The Bionomics Of Zele Chlorophthalmusand Its Bio-control Potentials To The Beet Webworm,Loxostege Sticticalis

Zele chlorophthalmus(Hymenoptera: Braconidae)is a widespread koinobiont parasitoid that have potential in controlling the beet webworm Loxostege sticticalis L.(Lepidoptera: Crambidae)larvae.The bionomics of this parasitoid and its potential use in the management of crop pests,however,has been less understood.Consequently,this has hampered the conservation of this species in field and the development of mass production technologies of this parasitoid.To further understand the biology of Z.chlorophthalmus,we investigated the characters of Z.chlorophthalmus about the morphology,eclosion,mating,oviposition period of and the relationship between supplement of nutrition and adult longevity;Three species of moth larvae were exposed to Z.chlorophthalmus to investigate the host preference,oviposition site preference on the host body,the fitness of the parasitoid offspring in different instars of the host larvae,and the effects of parasitization on the growth and amount of food consumed by the host larvae;The variations parasitic rate,life history parameters of Z.chlorophthalmus under different temperature regimes,including the parasitic rate,developmental duration,pupal weight,pupation rate and adult emergence rate developmental threshold temperatures and effective accumulated temperature,and number of generation occurred annually of Z.chlorophthalmus in the major distributed areas of L.sticticalis.The main results obtained as follows.1.The morphology and biological characteristics of Z.chlorophthalmusThe female and male wasps were similar in body length ranged from 5.8 to 8.4 mm.The adult wasps were usually emerged during daytime,between 06:00 to 14:00 h,and the male wasps were emerged 2-3 days earlier than the females.The sex ratio(female/male)was about 0.73:1.The female wasps mated with male wasps soon after emergence.The mating duration was short and generally lasted between 75-120 s.At 21℃,the longevity of wasps was significantly influenced by the types of nutrition supplied where wasps fed with 10% glucose solution had significantly longer life than those fed with 10% honey solution or water.The longevity of the virgin male and female(mean = 47.6 days)were significantly longer than those mated and oviposited(mean = 16.7 days).2.The parasitic behavior,relationship between the parasitoid wasp offspring fitness and host instars and changes in body size and food consumed by host larvaeFirstly,our result showed that Z.chlorophthalmus larvae were able to complete their development in the L.sticticalis larvae body.However,the parasitoid larvae were failed to develop in 3rd instar larvae of Mythimna separata and Spodoptera exigua.This suggested that the 3rd instar of M.separata and S.exigua may not the suitable host for Z.chlorophthalmus.Secondly,Z.chlorophthalmus females laid egg into the head,thorax and abdomen of the L.sticticalis larvae,but preferred the head and thorax.Thirdly,the parasitoid wasps preferred the 3rd and 4th instar larvae as host compared to 2nd and 5th instar of L.sticticalis.The parasitism rates of the 3rd and 4th instar of L.sticticalis were 67.3% and 61.9%,respectively.The developmental duration of the parasitoid larvae in the 3rd and 4th instar of L.sticticalis were significantly shorter(P < 0.05),and the pupation and emergence rates were significantly higher(P < 0.05)compared to larvae developed in the 2nd and 5th instar.These results implied that the 3rd instar larva of L.sticticalis is the most suitable host for the parasitoid wasp offspring and the selection of the 3rd instar L.sticticalis by adult Z.chlorophthalmus is concomitant with the greatest fitness of their offspring.The head width,body length and weight of the parasitized host larvae were significantly smaller(P < 0.05)than the un-parasitized larvae.The food amount consumed by the parasitized host larvae was about 30% less than that consumed by the un-parasitized larvae,and the differences was significant(P < 0.05).These results suggest that Z.chlorophthalmus can be a bio-control agent for the integrate management of L.sticticalis.3.The parasitism rate and the offspring fitness at different temperatures,developmental threshold,and generation number of Z.chlorophthalmus occurred in the fieldThe effects of temperature on the parasitism rate and fitness of Z.chlorophthalmus were investigated under 5 different temperatures(17,21,25,29 and 33℃)by using the 3rd instar L.sticticalis as host.The parasitism rate at 21 and 25℃(67.3% and 61.9%)were significantly higher than that at 17 and 29℃(22.50% and 44.67%).The parasitoid offspring were able to completed their development at temperature between 17-29℃.No parasitoid larvae develop in the host at 33℃,suggested the temperature maybe too higher for reproduction of Z.chlorophthalmus.The pupation rate and adult emergence rate of Z.chlorophthalmus at 21 and 25℃(> 70%)were significant higher than that at 17 and 29℃(P < 0.05),whereas the pupa weight of Z.Chlorophthalmus decreased with increasing temperature(19.69 mg to 14.31 mg).The longevity of the adult Z.Chlorophthalmus also decreased with increasing temperatures.For example,the longevity of the adults was 25.7 days at 17℃ but only 9.1 days when growth at 29℃.All these data indicated that 21-25℃ are the optimum temperature for the fitness of Z.Chlorophthalmus.The developmental duration for egg and larvae,pre-pupae and pupae stages,and whole generation decreased with increasing temperature within 17-25℃(50.8 days to 22.6 days),but prolonged at 29℃(23.50 days).Therefore,the relationship between temperature and developmental rate of Z.chlorophthalmus was better fitted in the Logistic model(coefficients of determinant,R2 were both greater than 0.935).Based on the calculation of developmental data obtained at different temperatures,the developmental threshold temperature and the effective accumulated temperature for the egg and larvae,pre-pupae and pupae stage,and whole generation of Z.Chlorophthalmus was 8.7,9.9 and 10.8℃ and 176.0,189.6,and 331.8 day·degrees,respectively.It was estimated that Z.Chlorophthalmus complete 2-5 generations annually in the major area of L.sticticalis as predicted by application of the developmental threshold and calculated annual thermal constant.