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).