Studies On Biological Of Cotesia Ruficrus Haliday (Hymenoptera:Braconidae)and Its Control Effectiveness On Cnaphalocrocis Medinalis Guenee

Cnaphalocrocis medinalis Guenee is one of the major pest of rice, and the utilization of parasitoids has been demonstrated to be an effective way to manage C.medinalisin. In this study,the invesitigation of parasitoid species of C.medinalis was conducted in Chongqing, and the dominating native parasitoid Cotesia ruficrus Haliday was subsequently studied, including the biological eharateristics of the parasitoid,the phenomenon of superparasitism and its control effectiveness of C. medinalis.The main results of this study are as followings:1. Invesitigation on parasitoids of C. medinalis in ChongqingIn this study, we found that there are abundant parasitoids of C.medinalis in Chongqing. Of 15 parasitoid species were founded, and 4 species were hyperparasites. The majority of parasitoids were single-parasitism, and the rest was poly-parasitism.The species and proportion of parasitoids varied with field locations. In these parasitoids, C. ruficrus and C.cypris were the main species parasiting C.medinalis larvae, and Brachymeria excarinataw as the main species parasiting C. medinalis pupae.2. The biology of C. ruficrusThe ontogenetic characteristics and development time of C. ruficrus was studied. Eggs with pedicles expanded during embryonic development.The larvae appeared to be hypermentamorphic with three instars. The newly hatched first instar larva has a weakly sclerotized head. The second instar larva had bladder. Three instar larva unthread host to cocoon. Pupa at the early stagewas larvae-like, and that at the middle and late stage were exarate pupa.The development period from egg to adult was about 10.5d.Parasitiods feeding on 20% honey had the longest longevity.The most adults adopted bisexual reproduction, but the unmated females adopted and rogenetic parthenogenesis.The oviposition peaked at two days after emergence.The oviposition only lasted for 2 to 3 seconds,the eggs were laid in the thorax of host.3. The Superparasitism of C. ruficrusThe number of the parasitoid cocoons increased with oviposition times, while the number of the dead parasitoid larvae increased when the host was parasitized 3 to 5 times. The survival rate of host before adult C. ruficrus emergence increased with the oviposition times, and reached to 50% when the host was parasitized 5 times. Ovipositing twice on host larvae are shown to be the most beneficial to the growth and development of C.ruficrus offsprings, and ovipositing thrice could cause superparasitism. Superparasitism prolonged the developmental duration,lowered the emergence rate and female/male ratio of the parasitoid offsprings. Laboratory experiments demonstrated that the superparasitism occurred in C.ruficrus when parasiting C.medinalis larvae. Female wasps, irrespective of the experience of oviposition,attacked hosts that had been parasitized by themselves or other individuals,and the superparasitism rate increased with the parasitoid density and inoculation hours.4. The control effectiveness of C. ruficrus on C. medinalisThe results showed that the 2～5th instars of host larvae could be parasitised, and the 3th instars was the most susceptible to parasitization compared with other instars (parasitic rate 66.67% and selection coefficient(0.42).The host age exerted significant effect on the growth of C.ruficrus offspring.With the host age increased, the development period from egg to cocoon shortened and reached the minimum of 8.47d when parasite 5 instar larvae,the number of the parasitoid cocoons firstly increased and then decreased,the offspring emergence rate was not significant different,the female body size of the parasitoid offsprings significantly increased,and the parasitoid offsprings of female laid eggs in the 5th instar larvae had the longest hind tibia length compared with those laid eggs in other instar larvae.The density of C.medinalis larvae affected the reproduction of C.ruficrus,which could be described with the functional response of Holling-Ⅱ model:Na=0.7575No/(1+0.7575 ×0.0694No).Instant attacking-ability was 0.06484,and processing one host larvae time was 1.224 hours. With the density of host increase, the parasitic capacity of C.ruficrus increased,but when host density was higher than 15 individuals, parasitism increase slowed and reached asymptote.The density of C. ruficrus also affected the parasitic effect, which could be fitted with Hassell-Varley modelling, and the equation was:a=0.1824P-0.34 The more C.ruficrus provided, the less host larvae were searched by parasitoids because of some interference acts of individuals.