Evaluation of the efficiency of insecticides in controlling the boll weevil in 2023

The 2022/23 cotton harvest had a total production of 7,69 million tons of seed cotton, and lint production was estimated at approximately 3,16 million tons. However, the same soil and climate conditions that favor the development of cotton farming also provide optimal conditions for the development and multiplication of approximately 250 species of insects and mites and, of these, around 60 species stand out for their ability to cause direct or indirect damage, and reaching pest status in some of the crop's development phases.

Although this large complex of pests is present throughout the development of the crop, none other leaves cotton producers as concerned as the boll weevil (Anthonomus grandis grandis (Boh.) (Coleoptera: Curculionidae). The infestation of this pest occurs in crops after the emergence of the first reproductive structures, the flower buds, or even before, remaining in tiguera plants among soybeans or corn grown before cotton and remaining until harvest. Productivity losses resulting from boll weevil infestations are the result of feeding and oviposition and, consequently, the development of larvae and pupae in flower buds and apples, which results in significant reductions in fiber production. As a result of the attack, up to 95% of productivity can be compromised if management actions are not adopted. However, even with the adoption of control measures in the Cerrado, losses of up to 15 arrobas/ha are estimated.

The 2022/23 cotton harvest had a total production of 7,69 million tons of seed cotton, and lint production was estimated at approximately 3,16 million tons. However, the same soil and climate conditions that favor the development of cotton farming also provide optimal conditions for the development and multiplication of approximately 250 species of insects and mites and, of these, around 60 species stand out for their ability to cause direct or indirect damage, and reaching pest status in some of the crop's development phases.

Although this large complex of pests is present throughout the development of the crop, none other leaves cotton producers as concerned as the boll weevil (Anthonomus grandis grandis (Boh.) (Coleoptera: Curculionidae). The infestation of this pest occurs in crops after the emergence of the first reproductive structures, the flower buds, or even before, remaining in tiguera plants among soybeans or corn grown before cotton and remaining until harvest. Productivity losses resulting from boll weevil infestations are the result of feeding and oviposition and, consequently, the development of larvae and pupae in flower buds and apples, which results in significant reductions in fiber production. As a result of the attack, up to 95% of productivity can be compromised if management actions are not adopted. However, even with the adoption of control measures in the Cerrado, losses of up to 15 arrobas/ha are estimated.

The behavior of ovipositing and completing development within the reproductive structures of the cotton plant, in addition to destroying the productive part of the plant, makes it difficult for the pest to come into contact with the insecticide, forcing the adoption of curative control practices (spraying) intended exclusively for the phase adult. However, adults are not easy targets, as they remain sheltered in the flower buds, being protected by the bracts and the crop canopy itself. Furthermore, after colonization, ovipositions become daily at a rate of three to ten eggs/day/female and consequently the next generation will emerge in daily emergencies, requiring consecutive applications to try to contain the “waves of infestation”. These serial applications result in a high number of interventions throughout the cultural cycle, with an average of 12 to 19 carried out, but, not infrequently, there are reports of up to 25 specific applications to control this pest.

Even though they are still the best options for weevil management, it is important to remember that the continuous use of insecticides, especially broad-spectrum insecticides, contributes to the rapid selection of resistant populations, as well as outbreaks of secondary pests, mainly due to pest pressure. selection and elimination of natural enemies. Due to these issues, since the 2015/2016 harvest, entomology researchers at IMAmt have annually monitored, through laboratory tests, the efficiency of insecticides that can be used to control the boll weevil in the Mato Grosso region. . This study gives rise to a technical circular that serves as a basis for choosing protocols by breeders, consultants and technical teams on farms.

The insects used in laboratory tests are obtained from reproductive structures with signs of attack at the end of the cotton harvest. These structures remain in the laboratory in 30x45x50 cm cages until the adults emerge. After emerging, the adults are transferred to 500 ml pots and kept for a period of five to ten days with availability of cotyledons, buds and/or honey paste and brewer's yeast (1:1), and then they are used in tests. of efficiency.

The efficiency test consists of immersing cotton leaf discs with 6 cm in diameter and flower buds in insecticide syrup, after which the discs are kept on paper towels and left at room temperature to evaporate the excess syrup, leaving only the dry residue of the insecticides to be tested. After drying the plant material, they are transferred to petri dishes along with the insects that are confined on the treated material for 48 hours, with mortality being counted after this period.

Similar to the results found since the beginning of monitoring, in this harvest (2022/23) boll weevil mortality was variable depending on the insecticide. Through statistical analysis, groups of insecticides were formed that presented similar averages.

Considering the group of insecticides with mortality above 80% and, therefore, composing the group of insecticides with greater efficiency under the conditions in which the test was carried out, the insecticides Malathion, Sponta, Pirephos/Legion and Suprathion presented 100% mortality and the Chaser, Marshal Star, Curbix, Politryn, Actara and Fipronil Nortox insecticides caused mortalities ranging from 80% to 98% (Figure 2).

While the insecticides Engeo Pleno S and Pirate differed from the previously mentioned insecticides and were classified as moderately efficient, having caused mortalities of 52% and 72,5%, respectively.

In the case of treatments with Talisman, Mustang, Bulldock, Bold, Hero, Talstar and Sperto, when used at the recommended doses, they did not exceed 50% mortality and can be considered inefficient for controlling the boll weevil (Figure 1 ).

According to the results, all insecticides composed only of pyrethroids or mixtures of two pyrethroids or pyrethroids with neonicotinoid showed efficiency below 55%. This low efficiency has been observed in toxicological surveys carried out by IMAmt since the 2015/2016 harvest. Furthermore, it is possible to notice that, with each harvest, the performance of three active ingredients belonging to the group of pyrethroids tested since 2014/2015, with the exception of small fluctuations, has been reducing (Figure 2).

The reduction in the efficiency of pyrethroids has been observed for some years by researchers, producers and consultants. However, it was only from 2018 onwards, with the documentation of resistance of boll weevil populations to beta-cyfluthrin (pyrethroid), that it was possible to understand the size of the problem: resistance was characterized as a mutation that confers insensitivity of the “target site” ( location of the nervous system where the insecticide acts). This type of mutation can confer resistance to the entire chemical group. This may explain the reduction in efficiency that has been occurring year after year for all tested pyrethroids.

The loss of effectiveness of pyrethroids for controlling boll weevil has direct impacts on the management of crop pests, both due to the fact that they are low cost, low toxicity, broad spectrum and because they participate in several mixtures recommended for spraying in cotton. In this way, the rational use of insecticides (following the principles of IPM), the rotation of active ingredients and the constant monitoring of populations regarding susceptibility to the main active ingredients used in cotton farming are essential devices for maintaining the effectiveness of available tools (insecticides) as well as playing a crucial role in managing the boll weevil's resistance to insecticides, delaying or even reversing ongoing developments.

The IMAmt carries out this test under laboratory conditions and emphasizes that its results should not be used as a recommendation for products to control the boll weevil. It should be noted that any insecticide used to control Anthonomus grandis grandis must be previously registered by the Ministry of Agriculture, Livestock and Supply (Mapa).

By Guilherme Gomes Rolim e Jacob Crosariol Netto (Instituto Mato-Grossense do Algodão); It is Karolayne Lopes Campos (UFRPE) 

Article published in issue 296 of Cultivar Grandes Culturas Magazine