A challenging pest capable of causing devastating losses in cotton cultivation, Helicoverpa armigera requires producers to integrate measures to combat it in a rational, effective and sustainable way. The use of bioinsecticides is one of the tools available to be associated with the complex management of this caterpillar.
Cotton cultivation in the Cerrado is favored by the conditions that the region offers, such as climate and flat land, which allow mechanization of farming. The socioeconomic importance of cotton is closely associated with the issue of controlling insect pests as this challenge is one of the highest cost factors in production. Since the 2012/2013 harvest, yet another pest with great destructive power began to worry producers. This is the Helicoverpa armigera, a caterpillar with the potential to cause devastating damage. The management of this pest has faced great difficulties. Among the integrated management actions suggested by the Ministry of Agriculture, Livestock and Supply (Mapa) to prevent and combat the pest is biological control.
With the objective of analyzing the effect of the bioinsecticide Diplomata, based on the HaSNPV virus, on the control of the caterpillar Helicoverpa armigera bioassays were conducted at the Agricultural Entomology Laboratory of the State University of Bahia, Campus IX, in Barreiras, Bahia, from September/2014 to January/2015, in BOD (25ºC 1ºC and 12 hours of photophase) for ten days. The daily mortality of first, second and third instar caterpillars was evaluated after 24 hours of feeding on cotton leaves sprayed with the product (80ml/ha). The statistical design was completely randomized, with five replications, each replication represented by ten caterpillars. Mortality data were subjected to analysis of variance using the Scott-Knott test (1974), at 5% probability. When comparing the instars for caterpillar control H. armigera there was an influence of the instar on the total mortality of caterpillars under the effect of the bioinsecticide (80ml/ha), with the 3rd instar being lower than the others at 73%, the 1st and 2nd instars did not show a statistically significant difference. The mortality peak occurred between the 4th and 5th day after ingestion of the occlusion bodies, the average mortality for the three instars was 64% at four days, 86,5% at seven days and 90% at ten days. Between the 1st and 2nd instars there was no statistical difference, these being effective in controlling the Helicoverpa armigera. The 3rd instar showed a statistical difference with a lower mortality rate than the others, but still maintaining effective control.
The caterpillars of H. armigera used in bioassays originated from eggs from adult breeding cages in the laboratory.
In bioassays using second and third instar caterpillars, when they were born, they were kept in boxes (pots) containing new cotton leaves as food until they reached the desired instar. The cotton leaves were obtained from plants grown in a greenhouse, cultivar BRS 335 (cultivar no. Bt) without pesticide residue.
Caterpillars of the first, second and third instars of H. armigera, and the commercial formulation of the bioinsecticide based on the HaSNPV virus (baculovirus) stored under refrigeration in a refrigerator (± 5°C).
For the bioassays, with the instars separately, a stock solution of 100ml of the HaSNPV virus (baculovirus) was prepared (dose = 80ml p.c./ha), in a spray volume equivalent to 200L of the commercial product/ha, using sterile distilled water . For the Witness treatment, only sterile distilled water was used. The solutions were then placed in individual hand sprayers. The treatments (T1 = HaSNPV Virus; T2 = Control) were then applied to the underside of cotton leaves, removed from the plant tip, in a single spray sufficient to cover the entire surface of the leaf with droplets. Then, the leaves from the treatments, with the sprayed side facing down, were placed in transparent cups in the amount of one leaf/cup, containing a caterpillar at the bottom (Figure 1). The cups identified with the treatments were closed with the lid, and then placed in bowls (40cm long, 20cm wide; 10cm high) containing moistened paper towel at the bottom to ensure high humidity inside the box.
To avoid contamination, cups from different treatments were not mixed in the same box (vessel). Then, the boxes were kept in a BOD chamber (25°C±1°C and 12 hours of photophase).
The cotton sheets from all treatments were replaced daily, during the evaluations, with others without the virus. The evaluations consisted of determining the daily mortality of caterpillars in the treatments, until the death or passage to the pupal stage of the last live caterpillar from the virus treatments.
The accumulated mortality values (%) of caterpillars in the Control treatment (without virus application) were not very relevant in all larval instars of H. armigera, being on the tenth day after the start of the test 6% for the first instar, 12% and 18% for the second and third larval instars, respectively (Figures 2 to 4). On the 5th day, there was a moment of intense mortality in treatments with HaSNPV virus. (Table 1).
The Witness cumulative mortality (%) values were used to determine the Abbott (1925) corrected mortality of virus treatments, and also as a reference to attest to the quality of the assay. Thus, it can be inferred, based on the reduced mortality of the Witnesses (Figures 2 to 4), that the mortality values presented by the product based on the HaSNPV virus (baculovirus) in Table 1 consistently express the control capacity of the product in the different larval instars of H. armigera, with reduced interference from the daily manipulation to which the caterpillars were subjected, and also from the environment represented by food, humidity, photoperiod, aeration and combination of these factors, among others.
In the first instar, mortality occurred from the first day, extending over the following days, highlighting the 4th day as the moment where the peak occurred with 45%. There was still mortality on the 5th day, which added to the previous day reached 68,3% of all caterpillars killed by the treatment (Figure 5).
In the second instar, mortality occurred from the second day, extending until the tenth. Despite being a little later, the peak also occurred on the 4th day with a value of 63,3% and the second highest peak on the 5th day with 15%, totaling 78,3% mortality (Figure 6).
Mortality in the third instar occurred on the first day, resuming on the third day, extending until the 10th and final day of evaluation. The peak occurred on the 4th day with a value of 31,3% and on the 5th day with 13,3%, totaling approximately 40% mortality, this value referring to half of the evaluation period (Figure 7).
The peak of mortality occurred between the 4th and 5th day after ingestion of the occlusion bodies. The average mortality for the three instars was 64% at four days, 86,5% at seven days and 90% at ten days. When comparing the instars for caterpillar control H. armigera, there was an influence of the instar on the total mortality of caterpillars under the effect of the HaSNPV Virus (80ml/ha) for the 3rd instar with a value lower than the others with 73%, the 1st and 2nd instars did not show a statistically significant difference.
Therefore, the highest mortality rate occurred from the 4th to the 10th day in 1st and 2nd instar caterpillars with 96% and 100%, respectively. The 3rd instar presented 73% mortality, lower than the other instars, but still maintained effective control.
Mikaele S. Silva, Marco A. Tamai, Ana Paula S. Lima, Ilzimara C. Ledo, Marcelo R. B. S. T. Andrade, Gracielia T. Oliveira, Universidade do Estado da Bahia; Mônica C. Martins, Círculo Verde Agronomic and Research Advisory
Article published in issue 201 of Cultivar Grandes Culturas.
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