Effect Of Short Period High Temperature On Pesticide Sensitivity Of Diamondback Moth (Plutella Xylostella (Linn.)) 3～(rd) Instar Larvae

ABSTRACTPesticides and short period high temperature are 2 most important factors that threaten the survival of diamondback moth (Plutella xylostella (Linn.)). Based on the survival ratio, growth rate and pesticide sensitivity, the effects, including both beneficial and harmful ones, on 3rd instar larvae of P. xylostella were studied. The results were as follows.The protection effect Pretreat the insects at 30℃for 2h, their LC to avermectin was 0．63 mg/L which was a little higher than that of those untreated ones and indicated an unsignificant protection effect (P>0．05) . Meanwhile, pretreat the tested insects at 30℃for the designed periods could increase their tolerance to avermectin at 0．28mg/L and 4h pretreatment at 35℃increased the survival at 1．13 mg/L or 2．25 mg/L avermectin. After being pretreated at 30℃, the LC of the tested insects to fipronil was 2．15 mg/L, much lower than that of unprtreated one s, which was 1．03 mg/L (P<0．05 ) . Two or four hours pretreatment at 30℃helped the tested insects to tolerate fipronil at 0．78 mg/L. Similarly, 2 or 4 hours pretreatment at 35℃helped the tested insects to tolerate fipronil at 1．56 mg/L.Treat the insects with 35℃for 4h and fipronil at 0．39mg/L or 0．78mg/L alternatively would significantly increase the larval pupation rate and adult emergence rate.The egg production increased in some treatments including 30℃lasted 8h follwed with 0．39 mg/L fipronil, 35℃for 4h with 0．78mg/L fipronil and 40℃for 4h with 0．39mg/L fipronil which indicated the stimulation effect of heat treatment on egg production.Adversary effect The sensitivity of the tested insects to avermectin unsignificantly increased after being treated at 30℃for 4 or 8h. The LC₅₀ of the tested insects to avermectin became lower after being treated at 35℃for 2h but that to fipronil decreased until the pretreatment period reached 4 h. When the pretreatment period was 8h at 35℃, the LC₅₀ of the insects to both avermectin and fipronil decreased especially that to avermectin. Pretreat the insects at 40℃for 4h, the LC₅₀ to fipronil decreased significantly.Treat the insects with 35℃and low dosage of avermectin alternatively could postpone the pupation time especially when the pretreatment period was 2h. Both the larval pupation rate and adult emergence rate decreased after higher temperature treatment especially at 35℃or 40℃. Also 35℃or 40℃alternatively treated with fipronil at low dosages extended the pupation time and decreased the larval pupation rate or adult emergence rate at 40℃.