Studies Of Effect Of Plant Essential Oils On Contact Toxicity Of Several Insecticide And Their Synergism Mechanism

1 Contact toxicity of several plant essential oil against diamondback moth(Plutella xylostella)The contact toxicity of essential oils of five plants Zanthoxylum bungeanum Maxim, Artemisia argyi Levl.et Vant,Pinus massoniana Lamb,Platycladus orientalis(Linn.) Franco,Artemisia annua L.were observed in vitro against 2-nd instar larvae of diamondback moth,Plutella xylostella(Fab)by dropping bioassay in the lab.The result showed that percentage mortality of larvae increased when applied with high concentration of essential oils after treatment 24h,Artemisia annua oil was the most effective among five plants, caused 51.73%mortality of the insects at 160μL·mL^-1concentration.Next was Platycladus orientalis,percentage mortality was 44.92%.however,it was only 7.65%and 8.60%when applied with these two kinds of essential oil at 10μL·mL^-1concentration.Under the same experimental conditions,Specific enzyme activity in the survivors treated with essential oils at10uL·mL^-1concentrations were tested by biochemical analysis.The result indicated that essential oils could inhibite activity of Acetylcholinesterase(ACHE)and Carboxylesterase(CatE).ACHE,CarE activity was high inhibited by Pinus massoniana,Z.bungeanum essential oils respectively,inhibition ratio was 23.94%,33.07%.respectively. The result indicated that some essential oils can inhibited activities of enzymes though they showed low contact toxicity to the larvae.The wax coat scanned by electron microscopy was wounded to some extent after treated with several plant essential oil,based on these results, studies on toxicity and synergism of several essential oils at sublethl concentration mixed with four kinds of insecticides including chlorpyrifos,fipronil,carbosulfan and abamectin against the diamondback moth larvae were carried out in the lab.2 Contact toxicity of Mixture of plant essential oil and insecticides against diamondback moth larvaeThe results of LC₅₀showed that all essential oils at 1μL·mL^-1could enhance the toxicity of chlorpyrifos to tested larvae,but fipronil could not.when the concentration of the essential oils was 10μL·mL^-1,the synergism ratios of Z.bungeanum,A.argyi,Pinus massoniana, Platycladus orientalis,A.annua at 10μL·mL^-1concentration were 1.88,1.97,4.35,4.31,3.26 fold to chlorpyrifos.1.28,1.75,1.42,1.84,1.17 fold to fipronil,0.63,1.55,0.86,1.63, 1.14 fold to carbosulfan,and 1.82,1.82,3.81,1.32,2.14 fold to avermectin respectively after larvae were treated in 48h.The results also indicated that the essential oil of Chenopodium ambrosioides at 10μL·mL^-1concentration could improve susceptibility of P. xylostella larvae to fipronil Synergism ratio was 2.27 fold than control one.LT₅₀of chlorpyrifos,carbosulfan,avermectin mixed with essential oil against larvae were much shorter than that of the pesticide without essential oil.3 Effects of mixture of plant essential oils and insecticide on some enzyme activities in diamondback moth larvaeInhibition ratios of specific enzyme activity of AChE and CarE were 24.56%and 76.47% respectively after larvae were treated by chlorpyrifos at 3000μg·mL^-1(LC₉₀)concentration for 48h,while AChE inhibition ratios were 52.63%～77.19%and CarE inhibition ratios were 52.63%～77.19%after larvae were treated with mixture of chlorpyrifos at same concentration and different essential oil at 10μL·mL^-1concentration.The results suggest that enzyme activities significantly decreased,which could be the main synergism mechanism of plant essential oil to chlorpyrifos.The larvae were applied with Chenopodium ambrosioides essential oil at 1uL·mL-1 and 10μL·mL^-1respectively for 2 hours.Than applied with fipronil at 3.66μg·mL^-1(LC₅₀)for 48h, the specific activities of three kinds of enzyme were significantly lower than that of larvae treated only with fironil.Effects of the two mixtures,carbosulfan mixed with Platycladus orientalis at 10μL·mL^-1 concentration(Platycladus orientalis·carbosulfan),avermectin mixed with Pinus massoniana essential oil 10uL·mL^-1concentration(Pinus massoniana·avermectin),on some enzyme activity in larvae body were persistently investigated by biochemical methods, the results showed that enzyme activities of larvae with different treatment method were gradually increased along with their growth.After treated by Platycladus orientalis·carbosulfan,the activity of AchE and CarE in tested larvae kept lower than that of carbosulfan treatment during many period,which may be one of the main synergism mechanism.However,GSTs activity was higher than the carbosulfan treatment during the whole experiments.After treated by Pinus massoniana·avermectin,the activity of AchE in survival larvae was higher than that of larvae treated only with avermectin during the whole experiments,while CarE and GSTs activity was lower than that of larvae treated only with avermectin during much different period of time.The final results suggest that activity changes of CarE and GSTs may be partly associated with synergism of Pinus massoniana essential oil to Avermectin.4 Effect of plant essential oils on penetration of insecticideGC—MS analysis showed that the extraneous chlorpyrifos remained 50%on tested larvae treated with chlorpyrifos(2000μg·mL^-1)alone,while it was only 10%～30%on which treated with mixture of chlorpyrifos and essential oils and 27%～48%on which treated with mixture of chlorpyrifos and Laurocapram after treatment for 4h。The results also showed that during the period of 4h to 20h after treatment,the chlorpyrifos residue quantity on tested larvae treated with chlorpyrifos alone decreased quickly,while that decreased slowly on larvae treated with chlorpyrifos mixed with essential oils at different concentration.Percentage of chlorpyrifos residue on tested larvae cuticular had no difference between different treatment after treated 20h.Laurocapram was a kind of penetrating agents, our research suggest that plant essential oils were super to Laurocapram to promote chlorpyrifos penertrating into larvae cuticular.Plant essential oil could significantly enhanced the cuticular penetration of chlorpyrifos to tested larvae,which may also be the main synergism mechanism,however,there was no significant difference between the same essential oil at different concentration.residue of fipronil on tested larvae cuticular treated with mixture of Pinus massoniana essential oil and fipronil was higher than that of treated with fipronil alone during first 8 hours,on the contrary,it was lower during the period of 8h to 16 h after treatment,the results indicated that Pinus massoniana essential oil could promote fipronil penetrating into tested larvae cuticular at last,however,essential oil at different concentration showed no significant difference on promoting penetration effect,the effect of Platycladus orientalis essential oil at different concentration on penetrating quantity of fipronil was same that of P. massoniana.C.ambrosioides can enhence fipronil penetrating quantity all the time,while other plant essential oils showed no promoting penetration effect to fipronil.Laurocapram also can promote fipronil penetrating the tested larvae cuticular after treatment 11h,and penetrating quantity increased when Laurocapram at high concentration. All results showed that the effects of plant essential oil on pesticide related not only with the varieties of insecticides,but also with the concentration of insecticides and essential oils and the treatment duration.