Productive and Economic Losses Caused by Dichelops melacanthus in Transgenic Bt Maize Bacillus thuringiensis

Abstract

Transgenic maize expressing Bacillus thuringiensis (Bt) toxin produces a crystal (Cry) protein toxic to caterpillars that is non-toxic to stink bugs. The objectives of this study were to identify the number of plants attacked and not by Dichelops melacanthus, to evaluate foliar damage through the number of punctures, to evaluate plant height, weight of grain production by corn plants attacked and not and economic loss. The research was carried out on a commercial production agricultural farm in an area of 700 m2. Eight areas were evaluated (10 m × 1.8 m) randomly distributed and in V6 physiological stage. Data were recorded, tabulated in Excel spreadsheet and statistically analysed by T Student test with 5% of significance for comparison of two independent groups. The results indicate that 80.7% of corn plants were attacked by D. melacanthus presenting punctures in their leaves. The average height for attacked plants was 41.2 ± 2.2 cm and 41.5 ± 3.3 cm for not attacked. A significant reduction in production of 23% was verified. The weight of grains of attacked plant was in average 3048 ± 319 g and 3956 ± 269 g in not-attacked plant, demonstrating that the damage caused by D. melhacantus reduces Bt corn productivity and loss of income of 98.93 US dollars per hectare.

1. Introduction

Bt corn is a type of transgenic corn that produces a protein of bacterial origin. The Cry protein, naturally produced by Bacillus thuringiensis, is toxic to defoliating caterpillars or stem borers, but not toxic to stink bugs. Maize is cultivated practically throughout the Paraguayan territory with different production systems and technological levels used. Some changes in the corn production system have contributed to the increase in productivity: the direct sowing system, the use of hybrids with high productive potential, the increase in cultivated areas in the second harvest, after the soybean harvest, and the use of genetically modified hybrids such as Bt maize [1].

In recent years, the main problem faced by maize producers is the difficulty of protecting the maize crop from leaf damage caused by Dichelops melacanthus (Hemiptera: Pentatomidae) during the first weeks after maize emergence. At this stage, the seedling is quite susceptible to leaf punctures caused by Hemipteran stylets during feeding.

Studies to estimate the economic threshold level for D. melacanthus in maize were carried out by Bianco [2] who estimated two insects per five plants of corn, which is equivalent to 2 stinkbugs/1.25 lineal m under field conditions. In addition, Gassen [3, 4] and Cruz et al. [5] recommend control measures for stinkbugs in the corn crop when two insects/m² are found, which is equivalent to one bug/lineal m. Both levels are higher than that found by Duarte [6] with 0.8 stinkbugs/m² equivalent to 0.4 insects/linear m under controlled population conditions.

What is worrying is that both Bt technology and seed treatment with neurotoxic insecticides fail to provide protection to the maize plant in the initial vegetative phase, this forces producers to be vigilant and take early control actions so that economic damages do not render production unfeasible due to foliar damage and the increase in production costs. The question that arises is: What percentage of attack can a commercial corn crop suffer and how much can it reduce the production of corn grains in weight and economic income?

However, these changes can trigger new problems and require constant studies for proper management. The occurrence of new pests or the increase of others that attack the crop can be seen as a direct reflection of alterations in the productive systems. The appearance of Dichelops melacanthus (Hemiptera: Pentatomidae), commonly known as green-bellied bug, is currently found in Brazil, Argentina, Paraguay, Uruguay, Bolivia, Colombia, Peru and Venezuela [7]. Infestation and damage occur at the seedling stage as a result of adult migration from surrounding crop debris or other plants within the field [8, 9, 10].

A transformation of great magnitude, such as the one experienced in our agriculture, should respond to a reasoned process, supported by knowledge and adequate technological management of each component of the production system. One of the fundamental pillars on which the cultivation of corn rests and that, therefore, directly influences the yields achieved, is pest control.

Knowledge of the population dynamics of these pests is important in the management of these organisms since through it the incidence is estimated and the management of the insect can be planned, which is essential when determining a control strategy that avoids the increase in the existing gap between potential returns and real returns [11]. Damage to maize from seedlings causes brown spots, leaf discolouration and twisting, reduced yield [12] or plant death [10, 13].

The realization of this work is based on the current concern about the high incidence of D. melacanthus in the corn crop, even with seed treatment and a series of insecticide applications during the crop cycle. This research tries to obtain real data from the field that can contribute to verify the level of damage caused by the attack of bugs in corn production and how this can affect the producer economically.

In this research, the objectives have been to evaluate the foliar damage caused by the attack of the green-bellied bug D. melacanthus on transgenic Bt maize, grain production, to identify the number of attacked and non-attacked plants. Quantify the number of punctures per plant, verify the height of the plant in the phenological state V6 of maize and determine the weight of grains of the attacked and unattacked plants to compare the yield.

2. Materials and methods

The study was carried out in the 2016 harvest at the Gredos Agricultural Farm, of the Martin and Martin Group, located 12 km from the city of Pedro Juan Caballero, in the Vice President Sánchez neighbourhood, on route 5 "General Bernardino Caballero”, Department of Amambay, georeferenced at 22°36′17″S 55°48′46″W. The property is located at approximately 640 m above sea level, the topography is flat and the area is used for extensive crops under a direct sowing system in rotation with soybeans, second-harvest corn, wheat and chia.

The region is characterized by having a transitional climate between a Mediterranean type and a frankly humid climate with an average annual temperature of 22°C and an average annual rainfall of between 1000 and 1200 mm per year, with the rainiest months being December and January, the less rainy months June, July and August. The soil of the place corresponds to an alfisol (Soil Taxonomy), with a clayey loam texture and shows a prolonged agricultural use of more than twenty-seven years of sustained production, with twelve years of production of cereals and other grains under the direct sowing system.

The study area was made up of eight areas randomly distributed over a 700 m² surface, the dimensions of the areas being 20 m wide and 35 m long with five rows of corn separated 0.45 m by 10 m long, with an average population of 108.2 plants. For comparison purposes, they were classified into plants attacked and not attacked by D. melacanthus.

The criteria adopted to classify the attacked plants consisted of the presence of three to four holes in the corn leaf blade that generally appear in three to four rows and are characteristic damages that form on the leaves after the puncture-puncture performed by the stylets from bed bugs. Meanwhile, for the plants not attacked, those maize plants that did not show the characteristic damage described were considered.

The number of Bt maize plants attacked and not attacked by D. melacanthus and the number of punctures per plant, height of the plant in the V6 phenological stage and the weight of grains of the attacked and non-attacked Bt maize plants have been quantified by D. melacanthus. The estimation of attacked and unattacked plants, number of punctures and height of the plant, was carried out 33 days after sowing, and the grain weight of the attacked and unattacked plants was carried out after harvest.

The calculation of the percentage of plants attacked and/or attacked was carried out through the analysis of relative frequency (hi) = fi/N, where fi corresponds to the number of times that a piece of data is repeated within the set and N, the total number of data from the set.

The unattacked corn plants were marked with a red tape, in all the areas attacked plants were found, that is, approximately 85% of the population was attacked by green-bellied bugs, even after making two applications of the insecticide Imidacloprid to control the pest, even stinkbugs continued to appear in the areas, so a third application of the product was made. In total, 866 Bt maize plants were evaluated.

In the demarcated areas, the number of punctures caused by the green-bellied bug was quantified, which was recorded by direct observation of the continuous perforations in each leaf and noted on a spreadsheet. The height of the plant was measured with a tape measure in the phenological stage V6 of maize, at that stage the plants presented a high attack by stinkbugs.

Once the maize reached commercial maturity, the ears were harvested manually. For this, the ears of corn were pulled out and placed in bags, which received an identification. Subsequently, the ears were exposed to the sun for 12 hours to reduce their humidity, and then the shelling and cleaning were carried out manually to eliminate impurities. The production was packed in the bags and identified according to their respective areas. Finally, the corn grains were weighed on a Profield brand four-digit precision scale.

The estimation of the corn yield was made by multiplying the average weight of the corn grains by the number of existing plants in the evaluated area, later, by the simple rule of three, the value of the corn yield per hectare was estimated. The calculation of the loss of income was made by multiplying the average weight loss of grams of corn per hectare by the commercial sale value of the product.

The data were arranged into two groups, a population of plants attacked by D. melacanthus and another population of plants not attacked by D. melacanthus. To verify the normal distribution, the data referring to the number of attacked and/or attacked plants as well as the height of the plants were subjected to the Shapiro Wilk test at 5% significance, the analysis showed that the data were homogeneous and met the requirements for the distribution. Use of parametric statistics, by and analyzed, using the Student’s T-test at 5% significance for comparison of two independent groups and verification of the existence of significant differences between treatments. The statistical program used was BioStat 5.3.

3. Results

It has been verified that of the 866 Bt maize plants evaluated in the vegetative physiological stage V6, 699 (80.7%) of the plants were attacked by the D. melacanthus stinkbug. The maize leaves presented evident damage by the presence of punctures or holes produced by the feeding of bed bugs with an average number of 6.1 punctures per plant. On the other hand, 167 maize plants (19.9%) were not attacked or did not present the characteristic symptoms described for D. melacanthus (Table 1).

Treatments	No. of plants (n = 866)	Average number of plants attacked and not attacked	Height (cm)
Attacked plants	699	87.4 ± 7.8 a	41.2 ± 2.2 a
Plants not attacked	167	20.8 ± 2.7 b	41.5 ± 3.3 a

Table 1.

Number of Bt maize plants and height of plants (cm) attacked by D. melacanthus and not attacked.

Average values followed by different letters in columns differ from each other by Student’s T-test at 5% significance.

The average number of D. melacanthus plants attacked was 87.4 ± 7.8 and 20.8 ± 2.79 not attacked, from an average population of 108.2 plants, considering the eight Bt maize areas evaluated in which there were between 100 and 117 plants.

The high value of maize plants attacked is due to the fact that the Cry protein, expressed by the Bt event, the seed treatment and the foliar application with neurotoxic insecticides were unable to provide protection to the maize plant from the attacks of D. melacanthus. On the one hand, the protein crystals of B. thuringiensis are non-toxic to pentatomidae bugs and on the other hand, the insecticide imidacloprid applied via seed and foliar was inefficient in controlling the population of pest hemipterans.

In relation to the variable height of the Bt maize plants, attacked or not by D. melacanthus, no statistically significant difference (p > 0.05) was verified between the populations. An average height of 41.5 cm was recorded for unattacked plants and an average height of 41.2 cm for attacked plants (Table 1).

These results reveal that the height growth of maize plants is not affected, however, the leaf area is compromised by the feeding punctures of the stink bugs, which in turn affect the photosynthetic capacity of the plant and the translocation of photoassimilates to the drainage region for the formation of corn kernels, which reduces their weight.

It has been verified that the average weight of grains per Bt maize plant not attacked by D. melacanthus was 3956 ± 269 g and for the attacked plants it was 3048 ± 199 g with significant statistical differences between them (p < 0.05). It has also been estimated that the average yield per hectare of the unattacked Bt maize plants was 2197.77 kg/ha and those attacked, 1693.33 kg/ha, thus the loss in maize production was 504.44 kg /ha due to damage caused by D. melacanthus. In this sense, considering the average value of corn in the corresponding harvest, the loss of income was 98.93 dollars per hectare (Figure 1).

Figure 1.

Average kernel weight ± SD (g) per Bt maize plant not attacked and attacked by Dichelops melacanthus. Average values followed by different letters differ from each other by Student’s T-test at 5% significance.

4. Discussion

According to the data referring to plants attacked and not, a high incidence of the green-belly bug has been verified in the Bt maize crop itself with applications of Imidacloprid insecticides (200 ml/ha). Probably the high occurrence of D. melacanthus is due to the resistance to the active principles applied during the development of maize. Another aggravating factor is that soybeans grown before corn also host a large number of bugs and the same chemicals are used to control them, which generates selection pressure for resistant biotypes.

No significant differences were observed in the height decrease of attacked and unattacked maize plants. This is compatible with those obtained by Crosariol Netto et al. [12], who observed that the transgenic plants did not show significant differences in relation to the height and respond differently to conventional non-Bt hybrids than if they can reduce plant height due to stink bug damage.

A high incidence and damage by D. melacanthus has also been verified in the V6 vegetative stage, which is consistent with the work of Copatti and Oliveira [14]. In their study, they show a high potential for damage to maize, which occurs in the initial state of development between V2 and V8. In other similar works, the reduction in plant height was not significant between conventional and transgenic hybrids, but rather it is a behaviour between varieties [15]. In another similar work, it is highlighted that the height of maize plants is not affected in any of the population and infestation of D. melacanthus [16].

As verified in this study, significant damage in maize is observed especially if the infestation occurs in the physiological stages V1 and V3, negatively affecting crop production [6], confirming a decrease in maize yield. (kg/ha) by increasing the number of green-bellied bugs per square meter, as observed in this research. In other similar works, the damage to the productivity components themselves is observed with field infestations of 2 and 4 bugs per m² [17]. Despite the reduction in plant height in conventional hybrids, no effects on productivity are observed, thus being the main factor in yield reduction due to a high infestation of D. melacanthus [12].

The same authors did not observe a relationship between the average weight of 100 corn kernels and the density of D. melacanthus. However, a negative relationship was revealed between the average weight of the spikes and the population densities of D. melacanthus. However, the grain yield decreases with the increase in the levels of infestation of stink bugs, evidencing that the increase in the population density of the insect reduces the weight and yield of grains in the corn crop. Bridi et al. [16] highlight that the reduction in productivity is 7.1% for each D. melacanthus added in 1 m², in a range of 0–4 stinkbugs per m².

Portela et al. [18] obtained similar results, where they verified that the green-bellied bug causes a greater intensity of reduction in the weight of the maize grain when compared to the brown bug Euschistus heros, evidencing that the first species potentially cause greater damage to this crop than the second.

Bridi et al. [16] highlight the reduction of grains of up to 3.96 grains per row, which represents a decrease of 12.3% for every 4 stinkbugs per m² compared to the absence of it. The length of the spike is also affected by the infestation of 3.16 stinkbugs, which causes a reduction of about 12% in relation to the size of the spike without the presence of the insect. In other similar works such as that of Cruz et al. [19], it is described that without the presence of the insect, the yield was 8048.43 kg/ha, while, in the presence of the insect, the grain yield was 6352.21 kg/ha, a difference of 21.07% or 1696.22 kg/ha.

According to Duarte et al. [6], it is possible to estimate that the level of economic damage for D. melacanthus in the corn crop is 8 bugs/m², population density above which pest control is economically justified. These results disagree with those obtained by Bridi et al. [16] where they indicate that between 1 and 4 bugs per m² significantly affects corn yield.

The insecticide commonly used to control bugs in corn is Imidacloprid. Research carried out by Chiesa et al. [8] show that this product can reduce the population density of the bug between 23.2 and 61.8% also that seed treatment with this insecticide does not efficiently protect against the attack of the bug D. melacanthus.

Albuquerque et al. [20] show that Imidacloprid may not be efficient when applied 8 days after the emergence of corn plants. Also, pre-emergence insecticide sprays have little effect on D. melacanthus, while post-emergence applications can achieve up to 80% control [21].

Another reason that may favour the high incidence of D. melacanthus is high temperature. The D. melacanthus bug performs better in high-temperature conditions (up to 31 ± 1°C) while constant temperatures of 19°C harm it. It has also been shown that Bt events such as transgenic soybeans do not affect their biology [22].

In several studies it has been shown that the treatment of corn seeds does not efficiently protect against the attack of D. melacanthus, also that the series of applications of chemical products in the vegetative and reproductive stages does not efficiently reduce the population density of the green-bellied bug. According to Modolon et al. [23], control plants without chemical seed treatments can present up to 100% of the plants attacked by D. melacanthus. On the other hand, Brustolin et al. [21] recorded up to 60% of plants attacked by D. melacanthus without seed treatments, while with treatments, the number of attacked plants can be reduced to 24%.

5. Conclusions

Most of the Bt maize plants (80.7%) were attacked by D. melacanthus. Which reduced productivity and economic income.

Dichelops melacanthus caused an average of 6.1 punctures per Bt maize plants, affecting grain yield by 23% and causing a loss of income of 98.93 US dollars per hectare.

Average kernel weight of Bt maize plants was reduced due to damage caused by D. melacanthus. However, the reduction in the average height of Bt maize plants was not significant.

In future research, it is recommended to evaluate the effect of the attack of the bugs in the reduction of leaf area and photosynthetic capacity that reduces maize production. Other control methods for D. melacanthus should also be evaluated.

Acknowledgments

National Council of Science and Technology (CONACYT, Paraguay) and National Program of Incentive for Researchers (PRONII), PROCIENCIA.

Conflict of interest

The author declares no conflict of interest.

Additional information

This work is the English translation of the article: Ferreira-Aguero MA, Benítez-Sánchez A, Velásquez JA, Vega-Britez GD, Lesmo-Duarte ND, Acosta-Resquín MF. Daños causados por chinche barriga verde Dichelops melacanthus en maíz transgénico Bacillus thuringiensis (Bt) [Internet]. Intropica. Universidad del Magdalena; 2020. pp. 66–71. Available from: http://dx.doi.org/10.21676/23897864.3938.