Insecticidal effectiveness of Piper retrofractum and Tephrosia vogelii extracts against Crocidolomia pavonana and Plutella xylostella and the safety of the extracts to Dialegma semiclausum

Most farmers rely heavily on the use of synthetic insecticides to control crucifer pests. Excessive use of insecticides may lead to some undesirable side effects to the environment and human health. In order to alleviate this problem, botanical insecticides could be used as alternative to conventional insecticides. The active fractions of ethyl acetate extract of Piper retrofractum (Pr)insecticidal activity in second-instar larvae of Crocidolomia pavonana and Plutella xylostella as well as for their safety to the adults of Diadegma semiclausum parasitoid. Fraction (fr) 2 of Pr from vacuum liquid chromatography (VLV) and fr2-4 of Tv from column chromatography (CC) had strong insecticidal activity of C. pavonana and P. xylostella. In the test with C. pavonana, the two fractions were more active by feeding than by contact. Based on LC50 at 72 hours since treatment (HST), fr 2 VLC Pr was 3.8 times more toxic than fr6 VLC Pr to C. pavonana. Besides, fr 2 VLC Pr was 2.4 times more toxic to C. pavonana than P. xylostella. On the contrary, fr 2-4 CC Tv was 1.8 times more toxic to P. xylostella than to C. pavonana. The mixture of fr2 VLC and fr2-4 CC Tv at concentration ratios of 8:5 and 5:1 was antagonistic to C. pavonana and P. xylostella, respectively. The mixture of fr6 VLC Pr and fr2-4 CC Tv, however, had additives to weak synergistic joint action against C. pavonana at equal test concentrations, the treatment with fr 2-4 CC Tv and its mixture with fr2 KVC Pr caused much lower mortality in D. semiclausum parasitoid adults than in its host larvae, P. xylostella. This suggests the selectivity of the test extracts in favour of the parasitoid. In contrast, an organophosphate profenofos, included in the study as a positive control, was much more detrimental to D. semiclausum than to P. xylostella. In a semifield experiment, fr 2-4 CC Tv had comparable effect to profenofos and bioinsecticide Bacillus thuringiensis in reducing the population of C. pavonana larvae on broccoli plant. Thus, T. vogelii extract preparation and, to a limited extent, its mixture with P. retrofractum extract hold the potential as alternative to conventional insecticides for the crucifer pests.