Mutual Manipulation In Development By Plutella Xylostella Linnaeus And Diadegma Semiclausum Hellen

Diadegma semiclausum Hellen (Hymenoptera: Ichneumonidae ) is one of major larval parasitoids of the diamondback moth (DBM), Plutella xylostella Linnaeus, an important insect pest of crucifer vegetable crops. The parasitoid is widely distributed in the north of China. The larval parasitism of DBM by this parasitoid in the field usually ranges from 40 % to 70 %, with the highest parasitism rates of more than 90%, indicating high practical potential of this parasitoid to be integrated in the management of DBM. As no detailed investigations of this parasitoid has been conducted, I carried out detail, quantitative studies on the biological and ecological characteristics of this parasitoid, with a population collected from Yunnan, China. The objective is to provide the necessary information for the evaluation and augmentation of this parasitoid, and for the study of the mechanisms responsible for community change of parasitoids associated with DBM in the field. The major results are sumarized as follows.1. The capacity of the parasitoid to parasitize the DBM larvae and host age preferenceMost attacks upon DBM larva by Diadegma semiclausum resulted in successfully parasitism, but few attacks did not. The parasitism rate by the parasitoid of the 2nd, 3rd and 4th instar DBM larvae were 87.61 %, 84.75 % and 65.52 %, respectively, when the host larvae in different instars were exposed separately to the parasitoid to be attacked one time. Although newly emerged adults could parasitize the 3rd instar DBM larvae with a parasitism rate of 61.67 %, their parasitism capacity to the DBM larvae in this instar increased with the increase of wasp age from 1 -day old to 5-days old, then decreased with the increase of wasp age from 7-days old on, when the host larvae in different instars were exposed separately to one female wasp for 6 hours. The parasitism rate of the DBM larvae and the amount of eggs laid at the 5th day by the wasp were 79.88 and 18.39. Diadegma semiclausum parasitized all four instars of DBM larvae, but preferred to parasitize 2nd and 3rd instars when it was exposed to the 2nd, 3rd and 4th instar DBM larvae concurrently. The preference indexes to the 2nd, 3rd and 4th instar were 0.37, 0.44 and 0.19, respectively. The capacity of the parasitoid to parasitize DBM larvae in the 4th instar decreased with the increase of developmental age of the host.2, Efffect of parasitism on the development and food consumption of the host larvaThe changes on development and food consumption of the parasitized larvae were investigated at 24 ℃. Compared with unparasitized host larvae, the parasitized larvae prolonged significantly their developmental time of both larval and prepupal stages, except the developmental time of the 2nd instar when initial parasitization occurred in the 2nd instar. When the host larvae were parasitized initially at the 4th instar, the duration of the parasitized host prepupa was almost six times more than that of unparasitized ones. When the host larvae were parasitized initially at the 2nd instar, the total food consumption of parasitized larvae were less than that of the unparasitized ones. However, there were no significant difference in the total food consumption between larvae parasitized initially at the 3rd or 4th instar and larvae unparasitized. DBM larvae, which had been exposed to parasitoid, had higher mortality than those that had not been exposed to parasitoid. The reason was discussed in the paper.3. The effect of host age on biological characteristics of Diadegma semiclausumThe effects of host larval instars of Plutella xylostella at initial parasitization on developmental time, rate of cocoon formation, and parasitism capacity of the resultant female wasp of Diadegma semiclausum, were observed at 24 ℃. The results showed that the developmental times both from egg to cocoom formation and from egg to adult emergence of parasitoid, which parasitized initially at the 4th instar larva, were significantly shorter than those initially parasitizing in the 2nd and 3rd instar larvae. The developmental time of the wasp from cocoon formation to adult emergence increased with the increase of stadium at parasitism, but no significant difference had been found. The number of total eggs laid by the parasitoid which parasitized initially in the 3rd instar larva was more than that by those parasitoid which parasitized initially in the 2nd and 4th instars larvae, although the parasitism capacity of the resultant female wasps at first day post-emergence was similar. However, host age at parasitization had no effect on the rate of adult emergence, cocoon formation. The final adult size of the parasitoids varied with the host instars at initial parasitization. The parasitoids starting their initial parasitization in the 4th instar host had the largest body size, follow by the parasitoids in the 3rd instar, and then the parasitoids in the 2nd instar. When no food was provided, the male wasp adults resultant from 2 or 3 instar...