of the corn earworm have kept the insect populations general below the economic threshold.

Farmers are being advised to monitor their soybean fields on a weekly basis for the insect. If the soybeans are still in the vegetative state, more than four caterpillars per linear meter of soybeans would justify an application of insecticide. After the soybeans enter the pod filling phase, more than two caterpillars per linear meter would justify an application. If the caterpillars get inside the developing soybean pod, they are more difficult to control.

In their demonstration plots scientists feel only one or two insecticide applications will be necessary to keep the insect below the economic threshold throughout the growing season. In contrast, it is estimated that farmers in Parana will average 5.5 insecticide applications on their 2013/14 soybean crop.

Scientists are also using these plots to monitor soybean rust as well. They feel that one fungicide application will be sufficient to control rust, but the average farmer in the state is expected to make two or three fungicide applications to control the disease.

The corn earworm was first discovered in Brazil last growing season in northeastern Brazil where it did considerable damage before scientists could even identify the insect. Some farmers in Bahia and Piaui have purchased enough insecticide to make up to nine applications, but scientists feel this is a gross over reaction to the actual situation in the field.