8 The Use of Unisexual Wasps in Biological Control
R. Stouthamer*
Laboratory of Entomology, Wageningen University, PO Box 8031, 6700 EH Wageningen, The Netherlands
Introduction
Biological control workers have long been fascinated by the phenomenon of unisexual reproduction. Timberlake and Clausen (1924) explained the possible advantage of a uni- sexual parasitoid over a form reproducing sexually by calculating the population increase of the sexual form compared with the unisexual form. The difference in popula- tion growth rate in such calculations can be astonishing (Fig. 8.1). In such calculations we assume that the unisexual wasps produce equal numbers of offspring to the sexual
forms and therefore over time the unisexual population should outcompete the sexual form since all the offspring of the unisexual form consist of females only.
The concept of choosing a unisexual mode of reproduction for the wasps to be used in biological control is interesting; however, up until now, this can only be applied in those cases where two modes of reproduction are present in a species. In the not too distant future, it may be possible to render sexual forms unisexual by infection with parthe- nogenesis-inducing (PI) microorganisms.
Natural infection with these bacteria is the
*Present address: Department of Entomology, University of California at Riverside, Riverside, CA 92521, USA.
© CAB International 2003. Quality Control and Production of Biological Control Agents:
Theory and Testing Procedures (ed. J.C. van Lenteren) 93
Abstract
Unisexual reproduction has long been seen as a clear advantage for wasps to be applied in biological control projects. The discovery that the mode of reproduction in parasitoid wasps may be manipulated from sexual to unisexual and vice versa will allow biocontrol workers to test the advantage of either mode of reproduction for biological pest control. Here a review is presented of the cases of unisexual reproduction found in parasitoid wasps. Unisexual reproduction is not rare among parasitoids; at least 150 cases of unisexual reproduction have been reported. The literature is reviewed for cases where both unisexual and sexual forms are used in the same control project to determine if the theoretical advantage of unisexual reproduction indeed materializes. Few cases can be used to test the presumed advantage of unisexuals. Some evidence is found for two advantages of unisexual reproduction: uni- sexuals are cheaper to produce in mass rearing than sexuals, and in classical biocontrol projects they are more easily established.
cause of unisexual reproduction in many Hymenoptera (Stouthamer, 1997), and initial experiments have shown that in some cases inter- and intraspecific transfers of these bacteria are possible (Chapter 9; Grenier et al., 1998; Huigens et al., 2000).
Two papers published in the early 1990s discussed the use of sexual versus unisexual lines in biocontrol. The first paper, by Aeschlimann (1990), suggested initially releasing unisexual forms, because they may be easier to establish. Subsequently sexual forms could be released to introduce genetic variation in the population. The generality of that idea was questioned by Stouthamer (1993), who argued that the sequence in which these two forms should be released depends on: (i) the type of biological control the release is intended for; and (ii) the den- sity of the hosts that are to be controlled.
In the following sections, I shall give an overview of the knowledge that we have gained about unisexual reproduction over the last 10 years and discuss work done specifically to test the merits of using either a sexual form or a unisexual form for biologi- cal control.
Causes of Unisexual Reproduction Two classes of causes are known for unisex- ual reproduction in Hymenoptera: (i) micro- bial infection; and (ii) other genetic mechanisms that allow unfertilized eggs to develop into females.
Over the last 15 years many species have been discovered that are infected with PI Wolbachia (Stouthamer et al., 1990b, 1993;
Stouthamer, 1997). These bacteria allow infected females to produce daughters from both fertilized and unfertilized eggs. In many species where PI-Wolbachiainfection is known, the infection has gone to fixation and all individuals in the ‘fixed’ population are infected females (Stouthamer, 1997). An example is the biocontrol icon Encarsia for- mosa(Zchori-Fein et al., 1992; van Meer et al., 1995). In a number of other species the infec- tion with PI Wolbachia is restricted to a smaller part of the population and both infected and uninfected individuals co-occur and gene flow still takes place between these two subpopulations (‘mixed populations’) (Stouthamer and Kazmer, 1994). Only wasps in genus Trichogramma populations are 94 R. Stouthamer
Generations
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Relative number of unisexual females in population 0 100 200 300 400 500 600 700 800 900 1000
Fig. 8.1. Relative size of unisexual population (= number of unisexual females/number of sexual females in generation n) if at generation 1 equal numbers of sexual and unisexual wasps are released. The sexual wasps produce offspring sex ratio (% females) of either 50% ________ or of 70% ... No other differences are assumed between the sexual and unisexual form; populations in exponential growth phase.
known to consist of both forms. The best- studied species is Trichogramma kaykaifrom the Mojave Desert (Stouthamer and Kazmer, 1994; Pinto et al., 1997; Huigens et al., 2000;
Stouthamer et al., 2001). The distinction between these two classes – fixed versus mixed – is important in regard to the influ- ence the Wolbachiamay have on the life-his- tory characters of the infected wasps.
In fixed populations we expect that there will be a selection for accommodation between the bacteria and their wasp hosts.
The evolutionary interests of both host and Wolbachiaare the same as long as the infec- tion is only passed on from the mother to her offspring. The wasp–Wolbachiacombina- tion that produces the most daughters will be selected for. In the case of infected indi- viduals occurring in a population with sex- ual individuals where gene flow between the sexual and unisexual individuals still occurs, the evolutionary interests of the Wolbachiaand the nuclear genes of the wasp are not the same (Stouthamer, 1997;
Stouthamer et al., 2001). Under such circum- stances the optimal sex ratio for the nuclear genes is a sex ratio involving at least some males, whereas for the PI Wolbachiathe opti- mal sex ratio is 100% female. This conflict in evolutionary interest of these two genetic elements can lead to an arms race between these elements, which may have as a by- product a reduced offspring production of the infected females. Other differences also exist between infected females from fixed versus mixed populations. When females from fixed populations are fed antibiotics, the males they produce are often unable to successfully mate with the females of their own line (Zchori-Fein et al., 1992;
Stouthamer et al., 1994; Pijls et al., 1996;
Arakaki et al., 2000). The most likely cause of this is an accumulation of mutations in genes involved with sexual reproduction in these infected lines. Such mutations are assumed to accumulate because they are not selected against any longer. Therefore, in populations where the infection has been fixed for a prolonged period, sexual repro- duction is no longer possible and sexual lines cannot be established from these forms.
Besides the influence of microbial infec- tion on parthenogenesis, there are also many cases of unisexual reproduction where this mode of reproduction is not sen- sitive to antibiotic treatment and therefore we assume that there is a genetic cause of unisexual reproduction. Examples of such unisexual reproduction are Venturia canescens(Speicher et al., 1965; Beukeboom and Pijnacker, 2000) and Trichogramma cacoeciae(Stouthamer et al., 1990a).
Unisexuals in Biocontrol
The incidence of unisexual reproduction among wasps used in biological control appears to be very high. In a sample of wasps used for biological control the per- centage of unisexuals was at least 15% (Luck et al., 1999). The cause for this high propor- tion of species carrying unisexual forms may be: (i) the proportion is not extreme but simply reflects the proportion of unisexual forms in nature; or (ii) the proportion is high because the establishment of a species in quarantine, in mass rearing (Stouthamer and Luck, 1991) and in the field is much more successful for unisexual forms than for sex- ual forms (Hung et al., 1988). If this latter reason is correct, then the high proportion of species that are unisexual in wasps used in biological control is simply a reflection of our inability to start cultures with only a few sexual individuals.
In Table 8.1, I give an overview of the cases of unisexual reproduction in para- sitoid wasps. Most of these cases were dis- covered when the wasps were studied for biological control purposes. In some gen- era, such as Aphytis, Encarsia and Trichogramma, the number of unisexual species is particularly high. This is most probably due to the relative importance of these groups in biological control and the awareness of the workers in this field of unisexual reproduction (DeBach, 1969). In other cases, it appears that certain host species seem to have a disproportionately high number of unisexual wasps attacking them; an example of this is weevils. The Use of Unisexual Wasps as Biocontrol Agents 95
96 R. Stouthamer
Table 8.1. Literature review of unisexual species or biotypes among parasitic Hymenoptera.
Taxa References
Ichneumonoidea Braconidae
Agathis stigmaterus Hummelen (1974) Apanteles cerialis Wysoki and Izhar (1981) Apanteles circumscriptus Shaw and Askew (1976) Apanteles pedias Laing and Heraty (1981) Apanteles thompsoni Vance (1931)
Centistes excrusians Loan (1963b)
Chelonus blackburni Platner and Oatman (1972)
Meteorus japonicus Clausen (1940); Li (1984); Fuester et al.(1993) Microctonus brevicollis Kunckel d’Herculais and Langlois (1891) Microctonus hyperodae Phillips and Baird (1996)
Microctonussp. Loan (1963a)
Microctonus vittatae Nagasawa (1947)
Perilitus coccinellae Balduf (1926); Wright (1978) Peristenus conradi Dayet al.(1992)
Peristenus howardi Dayet al.(1999)
Pygostylus falcatus Loan and Holdaway (1961); Loan (1963b); Milbrath and Weiss (1998)
Rogas unicolor Dowden (1938)
Aphidiidae
Aphidius colemani Tardieux and Rabasse (1988) Lysiphlebus ambiguus Rosen (1967)
Lysiphlebus cardui Nemec and Stary (1985) Lysiphlebus confusus Nemec and Stary (1985)
Lysiphlebus fabarum Rosen (1967); Nemec and Stary (1985) Lysiphlebus tritici Kelly and Urbahns (1908)
Ichneumonidae
Biolysia tristis Puttler and Coles (1962) Diadromus collaris Kfir (1998)
Gelis tenellus Muesenbeck and Dohanian (1927) Mesochorus nigripes Hunget al.(1988)
Polysphincta pallipes Clausen (1940) Sphecophaga burra Schmieder (1939) Sphecophaga vesparum Reichert (1911)
Thersilochus parkeri Kerrich (1961); Clancy (1969) Trathala flavoorbitalis Sandanayake and Edirisinghe (1992) Venturia canescens Speicheret al.(1965)
Chalcidoidea Pteromalidae
Mesopolobus diffinis Redfern (1976) Muscidifurax uniraptor Legner (1985) Spalangia erythromera Baker (1979) Eupelmidae
Anastatus pearsalli Muesenbeck and Dohanian (1927)
Eupelmus vesiculari Muesenbeck and Dohanian (1927); Phillips and Poos (1927) Aphelinidae
Aphelinus asychis Hartley (1922); Force and Messenger (1964) Aphelinus jucundus Griswold (1929)
Continued
Use of Unisexual Wasps as Biocontrol Agents 97
Table 8.1. Continued.
Taxa References
Chalcidoidea(Continued) Aphelinidae(Continued)
Aphytis aonidae Rosen and DeBach (1979)
Aphytis chilensis Rosen and DeBach (1979); Gottlieb et al.(1998) Aphytis chrysomphali Bartlett and Fisher (1950); Gottlieb et al.(1998) Aphytis comperei Rosen and DeBach (1979)
Aphytis diaspidis Zchori-Feinet al.(1995); Gottlieb et al.(1998) Aphytis hispanicus Gerson (1968)
Aphytis holoxanthus Rosen and DeBach (1979)
Aphytis lingnanensis Zchori-Feinet al.(1995); Gottlieb et al.(1998) Aphytis melinus Rosen and DeBach (1979)
Aphytis mytilaspidis Rosen and DeBach (1979)
Aphytis neuter Rosen and DeBach (1979)
Aphytis opuntiae Rosen and DeBach (1979) Aphytis phoenicus Rosen and DeBach (1979)
Aphytis proclia Sumaroka (1967)
Aphytis simmondsiae DeBach (1984)
Aphytis testaceus Rosen and DeBach (1979)
Aphytis vandenboschi Rosen and DeBach (1979); Titayavan and Davis (1988) Aphytis yanonenesis DeBach and Rosen (1982)
Azotus perspeciosus Pedata and Viggiani (1991) Azotus pulcherimus Viggiani (1972)
Encarsia citrina Flanders (1953a)
Encarsia formosa Speyer (1926)
Encarsia hispida Avilla et al.(1991) Encarsia inquirenda Gerson (1968) Encarsia lounsburyi Flanders (1953a) Encarsia meritoria Pedata and Hunter (1996) Encarsia pergandiella Hunter (1999)
Encarsia perniciosi Flanders (1953b) Eretmocerus mundus de Barro et al.(2000) Eretmocerussp. Hawaii Powell and Bellows (1992) Eretmocerussp. Hong Kong McAuslane and Nguyen (1996) Eretmocerus staufferi Rose and Zolnerowich (1997) Signiphoridae
Signiphora borinquensis Quezadaet al.(1973) Signiphora coquilletti Woolley (1984)
Signiphora flavella DeBachet al.(1958); Woolley (1984) Signiphora merceti DeBachet al.(1958)
Encyrtidae
Achrysophagus modestus Timberlake and Clausen (1924) Adelencyrtus odonaspidis Timberlake (1919)
Anagyrus subalbicornis Timberlake and Clausen (1924) Apoanagyrus diversicornis Pijlset al.(1996)
Blepyrus mexicanus Timberlake (1919) Chrysopophagus flaccus Timberlake (1919) Clausenia purpurea Rivnay (1942) Compariella unifasciata Clausen (1940) Encyrtus fulginosus Flanders (1943)
Encyrtus infelix Embleton (1904)
Habrolepis dalmani Clausen (1940)
Continued
98 R. Stouthamer
Table 8.1. Continued.
Taxa References
Chalcidoidea(Continued) Encyrtidae(Continued)
Habrolepis rouxi Flanders (1945, 1958) Hambletonia pseudococcina Carter (1937); Bartlett (1939) Microterys speciosus Ishii (1932)
Ooencyrtus fecundus Laraichi (1978)
Ooencyrtus submetallicum Wilson and Woolcock (1960a, b) Pauridia peregrina Timberlake (1919); Flanders (1959) Plagiomerus diaspidis Gordh and Lacey (1976)
Pseudoleptomastix squammulata Timberlake and Clausen (1924) Trechnites psyllae Slobodchikoff and Daly (1971) Tropidophryne melvillei Doutt and Smith (1950) Eulophidae
Ceranisus americensis Loomans and van Lenteren (1995)
Ceranisus menes Clausen (1940); Loomans and van Lenteren (1995) Ceranisus russelli Russell (1911)
Ceranisus vinctus Loomans and van Lenteren (1995) Galeopsomyia fausta Argovet al.(2000)
Nesolynxsp. Bueno et al.(1987)
Pedobius nawaii Muesenbeck and Dohanian (1927) Tetrastichus asparagi Russell and Johnston (1912) Tetrastichus brevistigma Berry (1938)
Tetrastichus cecidophagus Wangberg (1977)
Tetrastichusnr. venustus Teitelbaum and Black (1957) Thripobius semiluteus Hessein and McMurtry (1988) Mymaridae
Anagrus atomus Perkins (1905b)
Anagrus delicates Cronin and Strong (1996)
Anagrus ensifer Walker (1979)
Anagrus flaveolus Chandra (1980)
Anagrus frequens Perkins (1905b) Anagrus optabilis Perkins (1905b) Anagrus perforator Perkins (1905b) Anagrussp. nov. 1 Claridge et al.(1987) Anagrus takeyanus Gordh and Dunbar (1977)
Anaphes diana Aeschlimann (1986, 1990)
Polynema enchenopae Kiss (1986) Polynema euchariformis Clausen (1940) Trichogrammatidae
Megaphragma deflectum Takagi (1988); Loomans and van Lenteren (1995) Megaphragma mymaripenne Hessein and McMurtry (1988)
Trichogramma brevicappilum Pinto (1998) Trichogramma cacoeciae Marchal (1936)
Trichogramma chilonis Stouthameret al.(1990a); Chen et al.(1992) Trichogramma cordubensis Cabelloet al.(1985)
Trichogramma deion Bowen and Stern (1966); Stouthamer et al.(1990a) Trichogramma dianae Pinto (1998)
Trichogramma embryophagum Birova (1970)
Trichogramma evanescens Marchal (1936); Voegele and Russo (1981) Trichogramma flavum Marchal (1936)
Continued
interpretation of this list is difficult because it does not constitute an independent sam- ple of all parasitoid species. One might expect that the frequency of unisexual reproduction would be high particularly for solitary species and species with extremely small individuals, because for them the encounter between the sexes
might be the most difficult. Indeed several trichogrammatid and mymarid genera appear to be well represented. However, being small is not a prerequisite for unisex- ual reproduction, because some of the largest parasitic wasps species also have unisexual biotypes, e.g. Pelecinus polyturator (Johnson and Musetti, 1998).
Use of Unisexual Wasps as Biocontrol Agents 99
Table 8.1. Continued.
Taxa References
Chalcidoidea(Continued) Trichogrammatidae (Continued)
Trichogramma kaykai Stouthamer and Kazmer (1994); Pinto et al.(1997) Trichogramma oleae Pointelet al.(1979)
Trichogramma pintoi Wang and Zhang (1988)
Trichogramma platneri Stouthameret al.(1990a); Pinto (1998)
Trichogramma pretiosum Orphanides and Gonzalez (1970); Rodriguez et al.(1996) Trichogramma semblidis Pintureauet al.(2000)
Trichogramma telengai Sorakina (1987) Leucospidae
Leucospis gigas Berland (1934)
Pelecinoidea
Pelecinus polyturator Brues (1928); Johnson and Musetti (1998) Proctutropoidea
Amitus bennetti Viggiani and Evans (1992)
Amitus fuscipennis Viggiani (1991); Manzano et al.(2000)
Platygaster virgo Day (1971)
Telonomus dignus van der Goot (1915) Telenomus nakagawai Hokyo and Kiritani (1966) Telenomus nawai Arakakiet al.(2000) Cynipoidea
Hexacolasp. James (1928)
Hexacolasp. near websteri Eskafi and Legner (1974) Leptopilina austalis Werren et al.(1995) Leptopilina clavipes Eijs and van Alphen (1999) Phaenoglyphis ambrosiae Andrews (1978)
Bethylidea
Scleroderma immigrans Bridwell (1929); Keeler (1929a, b) Trigonalyidae
Taeniogonalos venatoria Weinstein and Austin (1996) Dryinidae
Gonatopus contortus Perkins (1905a) Gonatopus sepsoides Waloff (1974)
Haplogonatopus hernandazae Pilar Hernandez and Belloti (1984) Haplogonatopus vitiensis Clausen (1978)
Potential Advantages of Unisexuals As summarized by Stouthamer (1993), the advantages of unisexual wasps in biocon- trol are: (i) unisexual wasps have a poten- tially higher rate of increase than sexual wasps; (ii) unisexual wasps are cheaper to produce, all the wasps reared in mass rear- ing are females and only females are effec- tive in biological control; (iii) unisexual forms should be easier to establish in classi- cal biocontrol projects because they do not suffer from the Allee effect, i.e. a shortage of mating partners, which may limit the growth rate of sexual forms when wasp density is low; and (iv) for the same reason unisexual wasps may be able to reduce the host density to lower levels than the sexu- als, since low wasp densities may cause a reduction in the ability of females to find mates and therefore to produce daughters for the next generation.
Do unisexual wasps indeed have a higher rate of increase than sexual forms?
This will depend entirely on the number of female offspring produced per unisexual female versus per sexual female. Little is known about the number of daughters pro- duced by comparable sexual and unisexual females. In the case of Wolbachia-induced parthenogenesis, the relative offspring pro- duction of unisexual (infected) females dif- fers from that of the sexual females. This has been studied extensively in Trichogrammaspecies, where unisexual lines could be cured of their infection and ren- dered sexual (Stouthamer et al., 1990a).
When unisexual and sexual forms of the same line are compared, the offspring pro- duction of the sexual form is generally much higher when the unisexual form orig- inated from a population where both sexu- als and unisexuals co-occurred (mixed populations), while, if these comparisons were made using Trichogrammafrom popu- lations where the infection has gone to fixa- tion, no significant difference in offspring production could be found. In general, it appears that the influence of the infection
on offspring production is much higher in those cases where the infected and unin- fected wasps occur together (i.e. mixed populations) (van Meer, 1999). Similarly, there appears to be hardly any negative influence of the Wolbachia infection in species such as E. formosaand Muscidifurax uniraptor(Stouthamer et al., 1994).
These comparisons have been made in the laboratory using conditions where the wasps were given a surplus of hosts. In the field, the situation may be entirely different.
Even if the unisexual forms are capable of producing fewer offspring than the sexual forms in the laboratory, this may not be very important in the field. The number of hosts that a wasp encounters determines the num- ber of offspring produced and, as long as this number is below the maximal egg pro- duction of the unisexual line, all hosts encountered by both forms will be para- sitized (Stouthamer and Luck, 1993). Even when the hosts are more numerous than the maximum egg production (Mu) of the uni- sexual line, the number of daughters pro- duced by a unisexual female will be higher until the number of hosts encountered reaches the threshold T. If we define the sex ratio produced by sexual females as S, expressed as the fraction of daughters in the offspring, Tcan be derived as follows:
Mu= T×S, T= Mu/S
In the range of host densities of Muto T, the sexual form will kill more hosts per female than the unisexual form and yet the growth rate of the unisexual population will be higher than that of the sexual popula- tion. These three zones of host-encounter rates ( Mu, Mu–T, T) are useful values for making predictions about the relative usefulness of releasing unisexual versus sexual wasps for biocontrol (Fig. 8.2). As long as the host density is and remains such that the number encountered per female is larger than T, then it is more useful to release the sexual form. It will both have a faster rate of population growth and kill a higher number of hosts than the unisexual form. In the range of host densities between Muand T, the sexual form will kill a higher fraction of the host population but the rate 100 R. Stouthamer
of increase of the sexual form will be lower than that of the unisexuals. Finally, below Mu, the unisexuals and sexuals will cause the same number of hosts to be killed per female but the population growth rate of the unisexuals will be higher (1/Stimes as high per generation). If we assume that the wasp population is in an exponential growth phase, the criteria can be derived to determine the relative number of hosts killed over time in this tract; in the simplest case, the relative number of sexual females present of either form is given by (SV/Mu)n. The number of hosts killed per female equals V/Mu; therefore the relative fraction of hosts killed by the sexual forms in gener- ation nequals:
(SV/Mu)n×V/Mu= SnV1 + n/Mu1 + n.
Are unisexuals cheaper to produce?
A major part of the cost of producing para- sitoids for biological control is the cost of producing hosts. When unisexual wasps are used, all hosts result in female parasitoids and no hosts are wasted in the production of males. This should result in a reduction in production cost per female. While in species with a female-biased sex ratio the difference in production costs is not very large, in those species with sex ratios close to 50% the difference can be substantial. In addition, even in species that normally have a female- biased sex ratio, the sex ratio in mass rearing is often male-biased (Heimpel and Lundgren, 2000). Particularly for species used in inundative biological control, these Use of Unisexual Wasps as Biocontrol Agents 101
Host density expressed as number of hosts parasitized
Relative population growth rate
0 1 2 3
Mu T
Relative host-kill rate
Unisexuals: relative population growth rate and host-kill rate Sexuals: relative population growth rate
Sexuals: relative host-kill rate
Fig. 8.2. Relative population growth rate expressed as number of daughters per mother and the relative host-kill rate expressed as the relative number of hosts killed per mother of a unisexual form and a sexual form. The unisexual form produces 100% daughters but can only parasitize Mu(maximum egg production of the unisexual female) hosts, while the sexual form can parasitize Thosts (maximum egg production of sexual host). Sex ratio of sexual form is assumed to be 50% females.