| The tobacco whitefly, Bemisia tabaci (Gennadius), is a serious pest in worldwide. The population of B. tabaci abruptly increased in South China in 1996, and outbreaks in North China since 2000. As an important natural enemy of whiteflies, Encarsia formosa Gahan (Hymenoptera: Aphelinidae), is widely and successfully used for biological control in greenhouse. The majority of the experiment was accomplished in the Institute of Biological Control, Chinese Academy of Agricultural Sciences, and the Beijing Academy of Agriculture and Forestry Sciences. Various populations of E. formosa differ in their performances on different whitefly species. The development and survivorship of the American and British strains of E. formosa on nymphs of B. tabaci and different host plants, and under different temperatures were studied. The effect on adult emergence of storing the pupae of E. formosa at various temperatures was investigated.1. The American and British strains of E. formosa could parasitize all four nymph instars of B. tabaci. However, the percentages of parasitized were greatest on the 3rd instar and least on the 1st instar. The developing times of the two wasp strains from egg to adult were longest when the 1st instar nymph was parasitized and shortest when the 3rd and 4th instar (the British strain) or 4th instar nymphs were parasitized (the American strain). There were significant differences in parasitization rate and development time between the two strains when the 4th instar nymph was parasitized, but no differences in adult emergence. The longevity of the unfed female progeny that emerged from the 4th instar hosts was much longer than those from the 1st instar. When the 1st, 2nd or 3rd instar nymphs were parasitized, the host could feed and develop. However, when the 4th instar was parasitized, the development seemed to be inhibited.2. On five different host plants, tomato, cucumber, cabbage, eggplant and cotton, the total development period of the American and British strains was 16~18 and 17~20 days, respectively. The development times of the two wasp strains on different host plants were as follows, cabbage > cucumber > eggplant > tomato > cotton. There were clearly differences on tomato, cucumber, cabbage and eggplant, when the development times of the two strains were compared. However, there were no differences when developed on cotton. The percentages of parasitized were greatest on tomato, and least on cucumber. And the percentages of parasitized on different host plants were as follows, tomato > cotton > cabbage > eggplant > cucumber. There were no significant differences in percentage of parasitized on these 5 host plants when the two wasp strains were compared.3. At 18~33 , the total developmental periods of E. formosa from egg to adult emerging were decreased with the increasing of the temperature. At 18~23 , there were significant differences between the two wasp strains when the development time from egg to adult emerging and adult longevity were compared, however, no prominent differences at 28-33 . In the case of the American strain, the development was faster and the adult longevity was longer at lower temperature. The temperatures of development zero of immature stage, survival of adult and whole generation of the American strain were 13.1 , 12.3 , 12.8 , and the effectively accumulated temperatures were 177.23, 179.11 and 353.53 day-degrees, respectively. The temperatures of development zero of immature stage, survival of adult and whole generation of the British strain were 13.2 , 11.4 , 12.4 , and the effectively accumulated temperatures were 188.25,168.58 and 358.28 day-degrees, respectively. Thus the American strain could endure lower temperature conditions than British strain.4. The emergence rate of the wasps was positively correlated with the storage temperature, and negatively correlated with the length of the storage duration. At 5 , the percentage of the wasp emerging was 34% after storaged for 10 days, much lower than that of the control. At 10 , the percentag... |
