Effect Of Larval Density On Growth, Development And Defense Capabilities Of The Oriental Armyworm, Mythimna Separata (Lepioptera: Noctuidae)

Oriental armyworm Mythimna separata (Walker) (Lepidoptera:Noctuidae) is serious pest of of food crops in China and other Asian countries. It has caused a huge loss of grain production. In order to illuminate its regularity of occurrence and damage, clarify the major environmental physiological factors of the population dynamics and provide a scientific basis for its forecasting. Basing on a thoroughly review of the previous researches, effect of larvae raring density on growth, development and defense capability were systematically studied from the following aspects:development period, fecundity, cuticular color, resistance and survival rate to Exorista civilis Rondani parasite and (Metarrhizium anisopliae (Metch))Sor infection. Furthermore, PO activeity dynamics of larvae according to different stages and raring density were studied, and the role of PO activity in larvae melanism and immunity were also discussed. The main innovative results obtained are:Effect of larvae raring density on development period, survival rate, cuticular color and fecundity:Development period, body weight, motality, cuticular color and fecundity in Oriental armyworm M. seperata were investigated under different larval rearring densities(1,2,5,10,20 and 30 larvae/jar), and the impaction on this parameter was remarkable.1 larvae/jar had a significant longest development periods while 10 larvae/jar had the shortest, while those at the other treatments extended with the increasing of larval density. The preoviositional period of female was extend through larval density increased. The weight of pupa was also significantly different, with the heaviest one at the density of 1 larvae/jar (323.75mg), second is 10 larvae/jar's (286.91), others were about 230mg, while those at the other treatments were decrease with the increasing of larval density. Larval density play an important role in larval suvival rate, but not with larval density increases,1 larvae/jar had lowest mortality rate. The cuticular melanism (oplical density) was significant. OD had increase with the larval density. The fecundity of adult was also significantly different to the larval rearing density, with the highest one at the density of llarvae/jar (850.12) and the lowest one at density of 30 larvae/jar, while those at the other treatments were insignificantly different. Therefore, fecundity of adults was negatively related to the larval density.Effect of larvae raring density on the resistance ability, parasite position and survival rate to E. civilis parasite were reported in this research, and lower resistance ability and higher antibiotics in higher raring density were firstly identified. Larvae of 5th and 6th instar in two typical density,1 and 10 larvae/jar were used as our measure of host resistance to ectoparasitoids, the results show that the parasitism percentage of 10 larvae/jar was 51.5% and 52.5%, significantly higher than the other, the parasitism percentage were 26.0% and 40.0%. Tachinid fly's feeundity of 10 larvae/jar was significant higher than 1 larva/jar. Additionally, the oviposition preference of E. civilisto to positon of the host larvae by larval density also varies, in the lower density (71.43), larval abdomen was firstly chosen by tachinid fly while in the higher density (81.67), chest and head were firstly chosen. Finally, effect of host density on survival rate was notable, the survival rate in low density was significant higher than crowding density. For instance, the emergence rate of 10 larvae/jar (30.0) was lower than 1 larva/jar (50.0). On the contrary, the eclosion rate of moth was quite different.10 larvae/jar's (48.0) was significant higher than 1 larva/jar's (30.0), indicating immune ability to E. civilisto was significatant difference in different larval. These results firstly indicated that effect of larval density on parasitic behavior and survival rate of E. civilisto.Basing on a systematic research on the effect of larvae raring density to the anti-infection ability to M. anisopliae, resistance ability of the larvae significantly increased in higher raring density.than that in lower raring density. Larvae of 6th instar in 1,10,20 and 30 larvae/jar were used as our measure of host resistance to M.anisopliae, the results show that, the mortality and pupalation rate were remarkble difference:the results of using leaf immersion method show that the mortality of larvae will decrease with the increase of larva density and the difference is obvious. Although the mortality of larvae among density is varing across the concentration of M. anisopliae, armyworm larvae of high concentrations show stronger resistance towards the increase of high concentration M. anisopliae After the treatment, percentage of pupation of surviving larvae also shows the same tendency, namely the percentage of pupation will increase with the increase of density, the research called insect immersion method which is used to research the resistance and the ability of survival of the differenct denisity armyworm has the same results, namely the death rate of larvae that were infected by M. anisopliae decreases with the larval density decreasing. After infection, surviving percentage rate and emergence also decrease with the larva density decreasing. These results show that the disease-resistant ability of larvae will increase with the larval density increasing. Basing on the research of the PO activity dynamics according to larvae development, results showed that the PO activity of cuticular and haemolympho of the 6th instar larvae in higher density was significantly higher than ones in lower density, and the increasing PO activity were believed to be responsible for larvae melanism and higher resistance ability in higher density. The activity of PO of M. separata at different instars was studied and compared. The results showed that PO activity was different at different stages and instars in this insect. PO activity was gradually improved with the larval instars, the enzyme activity of 6th instar was highest and that of 2nd instar was lowest. The activity of PO in haemolympho and cuticle of M/. separata at different stages and instars was studied and compared, the results showed that PO activity of 6th instar haemolympho was significantly highest. PO activity in haemolympho and cuticul according to different density were studied, the enzyme activity of 30 larvae/jar was highest and that of 2 larvae/jar was lowest, PO activity was gradually improved with the larval density. The enzyme activity in cuticle had the same tendency with haemolympho's, the enzyme activity of 30 larvae/jar (499.86) was highest and that of 1 larva/jar was lowest (114.15), PO activity was gradually improved with the larval density and significant difference. These results were provide a scientific basis for density dependent melanism, resistance to parasites and pathogens.