Feeding preference of Spodoptera frugiperda on different sorghum genotypes Preferência alimentar de Spodoptera frugiperda em diferentes genótipos de sorgo

Arq. Inst. Biol., v.86, 1-6, e0992018, 2019 RESUMO: A resistência das plantas às pragas é uma característica de grande importância para a agricultura, pois reduz os custos com inseticidas e promove o aumento da produtividade, resultando em maiores lucros. Este trabalho foi desenvolvido com o objetivo de avaliar a preferência alimentar da lagarta Spodoptera frugiperda por diferentes genótipos de sorgo. O experimento foi conduzido no Laboratório de Entomologia da Universidade Estadual de Mato Grosso do Sul, na Unidade Universitária de Cassilândia, no período de março a junho de 2016. O delineamento experimental foi inteiramente ao acaso, com 10 repetições. Os tratamentos foram compostos por sete genótipos de sorgo: Agromen 50A40, Agromen 50A50, DOW 1G100, DOW 1G220, DOW 1G233, XB 6022 e LG 6310. As avaliações foram realizadas com lagartas de 1o instar. Anotou-se o número de lagartas que se estabeleceram nos genótipos aos 5, 10, 15, 20, 25, 30, 60 minutos e 24 horas após a infestação. Foi estimado o índice de preferência e a massa fresca de folha consumida. O genótipo Agromen 50A40 apresentou menor atratividade para a S. frugiperda dentre todos os genótipos de sorgo avaliados.


PALAVRAS-CHAVE:
Feeding preference of Spodoptera frugiperda on different sorghum genotypes INTRODUCTION Sorghum (Sorghum bicolor) is a species that presents high adaptability to different environments and has physiological characteristics that allow it to tolerate and develop satisfactorily in water deficient conditions when compared to other crops. Because of that, the crop has potential to be cultivated in the second crop in the Brazilian ecoregion called Cerrado (ALMEIDA FILHO et al., 2010). Currently, in Brazil, approximately 622 thousand hectares of sorghum are cultivated, with mean grain yield estimated at 2,667 kg ha -1 (CONAB, 2017).
Even with these characteristics, some relevant factors have caused significant damage to their production, as pests (ELLIOT et al., 2014). The fall armyworm, of the genus Spodoptera is one of the pests that cause significant damage, it is widely distributed in the world, causing significant damage, and among the 30 species described, half are pests of sorghum and several crops of economic importance. Among them, the Spodoptera frugiperda (Smith) stands out for feeding on over 80 species of plants including cotton, corn, and soybean (POGUE, 2002).
According to BARROS et al. (2010), the S. frugiperda feed on the leaves initially, consequently starting to consume grains in the initial stage of grain filling in certain cultures. Among the factors related to the control of this insect-plague, to reduce the damage caused in crops, several insecticides are used. By misusing these chemicals, the imbalance in the environment is becoming increasingly common and resulting in higher production costs (ANDRADE et al., 2016).
Several methods have been used to control the armyworm, and the diversification of sorghum genetic material has been of great importance in controlling the pest, and the plant resistance may be the most viable (CRUZ et al., 1998). Genetic improvement of sorghum, aiming at resistance to meet future grain and forage demand, can be an effective component of the integrated pest management program (ARUNA et al., 2015).
Considering the importance of controlling this pest, the objective of this work was to evaluate the feeding preference of the Spodoptera frugiperda on different sorghum genotypes.

Location
The experiment was carried out at the Entomology Laboratory of Universidade Estadual de Mato Grosso do Sul (UEMS), at University Unit of Cassilândia, from March to June 2016. The Spodoptera frugiperda caterpillars were obtained from the National Research Center of Maize and Sorghum (CNPMS -EMBRAPA), located in Sete Lagoas (MG).

Source of Spodoptera frugiperda
The S. frugiperda caterpillars were stored in the laboratory with artificial diet (KASTEN JUNIOR et al., 1978). The breeding and reproduction methodology proposed by PARRA (1986) was used. The conditions of breeding and reproduction were: temperature of 27 ± 1°C, relative humidity of 70 ± 10% and a photoperiod of 14 hours.

Plant preparation
Seven sorghum genotypes were used: Agromen 50A40, Agromen 50A50, DOW 1G100, DOW 1G220, DOW 1G233, XB 6022 and LG 6310. Sorghum genotypes were sown in pots with the volume of 5.0 dm -3 filled with soil collected in the 0 to 20 cm depth layer. The soil used is classified as Entisol (95 g kg -1 of clay, 50 g kg -1 of silt and 855 g kg -1 of sand).
The pots were irrigated so that the soil reached the field capacity. After that, ten seeds per pot were sown. The sowing depth was 3.0 cm. Five days after emergence (DAE) a thinning was performed leaving only one plant per pot.
At 15 DAE 150 mg dm -3 of nitrogen (urea) were applied, 300 mg dm -3 of phosphorus (single superphosphate) and 150 mg dm -3 of potassium (potassium chloride). Before to application, the sources were diluted in water and then applied. For irrigation,a daily application of 180 mL of water per pot was standardized.

Evaluations
The evaluations were carried out with 1 st instar caterpillars of S. frugiperda. The design was entirely randomized with ten replicates. Each arena (repeat) was formed by a 15 cm diameter Petri dish with damp filter paper placed on the bottom of the plate. The leaves of each sorghum genotype were collected at the pre-flowering stage.
The leaves were washed and dried with paper towel. Leaf rectangles of 3.0 × 3.0 cm (9.0 cm 2 ) were then cut longitudinally and parallel with the central vein. The rectangles were arranged circularly in the arenas and fixed with pins. At the center of the plate, 140 newly hatched caterpillars of S. frugiperda were released, and the arena was then sealed with the lid of the petri dish. The number of S. frugiperda caterpillars was recorded at 5, 10, 15, 20, 25, 30 and 60 minutes after infestation. It was also noted the number of caterpillars that settled in each treatment after 24 hours of infestation.
The preference index for newly hatched caterpillars of S. frugiperda was estimated, considering as a 100% index the genotype in which the largest number of S. frugiperda caterpillars was found 24 hours after infestation. The initial (before releasing) and final (24 hours after infestation) masses were obtained with an analytical balance. From the initial and final masses, it was possible to estimate the fresh mass consumed by the S. frugiperda of each genotype.

Statistical analysis
The obtained data were transformed into (x + 0,5)^1 /2 and then submitted to analysis of variance, and F test tested the significance of the mean squares obtained in the analysis of variance at the 5% probability level. The averages for genotypes were grouped by the clustering test proposed by SCOTT;KNOTT (1974) at the 5% probability level. The attractivity data were submitted to polynomial regression analysis as a function of time. The Student's t test was applied to evaluate the significance level of the regression coefficients.

RESULTS AND DISCUSSION
The highest percentage of fresh mass consumed by S. frugiperda was observed in genotypes XB 6022, DOW 1G100 and DOW 1G220, with consumption of 19.59, 19.20 and 16.15% of leaf sections, respectively. These results are showing feeding preference of the S. frugiperda for these sorghum genotypes. The genotype Agromen 50A40 presented the lowest percentage of fresh mass consumed (7.58%), thus revealing the preference of the S. frugiperda for the other genotypes studied, to detriment of this one (Table 1).
The feeding preference and leaf consumption by S. frugiperda may be related to the plant, the insect, and the environmental factors. These are ways of manifesting resistance, according to BUENO et al. (2006), who also reports that these genetic traits of the cultivar differ in resistance and are less likely to be related to the target pest and may also be related to age and part of the plant and the physiological stage. According to SMITH (2005), a plant ceases to serve as a host for the insect when there is no the insect-plant interaction, demonstrating the potential of this pest to choose another plant as the host, thus characterizing the antixenosis.
The S. frugiperda occurs more frequently in maize, which is more susceptible when compared to sorghum (CORTEZ, 1997) due to the insect's difficulty in feeding of sorghum. When the S. frugiperda feed, its maxillary palms and labratory gustatory receptors transmit information to the nervous system, which perceive the presence of chemical, physiological and / or morphological changes in the plant and transmit commands to the central nervous system of the insect, causing it to choose other plants to feed on (SMITH, 2005).
The number of S. frugiperda caterpillars attracted by the Agromen 50A40 and XB 6022 genotypes about the time after infestation presented a linear behavior (Fig. 1), while for other genotypes the number of S. frugiperda caterpillars did not change as the time progressed (Table 2).
The number of S. frugiperda caterpillars in the leaf sections in the Agromen 50A40 genotype showed a linear behavior, with a decrease in the number of caterpillars (Fig. 1A). These results indicate the non-preference of the S. frugiperda for this sorghum genotype. For genotype XB 6022, there was a linear increase in the number of S. frugiperda caterpillars in Table 1. Initial fresh mass, final fresh mass and fresh mass consumed from leaf sections of sorghum genotypes offered to Spodoptera frugiperda caterpillars for 24 hours. Cassilândia, Mato Grosso do Sul, 2016.

Genotypes
Initial fresh mass Final fresh mass Fresh mass consumed Means followed by the same letter in the columns belong to the same group by the SCOTT; KNOTT (1974) cluster test, at 5% probability; ns not significant by the F test; *significant at 1% by the F test; CV: coefficient of variation.     the leaf sections (Fig. 1B), indicating the S. frugiperda feeding preference for this genotype.
There was a reduction in the number of S. frugiperda caterpillars attracted by the Agromen 50A40 genotype when comparing the 5 th minute with the 60 th minute after infestation. The inverse occurs for genotype XB 6022. These results are a partial indication of the susceptible and resistant genotypes.
The different taste receptors of the insect can detect quantitative and qualitative variations on the chemical composition of the tested plant tissues, which depends on the different types of phytochemicals present in the plant, which have the effect of repelling the insect (SMITH, 2005).
There was no difference between sorghum genotypes regarding the number of caterpillars attracted in the first 25 minutes after infestation (Table 3). BOREGAS et al. (2013) found that in this stage of larval development of the insect there may occur interference in the adaptation and development of the insect in the host, that is, the caterpillars tend not to feed in the first minutes until finding a suitable host, and for this reason they migrate from one plant to another.
At 30 minutes, the genotypes Agromen 50A40, Agromen 50A50 and LG 6310, presented a smaller number of S. frugiperda caterpillars compared to the others. At 60 minutes, genotypes DOW 1G100 and XB 6022 presented the highest number of S. frugiperda caterpillars compared to the other genotypes. After 24 hours of infestation, the genotypes XB 6022, DOW 1G100, DOW 1G220 and DOW 1G233 presented a higher number of S. frugiperda caterpillars, indicating a higher feeding preference of S. frugiperda for these sorghum genotypes. The Agromen 50A40 genotype showed lower feeding preference for S. frugiperda after 60 minutes compared to the other genotypes.
The migration of S. frugiperda caterpillars from one genotype to another can be due to the stiffness of the leaves' Means followed by the same letter in the columns belong to the same group by the SCOTT; KNOTT (1974) cluster test, at 5% probability; ns not significant by the F test; *significant at 1% by the F test; CV: coefficient of variation.
epidermis, which increases with larger amounts of silica present in each plant, acting as a mechanical barrier, making it difficult to the S. frugiperda to feed (DJAMIN; PATHAR, 1967;CARBONARI;MARTINS, 1998).
With the feeding preference index, it is possible to verify that the S. frugiperda has a higher feeding preference for the genotypes XB6022 and 1G100, and less preference for the Agromen 50A40 genotype (Fig. 2). The lower feed preference of S. frugiperda for the genotype Agromen 50A40 can be attributed to the large amount of fiber present in the leaves (BOREGAS et al., 2013).

CONCLUSION
Among all sorghum genotypes evaluated, the Agromen 50A40 genotype showed less attractiveness by the Spodoptera frugiperda.