Effect of Temperature on the Toxicity of Biorational Insecticides against Sitophilus oryzae (Linnaeus) in Stored Wheat

Healthy adult cultures of S. oryzae were collected from grain markets of Faisalabad (Longitude: 73° 5’ 0” E; Latitude: 31° 25’ 0” N; Altitude: 192 m) in Punjab Province, Pakistan. Collected insects were reared on whole wheat grains at 26±1°C temperature and 70±5% relative humidity. Mixed sex and 2-3 weeks old S. oryzae adults were used in the experiments (Athanassiou et al., 2011). The study was approved by the University of the Punjab, Lahore, with the approval number No. D 1330/Acad.

Insecticide formulations used in the current study were: 240 SC spinosad [a microbial insecticide extracted from Saccharopolyspora spinosa], 1.9 EC emamectin benzoate [avermectin], 25 WG thiamethoxam [neonicotinoid], and 5.2 EC lufenuron [benzoylurea]. The grain commodity used in the bioassays was clean, untreated and uncontaminated wheat. These grains were purchased from grain markets of Lahore (Latitude: 31.5497◦ N; Longitude: 74.3436◦ E; Altitude: 216m).

Bioassays with all insecticides were done as described by Athanassiou et al. (2011) and Khan et al. (2016a) with few changes. Spinosad and emamectin benzoate were applied on wheat at five rates; 0.50, 0.75, 1.00, 1.50 and 2.00 ppm, whereas lufenuron and thiamethoxam were applied at 2.0, 4.0, 6.0, 8.0, 10.0 ppm, as a solution made in distilled water. For each insecticide at single dose rate, lots of 900g grains were required. To attain the required doses, the appropriate quantity of each insecticide was dissolved in distilled water and 90 ml solution of each insecticide was prepared. Then, on a tray a thin layer of grains for each insecticide were spread and treated with 9 ml of each insecticide dose separately and for uniform insecticide distribution grains were manually shaken for 5 min (Krishnamurthy et al., 2008; Athanassiou et al., 2011). Grains were allowed to dry before place in jars. From treated grain lot of individual insecticide, 100 g of grains were taken and placed in a plastic jar. In each jar, 30 adults of S. oryaze were added and the jars enclosed with a muslin cloth to prevent the insects escaping and also to facilitate aeration. The treated jars were maintained at 20°C, 25°C and 30°C. One jar containing grains treated with distilled water was served as control. All the bioassays were replicated thrice and mortality was counted after 21 days.

To find out the insecticide lethal concentration (LC50), data of mortality against each concentration of an insecticide with three replicates were collected and analyzed for probit analysis in SPSS software (16.0 version). At LC50, the toxicities of each insecticide were considered as different if the 95% confidence interval (CIs) did not overlap (Khan and Akram, 2014). For each tested insecticide, the ratio of higher LC50 to the lower LC50 was calculated as temperature coefficient. The temperature coefficient was considered as positive when the LC50 value lower at high temperature and negative when LC50 value is higher at low temperature (Musser and Shelton, 2005; Khan and Akram, 2014).

The toxicities of all four tested insecticides were found to be positively linked to the tested range of temperature (Table I). The results of spinosad according to LC50 values

1Total number of S. oryzae tested. 2Median lethal concentration. 3Temperature ratio of higher to lower LC50 value.

showed 1.34 and 1.97 folds increase in toxicity at temperature 25°C and 30°C when compared with toxicity at 20°C (non-overlapping of 95% CIs). The results also indicated the positive temperature coefficient with 2.65 fold for the tested temperature ranges. Similarly, as compared to the values at 20°C, the toxicity of emamectin benzoate based on LC50 values was increased 1.33 and 1.18 folds at temperature 25°C and 30°C, respectively. The temperature coefficient of tested ranges for emamectin benzoate was also positive with 1.59 fold increase. The toxicity of lufenuron also showed positive connection at all temperatures. The results when compared with 20°C (non-overlapping of 95% CIs) showed 1.21 and 1.36 fold increased in toxicity at high temperature. Findings of thiamethoxam showed a positive temperature connection with increase in toxicity from 1.69 to 1.77 folds at 25°C and 30°C when compared with toxicity at 20°C. The overall temperature coefficients for lufenuron and thiamethoxam were 1.64 and 3.00, respectively, for the tested range of temperatures.

In general, all tested insecticides showed positive temperature coefficients but the efficacy of thiamethoxam was maximum as thiamethoxam (3.00) > spinosad (2.65) > lufenuron (1.64) > emamectin benzoate (1.59).

In insects’ body, different metabolic activities which are responsible for the normal working of the nervous system and degradation of an insecticide are extremely dependent on temperature (Montgomery and Macdonald, 1990). Our results showed that the toxicity of spinosad was temperature dependent as it increased at 25°C and 30°C. These results are in accordance with Athanassiou et al. (2008) findings who reported increased mortality after spinosad treatment when the temperature was increased from 20°C to 30°C. One reason of mortality at high temperature may be that S. oryzae adults are more responsive at high temperatures compared to other insects. Spinosad act as a contact insecticide and at high temperature (30°C) which is close to the temperature required for their development (Khan et al., 2014) contact of adult weevils with insecticide increases, hence results in high mortality. Emamectin benzoate act as an activator of chloride channel by binding GABA receptors and disturb the nerve signals in arthropods (Grant, 2002; Khan et al., 2016b). Due to the binding of GABA receptors permeability of chloride ions increases within the cells (Rodriguez et al., 2007) as a result the transmission in nerves is reduced. The mortality of insects due to the insecticides depends upon different factors like temperature, dose rate and exposure time (Khan and Akram, 2017; Yasoob et al., 2017). The current study on the toxicity of emamectin benzoate showed a positive temperature connection with 1.33 and 1.18 fold increases at 20°C to 30°C temperature. Previously, Kavallieratos et al. (2009) evaluated the abamectin effectiveness against T. confusum, R. dominica and S. oryzae on wheat and maize at two temperatures (25°C and 30°C), and reported that efficiency increases with the increase in dose rate, temperature and insects’ exposure time.

Lufenuron (CSI) has been used to manage the larvae of various insect’s species like Lepidoptera (Saenz-de-cabezon et al., 2006) Coleoptera (Ahire et al., 2008), Diptera (Khan et al., 2016c) and Homoptera (Gogi et al., 2006). It is used on different crops including maize, cotton, and ornamentals. Lufenuron affects the moulting process and develops abnormalities by affecting the physiological processes (Sammour et al., 2008). We have shown 1.21 and 1.36 folds increase in toxicity of lufenuron at 25°C and 30°C temperature when compared with LC50 values at 20°C. Recent work on lufenuron done by Ali et al. (2016) showed that it is an effective chitin synthesis inhibitor against T. castaneum. The results of thiamethoxam showed that toxicity varies 1.69 and 1.77 folds at high temperatures when compared to the values at low temperature. These results are in accordance with the Arthur et al. (2004) findings in which wheat containing S. oryzae, and R. dominica was exposed with different doses at three temperatures and they concluded that mortality increased with increase in temperature, insecticide dose and insect exposure time. All the tested insecticides can better perform in a hot environment as compared to cold conditions. As in Punjab average temperature in June has been reported to be approximately 38°C (Anonymous, 2013). Hence, qualitative and quantitative losses due to S. oryzae in stored grain commodities can be controlled with appropriate management. This control could be achieved only with the selection of proper insecticide which is effective in the storage season. The present study highlighted that for highly efficient control of S. oryzae in wheat grain in warm environment, it is highly recommended to use biorational insecticides at high temperatures.

The efficacy of all insecticides was temperature dependent as it increased with increase in temperature. All tested insecticides revealed the positive temperature coefficient and may serve as a potential applicant to control S. oryzae in stored wheat under warm climatic conditions. However, further studies should be conducted at temperature ranges other than the studied ones in present study for the purpose to determine the highest temperature range (above 30oC) at which toxicity stop to increase.

The work presented here is part of PhD work of the first author (GM).

Authors have declared no conflict of interest.