The Effect of Photoperiods on the Insecticidal Activity of Hypericum perforatum Extract on the Third Larval Instar of Diamondback Moth, Plutella xylostella

Insecticidal activity of Hypericum perforatum extract towards third larval instars of the diamondback moth, Plutella xylostella in different concentrations including 5, 100, 1000, and 5000 µl/ml was tested at different photoperiods (12L:12D, 16L:8D, 8L:16D, 20L:4D and 4L:20D hours). The obtained values of LC10, LC25, and LC50 in normal photoperiod were 11, 74, and 622 µl/ml, respectively. The results showed a direct relation between insecticidal activity of hypericin and its concentration. Thus, values of LC10, LC25, and LC50 provided poor, fair, and good control, respectively. When these concentrations were tested in different photoperiods, results showed that the mortality of the test animals increased with increasing photoperiod in constant concentration. The mortality varied depending on light hours in photoperiods, with more light caused higher mortality. Results showed that the extract with a low concentration of 5 µl/ml was also effective in feeding indices of P. xylostella.