Effect Of Insecticides On Cotesia Plutellae

Effects of four insecticides on diamondback month were tested under laboratory condition. Four insecticides were methamidophos, fenvalerate, avermection , fipronil. Results indicated that fipronil was highly toxic to 2nd instar of F₁ larvae of Plutellae.xylostella, then were avermection and methamidophos, the toxicity of fenvalerate was lowest. Resistance levels to the four insecticides in P.xylostellae varied during different seasons. In general, LC50 of the four insecticides were high in Autumn and low in Spring.A pair of adults of Cotesia.plutellae were released into a cage to parasitize 2nd instar larvaes of diamondback moth 2h after larvaes of P. xylostella were feed with the four insecticides at sublethal doses, respectively. Results showed that both cocoon formation and adult emergence of C. plutellae decreased compared with control. Among different treatments conducted in different seasons, the rates of cocoon formation and adult emergence were only 10.3%¹5.2% and 54.8%⁷4.3% in methamidophos treatment, 14.8%¹8.7% and 75.0%⁸7.5% in fenvalerate treatment, 17.5%²5.1% and 80.1%⁸5.9% in avermection treatment, 5.8% 7.4% and 44.8%⁴7.5% in fipronil treatment, respectively. While the rates of cocoon formation and adult emergence in control were 27.2%³3.5% and 99.2% 100% respectively. In the four different treatments, the decrease of cocoon formation and adult emergence was not significant in avermection treatment, but significant in mthamidophos and fenvalerate treatment, in particular, extremely significant decrease was found in fipronil treatment. The depression on cocoon formation and adult emergence of C. plutellae by the four insecticides was fipronil > methamidophos > fenvalerate > avermection. Avermection was recommended to useto control diamondback moth during seasons when C.plutellae are relatively abundant in order to protect C.plutellae, while fipronil ,methamidophos and fenvalerate were thought to be avoid to use in the field for their high toxicity to C.plutellae.The rate of cocoon formation and adult emergence of C.plutellae varied when the larvaes of P .xylostella were fed with the four insecticides respectively in different time interval after host larvaes had been parasitzed. Compared with control, mthamidophos -. fenvalerate and fipronil caused significantly decrease of cocoon formation after three or five days that host larvaes were parasitized, fipronil and mthamidophos caused significantly the decrease of adult emergence when applied on the third day after larvaes were parasited .Although avermection caused decrease of the rate of cocoon formation and adult emergence of C.plutellae , there is no significant difference between different time intervals.Our results indicted that the four kinds of insecticides had caused the decrease of the cocoon formation and adult emergence to some extent, thus, when insecticides are applied at high dosage in field ,it not only can kill C.plutellae directly, but also can affect the cocoon formation and adult emergence of C.plutellae indirectly.