​Effective management of boll weevil

The cotton boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae), is one of the key pests of cotton, responsible for losses that can make it unfeasible or significantly alter the way in which cultivation is carried out. The adult of this insect is a beetle with a color ranging from brown- reddish and dark gray, reaching 3,2mm to 8mm in length (Figure 1). While the size can vary considerably depending on the nutritional condition of the host (buds or apples), the color of the integument changes with the age of the insect, with a predominance of grayish color in older insects.

The name "bicudo" refers to the fact that the insect's adults have an extended head, forming a characteristic rostrum or "beak" (Figure 1), common in many members of Curculionidae. The insect's mouthparts, used to damage reproductive structures, are located at the end of this "beak" or rostrum.

Cotton boll weevil adults can generally be differentiated from other "boll weevil" species due to the presence of two spine-shaped projections located on the femur of the anterior pair of legs, with the innermost spine being larger than the outermost spine. external.

The immature stages (eggs, larvae and pupae) are located inside the attacked structures, which include buds and apples. The eggs, slightly elliptical and opaque, are difficult to see, as they are around 1mm long and are deposited inside the attacked structures. The larvae are white, apodous and have a well-differentiated brown head capsule, reaching approximately 1cm at the end of their larval development (Figure 2). The pupae are white to cream in color and have a shape very similar to that of the adult, making it possible to visualize the face, legs and wings already at this stage (Figure 3).

Under Brazilian temperature conditions, the insect takes 14 to 22 days to complete its development cycle. This data provides an idea of ​​the number of generations that the pest can complete during just one cotton development cycle: on average, 3-4 generations, disregarding the overlapping of generations that may occur due to immigration from adjacent areas and births continuous weevils within the crop. Add to this the fact that each female boll weevil takes 4-5 days to begin oviposition, depositing around 100-300 eggs during her lifetime, and you will have an overview of the damage-causing potential of this boll weevil. insect.

In Brazil, this situation is even more critical or aggravated due to insects finding favorable environmental conditions for their development and a suitable host for a large part of the year, since "harvest" crops are followed by "off-season" (or overlapping) crops. in many regions. Additionally, the physiology of the cotton plant itself does not favor the management of the pest, given that the indeterminate growth habit promotes the continuous emission of reproductive structures. Therefore, it is not uncommon to find flower buds, flowers, apples and open bolls on the same plant, something that does not contribute to the lack of food supply and the pest's breeding site. Therefore, the sanitary void, one of the cultural control measures most used in pest management in general, which consists of the spatial or temporal unavailability of food in order to interrupt the pest's life cycle, requires special attention for implementation. This management measure has implications for the planting and harvesting calendar of crops, which varies between regions.

From a historical point of view, the relevance of this insect as a cotton pest is such that it justified the efforts made to eradicate it in the most relevant cotton production areas in the United States. In Brazil, after its entry in the 80s, the possibility of eradication was raised, following the example of the American initiative. However, this approach was disregarded after some time for operational reasons, mainly due to the amplitude of the planting calendar between the different producing regions of Brazil and due to the climatic conditions favorable to the pest, among other reasons. However, the entry of the pest was imperative in the changes that occurred in relation to the varieties cultivated and the change in the geography of production. These changes resulted in the replacement of semi-evergreen varieties by others with an annual cycle and large-scale cultivation in areas of the cerrado, to the detriment of the areas in which the crop had been cultivated until then.

From all of the above, one can get an idea of ​​why, even in cotton production areas where the boll weevil is considered eradicated or has a low prevalence, the insect is still considered a pest of great economic relevance. On many occasions, plants with vigorous vegetative growth present marginal productions. This situation is due to the fact that the pest concentrates its attack on the plant's reproductive structures (buds and apples) (Figure 4), contributing to a significant reduction in production due to quantitative (weight) and qualitative losses (change in fiber quality). In field assessments, it is possible to distinguish between structures showing only signs of feeding and those showing signs of oviposition. Oviposition punctures are normally elevated and contain the clear sign of the "sealing" of the oviposition hole made by the female (Figure 4). The injury resulting from feeding does not present a seal and shows visible openings that often contain pollen residues removed from the interior of the flower bud (Figure 1).

Crops infested with the boll weevil have a considerable number of buds falling to the ground, in addition to natural abscission, since after being attacked it is common for this structure to senescence within a week. Recognition of the pest's attack signal is very important, given that the indices used to make curative control decisions (chemical control) are based on the percentage of reproductive structures attacked, although this decision can also be based on monitoring adults collected in type traps Accountrap (Figure 5) containing the insect's synthetic pheromone. It is worth highlighting that although several broad-spectrum molecules are registered for the control of boll weevil, the peculiarity of the attack makes the different stages of development of the pest little exposed to the action of different biotic or abiotic control methods, making them little effective. Therefore, the uniqueness of this pest and its attack justifies, even more, the use of management measures in an integrated manner and, whenever possible, compatible and capable of acting in different phases of the insect's development cycle.

Recent studies, carried out at the University of Brasília (UnB) with colored fiber cotton grown without the use of agrochemicals and synthetic fertilizers, have revealed the importance of the premise of "integration" in boll weevil management. In these studies, the principles of host genetic resistance, behavioral control and chemical control were used in an integrated manner. Preliminary results showed that the BRS Aroeira cultivar, with white fiber, was less attacked by the boll weevil than the colored cultivars. These results were used in the planning of subsequent trials in which the use of this variety as a border for colored varieties was tested (Figure 6), with the aim of delaying the infestation of the latter, due to the colonization of crops by the boll weevil normally starting at the border. . Furthermore, mass collection was used using traps containing boll weevil pheromone compared to the control with neem oil-based formulation, in concentrations varying from 0,5% to 2%. Figures 7 and 8 show the average density and percentage of plants of the BRS Topázio variety attacked by the boll weevil in situations where pest management was carried out:

1) through mass collection with traps containing pheromone;

2) through the association between the traps and the borders cultivated with BRS Aroeira, respectively.

The asterisks indicate the moment at which the pest reached the control level, based on the intervention recommendation, when 10% of the plants are infested (show signs of attack). By analyzing the figures, it can be seen that the percentage of infestation in the plots, where the pest was managed exclusively through mass collection, was in the order of 70%, in the plots where the integrated use of traps was used with the white variety in the border, the maximum level of infestation was 40%. Something similar occurred in the plots whose management was carried out with increasing concentrations (0,5%, 1% and 2%) of a neem oil-based formulation as opposed to those whose management with the formulation was integrated with the cultivation of the white variety on the borders ( Figures 9 and 10). Thus, the level of infestation in areas managed exclusively with neem was much higher (around 50%) compared to those areas that were managed using integrated control (around 25%) of neem with Aroeira borders. The arrows in the graphs indicate the days and concentrations used in the spraying.

Therefore, it is evident that although the management measures employed were not sufficient to reduce the pest density below the control level, the integration resulted in a considerable reduction (by around 50%) in the infestation. These results prove the need to adopt multiple management measures to obtain more effective control of pests as harmful as the boll weevil. It is worth remembering that, whenever possible, such measures should be aimed at managing the different pest phases of the species or exploring the multiple possibilities of management measures available in the different tactics that make up the IPM, such as cultural, chemical, biological, physical, behavioral and others.

This approach is perfectly valid and applicable to other emerging and recently introduced pests in the country, especially when considering that, like the boll weevil, they are more difficult to be affected by traditional control methods (chemical control), a since they are also protected inside the reproductive structures. In this situation, the efficiency of chemical control is considerably reduced and the possibility of selecting resistant individuals increases, due to more frequent applications or continuous growth at the doses recommended for control. In this scenario, the same approach used when pest control was unsuccessful shortly after the introduction of organic insecticides becomes contemporary: the solution involves the integration of control measures and the integrated control of pest organisms.

Figure 1 - Boll weevil adult Anthonomus grandis Boheman (Coleoptera: Curculionidae). Photo: Jorge Braz Torres

Figure 2 - Boll weevil larva, Anthonomus grandis Boheman (Coleoptera: Curculionidae). Photo: Jorge Braz Torres

Figure 3 - Pupa of the boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae). Photo: Jorge Braz Torres

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Figure 4 - Bud (A) and apple (B) attacked by the boll weevil. The button (A) has a marked oviposition symptom. Photos: Jorge Braz Torres

Figure 5 - Trap Accountrap used to monitor cotton boll weevil adults, Anthonomus grandis Boheman (Coleoptera: Curculionidae). Photo: Tamiris Alves de Araújo

Figure 6 - Schematic representation of the plots that were cultivated with the white fiber variety (BRS Aroeira) surrounding the colored variety (BRS Topázio)

Figure 7 - Variation in boll weevil density, Anthonomus grandis Boheman (Coleoptera: Curculionidae), and the percentage of plants of the BRS Topázio variety attacked in areas where the insect was managed through mass collection with traps of the type Accountrap containing pheromone

Figure 8 - Variation in boll weevil density, Anthonomus grandis Boheman (Coleoptera: Curculionidae), and the percentage of plants of the BRS Topázio variety attacked in areas where the insect was managed through mass collection with traps of the type Accountrap containing pheromone + border with the white variety BRS Aroeira

Figure 9 - Variation in boll weevil density, Anthonomus grandis Boheman (Coleoptera: Curculionidae), and the percentage of plants of the BRS Topázio variety attacked, in areas where the insect was managed with a neem oil-based formulation

Figure 10 - Variation in boll weevil density, Anthonomus grandis Boheman (Coleoptera: Curculionidae), and the percentage of plants of the BRS Topázio variety attacked, in areas where the insect was managed with a neem oil-based formulation + borders with the white variety BRS Aroeira

Click here to read the article in Revista Cultivar Grandes Culturas, issue 177.