Lethal Interference Competition Between Encarsia Sophia And Eretmocerus Hayati (Hymenoptera: Aphelinidae)

Tobacco whitefly, Bemisia tabaci (Gennadius) (Hemiptera:Aleyrodidae), have many biotypes. After the introduction of B. tabaci B-biotype into China in 1990s, it has spread rapidly and broken out in several provinces in south China and caused severe damage to most of the economic important crops. Conversional chemical control methods are usually less effective due to the special characteristics (ex. tiny body, overlapping generations, high reproductive efficiency and hidden habit) of B. tabaci. However, control of B. tabaci in crop fields mainly relies on the pesticide nowadays and whiteflies have appeared resistant to many pesticides employed in their control due to excessively and continuously using pesticieds. In addition, pesticide does much harmful to the environment. So many contries lay more emphasis on integrate pest manegament (IPM). Moreover, biological control is very important in IPM. In this study, we introduced two parasitoids of B. tabaci biotye B, Encarsia sophia and Eretmocerus hayati from Texas in America. We studied host discrimination to the parasitized host by the two parasitoids in laboratory by means of reproductive biology and beheavious biology. Also, we investigated the two mechanisms of lethal interference competition-multiparasitism and host feeding in choice and non-choice condition. In addition, field cages experiment, using cotton as host plants, was conducted to determine whether the interaction of two species will affect the biological control of whitefly and how the interaction have effect on parasitoid population dynamics. The results and conclusions were as follows:1. Host discrimination of En. sophia and Er. hayati to parasitized hostWe studied discrimination ability on parasitized host by heterogenic and conspecific parasitoid and itself. The results indicated that both En. sophia and Er. hayati could accept parasitized host to lay eggs on it. In addition, destructive host feeding is one of the beheavior strategies for parasitoids to treat the parasitized host. However, whether experienced or not, oviposition seems not affect parasitoids'host discrimination abilities except En. sophia's conspecific discrimination.2. Mechanisms of lethal interference competition between En. sophia and Er. hayatiMechanisms of lethal interference competion were studied between En. sophia and Er. hayati. In addation, how the interspecies interaction influences conspecific and heterogenic population dynamics under choice (surplus heath hosts) and non-chocie (no surplus heath hosts) conditions was also examined in labortary after having a good understanding of host discrimination of parasitized host. The results indicated that in non-choice condition:En. sophia appeared to ruduce Er. hayati's progeny production via multiparasitism and host feeding. While Er. hayati reduced En. sophia progeny production only by multiparasitism. Second-introduced females would win the competition. More over, En. sophia reduced conspecific and self progeny production via host feeding. While Er. hayati reduced conspecific progeny production by multiparasitism and host feeding. There is no interference on itself progeny production. In choice condition:mechanisms of En. sophia's interference on Er. hayati is host feeding. While multiparasitism appeared to be the mechanism by which Er. hayati interfer En. sophia. Er. hayati won the competition consistently independent of the order of introduction. Both En. sophia and Er. hayati have no effect on the conspecific and self progeny production. We assumed that both parasitoids had the potential to directly affect the other through multiparasitism. En. sophia interfered intraspecifically as well as interspecifically by host feeding on parasitized hosts. Host density could affect the outcome of the inter-and intra-competion.3. Evaluation of field cage experiments with En. sophia and Er. HayatiCotton field cages experiments were conducted to determine the effectiveness of the two parasitoids on whitefly control in alone and combination treatment. The improvement of parasitoids'population dynamics and whether the inter-action between two species will affect host supression or not are considered. Our results indicated that both Er. hayati and En. sophia were effective species in controlling B. tabaci population. Er. hayati population had a rapid growth rate while En.sophia population is relative slower at the beginning stage after release. However, population of both species became stable at the last period of sampling dates. The percent parasitism of two parasitoids varied with sampling dates. Er. hayati's percent parasitism peaked at 29.8% while En.sophia's peaked at 28.2%. Er.hayati reduced the density of whitefly 67.5% and En.sophia reduced the density of whitefly 49.9% successfully at the end of sampling date compared to no parasitoids treatment. At the same time, the combination of two parasitoids provided a significant level of whitefly suppression. There was a significant decline (81.4%) of whitefly density in the combination treatment and the total percent parasitism of two parasitoids peaked at 49.9%. Also, two parasitoids population dynamic in the combination treatment was similar to Er.hayati population dynamic in the alone treatment. The existence of competitor could promote the population of En. sophia but suppressed the population of Er. hayati. These results demonstrated competitive interactions among the two parasitoids of B. tabaci under field manipulations but indicate that, competitive interaction among parasitoids do not affect host population suppression but achieve higher level of biological control.4. ConclusionIt was presummed that both En. sophia and Er. hayati could accept parasitized host to lay eggs on it and there exist competion between the two species. Multiparasitism and host feeding are two mechanisms of lethal interference competition between En. sophia and Er. hayati under choice and non-choice condition. Both En. sophia and Er. hayati were effective species against B. tabaci under field condition. The existence of competitor could promote the population of En. sophia but suppressed the population of Er. hayati. But the combination treatment showed the most effective suppression. That is competitive interaction among parasitoids do not affect host population suppression but achieve higher level of biological control.