Introduction

Maize (Zea mays L.) is cultivated practically throughout Mexico, in diverse climatic and soil conditions and its variety is such that the country has been considered the center of origin and diversity of this crop (^{Kato et al., 2009}). In the state of Guanajuato, the Spring-Summer cycle is the one with the largest sowing surface and the environmental conditions are characterized by long periods of drought and high temperatures, which make the trips (Frankliniela occidentalis) the most important pest in the crop of corn (^{Castresona et al., 2008}). The trips of the flower, is one of the most important agricultural pests in the world (^{Zhao, 2013}) and initiate its damage during the first 20 to 25 days since the plant emerges, in some cases the damage can be so severe that the plant can present the symptom of lack of moisture (^{CESAVEG, 2008}). This pest feeds, oviposits and transmits viruses in plants, which causes alterations in plant tissue as scars as well as other imperfections (^{Kay and Herron, 2010}). If there is no control, the attack continues until the plant finishes growing, but the real damage causes it in the first days of emergency (^{Castresona et al., 2008}).

The use of insecticides has been the main strategy used to control F. occidentalis (^{Bielza et al., 2007}), presenting difficulties in their control due to their behavior, since the nymphs are found in the foliage, the pupae in the Soil, and the adult has great mobility (^{Lopes da Silva et al., 2003}; ^{Helyer and Brobyn, 2008}). The high pressure to insecticides has led to the development of resistance to the main groups of insecticides (^{Zhang et al., 2008}). Since the mid-1980s, resistance in populations of F. occidentalis has been the subject of research after its establishment and propagation in greenhouses and outdoor production (^{Lewis, 1998}; ^{Bielza et al., 2007}).

The first reports of resistance were associated with toxaphene applied to cotton (^{Race, 1961}; ^{Bielza et al., 2007}). While several studies have shown that populations of F. occidentalis in different parts of the world, have developed resistance to insecticides such as diazinon, methomyl, cypermethrine, permethrin, fenvalerate and imidacloprid (^{Guangyu et al., 1995}; ^{Kontsedalov et al., 1998}; ^{Bustillo, 2009}) since that time, there have been cases of lack of efficacy in insecticides of the main chemical products (^{Herron and James, 2007}); which shows that chemical combat must be conducted carefully (^{Rodríguez et al., 2003}).

From this point of view, it is interesting to study the efficacy of new active materials and the form or period of application for the control of this pest. An alternative is the treatment of seeds, which is the application of biological, physical and chemical techniques and agents that provide the seed and the plant protection against the attack of insects and diseases in the early stages of cultivation (^{ISF, 2007}). Seed treatments can be used as primary tools in a successful Integrated Pest Management plan for sustainable agriculture, because the pest is controlled with lower amounts of active ingredient per hectare and are not released into the atmosphere (^{FAO, 2012}). Therefore, the objective of this research was to evaluate the biological effectiveness of different insecticides applied to corn seed for the control of trips (Frankliniella occidentalis).

Results and discussion

In Table 2 the Anva is presented for the density of F. occidentalis in which a significant effect (p≤ 0.01) is observed for treatment, week of sampling and treatment interaction per week. The coefficient of variation was less than 22% and a coefficient of determination R² of 0.823 indicating that there is 82% validity in the results.

In the Table 3 and 4 show the average squares for the samples taken in eight weeks, where a highly significant effect (p≤ 0.01) of the treatments evaluated in weeks 1, 2 and 8 is observed depending on the incidence of trips per plant, while in weeks 3, 4, 5, 6 and 7 the effect of the treatment was not observed.

Next, the results obtained in weeks 1, 2 and 8 are shown, which was where the experiment showed differences between treatments. For week 1, treatment with fipronil and acephate at -50% DC were those with the highest density with 3.09 and 2.82 trips per plant. Whereas, treatment with thiamethoxane + lambda cyalotrine to DC had the lowest density with 1.64 trips per plant (Table 5). These values are similar to those reported by ^{Arias and Adrian (2012)} in the cultivation of soybeans who used thiamethoxane + lambda cyalotrine at a dose of 200 mL ha^-1, which presented a density of 1.6 trips per plant at 11 days later to the emergency.

On the other hand, in relation to the treatments that presented a higher incidence, our results are superior to those reported by ^{Beltran et al. (2004}) in the control of Frankliniella schultzei in the cultivation of cotton using thiamethoxan in a dose of 200 g of eg. in 100 kg of seeds, where a density of 0.2 trips per plant occurred 15 days after emergence. For Fipronil the results are lower than those reported by ^{Bustillo (2009)} in the asparagus crop, which presented a density of 11.4 trips per plant at 11 days after emergence. In this sense, we can mention that the best results with the product thiamethoxan + lamda cyhalothrin are due to the thiamethoxan adheres and penetrates quickly to the seed which allows stability and bioavailability of the active ingredient (^{Valarezo and Loor, 2013}).

In week two the highest values of trips density were for the control treatments and thiamethoxan + lambda cyalotrine at -50% DC with values of 3.4 and 3.32 respectively. While the lowest value was observed in the thiodicarb treatment to DC with 1.85 trips per plant, there was a reduction in the population of 54.63% between the treatments with higher and lower density (Table 6). In relation to the thiodicarb product, our values are higher than those reported by ^{Beltran et al. (2004}) for the control of Frankliniella schultzei in cotton, where they observed a density of trips of 0.87 per plant 20 days after emergence. In this regard ^{Valarezo and Loor (2013)} mention that thiodicarb is an insecticide with rapid and residual action for the treatment of the seed, protecting it from the sowing until the first days after its germination.

For week 8, in Table 7 it can be observed that the thiodicarb treatment to DC was the one that presented the highest density of trips per plant (1.71) and the treatments bifenthrin + imidacloprid at -50% DC, bifenthrin + imidacloprid and acephate at DC expressed lower density of trips (1, 1.1, 1.1 and 1.18, respectively), there being a difference of 41.84% of trips incidence among the treatments that reported the highest and lowest values in week 8 of evaluation.

These results are in contrast to those reported by ^{Machaca (2012)} in the control of trips (Trips tabaci L.) in the cultivation of onion using imidacloprid at a dose of 50 mL per L in which an average of 2.88 of Trips tabaci was observed per plant at 63 days after the application. While, ^{Larral and Ripa (2007)} reported a presence of zero Heliotrips haemorrhoidalis 60 days after application in avocado plants (Persea amaricana Mill) using imidacloprid at a dose of 90 mL hL^-1.

On the other hand, Table 8 shows the behavior of the means in the density of trips at eight weeks of sampling; where thiamethoxane + lambda cyalotrine treatment to DC was the one that reported the lowest density of trips per plant with 1.53, followed by thiodicarb treatment of high dose with value of 1.62, while the control was the one with the highest value with 2.14, followed by thiodicarb at -50% DC with 1.97 trips per plant. A difference of 28.31% can be observed between the highest and the lowest value. These results differ from those reported by ^{Larral and Ripa (2007)} in a study carried out on the chemical control of Heliotrips haemorrhoidalis in avocado cultivation (Persea amaricana Mill) using thiamethoxan at a dose of 20 g hL^-1 who reported a null presence of H. haemorrhoidalis 60 days after application.

The effectiveness of thiamethoxan + lamda cyhalothrin in the study we can mention that it is an effective insecticide to control this pest (^{Shan et al., 2012}), and is considered a second generation nicotinoid (Maienfisch et al., 2001; ^{Naun et al., 2003}), which act on the central nervous system, specifically on the nicotinic acetylcholine receptor (Domínguez, 2014).

Neonicotinoids are widely used in the treatment of seeds (^{Proietto et al., 2013}), since their distribution in the seed and in the plant, is slowly metabolized and is available for a long period of time close to 30 days (^{Clavijo, 2008}).

In the Figure 1 shows the reported results for the density of trips per week, week 2 is observed to have the highest number of trips, with an average of 2.75 trips per plant, followed by week 1 and 3 with values of 2.45 and 2.47 respectively. While week 7 was the one that presented the lowest density with 1.16 trips per plant, there is a difference of 57.78% between week 2 and week 7. These results agree with those reported by Díaz (1994) where it mentions that the attack more severe trips (F. occidentalis) is mainly during 15 to 25 days after the plant has emerged.

Figure 1 Behavior of the means in the density of trips in corn per week, during eight weeks of sampling. 

In Figure 2, the behavior of the density of trips in relation to the climatic conditions during the sampling period is presented, where it can be observed that in weeks 1, 2 and 8 the highest values of maximum temperature were presented with 29.97, 29.69 and 27.83 °C respectively, also this week the highest density of trips per plant (2.45, 2.75 and 1.23). Regarding ^{Ascencio (2000)}, he points out that high temperatures and absence of rains (Valenzuela-García et al., 2010), favor the increase in the population density of trips, for which it is considered that temperature and precipitation are two abiotic factors that interact on the development of trips populations, since there is a highly positive correlation between air temperature, soil temperature, and relative humidity with the population density of trips (^{Driutti, 2000}).

Figure 2 Behavior of the climatic conditions (maximum temperature, minimum temperature, relative humidity and accumulated precipitation) and the density of trips in a sampling period of eight weeks. 

On the other hand, minimum temperatures were reported in weeks 1, 2, 3 and 5 with 16.7, 15.2, 15.6 and 15.5 °C respectively. And the accumulated precipitation presented the highest values for weeks 3, 4, 5, 6 and 7 with 18, 38.4, 42, 17.2 and 20.4 mm. These conditions favored the lower density of trips with 2.47, 1.9, 1.42, 1.29 and 1.16 respectively, because rainfall is an important factor for the control of trips as it cleans the leaves causing the trips to fall to the ground (Valle et al., 2003), which meant that the sampling in these weeks did not show significant differences between treatments in the density of trips, since together with the temperature, the number of hours of light interacts negatively in the population development of trips (^{Drutti, 2000}).

According to the data shown in Figure 2, in week two the climatic conditions were presented with values of a maximum and minimum temperature (29.69 and 15.21 °C), a relative humidity of 61.66% and a cumulative rainfall of 5 mm, which allowed this week to have the highest incidence of trips in corn plants, these conditions affected insect populations, resulting in a decrease in the reproductive rate. In this sense, it has been established that temperature is the main environmental condition that interacts in the population development of trips (^{Driutti, 2004}).

Conclusions

The lower density of F. occidentalis in the maize crop was presented with the Thiametoxan + lambda cyalotrina treatment in commercial dose (0.943 mL L^-1) applied to the seed, with an average of 1.53 trips per plant, exceeding 28.62% to the absolute witness.

At 15 days after corn germination, the highest density of trips per plant was found with 2.7552.

The chemical treatment of corn seed reduces the density of F. occidentalis during the 45 days after germination.

When temperatures are high and precipitation is low it helps that trips populations increase in the plant, on the other hand, if the precipitation is high it has a positive effect on the control of F. occidentalis, since it is the lava of the plant.