Hidden and abundant: cotton boll weevil management

The cotton boll weevil, anthonomus grandis Boheman, 1843 (Coleoptera: Curculionidae), is the main pest of cotton crops in Brazil, responsible for productivity losses of up to 75%, when not managed. This insect remains for most of its cycle (egg, larva and pupa stages) inside the cotton flower buds and apples, which makes controlling the pest difficult. In fact, this characteristic protects the development phases of the boll weevil from the action of natural enemies and, mainly, from insecticide applications. Thus, cotton boll weevil adults are practically the targets of applications.

The boll weevil colonizes cotton crops as soon as the crop emits the first flower buds, preferred structures for feeding and oviposition. This colonization occurs through migrations of the pest from areas of refuge (forests, banks of dams, rivers, etc.). Colonization can also be the result of boll weevils kept on remaining plants during the off-season, on roadsides and on roadsides, cotton bush plants and cotton regrowth in plots, due to incorrect destruction. In times of high pest pressure, colonization of crops is observed while still in the vegetative phase.

The high population levels of the pest in crops have been influenced by pest management in the previous harvest. This is because the boll weevil that is colonizing the area is that leftover from the last harvest. Thus, it is expected that by carrying out good pest management in the previous harvest – especially at the end of the cotton cycle – and well-executed destruction of ratoon crops, the population that will colonize the subsequent crop will be reduced.

Unfortunately, in all places where cotton is grown in Brazil, the boll weevil is present, causing damage. The last region where this pest was reported in the country was Sapezal, in the northwest of Mato Grosso, in 2007. Currently, the boll weevil is already completely widespread in the region, demonstrating its potential to colonize new areas.

Since 2006, the team from the Instituto Mato-grossense do Algodão (IMAmt) has been carrying out a project to monitor and control the boll weevil in Mato Grosso. This work has been financed by the Brazilian Cotton Institute (IBA) and allows monitoring to be carried out in all producing regions of Mato Grosso, starting in the off-season and being maintained until January. The result has been the establishment of strategies that allow good management of the pest in the State, reducing the population in years of high pressure.

Through monitoring, an increase in the boll weevil population was observed in this last harvest, with an increase in the capture of the pest in traps. The results indicate an average above 10 BAS (boll weevils/trap/week) in the regional centers South (Rondonópolis and region), Center (Campo Verde and region) and Central East (Primavera do Leste and region), with the index above 2,0 individuals/trap already puts the crop on red alert.

An example lies in the Central East Regional Center, which after reducing the rates in the 2013/2014 harvest (average of 1,9 BAS), recorded an average percentage of 10,35 BAS in monitoring in the pre-harvest 2014/15, which corresponds to an increase of 541% (Figure 1). These numbers have worried the entire cotton production chain, since the off-season, pre-harvest and beginning of cultivation would be times of lower occurrence of the pest.

Aggravating factors

 Several factors have contributed to a favorable environment for the boll weevil and its maintenance in the areas, and even favored its multiplication and consequent growth in this harvest. One of them lies in the poorly carried out destruction of the cotton ratoon, which results in the regrowth of plants in the areas, or even the presence of tigueras plants in soybean crops and/or other areas.

 Another aspect is the introduction of varieties with transgenic events resistant to the herbicide glyphosate, which has made the chemical destruction of crop residues difficult, as this herbicide was a tool used to eliminate ratoons, together with the 2,4D herbicide.

The rainy period during harvest times and sanitary emptiness, shortening the window for chemical or mechanical destruction, also made it difficult to destroy cultural remains in the last harvest. Likewise, the improper wrapping and transportation of seed cotton has generated host plants in the off-season, on the side of local roads and highways.

Another circumstance that has contributed to the presence of the boll weevil is the use of transgenic cotton varieties resistant to caterpillars (Bt cotton). As these materials provide satisfactory control of some caterpillars, especially in the initial phase of the cotton plant, there has been a reduction in pest monitoring. Consequently, the lack of this initial monitoring has allowed some pests to escape. Furthermore, the use of these transgenics also resulted in a reduction in the number of insecticide applications at the beginning of the harvest. Indirectly, early-stage applications to control caterpillars controlled the boll weevil and helped maintain the population at lower levels.

The current cultivation system, with cotton of 1^a e 2^a harvest, the planting window has increased and, therefore, the crop has spent longer in the field. This results in food being available for the boll weevil for a longer period of the year, favoring its multiplication. The longer period of plants in the field also reduces the sanitary void window, making the destruction of crop residues unfavorable.

The reduction in options for carrying out chemical control also worsens the situation, given the ban on the use of synthetic insecticides considered efficient in controlling the boll weevil, such as endosulfan and methamidophos. The adoption of management measures, in an uncoordinated and independent manner among farmers, results in different ways of conducting farming and favors the pest.

Due to the pressure from the boll weevil at the beginning of the cotton harvest, producers must be aware of the importance of initial actions to control the pest. If at this first moment, in the vegetative phase, monitoring and control in the plots are not rigorous and effective, the producer has a high risk of suffering failures in weevil control, causing severe damage to his crop.

The adoption of management measures in the initial phase of the crop (from before pre-planting until flowering) is essential for monitoring, knowing the pest situation in the plots and controlling the initial infestation. The IMAmt team is implementing an effective boll weevil control project in Mato Grosso, with the main objective of supporting, organizing and leading producers to use management measures to reduce the pest's population. Several management actions, which involve the entire production process, have been recommended.

Ten important actions in the initial phase of cultivation (Box)

1- Survey of the infestation history in the plots, mapping the known foci of the boll weevil (entry and exit doors), identifying possible areas of refuge. With the identification of these foci, plan, anticipate and execute different control measures.

2- Monitoring of soybean areas that follow cotton, checking the presence and intensity of cotton ratoon or cotton bush, and, in areas with high intensity, including insecticide in the soybean desiccation.

3- Installation of pheromone traps (Grandlure) 30 days before planting and keep them until the first flower buds are emitted. The installation must be carried out on the perimeter of the plots, at a distance of 150 meters from each other, with pheromone exchange every 15 days. These traps must be monitored weekly and the data organized to monitor the pest's population fluctuation.

4- Plantings must be carried out with a concentrated sowing calendar, mainly in neighboring plots.

5- Train pest monitors to monitor traps and visual inspection.

6- Carry out insecticide applications with ideal and regulated equipment, at appropriate times, prioritizing the techniques recommended by Application Technology.

7- Border applications must be started from the appearance of the third leaf, and continued until the end of the harvest. The application must be carried out sequentially, every five days, and the use of pyrethroid insecticides must be avoided, so as not to cause outbreaks of mites, whiteflies and aphids. After flowering, pay special attention to monitoring, before starting applications across the entire area.             

8- Applications in areas in the vegetative phase must occur in accordance with monitoring (data obtained from traps). As a suggestion: having up to two traps with boll weevils, apply them within the radius of action of the trap and neighboring traps; above two weevil traps, apply to the entire plot.

It is worth mentioning that the boll weevil capture history/trap/week (BAS number) determines the definition of infestation zones, assisting in applications from the emergence of the first floral bud (B1). The classification of infestation zones follows the following parameters:

Green area – 0 BAS, no application required;

Blue Zone – 0 to 1 BAS, make an application;

Yellow zone – 1 to 2 BAS, make two sequential applications, five days apart;

Red zone – above 2 BAS, make three sequential applications, with an interval of five days.

 9- Monitoring inside the plots, through visual inspection of the plants, must be carried out from the initial phase.

10- Carrying out joint actions and exchanging information between producers. This will contribute to a better understanding of the boll weevil attack and its management on each property, as well as standardizing good actions.

 

Management measures, from monitoring to pest control, must be adopted throughout the cotton production chain. To be successful in controlling the boll weevil, actions must be operated in a coordinated manner and applied collectively by region. These actions will contribute to coexistence with the pest, facilitating management, resulting in lower production costs and greater productivity.  

 The article appears in issue 191 of Cultivar Grandes Culturas.