ABSTRACT
Thrips, Thrips tabaci Lind. (Thysanoptera: Thripidae), is a key pest of garlic in Brazil. This study aimed to evaluate the influence of spacing on T. tabaci infestation of a garlic crop. The experiment was conducted at the experimental farm of the Goiano Federal Institute (Instituto Federal Goiano – IF Goiano) - Campus Urutaí. The experimental design was a randomized block with nine treatments and spacing in single rows (0.20 × 0.10 m, 0.25 × 0.10 m and 0.30 × 0.10 m) and double rows (0.20 × 0.10 × 0.10 m; 0.20 × 0.15 × 0.08 m; 0.25 × 0.10 × 0.10 m; 0.25 × 0.15 × 0.08 m; 0.30 × 0.10 × 0.10 m; and 0.30 × 0.15 × 0.08 m). The Chonan 15 genotype was used in three replicates. At 30, 50 and 70 days after emergence (DAE), evaluations on the following features were carried out: leaf length, number of leaves, angle between the two central leaves, damage and number of thrips per plant. At the end of the crop cycle, the plants were harvested. Individual cured bulb mass, estimated commercial yield, number of cloves per bulb and bulb diameter were also evaluated. Spacing influenced the angle between the central leaves, with spacing of 0.30 × 0.15 × 0.08 m, among the least dense spacing, producing the widest angle (26.96o), suffered one of the highest degrees of insect damage. A spacing of 0.25 × 0.10 m had the highest number of thrips per plant (2.99). Spacing influenced production, with the highest yield occurring with a spacing of 0.25 × 0.15 × 0.08 and 0.30 × 0.10 × 0.10 m (14.37 and 13.51 t/ha, respectively). The least dense spacing produced the largest bulb diameter (42.80 mm) and mass (30.66 g).
Key words: Allium sativum, insect, Thrips, plant architecture, non preference.
Thrips tabaci Lindeman 1889 (Thysanoptera: Thripidae) is a key pest of garlic in Brazil, due to the dry climate, which encourages the occurrence of this insect. The pest can reduce crop yield by up to 61 percent if not controlled (Maranhão and Menezes, 1991; Silva et al., 2003; Gonçalves and Vieira Neto, 2011). The attacked plants develop lesions on leaves that cause yellowing and premature dryness, highly infested leaves appear twisted, as if they were burned, before subsequently falling (Souza and Macêdo, 2009). This pattern occurs because the thrips scrape the leaf surface to feed on the sap, facilitating the emergence of various diseases, including
those caused by viruses (Gallo et al., 2002).
The most widely used control method is chemical control, which promotes biological imbalance, increases the risk of environmental contamination, insect resistance and increases production costs. Alternative controls such as changes in plant arrangement, spacing and planting density are viable ways of reducing losses and may contribute to the integrated management of thrips in the garlic crop (Gallo et al., 2002; Loges et al., 2004).
Structural characteristics such as plant architecture, angle between the two central leaves and leaf length influence the population of T. tabaci in onion crops (Loges et al., 2004). Plants with a greater central angle are resistant to thrips by antixenosis, but few studies con-ducted confirms, whether that smaller number of insects on plants with a higher central angle is due to more efficient control effected by natural enemies, insecticides or abiotic agents (Loges et al., 2004). The aim of this study was therefore to determine the influence of plant spacing in single and double rows on the thrips population density in a garlic crop.
The experiment was conducted at the experimental farm of the IF Goiano – Campus Urutaí in Urutaí, Goias State, Brazil (17° 19'13'' S, 48° 12'37' 'W. 697 m altitude). During the experiment, the cumulative rainfall was 12 mm, the maximum temperature reached 36°C, and the minimum temperature was 8°C. The planting of Chonan 15 genotype cloves was done in May, 2012, with nine treatments adopted at different spacings: in single rows (0.20 × 0.10 m, 0.25 × 0.10 m and 0.30 × 0.10 m) and in double rows (0.20 × 0.10 × 0.10 m, 0.20 × 0.15 × 0.08 m, 0.25 × 0.10 × 0.10 m, 0.25 × 0.15 × 0.08 m, 0.30 × 0.10 × 0.10 m and 0.30 × 0.15 × 0.08 m). The experimental design was conducted in randomized block design with three replicates. The experimental plot (2.5 m × 1.2 m) had a working area of ​​0.25 m2, comprising the central row. Irrigation was performed by a conventional sprinkler, and other cultural practices proposed by Filgueira (2008) were adopted, with the exception of use of chemicals for pest control after planting.
Evaluations were performed 30, 50 and 70 days after emergence (DAE), with five plants per plot randomly selected to evaluate the following features: leaf length (LL), corresponding to the length of the sheath to the apex of the largest leaf, expressed in centimeters; angle between the two central fully developed leaves (ANG), taken from the central axis of the plant using a protractor and expressed in degrees; total number of leaves (TNL); damage caused by thrips attack (DAMAGE), using a rating scale for the lesions caused by thrips, with a score of 0 indicating leaves with no attack symptoms, score 1: leaves with a few chlorotic spots, score 2: leaves with many chlorotic spots, score 3: leaves with chlorotic spots and early desiccation, and score 4: leaves with chlorotic spots and advanced desiccation (adapted from Albuquerque, 2004); and number of thrips on the two fully developed central leaves (NYMPH).
To perform the insect count, garlic plants were cut close to the ground, placed in a labeled plastic bag, brought to the Agricultural Entomology Laboratory and stored in cold storage to reduce thrips movement. The number of insects per plant was then counted. For identification of the adult Thrips the methodology proposed by Monteiro et al. (2001) was adopted.
At the end of the crop cycle, the plants were manually harvested. The harvested plants were submitted to a curing process, in the shade, for a period of 20 days. Subsequently, the bulbs were cleaned, the shoots were cut to 1.0 cm, and the roots were removed. Regarding features associated with production, the individual mass of the cured bulbs (IMB), estimated commercial yield (ECY), cloves per bulb (CPB) and diameter of bulb (DIA) were all evaluated. The data were transformed into (X + 0.5)1/2 for analysis. The results were submitted to an analysis of variance using the F-test, and the treatment means, when significant, were compared using the Duncan test at 5% probability. A correlation analysis among the number of T. tabaci and the angle between the two central leaves in the different planting arrangements was per-formed. Statistica software version 7.0 (Statsoft, 2004) was used.
The TNL means relating to the different spacings were not significantly different from each other, and spacing did not affect the number of leaves per plant (Table 1). Regarding evaluation periods, there was no difference between 30 and 50 DAE, but at 50 and 70 DAE, there were significant differences, with a mean increase of 0.46 leaves per plant. The values ​​found for the number of leaves were below those obtained by Mota et al. (2005), who evaluated morphological similarities of different cultivars of garlic, with the Chonan cultivar exhibiting a mean of 7.83 leaves per plant.
With regard to damage caused by T. tabaci, in the first evaluation at 30 DAE, significant differences were not observed between treatments due to the absence of leaf symptoms, but from 50 to 70 DAE, there was an increase from 0.81 to 3.24 in terms of damage (Table 1). This increase was due to the longer period of plant exposure to pest attack. There was also a difference in damage score for the different spacings, with the 0.20 × 0.15 × 0.08 m and 0.30 × 0.15 × 0.08 m spacings obtaining the highest scores and the 0.30 × 0.10 × 0.10 m spacing obtaining the lowest score (Table 1).
Leite et al. (2004) observed an increase in the percentage of leaf area damaged by T. tabaci in different onion cultivars. In the Aurora cultivar, the percentage varied among 1.04, 6.07 and 17.98 percent in June, July and August, respectively. These observations coincide with the tendency toward increasing damage values in the garlic plant. The number of thrips per plant increased from 0.72 to 3.50 thrips per plant between 30 and 50 DAE, but there was a drop in population between 50 and70 DAE, reducing to 1.16 thrips per plant (Table 1). These data contradict Ramiro's (1972) report that the incidence of T. tabaci in the onion crop is initially uneven and grows gradually, with the largest population at the end of the crop cycle. This finding can also be observed in a study by Leite et al. (2004), who obtained a gradual increase in the thrips population on Conquista onions, varying among 0.00, 2.13 and 6.35 insects in the months of June, July and August, respectively. Zamar et al. (2007) also found an increase in the thrips population of garlic plants as the end of the crop cycle drew near.
There was no difference in pest population with regard to spacing, with the largest infestation in the 0.25 × 0.10 m spacing, at 2.99 thrips per plant, and the lowest in the 0.30 × 0.10 × 0.10 m and 0.30 × 0.10 m spacings, 1.00 and 1.29 thrips per plant, respectively. These results corroborate those of Leite et al. (2007) who evaluated the incidence of insects and disease in onion cultivars and found mean values ​​of 2.88 thrips per leaf. In this study, the spacing with the largest thrips populations did not coincide with the spacing that had the greatest damage caused by thrips.
Plant spacing affected the angle between the central leaves, with less dense spacing, such as 0.30 × 0.15 × 0.08 m, having larger angles and denser spacing, such as 0.20 × 0.10 × 0.10 m and 0.20 × 0.10 m, having smaller angles (Table 1). The angle values ​​found are similar to those found by Mota et al. (2005), who evaluated morphological similarities of different garlic cultivars and found an angle of 19.76° in the Chonan cultivar.
An interaction between central angle and number of thrips per plant was observed in the 0.25 × 0.10 m spacing, which had one of the smallest angles and showed the highest infestation of thrips per plant. The spacing of 0.30 × 0.15 × 0.08 m, which had the largest angle also had the higher thrips population. These data corroborate those of Silva (2011), who noted the same central angle and same thrips population in different onion cultivars. Using linear regression analysis to compare the number of thrips and the angle of the central leaves (Figure 1), a significant difference can be observed with a positive linear correlation, indicating that a reduction in the former factor involves an increase in the latter. The relationship between central angle and thrips number is not well established in the literature on T. tabaci infestation, but the data corroborate those of Loges et al. (2004), who found higher thrips infestations in onion plants with smaller leaf angles. Higher T. tabaci infestation in treatments with lower leaf angles may be due to better shelter for the insects, protecting them from natural enemies and the action of insecticides also minimizing adverse environmental conditions (Jones et al., 1935; Loges et al., 2004).
There was a reduction in the angle between the central leaves in the different evaluation periods, with a significant reduction from 23.92° at 50 DAE to 18.37° at 70 DAE (Table 1). This reduction may be due to the increased total number of leaves per plant, thereby reducing the angle between the central leaves. This finding is in accordance with those of Loges et al. (2004), who found a reduction in angle due to an increase in number of onion leaves.
Another variable that spacing did not affect was leaf length (LL), with no significant differences (Table 1). Resende et al. (2013), evaluating morphological aspects of garlic clones under field conditions, found a leaf length of 51 cm in the Chonan cultivar, which is similar to that found in the present study. Loges et al. (2004) suggested there was a relationship between plant height and numbers of thrips, as taller plants have heavier leaves and thus a larger angle between the central leaves, but this was not observed in the present study due to the similarity of leaf length. Regarding the different evaluation periods, there was a difference between 30 and 50 DAE, with leaf growth of 13.91 cm, but there was no significant difference between 50 and 70 DAE.
Production components and number of cloves per bulb were not affected by spacing (Table 2) because these are characteristics linked to the cultivar and not to planting arrangement (Silva and Silva, 2009). Bulb diameter was influenced by spacing, with one of the least dense spacings of 0.30 × 0.10 m having the largest bulb diameter, 42.8 mm, and one of the most dense spacings, 0.20 × 0.10 × 0.10 m, having one of the smallest diameters.
Average bulb mass was also influenced by spacing. One of the least dense spacings had the highest individual bulb mass, which was the spacing of 0.30 × 0.10 m with a mass of 30.66 g. One of the most dense spacings, 0.20 × 0.15 × 0 08 m, yielded one of the lowest masses. These results suggest that a less dense planting arrangement produces bulbs with greater mass and larger diameter. In relation to yield, the spacings that stood out were 0.25 × 0.15 × 0.08 m and 0.30 × 0.10 × 0.10 m, with yields of 14.37 and 13.51 t/ha, respectively. Souza and Carmo (2012) found that the best spacing for garlic crop planting was 0.25 × 0.10 m and 0.25 × 0.15 m, with yields of 11.52 and 11.73 t/ha, respectively, values lower than those found in this study.
The yield achieved in this study was higher than that found by Resende et al. (2013), who achieved an average Chonan cultivar yield of 6.7 t/ha, except when using 0.20 × 0.10 m spacing. The spacings with the highest yields coincide with those with the lowest thrips populations. Spacing influenced the angle between the two central leaves, a feature associated with plant insect resistance. Production was not influenced by the number of thrips per plant due to the low insect population, which was influenced by spacing. In larger spacings, larger bulb diameters and mass were obtained, but the number of cloves per bulb was the same in all spacings.
The authors have not declared any conflict of interesta
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