Effect Of Four Biological Insecticides On Activity Of Juvenile Hormone Esterase In The Diamondback Moth, Plutella Xylostella

The diamondback moth (DBM), Plutella xylostella (L.), is a major cosmopolitan pest of brassica and other crucifer crops all over the world. Pesticide use has been the most important way in DBM control, but DBM has become highly resistant to almost all pesticides. So, it is in great necessity to find new ways to manage DBM. In this thesis, the toxicities of4biological insecticides on DBM were tested and their effects on juvenile hormone esterase activity were studied. Also the interaction between3-octylthio-1,1,1-trifluoro-2-propanone (OTFP) and the involved insecticides were investigated. The results were as follows:(1)The toxicities of the4biological insecticides on DBMThe toxicity test results showed that the LC50s of avermectin, spinosad, azadirachtin and Bt on the3rd instar larvae of DBM were0.14mg/L,0.06mg/L,23.33mg/L and0.48×106IU/L, respectively and those on the4th instar larve were0.24mg/L,0.11mg/L,42.62mg/L and0.68x106IU/L, respectively.(2) The effect of the4biological insecticides on juvenile hormone esterase (JHE)Based on the results, the effect of avermectin, spinosad, azadirachtin and Bt on JHE on DBM could be devided into3types, which were inhibition-stimulation-inhibition type, stimulation-inhibition type and stimulation-inhibition-stimulation type, and it varied with insecticide and the instar of the insects. Among them, the effect of avermectin on the3rd and4th instar larvae and that of Bt on the4th instar larvae all belonged to the type1, that of spinosad and Bt on the3rd instar larvae belonged to type and that of neem on both the3rd and4th instar larvae belonged to type3.(3) Interaction between OTFP pretreatment and the insecticidesThe interaction between OTFP and the insecticides could result in synergism, addition or antagonism which depended upon the pretreatment duration of OTFP, type of pesticides as well as their concentrations, the instar of the insects and the expermental duration. Besides, the interaction result was not in accordance with the effect of the insecticides on JHE. For example, the interaction between OTFP pretreatment for24h and azadirachtin on the4th instar larvae resulted in antagonsim, but at the set2concentrations, which were15mg/L and30mg/L, neem could inhibit the JHE activity significantly.