Cotton bollworm management

Common practice on farms, mixing tank can cause problems such as clogging of spray nozzles and even reducing the effectiveness of the products. When this process involves mixing biological insecticides with chemical pesticides the effects on performance of caterpillar control in cotton cultivation may become even more more evident.

Cotton cultivation is among the most important in the Brazilian agricultural production scenario. Every year the planted area has increased, which generates the need for development of tools that optimize crop management and are viable for the farmer. One of these tools is tank mix.

Often the practice of tank mixing causes product incompatibility problems, which can form precipitates, clog spray nozzles and even reduce the product effectiveness. This fact tends to be more evident when working with a mixture of pesticides and biological control products. With the emergence of biological insecticides, many producers still continue to adopt this mixing practice without being sure that the reactions that occur between the defensive and biological control agent, which are generally bacteria Bacillus thuringiensis (Bt), do not cause damage, decreasing its efficiency on caterpillars.

Given the common practice adopted in recent harvests and the lack of knowledge about the effect of fungicides on bacterium Bt, a test was carried out with the objective of evaluating the efficiency of a biological insecticide based on B. thuringiensis in mixture with fungicides to control caterpillars in cotton crops.

O test was installed under field conditions at the Assist Experimental Station Agricultural Consultancy and Experimentation, in the municipality of Campo Verde, Mato Thick. The cotton variety used was FM 944 GL sown on 27/01/2015. The design was a Randomized Block Design (DBC) with 6 treatments (insecticides, fungicides and mixtures) and 4 repetitions. The experimental plot was made up of 4 cotton lines spaced 90cm apart, 5 meters apart. of lenght. 10 plants/plot were evaluated (Table 1).

The products were applied twice times with an interval of 7 days. The first application took place on 15/05/2015 (stadium C4) and the second in 22/05/2015 (stadium C5) with the aid of a backpack air sprayer induced with constant pressure. O spray volume was 130 liters per hectare, with a compound application bar with 6 fan-like points, with a space of 50 cm.

The evaluations for determination of the number of caterpillars were carried out before applying the treatments (prior) and at 3 days and 7 days after first application and at 3, 7, 10 and 14 days after the second application. 10 plants were evaluated in each plot, with scanning of the entire plant (careful observation in all parts of the plant) to identify and count the caterpillars present. To the main caterpillars found and evaluated were helicoverpa spp e Curuquerê.

FOR helicoverpa he was possible to observe that at 3DA1A (3 days after first application) there was no statistical difference between treatments. However, the treatment he received with Spinosad (0,15 L/ha), presented greater control efficiency with 75%. In evaluating the 7 DA1A (7 days after first application) it was found that treatments with Spinosad and Bacillus thuringiensis applied alone were statistically superior to treatments where were applied in a mixture with fungicide. Even with low numbers of caterpillars it was possible to differentiate the treatments, where the two best presented efficiency above 80% control (Table 2).

B. thuringiensis applied alone showed good caterpillar control. However, in mixture with Tetraconazole, Fentin Hydroxide and Mancozeb it was not possible to observe this efficiency. Probably, the addition of fungicides to the B. thuringiensis may have caused some reaction that interfered with the effectiveness of this biological insecticide.

In the evaluations after the second application, it was possible observe again that there was a difference between the treatments. At 3DA2A (3 days after the second application) it was again found that Spinosad and B. thuringiensis applied alone were more efficient in controlling helicoverpa (Table 3).

At 7, 10 and 14 DA2A, Spinosad It is B. thuringiensis presented control efficiency similar to each other and distinguished from B. thuringiensis + Tetraconazole, B. thuringiensis + Fentin hydroxide It is B. thuringiensis + Mancozeb which They were equal to each other and had the same behavior as the witness. Those results demonstrate that the addition of products affected the behavior of biological insecticide for caterpillar control helicoverpa spp.

For the leafworm caterpillar two assessments were carried out, at 10DA2A and 14DA2A. In evaluating the 10DA2A it was found that treatments where the plant-based product was applied B. thuringiensis and Spinosad were similar to each other, differing from mixtures of B. thuringiensis + Tetraconazole and B. thuringiensis + Hydroxide fentin which were statistically superior to the mixture of B. thuringiensis + Mancozeb which was statistically equal to the control (Table 4).

At 14 DA2A (days after second application), as observed for helicoverpa, it was observed that there was an effect of mixing the biological insecticide with fungicides, where these mixtures were inefficient in controlling the caterpillar, being statistically equal to the control. This shows that there is some effect of fungicide on the bacteria contained in the insecticide based on B. thuringiensis.

According to Knaak et al. (2003), the presence of emulsifiers and other concentrated additives used in preparations can generate compatibility problems with entomopathogens. In addition of emulsifiers, it was possible to observe that the water-dispersible powder fungicide (WP) and suspension concentrate (SC) can also cause problems with compatibility with the entomopathogen.

It was possible to conclude that the treatments that received Spinosad and B. thuringiensis presented the best levels of control for caterpillars curuquerê and helicoverpa. The addition of the fungicides Tetraconazole, Fentin Hydroxide and Mancozeb showed antagonistic effect on Bacillus thuringiensis when mixed with biological insecticide. It was found that the mix of B. thuringiensis with these fungicides tested negatively influence the control of helicoverpa and Curuquerê.

Article published in issue 217 of Cultivar Grandes Culturas, June 2017.