Brazilian soybean growers worried about two threatening pests

The recent weather in Mato Grosso has generally been favorable for the development of the soybean crop, but it has also been favorable for soybean pests as well. Farmers in the state are worried about the early onset of soybean rust in the state and the appearance of a new leaf-eating worm called by various names including "cotton worm" or "apple worm".

Soybean rust - There have been nine cases of soybean rust confirmed in the state and farmers are concerned because the soybean planting was spread out over a long period of time due to the irregular nature of the showers during September and October. Farmers who were lucky enough to receive rainfall in September went ahead and planted their early maturing soybeans while other farmers had to wait up to a month before they received enough moisture to plant.

If the early maturing soybeans are infected with rust when the crop is being harvested, the spores can be spread easily during the harvest process to later planted fields where the soybeans are still susceptible to the disease. With this in mind, scientists have advised farmers to continue applying fungicides until the crop is mature and not to try to economize by skipping the last application. If they do that, it could make the disease worse for neighboring fields.

If farmers just want to prevent rust from entering into their soybean fields, it would require two or three fungicide applications. In more severe cases where the disease is already present in the field, it might require four or five applications. Each fungicide application can cost the equivalent of approximately one sack of soybeans per hectare or just short of one bushel per acre.

Leaf-eating caterpillar - Soybean farmers in Mato Grosso and several other Brazilian states have discovered a new leaf-eating caterpillar attacking their soybean fields. They are calling the new pest either "cotton worms" or the "apple worms". It is a common pest where cotton is grown, but rarely if ever seen attacking soybeans. This new pest has been found in the states of Mato Grosso, Mato Grosso do Sul, Goias, Minas Gerais, and Sao Paulo. If left unchecked, these leaf-eating caterpillars have the capacity to defoliate the crop within five days.

The caterpillars are being found in areas where cotton is not grown and agronomists are having a hard time explaining why this pest appeared in such numbers this growing season. It's possible that the insect is just migrating from one region to another, but that seems to be to simple of an explanation especially since the worm has been found in a wide geographic region.

An agronomist in the state of Sao Paulo theorizes that the increase of this pest is potentially connected to the increased use of Bt corn hybrids. Three quarters of the corn grown in Sao Paulo is Bt corn which does not require insecticide applications to control insects. The resulting decrease use of insecticides has allowed more rare species of insects to multiply freely and then to migrate into soybean fields. Scientists continue to examine the situation and additional possible explanations may be forthcoming.

Farmers with this problem are being forced to make one or possibly two additional insecticide applications at a cost of approximately R$ 60 per hectare (approximately USD 12 per acre) for each application depending on the chemical used. The cost of each additional application is equivalent to approximately one sack of soybeans per hectare or about one bushel per acre.

One of the problems with this new pest is that farmers are not allowed to use insecticides that have successfully controlled the insect in cotton fields if the chemical is not registered for use in soybeans. Therefore, farmers are using insecticides registered for soybeans not knowing if it will do an adequate job in controlling the insect.