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Revista Ceres

Print version ISSN 0034-737XOn-line version ISSN 2177-3491

Rev. Ceres vol.66 no.4 Viçosa July/Aug. 2019  Epub Oct 03, 2019

https://doi.org/10.1590/0034-737x201966040008 

Plant Health

Tank mixture of pesticides and foliar fertilizers for Triozoida limbata control in guava trees (Psidium guajava L.)1

César Henrique Souza Zandonadi2  * 
http://orcid.org/0000-0003-3776-1682

Thales Cassemiro Alves2 

Heli Heros Teodoro de Assunção2 

Guilherme Sousa Alves2 

Sergio Macedo Silva3 

João Paulo Arantes Rodrigues da Cunha2 

2 Universidade Federal de Uberlândia, ICIAG, Uberlândia, Minas Gerais, Brazil. cesarzandonadi@ufu.br; thalescalves@hotmail.com; heli_heros@hotmail.com; guilhermeagro43@yahoo.com.br; jpcunha@ufu.br

3Universidade Federal do “Vale do Jequitinhonha e Mucuri”; Campus Unaí, MG, Brazil. sergiomacedosilva@yahoo.com.br


ABSTRACT

Although tank mixture of pesticides and foliar fertilizers is common practice in agriculture, further clarification and scientific evidence is needed to support regulation. Thus, the objective of this work was to evaluate the effect of tank mixing an organosilicon adjuvant and manganese foliar fertilizer on the insecticide imidacloprid effectiveness against Triozoida limbata in guava trees. The experimental plot consisted of four consecutive trees in the same row subdivided into 4 quadrants. The experiment was arranged in a randomized block design with split plots, with four replications. Treatments were as follows: T1 - Imidacloprid (Imid.); T2 - Imid. + Polyether-polymethyl siloxane copolymer (Sil.); T3 - Imid. + MnSO4; T4 - Imid. + Sil. + MnSO4; and T5 - Control (no application). Physical-chemical characteristics, spray deposition on the leaves and losses to the soil, guava psyllid percentage of infestation and nymph’s number were evaluated. The addition of foliar fertilizer into the mixture decreased the pH and the surface tension and increased the electric conductivity and the viscosity of the insecticide solutions. The silicon adjuvant decreased the surface tension and increased the viscosity and the pH. The tank mixture of organosilicon adjuvant and manganese foliar fertilizer has no influence on the efficacy of the insecticide.

Keywords: guava psyllid; physical-chemical characteristics; neonicotinoids; foliar fertilizer; organosilicon

INTRODUCTION

Guava (Psidium guajava L.) stands out among the Brazilian tropical fruit species mainly because of its flavor and nutritional value. To improve fruit quality and increase the production, growers must overcome obstacles such as orchard management, problems with fertilization, application technologies, as well as the high incidence of diseases and pests. One of the main problems for guava production is the insect known as guava psyllid (Triozoida limbata - Hemiptera: Triozidae) (Galli et al. 2014; Barbosa & Lima, 2010).

The characteristic symptom of guava psyllid attack is the curling of the edges of the leaves caused by feeding of colonies of nymphs. Leaves can fall from this attack, compromising the production (Barbosa et al., 2001; Gallo et al., 2002). According to Colombi & Galli (2009), the importance of this psyllid has increased probably because of the adopted production system, with more irrigation and tree pruning, which favors the psyllid population to grow due to the abundant number of new sprouts.

‘Paluma’ is one of the most planted cultivars in Brazilian orchards, mainly because of its capacity to produce fruits for both industry and fresh consumption (Farias et al., 2017). This cultivar is not resistant to attacks of psyllid, which became one of the main problems for its production (Barbosa & Lima, 2010).

The use of pesticides is frequent during the guava cycle, mostly as tank mixtures, a common practice in Brazilian farms to ensure productivity and reduce application costs. The main problem is the unexpected effects that different product combinations can cause to the application (Gazziero, 2015).

Neonicotinoid insecticides are remarkably effective against sucking insect pests and have been shown to effectively control the guava-psyllid (Barbosa et al., 2001). However, Lima & Gravina (2009) described a different effect on this pest, as they found that this product was inefficient on a high density of infestation after some time. Besides the decrease in the efficiency of the products, the association of different compounds within the spray tank can change their effectiveness (Khaliq et al., 2012).

Tank mixing has been a very common practice in farming, especially with the addition of adjuvants and foliar fertilizers. However, the effects of these blends have not been well understood. Physicochemical properties are altered with these mixes and may influence the efficacy of plant protection products. The adjuvant composition and formulation can also affect physicochemical characteristics of the spray, mainly pH, surface tension, and viscosity (Cunha & Alves, 2009), which justifies the need for further studies on the behavior of these properties in relation to certain mixtures and the possible biological effects.

The penetration and physiological effect of leaf-applied nutrient sprays involves a series of intricate mechanisms ranging from the mode of application to the physicochemical characteristics of the solution, the prevailing environmental conditions, or the target plant species. There are many processes involved, which make it difficult to develop new strategies to optimize the efficiency of foliar sprays under different conditions during the growing of the crops (Fernandez & Eichart, 2009).

Adjuvants are added to the mixture to enhance the efficiency of the solution in different ways. They can improve greately the spreading of the droplet and the wetting of the spray on the target (Cunha et al. 2010A), as well as influence the penetration through the cuticle (Wang & Liu, 2007). However, despite of the advantages of adjuvants, more information is needed about their association with foliar fertilizers and the effects on the efficiency of biological molecules such as pesticides.

Therefore, the objectives of this work were to evaluate the effect of tank mixing an organosilicon adjuvant with a manganese foliar fertilizer on effectiveness of the insecticide imidacloprid in the control of T. limbata in guava trees.

MATERIAL AND METHODS

Experimental site

The present work was carried out in a guava orchard (P. guajava L.), cultivar ‘Paluma’, at “Água Limpa” experimental farm (19°6'16,49"S and 48°20'54,38"W; 795 m altitude), belonging to the Universidade Federal de Uberlândia (UFU), Uberlândia - MG - Brazil. According to Köppen classification, the area is characterized as Aw (tropical, hot humid area with cold and dry winter).

A production area was selected with 80 plants (nine years old) spaced 5.0 m between rows and 3.0 m between plants. The total experimental area was about 1000 m2. The experimental plot consisted of four consecutive trees in the same row with each tree subdivided into 4 quadrants (Q1, Q2, Q3 and Q4). The net plot consisted of the two inner trees and the borders were formed by the first and the fourth trees. Each block was divided in 5 plots arranged linearly.

The treatments consisted of applications of 600 L ha−1 of the mixture at 0.46 km h−1, in duplicate (two times of application). Q1 and Q3 were allocated along the row and Q2 and Q4 were across the row (Figure 1). During the treatments, a plastic protection was used to avoid product drift to adjacent plots in the direction of application.

Figure 1: Detail of the experimental plot used for the application of the treatments and the direction of application used in the subplots. 

The first period (1st application) was conducted in the 2017/2018 harvest, in December 14th 2017, a period of high infestation of guava psyllid. The second period was in April 13th 2018 (2nd application), after harvesting the fruits, the experiment was repeated in the same area, following the same methodology.

Treatments

Treatment solutions were prepared with a manganese salt (Manganese sulfate - MnSO4), an adjuvant (polyether-polymethyl siloxane copolymer), and an insecticide (Imidacloprid), at the Agricultural Mechanization Laboratory (LAMEC), of the Federal University of Uberlândia (UFU). The product specifications are presented in Table 1.

Table 1: Specifications of pesticides and fertilizers (insecticide, adjuvant, and foliar fertilizer) used for the treatments and evaluation of tank mixture 

Product Active ingredient Function Concentration *Formulation Dose
Provado® SC 200 Imidaclopride (Imid) Insecticide 200 g L-1 SC1 2.5 mL plant-1
Break Thru® Polyether-polymethyl siloxane copolymer (Sil.) Adjuvant 1000 g L-1 SC2 0.1 % v v-1
Manganese sulfate Manganese sulfate (MnSO4) Foliar fertilizer 30 % PW 0.05 %

*SC1 - Suspension concentrate; SC2 - Soluble concentrate; PW - Powder.

Treatments were as follows: T1 - Imidacloprid (Imid.); T2 - Imid. + Polyether-polymethyl siloxane copolymer (Sil.); T3 - Imid. + MnSO4; T4 - Imid. + Sil. + MnSO4; and T5 - Control (no application).

Experimental conduction

The applications were performed using a pneumatic backpack sprayer (Stihl® SR450, 14 L tank with a 2900W engine). Droplets are formed by action of the wind and as a function of the setting of the orifice, which is the output of the spray (not using a hydraulic nozzle). The sprayer was run at half engine speed to reduce drift and overload, with a flow rate of 1.45 L min-1.

The treatments were applied at right angles to the line of rows (Q2 and Q4 received direct applications) on both sides of the tree (Figure 1), and at a distance of approximately 1.5 m between the sprayer and the tree. To avoid contamination of plots, a plastic canvas was used during the applications to cover the adjacent area. After the applications, the samples were collected (leaves and petri dishes) and analyzed at LAMEC.

During the experiments, the environmental conditions were monitored. Data on the first period were: temperature (°C) between 23.7 - 27.5; humidity (%) between 60.5 - 70.0; and wind speed 4.5 - 11.7 (km h-1). Data on the second period were: temperature (°C) between 23.9 - 29.1; humidity (%) between 55.6 - 75.6; and wind speed 0.1 - 5.6 (km h-1).

Evaluations

Physicochemical evaluations

Different tank mixes were prepared with the pesticide to evaluate the following physical-chemical characteristics: density, pH, electric conductivity (EC), viscosity (Visc.), and surface tension (ST). As a way of comparison, distilled water was used as negative control. The procedures of the evaluations are described elsewhere (Cunha et al., 2010B). The evaluations of physical-chemical properties were carried out at LAMEC.

Application technology evaluation

To evaluate the application technology, each plot was formed by four trees and each tree was subdivided into four quadrants.

The net plot consisted of the two inner trees, from which we collected two leaves per quadrant, resulting in 16 leaves per plot. The leaves were collected immediately after the applications on the middle third of the trees, in the middle of the crown.

The evaluations of leaf deposition and the losses to the soil were described elsewhere (Tavares et al., 2017).

Pest evaluation

To check the psyllid infestation level, the area was sampled on the day before the first application. The evaluations were performed on days 7, 12, and 14 after the application (Daa).

The two inner trees in the plot were sampled for efficacy, as the first and the last tree were considered as borders. The damage threshold of guava psyllids was the point at which 30% of the leaves were damaged, justifying the spraying for pest management in the area. The procedures for sample evaluations were described by Tavares et al. (2017).

Psyllid nymphs were counted on each leaf, and the mean of the plot was calculated using a digital microscope (Dino-lite pro model: AM - 413ZT) with a 200x magnification,

+Experimental design and statistical analysis

The experiment was arranged in the split-plot randomized block design, with five treatments and four replications. The treatments were assigned to the plots and the quadrants (Q1, Q2, Q3 and Q4) to the subplots. Data were subjected to Shapiro Wilk’s normality test for normal distribution of errors and Levene's test for homogeneity of variances, at 0.01 of significance.

The “F” test was performed to determine levels of significance at 0.05 and 0.01 for the analysis of variance. When the tests detected significance, the means were compared by the Scott-Knott’s test at 0.05 level of probability. When necessary data was transformed by √(x+1). All analyses were performed using the SPSS software.

RESULTS AND DISCUSSION

Physicochemical evaluations

The physicochemical characteristics of the treatments changed as a function of each product added (Table 2). The adjuvant caused no change in EC in the treatment with only insecticide and provided a small increase in pH. On the other hand, the foliar fertilizer reduced the pH and increased EC significantly. The density increased as a function of the addition of the products to the mixture, however, the changes were small. Andrade et al. (2013) found that some of these characteristics, mainly the pH, were influenced by the addition of foliar fertilizers.

Table 2: physicochemical characteristics of the treatments 

Treatment Density (g L-1) pH EC (µS cm-1)+ Visc. (mPa s-1) ST (mN m-1)
Imid 1.026 C 6.32 C 4.00 D 0.94 E 50.75 B
Imid+ Sil. 1.029 B 7.27 A 4.75 D 1.06 B 25.50 C
Imid+ MnSO4 1.034 A 4.55 D 1729.00 B 1.02 C 26.50 C
Imid+ Sil. + MnSO4 1.034 A 4.30 E 1961.50 A 1.13 A 23.75 D
Water 1.024 D 6.85 B 16.25 C 0.99 D 71.50 A
CV 1.31 2.55 1.91 0.91 3.07
F 15242.222* 329.026* 14700.377* 230.179* 1199.713*
Flevene 4.785ns 3.607ns 1.877ns 0.458ns 0.769ns
SW 0.956ns 0.920ns 0.934ns 0.946ns 0.939ns

+EC: data transformed √(x+1); CV - Coefficient of variation; F- values of calculated F for the different treatments. SW - Shapiro Wilk test. ns;* - non significant; significant at 0,05.; Means followed by the same letter are not significant different by the Scott Knott’s test (p ≤ 0.05).

The decrease in the pH of the spray mixture after the addition of MnSO4 is due to the dissociation of this compound, releasing SO4 -2 ions in the solution, which in turn provides H + ions, making the pH acidic. The increase in EC due to the addition of MnSO4 in the two treatments can also be explained by the dissociation of the inorganic salt that provides free H+ ions into the solution and are capable of conducting the electric charge.

For certain herbicide molecules, the decrease in the pH of the spray solution is crucial to maintain product efficacy. During the mixing process, the presence of inorganic salts such as foliar fertilizers cause incompatibility between molecules and loss of efficacy (Bernards et al., 2005), which is not yet relevant to other products, like neonicotinoids.

The product mixture reduced the surface tension in relation to water, with emphasis on the foliar fertilizer and the adjuvant, which resulted in the lowest values. The foliar fertilizer had the potential to decrease the surface tension, without using the adjuvant. Again, the dissociation of MnSO4 can explain the reduction in the surface tension. This inorganic compound can reduce the strong intermolecular interactions inside the water molecule during the hydro-dissociation, because it provides free H+ ions to solution.

According to Iost & Raetano (2010) silicon adjuvants were more efficient in reducing the surface tension. This reduction was more pronounced due the association with MnSO4, which promotes greater spray droplet spread on the target and, therefore, can favor its absorption.

The viscosity increased with the addition of the fertilizer and the adjuvant to the insecticide. Higher viscosity of the spray solution results in larger droplets. Thus, the addition of ions to the solutions had directly influence on these characteristics.

Application technology

The variables analyzed for application technology showed that foliar deposition was significant (P < 0.05) for treatments and quadrants only for the first period of application (Table 3). However, losses to the soil were significant between treatments and quadrants for both applications.

Table 3: ANOVA summary for application technology 

1st application 2nd application
Deposition Loss to the soil Deposition Loss to the soil
Ftreat 4.776* 1.822 ns 2.031 ns 4.473*
Fquad 7.719* 1.723 ns 0.754 ns 5.839*
Ftreat*quad 0.345ns 2.416* 0.715 ns 2.991*
Flevene 2.234 ns 1.813 ns 2.286 ns 2.077 ns
SW 0.982 ns 0.975 ns 0.974 ns 0.948 ns

F- Values of calculated F for different treatments. SW - Shapiro Wilk’s test. ns;* - non significant; significant at 0.05.

The tracer deposition in the first application was the highest in the treatment with only insecticide, differently from the others, that presented the same deposition pattern (Table 4).

Table 4: Foliar tracer deposition (µg cm-2) after treatment applications (First application) 

Treatment Deposition (µg cm-2)
Imid 10.33 A
Imid+ Sil. 7.08 B
Imid+ MnSO4 7.61 B
Imid+ Sil. + MnSO4 5.54 B
CVt 47.80

CV - Coefficient of variation; t - values of treatment; Means followed by the same letter are not significantly different according to Scott Knott’s test (p ≤ 0.05).

When the products were added to the insecticide, the surface tension was drastically reduced (Table 1). When the leaves were sprayed, the droplets could stay over the leaf, adhered, spread, or even runoff. According to Van Zyl et al. (2010), depending of the surfactant concentration, the values of surface tension could become lower and then cause excessive spreading with droplet runoff. This could justify the lower deposition values in the treatments that had more of the other products than the insecticide.

The deposition was higher in Q2 and Q4 as expected (Table 5), mainly because the direction of the application and the leaves of Q1 and Q3 overlapped by leaves of the border trees. Tavares et al. (2017) found similar results when evaluated electrostatic application in guava trees. The authors observed that the quadrants that received direct application had more deposition than those that did not receive it.

Table 5: Foliar deposition of tracer (µg cm-2) on each quadrant (First application) 

Quadrant Deposition (µg cm-2)
1 6.24 B
2 11.08 A
3 4.85 B
4 8.39 A
CVq 53.08

CV - Coefficient of variation; q- values of quadrant; Means followed by the same letter are not significantly different by the Scott Knott’s test (p ≤ 0.05).

At the first application, the treatments presented almost the same pattern of spray losses to the soil in all quadrants, except the lowest loss in Q2, for the treatment with all products and the treatment Imid + MnSO4 (Table 6). Then as well, at the second application, Imid +Sil and the treatment with all products presented similar losses to the soil in all quadrants. The treatments with only the insecticide had more losses in Q1, but when the adjuvant was added, the losses became higher in Q3 and Q4 (Table 5).

Table 6: Spray loss (µg cm-2) to the soil (1st and 2nd application) 

1st application
Treatments Quadrants
1 2 3 4
Imid 0.52 Aa 0.51 Ab 0.54 Aa 0.68 Aa
Imid+ Sil. 0.63 Aa 0.26 Ab 0.30 Aa 0.56 Aa
Imid+ MnSO4 0.83 Aa 0.81 Aa 0.46 Aa 0.65 Aa
Imid+ Sil. + MnSO4 0.45 Aa 0.17 Bb 0.78 Aa 0.53 Aa
CVt 77.31
CVq 44.79
2nd application
Treatments Quadrants
1 2 3 4
Imid 1.08 Aa 0.46 Ba 0.74 Ba 0.58 Ba
Imid+ Sil. 0.38 Bb 0.19 Ba 0.89 Aa 0.68 Aa
Imid+ MnSO4 0.22 Ab 0.29 Aa 0.57 Aa 0.48 Aa
Imid+ Sil. + MnSO4 0.21 Ab 0.16 Aa 0.25 Ab 0.17 Aa
CVt 88.71
CVq 49.44

CV - Coefficient of variation; t -treatment values; q-quadrant values; Means followed by the same capital letter in the row and small letter in the column are not significantly different by the Scott Knott’s test (p ≤ 0.05).

Tavares et al. (2017) found similar results when evaluating the standard application in guava trees with the same equipment and spray volume.

Pest evaluation

The variables analyzed to evaluate pest attack (nymph number and infestation level) were significant (P < 0.05) as a function of the different evaluation period as Table 7 shows.

Table 7: ANOVA summary for pest evaluation 

Nymph number (average)
1st application 2nd application
0 Daa 7 Daa 12 Daa 14 Daa 0 Daa 7 Daa 12 Daa 14 Daa
Ftreat 1.694ns 0.797ns 1.014ns 3.248* 1.379ns 3.001ns 5.578* 10.709*
Fquad 0.967ns 0.137ns 3.011* 7.121* 6.648* 5.856* 1.989ns 2.498ns
Ftreat*quad 2.070* 0.904ns 1.012ns 0.880ns 1.656ns 0.834ns 0.482ns 0.640ns
Flevene 2.057ns 2.700ns 1.419ns 0.836ns 1.678ns 2.359* 5.640* 6.871*
SW 0.947* 0.985ns 0.980ns 0.948* 0.972ns 0.971ns 0.815* 0.915*
Psyllid infestation (%)
1st application 2nd application
0 Daa 7 Daa 12 Daa 14 Daa 0 Daa 7 Daa 12 Daa 14 Daa
Ftreat 0.917ns 2.248ns 8.917* 17.593* 0.422ns 4.913* 4.608* 8.004*
Fquad 0.105ns 4.056* 1.947ns 0.678* 0.717ns 7.686* 2.400* 2.560ns
Ftreat*quad 1.369ns 0.574ns 1.113ns 1.202ns 1.111ns 1.202ns 1.403ns 1.395ns
Flevene 1.200ns 2.059ns 2.548* 2.007ns 1.718ns 1.532ns 2.221* 1.540ns
SW 0.945* 0.979ns 0.972ns 0.971ns 0.972ns 0.975ns 0.973ns 0.951*

F- Values of calculated F for different treatments. SW - Shapiro Wilk’s test. ns;* - non significant; significant at 0.05.

At the first application, the percentage of infestation at 7 Daa was higher in Q1 and Q3, whereas at the second application, the percentage of infestation was higher in Q2 and Q4. These results have a relation with the deposition of the tracer in the quadrants, showing that, in this case, the quadrants with more tracer deposition presented a reduction in the percentage of infestation.

In the second period of the experiment, the plants reduced the number of leaves because of the climate and the overlapping of the branches of the neighboring trees in Q1 and Q3, which had not happen in the first period because the size of the trees (Table 8).

Different from the infestation level, the number of nymphs was greater in Q2 and Q4 in the first period of application (Table 9). At the second application, at 0 Daa, nymph distributions were similar in all quadrants. However, at 7 Daa the number of nymphs reduced in Q1 and Q3 and increased in Q2 and Q4 (Table 9).

Table 8: Psyllid infestation (%) in different tree quadrants 

Quadrants 1ª application
0 daa 7 daa 12 daa 14 daa
1 37.50 48.75 B 36.25 27.50
2 32.50 26.25 A 33.75 35.00
3 37.50 47.50 B 40.00 27.50
4 35.50 22.50 A 20.00 22.50
CVt 84.75 70.91 59.76 63.25
CVq 68.97 72.71 86.00 88.02
Quadrants 2ª application
0 daa 7 daa 12 daa 14 daa
1 42.50 27.50 A 23.75 A 22.50
2 55.00 58.75 B 37.50 B 35.00
3 52.50 22.50 A 21.25 A 16.25
4 52.50 41.25 B 37.50 B 28.75
CVt 73.11 46.05 90.89 69.13
CVq 57.85 69.83 83.91 68.02

CV - Coefficient of variation; t -treatment values; q-quadrant values; F- values of calculated F for different treatments. ns;* - non significant; significant at 0,05.; Means followed by the same letter in the column are not significantly different, by the Scott Knott’s test (p ≤ 0.05).

Table 9: Psyllid Nymph (average) according to each different tree quadrant 

Quadrants 1st application
0 daa 7 daa 12 daa 14 daa
1 3.25 2.05 2.60 A 2.95 A
2 2.10 2.45 5.80 B 8.60 B
3 2.10 2.00 2.85 A 2.55 A
4 2.25 2.40 4.15 B 6.70 B
CVt 73.24 79.14 57.89 68.85
CVq 75.12 25.16 71.31 57.50
Quadrants 2nd application
0 daa 7daa 12 daa 14 daa
1 10.35 2.25 A 0.70 1.45
2 7.90 16.50 B 2.40 3.00
3 7.79 3.90 A 0.65 0.85
4 3.35 13.90 B 2.35 2.00
CVt 64.62 69.27 59.79 68.88
CVq 53.12 53.06 86.00 72.10

CV - Coefficient of variation; t -treatment values; q-quadrant values; F- values of calculated F for different treatments. ns;* - non significant; significant at 0,05.; Means followed by the same letter in the column are not significantly different by the Scott Knott’s test (p ≤ 0.05).

Marcelino & Barbosa (2016) found that T. limbata adults showed moderate to high aggregated distribution in all development phases of guava, independent of the average size of the population, which could justify the higher number of nymphs in the quadrants 2 and 4, which had more leaves.

Figure 2 shows that the number of nymphs where similar after the application of the treatments. At 14 Daa, the number of nymphs had increased in the control treatment, differing from the others that had the same averages, showing that the insecticide was effective against the insects up to that time.

Figure 2: Mean nymph number after the treatment applications (1stperiod).ns;* - non significant; significant at 0.05.; Means followed by the same letter are not significantly different by the Scott Knott’s test (p ≤ 0.05). 

At the second application, the number of nymphs fluctuated up to 12 Daa. It was only at 14 Daa that the treatments presented difference from the control. In this application, the number of nymphs had been reduced from the treatments with the insecticide, except when mixing it with the foliar fertilizer, showing a difference between the others and the control as well (Figure 3).

Figure 3: Mean nymph number after the treatment applications (2nd period).ns;* - non significant; significant at 0.05.; Means followed by the same letter are not significantly different by the Scott Knott’s test p ≤ 0.05. 

Galli et al. (2014) found that cultivars “Paluma” and “Rica” were the most attacked by psyllid compared with different commercial accesses and others in test. The percentage of damage were higher than 50% during all the experiment.

The damage threshold of 30% was reached and the applications were necessary in both periods (Figures 4 and 5). These levels reduced at 12 and 14 Daa, except for the control (Figure 4) in the first period and from day 7 to 14 Daa in the second application (Figure 5). In both periods, the damage threshold was reduced to below the recommended level for another application, which justify only one application in each period.

Figure 4: Leaves infested (%) by psyllid in guava trees (1st period).ns;* - non significant; significant at 0.05.; Means followed by the same letter are not significantly different by the Scott Knott’s test (p ≤ 0.05). 

Figure 5: Leaves infested (%) by psyllid in guava trees (2nd period).ns;* - non significant; significant at 0.05.; Means followed by the same letter are not significantly different by the Scott Knott’s test p ≤ 0.05. 

The first application reduced the percentage of infestation in all treatments, but the treatment with only insecticide reduced it more than the others (Figure 4). However, in the second application, all the treatments reduced the infestation to the same level, becoming only different from the control (Figure 5). As we can see in Table 4, the foliar deposition of the treatment with only insecticide was greater than the other treatments, which explains the difference between the treatments in the first application.

When the insecticide imidacloprid was applied on high density levels (higher than 50%) and in the interval of 15 days between applications, there was no reduction of the infestation levels above the damage threshold (Lima & Gravina, 2009).

CONCLUSION

The addition of foliar fertilizer in the mixture reduced the pH and surface tension and increased the electric conductivity and viscosity of the insecticide solutions. The silicon adjuvant reduced the surface tension and increased the viscosity and the pH. The quadrants that received direct application (2 and 4) presented higher spray deposition. All the treatments and quadrants presented almost the same spray losses to the soil.

The number of nymphs as well as the infestation level decreased with the treatment applications. The tank mixture of the adjuvant organosilicon and the manganese foliar fertilizer had no influence on the efficacy level of the insecticide.

ACKNOLEDGEMENTS

The authors thank CNPq, CAPES, and FAPEMIG for the financial support.

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Received: August 29, 2018; Accepted: May 15, 2019

*Corresponding author: cesarzandonadi@yahoo.com.br

1

This work is part of the thesis of the first author.

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