The Effects Of Two Aphelinid Parasitoids At Low Bemisia Tabaci MEAM1 Densities And Optimizing Release Criteria In Greenhouse-Grown Tomatoes

Bemisia tabaci?Gennadius??Hemiptera,Aleyrodidae?Middle East Asia Minor 1?MEAM1?is a noxious invasive pest in China and has consistently been causing extensive losses to valuable crops.Biological control is a potential management option and can be effective solution of the problems caused by this devastating pest.Among the Aphelinid parasitoids,Eretmocerus hayati?Zolnerowich and Rose??Hymenoptera:Aphelinidae?which is a solitary parasitoid and Encarsia sophia?Girault and Dodd??Hymenoptera:Aphelinidae?which is a heteronomous hyperparasitoid,have proven their worth to control B.tabaci MEAM1 and are often regarded as potential biological control agents.However,for the effective suppression of B.tabaci MEAM1 by utilizing these parasitoids,the understanding of the interactions among parasitoids and hosts under different circumstances is of vital importance.The main objective of this thesis was to evaluate the performance of En.sophia and Er.hayation low B.tabaci MEAM1 densities and to investigate the effects of various release criteria?varying release ratios and release frequencies?on biological control of B.tabaci MEAM1 on greenhouse grown tomato as well as to investigate the effects of different host densities and exposure durations on behavioral preference of Er.hayati in laboratory.The evaluations of the En.sophia and Er.hayati on varying low B.tabaci MEAM1 densities on tomato Solanum lycopersicum L.?Solanaceae?were conducted during 2016 in greenhouses situated at Langfang Experimental Station,Hebei province,China.The decrease in the B.tabaci MEAM1 nymphal densities on plants significantly decreased the parasitism,host feeding and total mortality by both parasitoid species.The females of both parasitoid species preferred to oviposit in than host fed upon the whitefly nymphs at studied lowest host density level.Er.hayati performed better by parasitizing significantly more nymphs and causing significantly higher nymphal mortality at all the low host densities,and by host feeding upon the similar number of hosts as En.sophia at studied lower host densities.Then the evaluations were extended with the parasitoid Er.hayati performing better at low densities,to investigate the effects of varying adults and nymphal densities as well as infestation durations of B.tabaci MEAM1 adults on the behavioral response of Er.hayati in laboratory settings during 2017-2018.The infestation of 60 adults per tomato plant of B.tabaci MEAM1?1:1 sex ratio?infesting for 24 hours were found to be required by the parasitoid to alter their behavioral preference and parasitoids were significantly attracted to this density as well as all studied higher densities when offered with clean tomato plant as choice.Increase in the infestation duration of B.tabaci MEAM1 adults significantly increased parasitoids attraction to plants and Er.hayati selected the plants infested for higher durations than offered plants infested by whiteflies for relatively lower durations.Er.hayati was also more attracted to the plants with higher nymphal densities of B.tabaci MEAM1,however the increase in density from 0 to 30 nymphs per plant didn't significantly attract Er.hayati.With the aim to establish the better release criteria?release ratios and release frequencies?of heteronomous parasitoid En.sophia for the control of B.tabaci MEAM1,the study was conducted in greenhouses of Langfang Experimental Station,Hebei province,China during 2016 and 2017.During 2016,the efficacy of three release ratios?parasitoids:hosts=1/5,1/10 and 1/15?were tested on whiteflies infested tomato plants in greenhouse cages.En.sophia released at 1/10 release ratio had the significantly high parasitism compared to other two release treatments.However,the whiteflies densities(3^rd+4^th instar nymphs)did not have significant differences among the three release ratio treatments.Therefore in 2017,the efficacy of parasitoid En.sophia released at constant optimum release ratio?1/10,obtained by 2016 study?but with varying release frequencies on weekly intervals was investigated during early spring-summer conditions on greenhouse grown tomato.The En.sophia had a significantly higher parasitism on B.tabaci and better suppressed the host density effectively when released at weekly release frequency of 3?released repeatedly for 3 consecutive weeks?than other 1 and 2 release frequencies.Whitefly densities were as much as 3.5 times lower in optimum release frequency cages than control treatment?no parasitoids release?by the peak week.In conclusions,the current study suggested that when the average host densities are10 nymphs/leaf or below of B.tabaci MEAM1,to enhance whiteflies suppression,alternative strategies might be required e.g.continuous low number release of parasitoids by banker plants to retain the parasitoids by non-host food.Moreover,Er.hayati also utilizes the whitefly induced plant volatiles and attraction of parasitoid to infested plants increases with increase in B.tabaci MEAM1 densities and infestation duration.En.sophia released at1/10 ratio?parasitoid:host?for 3 consecutive weeks could achieve efficient suppression of B.tabaci MEAM1.