A study on the biological and physiological traits of Bactrocera dorsalis, with special reference to its invasion potential into the Western Cape of South Africa

Thesis (PhDAgric)--Stellenbosch University, 2018.ENGLISH ABSTRACT: Bactrocera dorsalis (Hendel) is of Asian origin and is present in the northern and north-eastern parts of South Africa, but is still absent in other areas of the country including the Western Cape Province. The Western Cape Province is the largest producer of deciduous fruit in South Africa, exporting 41% of the deciduous fruit grown in the province. South Africa earned about R7 billion in export revenue from deciduous fruit exports in 2015. Currently, Ceratitis capitata (Wiedemann) and Ceratitis rosa s.l. Karsch are economically the most important fruit fly species on deciduous fruit in the Western Cape Province of South Africa. However, there is currently a lurking threat of potential introduction of B. dorsalis in Western Cape Province and this is of great concern to the deciduous fruit industry. Bactrocera dorsalis has shown remarkable range expansion over the past 10 years within Africa, adapting to different climatic conditions. Bactrocera dorsalis was also found to be able to out-compete a number of Ceratitis species in Africa. The aim of this study was to determine the invasive potential of B. dorsalis in the Western Cape Province of South Africa. The thermal biology, utilisation of deciduous fruit and the competitive ability of B. dorsalis were studied. A simple morphology based identification tool for B. dorsalis larvae was also developed in order to aid in early detection of the pest. A detailed assessment of acute high and low temperature survival ability of four life stages of B. dorsalis and the plasticity thereof was carried out to test the hypothesis that traits of the thermal niche have contributed to the species‟ invasion ability. The extreme low and high temperatures at which c. 20% of the population of B. dorsalis survived were determined to be -6.5°C and 42.7°C, respectively, when using 2 h exposures. The egg stage was found to be the most resistant life stage to both high and low temperatures with 44 ± 2.3% and 60 ± 4.2% surviving the low and high discriminating temperature treatments respectively. The potential for adult hardening responses to mediate tolerance of extremes was also considered using a diverse range of acute conditions (using 2 h exposures to 15°C, 10°C and 5°C and 30°C, 35°C, 37°C and 39°C as hardening temperatures, and some treatments with and without recovery periods between hardening and discriminating temperature treatment). The results of these studies showed that although some significant hardening responses could be detected in certain treatments (e.g. after exposure to 37°C and 39°C), the magnitude of this plasticity was generally low compared to two other wide-spread and more geographically-range-restricted con-familial species, C. capitata and C. rosa. In other words, B. dorsalis adults were unable to rapidly heat- or cold-harden to the same extent as the other Ceratitis species examined to date. These results suggest a narrower thermal niche in B. dorsalis compared to these Ceratitis species - in both basal and plastic terms - and suggests that its geographic distribution might be more restricted as a consequence. The larval stage of fruit flies is the most commonly intercepted life stage, and identification of this stage using traditional morphological methods such as identification keys is difficult. This study investigated the use of shape analysis, a morphometric method, to identify the third instar larvae of four tephritid species commonly intercepted in fruit destined for export. Larval specimens of laboratory reared B. dorsalis, C. capitata, C. rosa s.s. and Ceratitis cosyra (Walker) were used. The mandibles of third instar larvae of all species were dissected out, dehydrated and mounted in Euparal. Images of the mandibles were captured and analysed using Elliptical Fourier Descriptors (in the SHAPE v.1.3 analysis programme). According to the cumulative eigenvalues, the first two Principal Components (PCs) contributed the most (65%) to the shape change. The first PC separates C. rosa s.s. and C. cosyra from C. capitata and B. dorsalis. Ceratitis capitata and B. dorsalis were separated by the second PC. This study showed that morphometrics, in the form of shape analysis of the mandibles, can be used in combination with measurements of the mandibles to distinguish between third instar larvae of B. dorsalis, C. capitata, C. rosa s.s. and C. cosyra. Nutritional stress and population density are some of the factors that can contribute to morphological changes in insects. This study evaluated the effect of four different fruit crops mainly cultivated in Western Cape Province, South Africa: Prunus persica (L.) Batsch, (Nectarine), Prunus domestica L., (Plum), Malus domestica Borkh., (Apple) and Pyrus communis L., (Pear) on the wing shape of B. dorsalis and C. capitata, the dominant fruit fly pest on deciduous fruit in the region. Geometric morphometric tools were used to compare the relative positions of landmarks on the wings of the flies. The results show significant differences in the shape of wings between males and females of both species, indicating sexual dimorphism. The distances between corresponding landmarks among the averaged wings of B. dorsalis and C. capitata varied highly significantly between individuals that were reared on nectarine, plum, apple and pear. It is as yet unclear how these results translate into fly fitness, but observing significant shape changes resulting from nutritional factors warrant further investigation. The development, reproduction and survival of B. dorsalis and C. capitata on main deciduous fruit types cultivated in the Western Cape were studied. For both species, adult emergence was over 90% on all crops, except for C. capitata on apple, which was at 84%. The ratio of male:female flies was about 50:50 for both species on all the fruit types. Bactrocera dorsalis had a higher net reproductive rate (Ro) on all deciduous fruit tested compared to C. capitata. The value of Ro was the lowest for C. capitata on apple and highest on plum. For B. dorsalis, Ro was lowest on nectarine and highest on pear. Bactrocera dorsalis adults generally lived longer than those of C capitata, irrespective of the fruit types that they developed from. These results indicate that all the fruit types tested were suitable for both B. dorsalis and C. capitata to complete their life cycles. The long period of egg production on apple and the high numbers of eggs deposited on pear makes these fruit types ideal bridging hosts for B. dorsalis to survive until other hosts become available. Interspecific competition regulates the distribution and abundance of a number of phytophagous insects. Ceratitis capitata is currently the dominant species on deciduous fruit in the Western Cape Province of South Africa. Studies were conducted to quantify adult and larval interactions between B. dorsalis and C. capitata on four deciduous fruit types (nectarine, plum, pear and apple). When B. dorsalis and C. capitata adults were evaluated separately, they infested deciduous fruit at more or less the same rates based on the number of pupae reared from the fruit. The only exception was on plum where B. dorsalis produced significantly more pupae and consequently adults from the fruit compared to C. capitata. When adults of the two species were mixed within a confined space, Bactrocera dorsalis was able to out-compete C. capitata in most treatments and crops. Ceratitis capitata was only able to out-compete B. dorsalis on pear. The highest mean numbers of adults of both species emerged from nectarine and plum, with the lowest number emerging from pear. The larvae of C. capitata were more successful in completing development than those of B. dorsalis when present in mixed ratios as larvae of the two species in plum. Ceratitis capitata larvae developed faster than B. dorsalis on all deciduous fruit types tested. Bactrocera dorsalis larvae were able to complete development more successfully in apple than C. capitata in the larval competition experiments. The competition studies between B. dorsalis and C. capitata demonstrated that on deciduous fruit, competition between the two species would be in favour of B. dorsalis at the adult stages (ovipositing females) and, depending on fruit types, in favour of C. capitata at the larval stages. Overall the probability of B. dorsalis invading the Western Cape and displacing C. capitata in deciduous fruit is bigger than the opposite happening. In case B. dorsalis becomes established in the Western Cape, the populations of the pest will probably be reduced to undetectable levels during the winter, with a bloom in the population in summer. As B. dorsalis completes more life cycles in Western Cape, it will probably adapt to the local conditions and become a bigger problem for fruit growers. Fruit like apple and pear are not good hosts for C. capitata, but might be better hosts for B. dorsalis, since B. dorsalis deposited a significantly higher number of eggs on pear, lived longer and produced low numbers of eggs over a long time on apple. This could increase the cost of spray programmes, since fruit types with low incidence of spraying could in the presence of B. dorsalis require more frequent control interventions.AFRIKAANSE OPSOMMING: Bactrocera dorsalis (Hendel) is van Asiese oorsprong, kom in die noordelike en noord-oostelike dele van Suid Afrika voor, maar kom nog nie in die res van Suid Afrika (insluitend die Weskaap) voor nie. Die meeste sagtevrugte word in die Weskaap provinsie van Suid Afrika geproduseer, waarvan 41% van die produksie uitgevoer word. In 2015 het Suid Afrika ongeveer R7 biljoen uit sagtevrugte uitvoere verdien. Ceratitis capitata (Wiedemann) en Ceratitis rosa s.l. Karsch is die twee vrugtevliegspesies met die grootste ekonomiese impak op die sagtevrugtebedryf in Suid Afrika. Die moontlikheid dat B. dorsalis na die Weskaap kan versprei en daar vestig is „n bedreiging vir die sagtevrugtebedryf. Die gebiede waar Bactrocera dorsalis in Afrika voorkom het uitgebrei en vergroot oor die laaste 10 jaar soos die vlieg by verskillende klimaatstoestande aangepas het. Bactrocera dorsalis was ook in staat om populasies van „n aantal Ceratitis spesies te verdring in Afrika. Die doel van die studie was om die potensiaal van B. dorsalis om die Weskaap provinsie van Suid Afirka in te dring en daar te vestig, te bepaal. Die vermoë van B. dorsalis om by verskillende temperature aan te pas, die vermoë van die vlieg om sagtevrugte as gasheer te benut en die vermoë om met ander vliegspesies te kompeteer om vruggashere is bestudeer. „n Gebruikersvriendelike basiese identifikasietegniek vir die identifikasie van B. dorsalis larwes is ook ontwikkel om te help met die vroeë opsporing van die vliegspesie. „n Gedetailleerde evaluering van die vermoë van vier verskillende lewensstadia van B. dorsalis om akute hoë en lae temperature te oorleef is uitgevoer om die hipotese te toets dat sekere eienskappe van die temperatuur nis kon bydra tot die spesie se indringingsvermoë. Die uiterste hoë en lae temperature waarby c. 20% van die B. dorsalis populasie sal oorleef, is bepaal deur die vlieë bloot te stel aan -6.5°C en 42.7°C vir 2 uur. Die eiers was die lewensstadium wat die meeste bestand was teen lae en hoë temperature met 44 ± 2.3% wat die uiterste lae temperature oorleef het en 60 ± 4.2% wat die uiterste hoë temperatuur oorleef het. Die potensiële vermoë van die volwassenes om beskermingsmeganismes teen uiterste temperature to ontwikkel is getoets oor „n reeks temperature (blootstelling vir „n 2 uur tydperk aan 15°C, 10°C en 5°C asook 30°C, 35°C, 37°C en 39°C, met en sonder herstelperiodes). Die resultate toon dat alhoewel B. dorsalis wel „n beskermingsrespons ontwikkel het (byvoorbeeld na blootstelling aan 37°C en 39°C), was die omvang daarvan laer as die van die twee ander wydversreide en meer geografies beperkte C. capitata and C. rosa. Bactrocera dorsalis was nie in staat om „n hitte-of kouebeskermingsrespons te ontwikkel tot dieselfde mate as wat die ander twee spesies dit kon doen nie. Die geografiese verspreiding van B. dorsalis mag as gevolg daarvan meer beperk wees. Vrugtevlieë word meestal in die larwale stadium in vrugte gevind en die identifikasie van larwes deur middel van morfologiese sleutels is moeilik. In hierdie studie is gepoog om die vorm van die mandibels van die derde instar larwes met behulp van morfometriese tegnieke te analiseer en vergelyk. Die mandibels van die vier vrugtevliegspesies wat die meeste in uitvoervrugte gevind word is vergelyk. Die larwes van B. dorsalis, C. capitata, C. rosa s.s. en Ceratitis cosyra wat in die laboratorium geteel is, is gebruik. Die mandibels van die derde instar larwes is deur disseksie verwyder en in Euparal op voorwerpglasies monteer. Fotos is van die mandibels geneem en die fotos is geanaliseer deur die “Elliptical Fourier Descriptors” (in die SHAPE v.1.3 analise program) te gebruik. Na aanleiding van die kumulatiewe “eigen”waardes, dra die eerste twee hoofkomponente [Principal Components (PCs)] die meeste (65%) tot die verskille in vorm van die mandibels by. Die eerste PC skei C. rosa s.s. en C. cosyra van C. capitata en B. dorsalis. Ceratitis capitata en B. dorsalis word deur die tweede PC van mekaar geskei. Hierdie studie het getoon dat morfometrie, in die vorm van die analise van die vorm van die mandibels, in kombinasie met afmetings, gebruik kan word om tussen die derde instar larwes van B. dorsalis, C. capitata, C. rosa s.s. en C. cosyra te onderskei. Voedingsstres en populasiedigtheid is faktore wat kan bydra tot verandering in die morfologie van insekte. Hierdie studie het die effek ondersoek wat vier vrugtegewasse wat in die Weskaap Provinsie van Suid Afrika verbou word [Prunus persica (L.) Batsch, (Nectarine), Prunus domestica L., (Plum), Malus domestica Borkh., (Apple) en Pyrus communis L., (Pear)] op die vlerkvorm van B. dorsalis en C. capitata (die dominante vrutevliegplaag op sagtevrugte in die area) het. Geometries-morfometriese metodes is gebruik om die posisies van gekose punte op die vlerke van vlieë te vergelyk. Die resultate toon dat daar betekenisvolle verskille tussen die vlerke van mannetjies en wyfies voorkom by beide spesies, wat op geslagsdimorfisme dui. Die gemiddelde waardes van die afstande tussen die verskillende gekose punte op die vlerke van B. dorsalis en C. capitata verskil hoogs betekenisvol tussen individue wat hulle lewenssiklus voltooi het op nektarien, pruim, appel en peer. Dit is nog onbekend watter effek hierdie verandering in vlerkvorm as gevolg van die verskille in voedingsfaktore op die lewenskragtigheid van die vlieë het en verdere navorsing is nodig. Die vermoë van B. dorsalis and C. capitata om op die hooftipes sagtevrugte wat in die Weskaap verbou word voort te plant en te oorleef, is ondersoek. In die geval van beide spesies het 90% van die papies in volwassenes ontwikkel, met die uitsondering van net 84% van die C. capitata papies wat op appel tot volwassenes ontwikkel het. Die verhoudig van mannetjies:wyfies was ongeveer 50:50 vir beide spesies op al die vrugtipes. Die netto reproduktiewe koers (Ro) van Bactrocera dorsalis was hoër as die van C. capitata op al die gewasse. Die Ro vir C. capitata was die laagste op appel en die hoogste op pruim. Die Ro vir B. dorsalis was die laagste op nektarien en die hoogste op peer. Vowasse Bactrocera dorsalis vlieë het oor die algemeen langer geleef as C. capitata, ongeag van die vrugtipe waarop hulle ontwikkel het. Hierdie resultate toon dat al die vrugtipes geskik was vir B. dorsalis en C. capitata om hulle lewenssiklus op te voltooi. Die hoë aantal eiers wat op peer waargeneem is en en die langer tydperk van eierproduksie op appel deur B. dorsalis maak hierdie twee vrugsoorte ideale oorbruggingsgashere totdat ander gashere beskikbaar word. Interspesie kompetisie beheer die verspreiding en getalle van verskeie plantvoedende insekte. Ceratitis capitata is op die oomblik die dominante vrugtevliegspesie op sagtevrugte in die Weskaap provinsie van Suid Afrika. Studies is uitgevoer om die interaksie tussen die larwes en volwassenes van B. dorsalis and C. capitata op vier soorte sagtevrugte (nektarien, pruim. peer en appel) te bepaal. In eksperimente waar die interaksie van volwasse B. dorsalis en C. capitata apart geëvalueer is, was die vlakke van besmetting ongeveer dieselfde, gemeet aan die aantal papies wat voorgekom het. Die enigste uitsondering was dat B. dorsalis betekenisvol meer papies en volwassenes as C. capitata op pruim voortgebring het. In eksperimente waar volwassenes van die twee spesies in „n beperkte ruimte geplaas is, was B. dorsalis die sterkste kompeteerder in meeste behandelings en op meeste vrugsoorte. Ceratitis capitata was slegs op peer die sterkste kompeteerder. By beide spesies het die hoogste gemiddelde aantal volwassenes op nektarien en pruim uitgebroei, met die laagste aantal op peer. Die larwes van C. capitata was in staat om meer susesvol te kompeteer met die van B. dorsalis wanneer hulle saam in verskillende verhoudings in pruime geplaas is. Die larwes van C. capitata het vinniger as die van B. dorsalis ontwikkel op al die soorte vrugte wat getoets is. In die eksperimente waar die larwes van die twee spesies teen mekaar gekompeteer het, kon die larwes van B. dorsalis hulle lewenssiklus meer suksesvol op appels voltooi as die van C. capitata. Die kompetisie eksperimente tussen B. dorsalis en C. capitata op sagtevrugte het getoon dat B. dorsalis die beter kompeteerder is in die volwasse stadium terwyl die larwes van C. capitata beter kompeteerders was op meeste van die vrugsoorte. In die geheel gesien is die moontlikheid dat B. dorsalis pes status in die Weskaap sal bereik groter as die kans dat dit nie sal gebeur nie. Wanneer dit gebeur sal die getalle van die populasie waarskynlik onopspoorbaar laag wees in die winter, met „n opbloei in getalle in die somermaande. Soos wat die B. dorsalis populasie meer lewenssiklusse onder die plaaslike toestande voltooi, sal dit waarskynlik by die plaaslike toestande aanpas en „n goter probleem vir produsente word. Kernvrugte soos appel en peer is nie goeie gashere vir C. capitata nie, maar mag beter gashere vir B. dorsalis wees aangesien B. dorsalis betekenisvol hoër getalle eiers op peer gelê het en oor „n lang tyd lae getalle eiers op appel gelê het. Dit kan die koste van spuitprogramme verhoog, aangesien vrugtipes wat vantevore minder bespuitings nodig gehad het, nou meer dikwels gespuit sal moet word