Cotton boll weevil combat strategies

The main pest of cotton, the boll weevil Anthonomus grandis is responsible for losses close to 360 million dollars per year in Brazil. Combating it requires permanent attention, intense management and the absence of cotton plants in the off-season, so that it is possible to live with this insect without resulting in heavy losses.

Of all the pests that attack cotton in Brazil, the boll weevil [Anthonomus grandis (Coleoptera: Curculionidae)] is undoubtedly the main one. Characteristics of the biology, ecology and behavior of this insect impose a great challenge on the producer. This is because even with the use of control measures, if they are not well planned and adopted in their entirety as recommended by the management program, there will still be productivity losses. A survey recently carried out by the Entomology team at the Instituto Mato-grossense do Algodão (IMAmt) revealed that cotton producers in Brazil lose approximately 375 dollars per hectare/year to the boll weevil, including losses caused by the pest and management costs (insecticides, traps, weevil-killing tube, destruction of tigueras, regrowth, etc.). Extending this to the total cultivated area, Brazil loses approximately 360 million dollars/year to the boll weevil (production losses + management costs), amounts that justify any investment related to planning to reduce the pest's population.

 Losses caused by the boll weevil are high due to the attack on the reproductive structures of the cotton plant, both for feeding and reproduction. The insect completes its development by going through the egg, larva, pupa and adult stages (Figure 1), the first three of which occur protected inside the reproductive structures (e.g. flower buds). This way, only adults are exposed and susceptible to spraying. The adult is easily identified as it is a beetle from the Curculionidae family, and can be four to ten millimeters long, has a long and curved rostrum (mouthparts), which normally measures half the length of its body, and its color can vary. light brown, greyish brown to dark brown.

Usually, the boll weevil enters the cotton crop when the plants begin the reproductive phase, that is, as soon as the plants emit the first flower buds, and when it becomes established in the area due to lack of control or adoption of measures Ineffective control, infestations persist until the end of the crop cycle. From colonization onwards, the dispersion of the boll weevil within the plot is related to future generations/multiplication, and the progress of this movement is influenced by the management adopted. Monitoring via pheromone traps or monitoring via monitors on the edges and inside the plots is a key activity for correct decision-making for boll weevil control. This is because the success of chemical control is related to the application of insecticides at the time of colonization indicated by monitoring. Therefore, it is recommended that insecticide applications on plots showing infestation after detection of the pest are sequential, normally in three applications with an interval of five days between them. These applications are important to successfully control emerging adults, which occurs constantly (emergency flows), as well as to control individuals that escape previous spraying.

Boll weevil management is not limited to monitoring and correct insecticide applications. It is necessary to adopt management measures throughout the year, that is, from the established culture, at the end of the cycle, harvest and, even, in the off-season (post-harvest). In this way, it is possible to affirm that boll weevil management does not depend solely on one tool, such as insecticides, but rather on a sum of actions that will result in less pest pressure in subsequent harvests and, consequently, lower losses.

Off-season management must be strictly adopted by everyone, especially in relation to spontaneous cotton plants during this period, which must all be eliminated, whether tigueras, ratoons and/or regrowth, regardless of where they are found. As the boll weevil has few hosts that are used in its food and, in Brazil, to date, there are no reports of it reproducing in other plant species other than cotton, the elimination of cotton plants in the off-season is a measure essential for reducing the boll weevil population, thus ensuring a smaller population to infest crops in subsequent harvests. It is worth noting that the practice of destroying cotton plants outside the growing season is supported by law, and it is a duty to comply with the cotton plant's health status.

Cotton producers in Mato Grosso, responsible for more than half of the production of all cotton in Brazil, must remain vigilant to the recent changes in the Normative Instruction (SEDEC/INDEA-MT Nº 001/2016) which deals with the standards for dates of planting, sanitary emptying, destruction of ratoon crops, boll weevil control and others. In particular, in relation to the changes in the sanitary vacancy dates, which were regionalized, dividing Mato Grosso into two regions, with Region 1 being integrated by the South, Central-East and Center Regional Nuclei; and Region 2 by the Northwest, Middle North, Central North and North Regional Nuclei. This division is presented in Figure 1. The dates for the sanitary vacancy in each region were established as follows:

Region 1 – between October 1st and November 30th;

Region 2 – between October 15th and December 14th.

Another important point that producers should pay attention to is the term used for assessment. Previously, a “live plant” of cotton was used, with the producer being fined when any live plant was found on his property during the period of the sanitary void, while the current term is “plant with phytosanitary risk”, the definition of which is: tigueras cotton plants above from stage V3 and resprouting plants (ratoons) with more than four leaves per shoot or presence of reproductive structures.

With the exception of cotton crops, properties must keep areas free of plants with phytosanitary risk at any time of the year.

Summary of practices to reduce the boll weevil population and losses 

- Carry out good pest management during the crop development period, maintain this management until the end of the harvest, with monitoring and correct applications of insecticides, even at the end of the cotton cycle, not putting all the work carried out during the harvest to waste ;

- Seek to maintain areas with no boll weevil host in the off-season, by eliminating cotton plants from any location (plots, roadsides, cotton gins, feedlots, etc.), also eliminating cotton plants present in plots cultivated with soybeans and corn;

- Carrying out management measures in a joint and coordinated manner between cotton properties in order to seek standardization of good actions.

In relation to working together in the cotton production chain, IMAmt, through the Effective Control of Boll Weevil project in Mato Grosso, has created Cotton Technical Groups (GTAs), with the purpose of promoting the interaction of members of this production chain and, therefore, exchange information, standardize actions, technical visits, among other actions that contribute to the successful management of boll weevils and other pests. To date, in Mato Grosso, nine GTAs have been formalized: Sapezal, Sorriso, Nova Mutum, Lucas do Rio Verde, Diamantino/Deciolândia, Campo Novo do Parecis, Rondonópolis (Serra da Petrovina region), Primavera do Leste and Campo Verde. To facilitate dialogue and interaction among members, monthly meetings are held in each location and, despite the recent formation of GTAs, considerable progress has already been observed in relation to boll weevil management, especially with regard to the destruction of ratoons and cotton tigueras, monitoring, sequential applications of insecticides, transportation of bales and cottonseed, among others.

Although the “dry” climate was unfavorable for the boll weevil during the 2015/2016 harvest, contributing to smaller populations in the final phase of the harvest, it is necessary to remain alert and carry out all recommended actions. Unfortunately, there has not been much news in relation to boll weevil management. Successful management measures recommended since the entry of this pest in Brazil are the same as those currently used, so what results in greater or lesser management success is the austerity with which these practices are implemented.  

Eduardo Moreira Barros, Jacob Crosariol Netto, Instituto Mato-grossense do Algodão (IMAmt); Jorge Braz Torres, Guilherme Gomes Rolim, Federal Rural University of Pernambuco (UFRPE)

Article published in issue 208 of Cultivar Grandes Culturas.