Identification and evaluation of Trichogramma parasitoids for biological pest control

I.M.M.S. e Silva

Research output: Thesisinternal PhD, WU

Abstract

<p>Egg parasitoids of the genus <em>Trichogramma</em> are used as biological control agents against lepidopterous pests. From the 180 species described world-wide, only 5 have large scale application. The development of better methods to select other <em>Trichogramma</em> species/strains is necessary for a more effective use of these wasps against target pests. The main aim of this thesis is to investigate criteria and methods for identification and selection of <em>Trichogramma</em> species/strains for biological control. In <strong>chapter 1,</strong> I give a short review on biology, research and use of <em>Trichogramma</em> , state the research aims and present the outline of the thesis.</p><p>One of the first problems researchers are confronted with when working on <em>Trichogramma</em> is their identity. Along with the classical methods, simple and practical methodologies of species identification are needed. The use of molecular methods to characterise and identify five Portuguese <em>Trichogramma</em> species is described in <strong>chapter 2</strong> . These methods are: 1) electrophoresis of PCR amplified DNA internal transcribed spacer 2 (ITS-2), followed by digestions by means of restriction enzymes and 2) esterase electrophoresis. Five <em>Trichogramma</em> species occurring in Portugal, <em>T. bourarachae, T. cordubensis, T. evanescens, T. pintoi</em> and <em>T. turkestanica</em> can be easily distinguished from each other by means of dichotomous keys that were constructed based on the two methods.</p><p>The two methods may be used together with other identification methods for field specimens or be used on its own for detection of possible laboratory contaminations. The specificity of mating attempts was investigated as an additional tool to quickly distinguish between <em>Trichogramma</em> species ( <strong>chapter 3</strong> ). Although mating attempts occurred more often between conspecifics, they were not always species-specific. Two compounds that might be involved in mate-specific recognition in <em>Trichogramma</em> were characterised. These two compounds are produced by virgin females and were not found in mixed groups of males and presumably mated females or in groups of males only. We concluded that the specificity of mating attempts cannot be used as a general taxonomic tool for <em>Trichogramma</em> .</p><p>From the point of view of biological control only the females are useful, because they kill the host eggs. We know of several <em>Trichogramma</em> species in which virgin females produce female offspring (thelytoky). In most of these cases thelytoky is related to the presence of <em>Wolbachia</em> , an endosymbiotic bacterium. In some cases all female wasps in a population are infected with the bacteria (fixed population), while in other cases infected and uninfected females coexist (mixed population). The hypothesis that in fixed populations the effect of the bacteria on <em>Trichogramma</em> female fitness is less negative than in mixed populations was tested in <strong>chapter 4</strong> . In these experiments two fixed <em>Trichogramma</em> lines of <em>T. cordubensis</em> and <em>T. oleae</em> and four mixed lines, one of <em>T. deion</em> and three of <em>T. kaykai</em> , were tested for several female fitness parameters.</p><p>Each of these original thelytokous lines was compared with a laboratory-generated sexual (=arrhenotokous) line, which had been produced by killing the <em>Wolbachia</em> bacteria using antibiotics. Daughter production was higher for the thelytokous fixed lines than for their sexual counterparts, while the opposite was found for three of the four mixed lines studied. As expected, the negative impact of <em>Wolbachia</em> on total offspring numbers was much more severe for the mixed lines than for the fixed ones.</p><p>In addition to the infection status of the wasp populations, fixed <em>versus</em> mixed, there are other factors that can influence the reproductive success of the wasps. One of them is the females' ability to search for eggs to parasitize. The dispersal of two thelytokous lines of <em>T. cordubensis</em> was assessed in a laboratory chamber and in a greenhouse ( <strong>chapter 5</strong> ). This chamber was especially designed and tested as a tool for <em>Trichogramma</em> screening. The <em>Trichogramma</em> line which dispersed more in the chamber also dispersed more in the greenhouse, suggesting that the chamber might be a useful tool for a pre-evaluation of <em>Trichogramma</em> strains. Next, the biological control potential of <em>Wolbachia</em> -infected and of uninfected <em>Trichogramma</em> was investigated ( <strong>chapter 6</strong> ).</p><p>Fecundity and dispersal of <em>T. cordubensis</em> and <em>T. deion</em> were measured in the laboratory and in the greenhouse. Sexual lines had a higher fecundity than their thelytokous counterparts. The arrhenotokous lines dispersed more in the laboratory chamber but the opposite effect or no difference was found in the greenhouse. Calculations show that by releasing hundred adult wasps of both lines, thus including arrhenotokous males in the sexual line, more eggs are parasitised by the thelytokous wasps. Therefore, in spite of their lower individual female fecundity, thelytokous lines have a better potential for biological control than their sexual counterparts.</p><p><em>Trichogramma</em> females do not only parasitise lepidopteran eggs but also parasitise eggs of some beneficial insects. To determine which <em>Trichogramma</em> species is the least detrimental to the beneficial insect <em>Chrysoperla carnea</em> , the degree of successful parasitisation of <em>Helicoverpa armigera</em> (a pest insect) and <em>Chrysoperla carnea</em> , by the five Portuguese <em>Trichogramma</em> species was studied in the laboratory ( <strong>chapter 7</strong> ). The fraction <em>H. armigera/C. carnea</em> eggs parasitised differed between <em>Trichogramma</em> species. The average number of parasitised eggs per female in 24 h by <em>T. pintoi</em> and <em>T. bourarachae</em> was 10 of <em>H. armigera</em> and about 0.5 of <em>C. carnea</em> . For the other three <em>Trichogramma</em> species ( <em>T. cordubensis</em> , <em>T. evanescens</em> and <em>T. turkestanica</em> ) these averages varied from 6 to 11 <em>H. armigera</em> eggs and from 3 to 4 <em>C. carnea</em> eggs.</p><p>Total adult offspring production, contacts with hosts, secondary clutch size and sex ratio of each <em>Trichogramma</em> line were determined as well. The results obtained show that sympatric <em>Trichogramma</em> may parasitise target and non-target species in different proportions. If this difference corresponds to the field situation, simple laboratory tests could be performed to select not only effective biological control agents, but also species that are the least detrimental to non-target hosts. <em>Trichogramma bourarachae</em> and <em>T. pintoi</em> are the most promising candidates for the control of <em>H. armigera</em> in agroecosystems where Chrysopidae form an important part of the natural enemy assemblage. In <strong>chapter 8</strong> , the most important research findings are discussed in a broader context.</p>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • van Lenteren, Joop, Promotor
  • Stouthamer, R., Promotor
Award date17 Sep 1999
Place of PublicationS.l.
Publisher
Print ISBNs9789058080745
Publication statusPublished - 1999

Keywords

  • trichogramma
  • parasitoids
  • thelytoky
  • identification
  • biological control
  • insect pests
  • lepidoptera

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