Effect of Three Bean Species on the Development and Reproduction of a Population of the Parasltold,Encarsia bimaculata,on the Whitefly,Bemisia tabaci

Abstract

Developmental time, parasitism, emergence, longevity, fecundity and demographic parameters of population of Encarsia bimaculata Heraty and Polaszek (Hymenoptera: Aphelinidae), a parasitoid attacking Bemisia tabaci (biotype B) (Gennadius) (Homoptera: Aleyrodidae) infesting soybean, Glyine max L. (Merr), cowpea, Vigna unguiulata L. and garden bean, Phaseolus vulgaris L. (Fabeles: Fabaceae) were quantified and compared. Encarsia bimaculata was able to complete its life cycle independent of the B. tabaci instar parasitized. However, parasitoid development was significantly slower when first (19 d), second (15 d) instars or pharate adults (14 d) were parasitized compared to the third (13 d) or fourth (13 d) instars. Consequently, percent parasitism was higher when the third (51%) or fourth (46%) instars were parasitized compared to the first (22%), second (25%) instars or pharate adults (36%) of B. tabaci. Similarly, percent parasitoid emergence was significantly higher when third (83%) or fourth (76%) instars were parasitized compared to when the first (34%), second (64%) or pharate adults (54%) were parasitized. Host plant species significantly influenced egg to adult developmental time, percent parasitism and the day on which E. bimaculata nymphs hatching from eggs was first observed. More nymphs were parasitized on cowpea (40%) followed by garden bean (36%) and soybean (32%), while percent hatching was significantly higher on soybean (76%) followed by cowpea (68%) and garden bean (42%). Adult parasitoid females lived an average of 6.7 d on soybean, 7.6 d on cowpea and 7.2 d on garden bean and laid a lifetime average of 27 eggs on soybean, 31 eggs on cowpea and 30 eggs on garden bean. The daily mean fecundity of E. bimaculata was not significantly different on the three bean species. Life table parameters showed that the net reproductive rate (R(o)) was 14.50, generation time (T(c)) was 17.16, intrinsic rate of natural increase (r(m)) was 0.16, finite rate of growth (lambda) was 1.17 and doubling time (T(d)) was 4.44 for parasitoids on soybean. On cowpea, R(o) was 15.32, T(c) was 18.59, r(m) was 0.15, lambda was 1.16 and T(d) was 4.72, while, on garden bean, R(o) was 8.95, T(c) was 19.28, r(m) was 0.11, lambda was 1.12 and T(d) was 6.08. Given these life table parameters, higher population build up of the parasitoid will be expected on cowpea and soybean, respectively, compared to garden bean. Thus, for an effective augmentative release program involving E. bimaculata for the control of B. tabaci, it is important to take into consideration both the host stage of B. tabaci and the nature of the host plant on which it is developing.