Background

European corn borer has a widespread host range, attacking practically all herbaceous plants. Their generation number depends on climatic factors, and there is a considerable adaptation for climate conditions even within strains. Ostrinia nubilalis is one of the serious pests among corn borers that infest grain corn causing severe reductions in corn production. Their development, survival rate, geographical distribution, and population density are strongly influenced by either insecticides or the prevailing temperature in the field (Ebeid, 2012; Ebeid et al., 2017).

Nanotechnology is a new environmental field that could be effective in reducing the cost of some pesticides and might be helpful in keeping the environment from pollution (Chinnamuthu and Boopathi, 2009).

The biological development depends on the received amount of heat units “Degree thermal-Days” (DD) to complete one generation as well as the lower threshold temperature for each insect, where no development occurs below that level (Roy et al., 2003). These values may be used in predicting insect activity during the growing season (Ebeid et al., 2015).

The conducted life table studies on corn borer populations perhaps provide the description for the population developmental rate under different temperatures, as well as prediction for population technique of the pest. Also, to develop the mass-rearing dynamics of the most important biological control agents to be used in Integrated Pest Management (IPM) program (Sedighi et al., 2016). Among the life table parameters are the rate of intrinsic (rm), the rate of net reproduction (R0), and the mean generation time (Chi and Su, 2006).

The goal of this paper is to compare between the two different strains of O. nubilalis according to their localities and their effect by challenger/silica nanoparticle formulation treatment, with respect to their developmental progress and change in temperature, life table, and to determinate the zero of developmental values for the different stages of O. nubilalis strains to detect the change that occurred during the emergence of adult.

Materials and methods

Tested conditions

Four constant temperatures (15, 25, 35, and 45 °C) at 70 ± 2% RH are used to determine the thermal requirement as lower temperature threshold (T0), heat unit (DD), and life table for European corn borer, O. nubilalis.

Tested material

Silica nanoparticles/challenger formulation that applied was manufactured by the Refractories, Ceramics and Building Materials Department, National Research Centre, Giza, Egypt. Silica nanoparticles were served as a carrier for challenger 36% SC, a commercial formulation of chlorfenapyr pesticide, manufactured by BASF-The Chemical Company, New Jersey, USA. The recommended concentration (0.5%) with two drops of Tween-80, as dispersant, was added and applied to larval diet (corn stalk) for 3 days and then feed on untreated food till pupation. Treatment was carried out by using dipping technique for 2 min and then left to dry (Ebeid et al., 2013). Treated larval stages were examined daily under the effect of different constant temperature till adult emergence.

Experimental insects

Two strains of European corn borer, Ostrinia nubilalis larvae, were collected from two Governorates (Sharqia and Qena Governorates), which represent different climatic conditions, were transferred into glass jars lined with moist saw dust to laboratory, and reared on corn tassel emerged from the whorl then stalk and ear at constant conditions (35 ± 2 °C and 75 ± 5% RH) till adult emergence and oviposition.

Egg masses were collected in Petri dishes (10-cm diameter) that lined with moist filter paper. Three replicates of egg (each has 30 individual) were conducted for each temperature. The developmental progress of each larval instar feeding on stalk treated with challenger/silica nanoparticles (0.5% concentration) of the two tested strains was monitored until adult emergence at abovementioned constant temperatures.

A linear regression diagram was used to determinate the relationship between development rate and tested temperatures within the linear range, where the rate of development is calculated as the percent of inverse of the number of days required for development at that temperature. The resulting regression equation (Y = a + bx) where x is the temperature and Y is the developmental rate used to estimate the lower temperature threshold “zero of development” (x-intercept method T0 = − a/b) (Campbell et al., 1974).

Thermal unit (DD) accumulations for each tested temperature of the tested insects were calculated according to Campbell et al. (1974).

To construct the life table, the initial number of treated female, the fraction of egg reached maturity, and the actual female age were calculated. Moreover, the life table used to compare the impact of different temperatures on the population growth of O. nubilalis from the two localities is according to Chi and Su (2006) and Tuan et al. (2014).

Statistical analysis

Student t test (independent samples t test) was carried out through the SPSS computer program to discriminate differences between the two strains. The raw data were analyzed according to “TWOSEX-MSChart” computer program (Chi and Su, 2006).

Results

Developmental progress of Ostrinia nubilalis treated with silica nanoparticles at different constant temperature

Silica nanoparticles form affected the development of two treated strains of O. nubilalis at four constant temperatures, which revealed increase in developmental rate from egg to adult with increases in temperature (Fig. 1).

Fig. 1
figure 1

Threshold temperature of different developmental stages of O. nubilalis

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Duration of different immature-treated stage

At the prevailing temperature, the duration of all developmental stages (except prepupal stage at 15 °C and pupal stage at 45 °C) of the Sharqia strain was significantly higher than that of the Qena strain (Tables 1 and 2).

Table 1 Thermal requirements for treated O. nubilalis two strains from two different localities
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Table 2 Duration of different treated stages of O. nubilalis under different constant temperatures
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Developmental progress and thermal conditions of different strains

Egg stage

With respect to the Sharqia strain, incubation period is significantly higher than that of the Qena strain, causing lower rate of development and consequently higher threshold temperature. The linear regressions revealed lower development threshold (T0) were 12.33 and 9.8 °C for the Sharqia and Qena strains, respectively. The decreasing of incubation period with increasing temperature leads to high rate of the development from 15 to 45 °C. Estimation of average thermal constants for egg stage was 123.34 DD and 110.68 DD for the Sharqia and Qena strains, respectively (Table 1 and Fig. 1).

Larval stage

The same trend was observed in case of treated larval stage, where the lower threshold temperatures (T0), and thermal constants for larval stages of O. nubilalis of the Sharqia strain were (12.09 °C and 714.47 DD) as compared with that of the Qena strain (11.67 °C and 534.63 DD) (Table 1 and Fig. 1).

Prepupal stage

The insignificant differences between the prepupal stage of O. nubilalis of two localities at 15 °C lead to threshold temperatures (T0) for tested prepupal stages of the Sharqia strain (10.06 °C) become nearly the same value to that of the Qena strain (9.90 °C) (Fig. 1). They were recording thermal units of 90.28 and 69.01 DD for the Sharqia and Qena strains, respectively (Table 1).

Pupal stage

The treated pupal stage duration behaved the same trend, i.e., decreased with the increasing temperature. This decrease leading to the increase of the rate of development that affects the average thermal unit of the Sharqia strain to reach 209.5 DD, while the Qena strain recording 206.71 DD (Table 1). The lower threshold temperatures (T0) for pupal stages of Sesamia cretica were (12.4 °C) for the Sharqia strain which was higher than the Qena strain (9.72 °C) (Fig. 1).

Life table parameters

Table 3 illustrated the effect of the intrinsic rate, the rate of net reproduction, and the finite rate for Sharqia treated strain with the increase of the temperature from 15 to 45 °C, while both average generation time and doubling time showed a contrary trend with respect to that of Qena. Moreover, the mean generation time for the Sharqia strain showed higher values than that for the Qena.

Table 3 Life table parameters for Ostrinia nubilalis under different constant temperature
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On the other hand, the life table parameters for O. nubilalis of both localities clarify that the mean generation time Tc was shorter when female reared on 45 °C.

Discussion

Thermal fluctuation may vary between species, populations, development stages, other ecological factors, and treated or untreated food source (Bueno et al., 2009). Temperature is the most vital factor affecting the rate of development of insects (Haile et al., 2002). The knowledge of thermal constants and lower development thresholds provides creative prediction model to determine the developmental rate of a particular species of arthropod and timing pests’ control in different environments (Quesada-Moraga and Santiago-A’lvarez, 2000).

Our results were in accordance with that reported by Hassan et al. (2008) who mentioned that the long geographical distance between Qalubiya and Qena and the difference in climatic conditions as well indicating that there was high genetic differentiation between two Sesamia cretica strains. The developmental rates of the undeveloped stages of O. nubilalis between the two treated strains varied due to the difference of prevailed temperature in the two localities. These findings are in accordance with that of Rahman and Khalequzzaman (2004) on stem borers and contradict with that obtained by Calvin et al. (1991) on Ostrinia nubilais. Our results revealed highly significant differences within each treated stage of the same geographical strain and different tested temperature. This probably may be due to the differences in temperature fluctuation; and these results are similar to that of Haile et al. (2002). It was noticed that the decrease in the developmental period with increasing temperatures causes an increase in the developmental rate as mentioned by Haddad et al. (1999). An opposite trend was reported in case of other pyralid as rice stem borers (Rahman and Khalequzzaman, 2004). In our result, Qena treated strain showed a higher value for zero of development than the Sharqia strain; these findings agreed with that of Orang et al. (2014).

The biology of insects requires the determination of their life table parameters as the net reproductive rate being higher in the Qena strain at 15, 25, and 35 °C than in Sharqia leading to decrease in doubling time and slightly rising both the finite rate and intrinsic rate for O. nubilalis. The previous data matched with that reported by Sedighi et al. (2016) on S. cretica.

The mean generation period and doubling time may be decrease by increasing the temperature as mentioned by some authors (Ebed, 2008; Firake and Khan 2014).

Conclusion

It could be concluded that the duration and thermal requirements of two populations of the silica nanoparticle-treated strain of Ostrinia nubilalis (Hüb.) from different localities and its development at four constant temperatures (15, 25, 35, and 45 °C) were significantly different between each other. The developmental time increased significantly with the increasing temperature within the range of both strains. Such information and life table are crucial to predict the seasonal abundance of the adult O. nubilalis in the field, which is essential for the successful implantation of control measures and enables us to determine the best time for controlling with better performance.