Rats attack soybean and corn crops

Over the past few months, Brazilian crops have suffered in different ways. The start of the 2023/24 soybean harvest was marked by excessive rainfall in regions in the south of the country and scarce in the center-north, which led to delays in sowing and non-ideal development in different areas. In addition to climatic conditions, biotic factors, such as pests and diseases, which often cause irrecoverable damage, also exerted strong pressure on the crop.

In this aspect, the occurrence of unusual organisms attacking soybeans and corn has been observed in different regions of the country. Throughout the period of soybean reproductive development, rural producers, researchers and consultants observed areas where the pods were attacked in a peculiar way: eaten grains and ripped out pods, often found in piles close to the attacked plant. Initially, it was believed that crickets or other chewing insects were causing these injuries. However, with further investigation and observations, it was identified that those responsible for the injuries were rodents.

The occurrence of these animals observed in crops in a more intensified form is recent, so it is not yet known which species or species are present, but it is believed that they all belong to the genus Rattus. Furthermore, they can be classified into different categories according to their geographical origin. For example, the black rat originates from Asia, but has been introduced to other parts of the world, including Brazil. The colony rat originates from Europe, but today it is found in different regions of the world, which may result in different species occurring in macro-regions.

The digestive system of these rodents is adapted to feed on a variety of food sources, including grains, seeds and even insects. Its reproduction is a remarkable characteristic due to the fact that it occurs throughout the year and very quickly: a pregnancy lasts around 21 days. The cubs are born blind and hairless, but they grow quickly and are capable of reproducing at two to three months of age. This results in a high reproductive rate and a rat population that can grow rapidly in the occupied region. As for behavior, they are nocturnal and crepuscular animals, being active from dusk to dawn, which often makes it difficult to see them and confirm that they are responsible for attacks on crops.

Initially, the occurrence of rodents has been observed regionally. In 2017 there were reports of rat attacks on soybean crops in the state of Mato Grosso do Sul, in the Amambai region, where they attacked pods to feed on the grains. In the 23/24 harvest, regions in the states of Paraná and Mato Grosso also reported the presence of these animals in the soybean harvest and, in a more intensified way, in corn crops that followed soybeans.

Rodents, in the case of soybeans, feed on the grains that are still in the pods and end up knocking them off the plants, which has still occurred in localized areas, but which can be worrying, as they feed on the pods that are final product. In corn fields, they look for the seed that was sown. This happens mainly at the beginning of seedling emergence, at a time when the roots are not yet well developed. As a result, the plant ends up falling over and as a consequence the stand is lost. Attacks in corn areas were identified in random disturbances throughout the crop.

In Mato Grosso, in the regions of Sorriso and Campo Novo do Parecis, it was possible to verify attacks in commercial and research areas. In addition to the fallen plants and eaten grains, it is possible to verify the presence of these animals through the burrows they make in the middle of the crops. They dig in the ground to lodge themselves, leaving the hole through which they enter and exit visible. These holes vary in diameter, which may occur depending on the species, size and number of individuals sheltering in the burrow.

In this scenario, there is a clear influence of one culture on another and the formation of the so-called “green bridge”, the same concept adopted to demonstrate, for example, the occurrence of insects from one culture to another, through plants weeds and tiguera plants. In the case of rodents, the green bridge is caused by the availability of grains. In the corn fields where the rats were found, the presence of soybeans resulting from harvest losses was also observed, as well as in the soybean fields, ears of corn from the previous harvest were observed, which are also sources of food in addition to the grains in pods and newly sown seeds.

Although this problem has been observed in specific regions, it is necessary to emphasize the importance of mitigating crop losses to prevent this situation from growing in size. So, the main question about this situation may be: how to control the rat population? At this time, there are still no proven tools for managing rodents in crops. There are commercial baits, based on active ingredients such as difethialone and brodifacoum, which are used in urban environments or closed environments. However, the use of these tools in crops needs to be understood for better positioning and to avoid environmental imbalance. Therefore, to mitigate the effects of the occurrence of rats, one must return to already known concepts and practices.

Grain harvest loss occurs in all crops and its impacts are multifaceted, directly affecting the economy, society and the environment. Considering that these losses begin even before the harvester enters the field, due to the occurrence of pests and plant lodging, for example, integrated management strategies and more assertive agricultural practices are essential to mitigate these impacts. By minimizing the amount of grain lost, there is less food availability for rodents, which leads them to look for other environments.

Other approaches include improving field maintenance and eliminating areas prone to rat breeding, such as piles of trash, debris and improper grain disposal. Climatic factors, such as the amount of rainfall, also influence the occurrence of these animals: larger amounts of rain end up flooding the burrows and forcing individuals to leave for other areas. However, there is not much that can be done about this factor. Furthermore, mainly for initial control of infestations when they still occur in smaller reboleiras, the use of traps (mousetraps) can be an alternative.

In addition to direct damage to crops, rats can serve as vectors of diseases and parasites that can affect both domestic animals and humans. For example, species in the genus Rattus can transmit leptospirosis, a serious bacterial disease that can cause kidney failure and death in humans. Furthermore, rats can also transmit intestinal and lung infections to stray animals, decreasing their market value and potentially causing death.

Mitigating the occurrence of rodents in crops should be seen as another activity within what we call integrated pest management, including the correct identification of the causative agent and using the best strategies available to combat it.

By Gabriela Vieira Silva (Fitolab – Agricultural Research and Development) and Adeney de Freitas Bueno (Embrapa Soya)

Article published in issue 298 of Cultivar Grandes Culturas Magazine