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To study the relationships between leaf nitrogen and the reproductive potential of diamondback moth, all reproductive parameters of this pest raised on two canola cultivars were evaluated. A standardized regression coefficient (β) was used as an index for nitrogen-reproduction relationship strength. The only difference between net fecundity rate and net fertility rate is hx’s effect, but the difference in their standardized regression coefficients was not significant [β=+0.934 (R2=0.87, F1,4=27.34, P=0.006) and β=+0.922 (R2=0.85, F1,4=22.825, P=0.009)]. Accordingly, gross fecundity rate and gross fertility rate differ only in hx’s effect, but the difference in standardized regression coefficients again was not significant [β=0.895 (R2=0.8, F1,4=16.159, P=0.016)-0.890 (R2=0.79, F1,4=15.266, P=0.017)=0.005]. As gross fecundity rate differs from net fecundity rate only in midpoint survivorship (Lx)’s effect, it is understood that survivorship could affect the plant nitrogen–fecundity relation considerably (standardized coefficients difference=0.044) and could be a critical parameter in insectplant interactions. But, the terms of reproductive parameters, i.e. Lxand hx, showed the same effect on the strength of nitrogen-fecundity regression statistically, even though Lx has been selected frequently by many researchers as an important fitness correlate. Measuring the hatch rate could be recommended in trophic interactions studies due to its being easier to apply, more robust, and quicker to accomplish than measurement of survivorship; however, it is important as an indicator in combination with brood size for determining the initial population size of an insect herbivore.
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