MT Foundation evaluates damage and caterpillar populations in cotton biotechnologies

Just like in the 2021/22 cotton harvest in Mato Grosso, the 2022/23 season has suffered from the occurrence of false caterpillars (Chrysodeixis includens), from the Spodoptera complex with Spodoptera eridania and Spodoptera frugiperda, in addition to Helicoverpa armigera. The incidence occurs mainly in crops planted with cultivars that do not have tools to control these pests, such as Bt technology. Research carried out by the Mato Grosso Agricultural Research Support Foundation (MT Foundation) and presented this month in Sapezal-MT, shows the reduction or absence of damage to transgenic cotton plants.

Materials capable of expressing Bt toxins (from Bacillus thurigiensis), for example, have control of the species of cotton leafworm (Alabama argillacea), false bollworm (C. includens), Helicoverpa armigera, apple caterpillar ( Chloridea virescens) and Spodoptera spp., all occurring in cotton crops. In the MT Foundation's research for this harvest, so far no population of caterpillars and, consequently, damage has been observed in Bt3, WS3 and GLTP cultivars. According to Lucia Vivan, entomologist and researcher at the institution, assessments will continue until the end of the cycle in order to check whether these pests will be present.

In non-Bt cultivars, in the same experiments, damage began with plant defoliation and also occurred in vegetative structures. The presence of S. eridania caterpillars began at the F1 stage, with greater damage to leaves and consumption of bracts, and lesser damage to floral buds. From the stages with flower buds and apples, infestations of caterpillars S. frugiperda and H. armigera intensified and caused damage to vegetative structures.

The specialist explains that the presence of S. frugiperda caterpillars was observed even in Bt2 cultivar, with damage to structures such as flowers, buds and apples of the cotton plant, however, at lower rates than non-Bt cultivar. “This is a demonstration that there is already a problem regarding the evolution of resistance to Bt proteins”, she highlights.

Refuge

The main way to avoid reducing the effectiveness of the technology is to adopt refuge areas in Bt crops. “Failure to adopt could lead to potential long-term risk. Pests can evolve resistance to Bt proteins, leading to a reduction in their effectiveness, meaning we will lose this tool”, warns Lucia.

To prolong the effectiveness of Bt proteins expressed in transgenic plants, it is necessary to adopt practices known as Insect Resistance Management (MRI) programs.

These programs are essential for transgenic plants and require the planting of conventional cotton at a maximum distance of 800 meters between plants, with 20% of the area being non-Bt material.

This area is called a “refuge area” and its objective is to allow the crossing of insects resistant to Bt proteins with insects susceptible to these proteins, resulting in susceptible offspring, thereby restoring the population's susceptibility to Bt cotton proteins.

Complete management

In addition to the use of Bt, WS3 or GLTP cultivars, pest control in cotton requires good chemical and biological control and integrated management practices such as monitoring, maintenance and preservation of natural enemies, seed treatment and establishment of a specific sowing period for cultivation in each region.

The researcher also emphasizes that chemical and biological controls must be used for pests that are not targets of Bt technology.