Host specificity assessments of Cotesia plutellae, a parasitoid of diamondback

P.J. Cameron1, G.P. Walker1, M.A. Keller2 and J.R. Clearwater3 1New Zealand Institute for Crop & Food Research Ltd, Private Bag 92169, Auckland, New Zealand 2Department of Crop Protection, University of Adelaide, South Australia 5064 3Horticulture and Food Research Institute of New Zealand, Private Bag 92169, Auckland, New Zealand. Current address, 63 Peter Buck Rd, Auckland.

Abstract Cotesia plutellae is being assessed as a potential biological control agent for introduction against Plutella xylostella in New Zealand. As the literature on C. plutellae provided variable assessments of its host specificity, further information was collected from the laboratory and field. Our field assessments in Fiji indicated that this parasitoid did not attack other in or around vegetable brassica crops. Laboratory tests on a colony of C. plutellae in South Australia, including simple no-choice experiments and flight tunnel choice tests, showed that the parasitoid could choose to oviposit in other Lepidoptera and that successful development rarely occurred. In New Zealand, similar laboratory tests of C. plutellae collected from Fiji revealed that it was capable of ovipositing and developing in seven other species of Lepidoptera. Host suitability was assessed by comparing the ability of the parasitoid to develop in P. xylostella and other species. Host acceptability was compared by assessing the flight of adults to test larvae on their host plants, and by comparing oviposition preferences. These experiments suggested that C. plutellae may parasitise Lepidoptera other than P. xylostella in New Zealand and indicate that further assessments are required to determine its potential impact in the field.

Key words: Cotesia plutellae, Plutella xylostella, host specificity, oviposition, development.

Introduction the host specificity of C. plutellae have included Cotesia plutellae Kurdjumov (Hymenoptera: consideration of the literature, assessments of its host Braconidae) is being considered as a candidate for range in the field, and laboratory experiments to define introduction into New Zealand to augment the existing behavioural and physiological measures of host range. parasitoids of diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Yponomeutidae). These Methods parasitoids, Diadegma semiclausum (Hellen) and Field survey Diadromus collaris (Gravenhorst), can provide high The specificity of C. plutellae in the field was rates of parasitism in the spring and early summer, but examined by collecting and rearing Lepidoptera larvae in dry periods of the summer DBM population from crucifers and adjacent weeds in areas of Fiji increases are often not controlled by existing natural where the parasitoid was known to be present (Walker enemies. Insecticide applications are then necessary et al., in press). Larvae were reared individually to (Beck and Cameron, 1992). C. plutellae was proposed allow any parasitoids to emerge, and representative for introduction because it kills DBM larvae at an Lepidoptera larvae and adults were retained for earlier stage than the existing parasitoids and therefore identification. Dr J.A. Berry and A.K. Walker identified may reduce feeding damage on host plants. This the hymenopterous parasitoids and J.S. Dugdale parasitoid may also supress DBM better than D. identified the Lepidoptera. Similar observations were semiclausum because it is more active at higher carried out in South Australia, although the apparent temperatures (Talekar and Yang, 1991). absence of C. plutellae minimised their value. The major question concerning the introduction of C. plutellae into New Zealand is its specificity to Laboratory assessments DBM and its possible effects on non-target species. Sources of : Laboratory experiments were Fitton and Walker (1992) point out that although C. performed in 1994 at the University of Adelaide with plutellae is widely assumed to be host specific, it has a culture of C. plutellae that had been obtained from been recorded from several other species of Dr N.S. Talekar in Taiwan. Separate experiments were Lepidoptera. Although for some early biological carried out at Crop & Food Research in Auckland with control introductions native alternative hosts were C. plutellae collected from Fiji in 1995. Both cultures considered as useful reservoirs (Cameron et al. 1993), were identified by Drs J.A. Berry and A.D. Austin, conservation of native species, including the few and voucher specimens have been deposited in the attractive native butterflies in New Zealand, is now New Zealand Collection at Landcare an important issue. In this study, our assessments of Research in Auckland.

Biologically-based technologies 85 C. plutellae was reared on DBM larvae fed on developing. The remaining test larvae were reared until cabbage. Adult parasitoids were held in 4 litre, parasitoid larvae emerged to form cocoons, or until ventilated, clear plastic containers at 21 °C and fed the test larva died. The comparative success of dilute honey solution on cotton wicks. From days 2 to parasitoid development in different test species was 5 after adult emergence, approximately 60, 2nd and also assessed by exposing six replicates of 8Ð12 test 3rd instar larvae on a small cabbage leaf were larvae to individual females in 4 litre cages for 3 h. In presented for parasitisation to 10 females (and a similar similar experiments, parasitoid females were provided number of males) for 3 h. The larvae were then reared with a choice between larvae of DBM or another test on cabbage, and parasitoid cocoons were collected species. The results were expressed as the number of after 11Ð13 days and if necessary stored at 15 °C in C. plutellae cocoons per number of larvae exposed to glass vials for up to 1 week. parasitism. Lepidoptera species to be tested were generally Flight tunnel tests: The acceptability of different collected as adults from light traps. Eggs were test species was assessed by observing the flight of collected from gravid females and larvae reared on adult female C. plutellae to larvae on excised leaves their usual host plant or on cabbage. The majority of in a flight tunnel. These behavioural tests were test insects were collected from Auckland (Table 1); initiated at the University of Adelaide using methods those from other sources were: Plutella antiphona developed by Keller (1990). The wind speed in the Meyrick (from Chatham Island), Australian Bassaris tunnel was set at 60 cm/s. Adult parasitoids were itea F. (Adelaide, South Australia), Australian Diarsia released at 70 cm from the test insects, and the intermixta Guenee (Devonport, Tasmania), Neumichtis experiments were run at 25 °C. At Crop & Food saliaris Guenee (Devonport, Tasmania). Research in Auckland, behavioural assays were Host suitability: The suitability of Lepidoptera continued using a flight tunnel based on the design of larvae of different species for development of C. Miller and Roelofs (1978). The tunnel was operated plutellae was tested by exposing individual larvae to at a wind speed of 50 cm/s and a temperature of 21 a single mated female in a 100 x 25 mm glass vial for °C. Test females were fed and mated but had no 5 minutes in an attempt to force oviposition. Larvae experience of Lepidoptera larvae prior to release in were chosen to match the approximate size of the 2nd the tunnel. Females were presented with larva-plant or 3rd instar P. xylostella used as controls. Larvae were combinations either alternately (no-choice) or presented to one female in a sequence, alternating simultaneously (choice test). Five to ten test insects control and test larvae, until the parasitoid failed to were placed on each plant 24 h prior to the experiments oviposit in DBM. The time taken to initiate oviposition to ensure the presence of some leaf damage. Plants was also measured. The success of oviposition was were presented as one or two excised leaves to provide checked by dissecting some test larvae after 48 h to a similar leaf area for each test. For the choice tests, determine if eggs had been deposited or larvae were the plants were placed approximately 15 cm apart

Table 1. Oviposition and development of Cotesia plutellae in test insects, measured as number per larvae exposed to parasitism

Family Ovipositions Eggs Cocoons Cocoons Test per exposure per oviposition per oviposition per exposure Yponomeutidae Plutella xylostella (DBM) 182/190 32/41 21/23 60/110 Plutella antiphona Ð Ð 7/8 21/72 Tortricidae Epiphyas postvittana 6/33 0/1 0/5 0/48 Pieridae Pieris rapae 11/32 0/11 0/3 0/60 Nymphalidae Basaris itea 5/40 Ð 1/12 Ð Basaris itea ex Australia 0/30 Ð Ð Ð Arctiidae Nyctemera amica 17/24 Ð Ð 7/22 Agrotis ipsilon Ð 0/3 5/12 1/50 Diarsia intermixta Ð 2/6 1/8 9/50 Diarsia intermixta ex Australia Ð Ð 2/31 Ð Graphania mutans 15/30 Ð 3/17 3/50 Graphania ustistriga Ð Ð Ð 30/50 Helicoverpa armigera 16/30 2/16 0/5 Ð Neumichtis saliaris ex Australia Ð Ð 0/20 Ð Spodoptera litura 14/24 1/8 0/17 0/47 Thysanoplusia orichalcea 4/10 Ð 0/4 Ð

86 Proceedings: The Management of Diamondback Moth and Other Crucifer Pests across the air flow, equidistant from the centre line, Field survey and their position was alternated between each test. Collection and rearing P. xylostella in the Suva region of Fiji in 1992 and 1995 indicated that C. plutellae Results and Discussion was common. It parasitised 74% of DBM in the 1995 Published host records survey (Walker et al. in press). Larvae of several Numerous field records suggest that C. plutellae is a Noctuidae and one Pyralidae were also collected from narrowly oligophagous parasitoid of DBM that brassicas and weeds in the same area. Although other occasionally parasitises other Lepidoptera species. The parasitoids were present, no C. plutellae were reared majority of records in Shenefelt’s 1972 summary of from 563 Spodoptera litura F., 43 Helicoverpa host records report DBM as the only host. The main armigera (Hubner), 17 Chrysodeixis eriosoma exception to this is the extensive list of hosts derived Doubleday (all Noctuidae) and 130 Hymenia from Wilkinson (1939) in his description of the recurvalis (F.) (Pyralidae). Crocidolomia binotalis parasitoid. Most of Wilkinson’s records appear to be (Pyralidae) was similarly not parasitised, but the based on host identifications associated with parasitoid presence of C. plutellae in the region that this specimens, and six of the hosts are based on single lepidopteran was collected was not confirmed. These records. After extensive rearing, Delucchi et al. (1954) results augment the previous observations of Lim considered that two of the species on Wilkinson’s list, (1982) (cited in Waterhouse and Norris, 1987) that C. Pieris rapae L. and Pieris brassicae L., were not binotalis and Hellula undalis were parasitised in the attacked by C. plutellae. In his surveys of parasitism laboratory but not in the field. of Hyphantria cunea Drury (Arctiidae), Bogavic (1953) recorded approximately 26 C. plutellae, Laboratory assessments equivalent to less than 0.01% parasitism. Together with Test species: Initial assessments in South Australia in Wilkinson’s records of five parasitised Diacrisia 1995 tested three native species. Two of these species, urticae Esp. (Arctiidae), these records suggested that Diarsia intermixta and Neumichtis saliaris further host assessments should include representatives (Noctuidae), are associated with brassicas (Common, of this family. Wilkinson’s (1939) report of 28 host 1990), whereas the yellow admiral Bassaris itea records for two species of Lymantriidae and six host (Nymphalidae) occurs on nettle (Urtica dioica) and is records for Malacosoma castrensis L. (Lasiocampidae) valued, particularly in New Zealand, as an attractive also warrant consideration. The most definite species. Test species in New Zealand included a near information on host association is that for Aglais relative of DBM, the endemic species Plutella urticae L. (Nymphalidae). Wilkinson (1939) received antiphona, which has a limited distribution on specimens of C. plutellae reared from A. urticae, and Cruciferae, particularly water cress, Nasturtium maintained a culture of this parasitoid on both DBM officinale (Dugdale, 1973). Seven test species that and A. urticae. He stated that “.... Apanteles plutellae occur on brassicas in association with P. xylostella Kurdj., is able to utilise both P. maculipennis and A. were Noctuidae (Table 1). Other test species from urticae as a host.” The potential for rearing C. plutellae brassicas were Pieris rapae (Pieridae) and Epiphyas on alternative hosts in the laboratory was also postvittana (Tortricidae). The remaining test species demonstrated by Wang et al. (1972) who showed that where Nyctemera amica (Arctiidae) collected from although the parasitoid preferred to oviposit in DBM ragwort (Senecio jacobaea) and B. itea (Nymphalidae), it survived better in the rice moth Corcyra cephalonica both representing families that include hosts of C. Stainton. Similarly, Lim (1982) (cited in Waterhouse plutellae previously reported in the literature and Norris, 1987) recorded that Crocidolomia binotalis (Wilkinson, 1939). There are no Lasiocampidae or Zeller and Hellula undalis Guenee were parasitised Lymantriidae in New Zealand so assessments of in the laboratory but not in the field. An additional species from these families were not relevant. field host record for C. plutellae was reported by Host suitability: In experiments with the Baloch et al. (1966) who recorded low rates of University of Adelaide culture of C. plutellae, the parasitism when assessing the potential of Oeobia parasitoid was induced to oviposit in D. intermixta, (=) verbascalis Schiff. (Pyralidae) for and from 31 oviposition attempts two cocoons were biological control of Noogoora burr, Xanthium produced (Table 1). Oviposition responses to N. strumarium L. saliaris were more difficult to obtain and no cocoons These literature records, together with general were produced from 20 oviposition attempts on this criteria for selecting test species, suggested three species. B. itea was unacceptable to C. plutellae to categories of test species: the extent that no oviposition responses were obtained. 1. Close relatives, i.e. Plutellidae. C. plutellae was collected from Fiji and imported for 2. Species on crucifers, especially Noctuidae. assessment in quarantine in Auckland. This culture 3. Species in the same family as hosts recorded in attempted to oviposit in all test species, but dissection the literature, i.e. Nymphalidae, Noctuidae, of larvae revealed that eggs were not deposited in E. Arctiidae, Pyralidae, Lymantridae, postvittana or P. rapae (Table 1). The oviposition rate Lasiocampidae. was highest in DBM and oviposition was initiated more quickly (data not shown) in this species. There was no clear difference between oviposition rates in

Biologically-based technologies 87 species other than P. xylostella, nor was the oviposition but plants with larvae of H. armigera or N. amica were rate related to success in cocoon formation. For as attractive as DBM. The rate of successful flights example, no cocoons developed from Spodoptera declined slightly in choice tests, but apart from reduced litura, but more than 50% of the larvae attracted flights to B. itea, C. plutellae showed no distinct oviposition attempts. This demonstrated that preference. Flights to H. armigera and N. saliaris, oviposition provided a poor estimate of the suitability previously demonstrated to be unsuitable for of a species, possibly because this response may be development, strongly suggested that C. plutellae is elicited by plant (cabbage)-associated factors. Of those attracted by cabbage volatiles, or by the volatiles from species where eggs were detected, both S. litura and damaged cabbage. This behaviour has also been Helicoverpa armigera were unsuitable for further observed in Cotesia rubecula by Agelopoulos and development. The rate of cocoon formation (Table 1) Keller (1994a, b) who reported that, although this indicated that six species were not hosts: B. itea, A. parasitoid did not distinguish between damage by host ipsilon and G. mutans were occasional laboratory or non-host Lepidoptera, the blend of volatiles emitted hosts; and G. ustistriga, N. amica, P. antiphona and from frass was different for DBM and Pieris rapae. D. intermixta were all suitable laboratory hosts for C. The development of behavioural measures that plutellae. Exposure of test species in choice tests reflect the host range of oligophagous species such as reduced the rate of cocoon formation (Table 2) C. plutellae will require further refinements of testing indicating that mixed host populations interfered with procedures. Whereas simple tests may be suitable for host location and reduced the probability of attack on demonstrating high degrees of specificity such as alternate species. found in C. rubecula (Cameron and Walker, in this Estimates of the development rate of C. plutellae proceedings), the specificity of C. plutellae is provided another measure of the suitability of some apparently determined more by behaviour than test species. In P. antiphona, parasitoids developed physiological compatibility. Testing procedures will from egg to cocoon at the same rate as in DBM. By therefore need to consider that because confinement contrast, D. intermixta and G. ustistriga required two disturbs the natural behaviour of parasitoids, laboratory more days to develop, and N. amica took four more tests will usually overestimate their host range in the days than DBM. field (Sands, 1993). To obtain more realistic measures Flight tunnel tests: Flight tunnel experiments of host range in the field, we also plan to extend our showed that C. plutellae females could fly to all collection and rearing of Lepidoptera from regions test combinations of insect and plant species (Table where C. plutellae is naturally present. 3). During successful flights females frequently exhibited characteristic casting behaviour as they Acknowledgements sampled the odour gradients produced by each source. We thank Anne Barrington, Tim Herman, Lionel Hill, In no-choice tests, fewer adults flew to B. itea on nettle, John LeSueur and Chris Winks for supplying test

Table 2. Development of Cotesia plutellae cocoons from tests insects compared with Plutella xylostella (DBM) in choice and no-choice experiments, measured as number of cocoons per larvae exposed to parasitism

Alternate test insect and plant Choice No choice Alternate DBM Alternate DBM on cabbage on cabbage Graphania mutans on cabbage 1/60 16/60 3/50 Nyctemera amica on ragwort 9/60 16/60 7/22 41/50 Spodoptera litura on cabbage 0/50 12/50 0/47

Table 3. Flights of Cotesia plutellae to test insect and host plant combinations compared with Plutella xylostella (DBM) in a flight tunnel

Number of flights per number of tests Alternate test insect and host plant DBM on Alternate test cabbage combination No choice test Bassaris itea on nettle 10/18 4/15 Helicoverpa armigera on cabbage 10/19 9/21 Nyctemera amica on ragwort 7/15 5/13 Choice test Bassaris itea on nettle 18/43 6/43 Nyctemera amica on ragwort 8/27 6/27 Diarsia intermixta on cabbage ex Australia 19/51 21/51 Neumichtis saliaris on cabbage ex Australia 13/30 14/30

88 Proceedings: The Management of Diamondback Moth and Other Crucifer Pests insects, and Darryl Jackman and Sarah Painter for Dugdale, J.S. (1973). The genus Plutella (Hyponomeutidae) rearing various Lepidoptera and assisting with in New Zealand and the family position of Circoxena experiments. Andy Austin, Jo Berry and Annette (Lepidoptera). New Zealand Journal of Science 16: Walker provided identifications of the parasitic 1 009Ð23. Fitton, M. and Walker R, A. (1992). Hymenopterous Hymenoptera, and John Dugdale and Lionel Hill parasitoids associated with diamondback moth: the identified the Lepidoptera. taxonomic dilemma. In Diamondback moth and other crucifer pests (ed. N.S. Talekar) pp. 225Ð232. References Keller, M.A. (1990). Responses of the parasitoid Cotesia Agelopoulos, N.G. and Keller, M.A. (1994a). Plant-natural rubecula to its host Pieris rapae in a flight tunnel. enemy association in the tri-trophic system Cotesia Entomologia Experimentalis et Applicata 57: 243Ð249. rubecula-Pieris rapae-Brassicaceae (Cruciferae) II: Miller, J.R. and Roelofs, W.L. (1978). Sustained-flight tunnel Preference of C. rubecula for landing and searching. for measuring insect responses to wind-borne sex Journal of Chemical Ecology 20: 1 735Ð48. pheromones. Journal of Chemical Ecology 6: 187Ð198. ÐÐÐÐ (1994b). Plant-natural enemy association in the Sands, D.P.A. (1993). Effects of confinement on parasitoid/ tri-trophic system Cotesia rubecula-Pieris host interactions: Interpretation and assessment for rapae-Brassicaceae (Cruciferae) III: Collection and biological control of arthropod pests. In Pest Control identification of plant and frass volatiles. Journal of and Sustainable Agriculture (ed S.A. Corey, D.J. Dall Chemical Ecology 20: 1 955Ð67. and W.M. Milne) pp. 196Ð199. Beck, N.G. and Cameron, P.J. (1992). Developing a reduced Shenefelt, R.D. (1972). Hymenopterum catalogus. Part 7, spray program for brassicas in New Zealand. In Braconidae 4. Dr W. Junk N.V s-Gravenhage. Diamondback moth and other crucifer pests (ed. N.S. Talekar, N.S. and Yang, J.C. (1991). Characteristics of Talekar) pp. 341Ð350. parasitism of diamondback moth by two larval Baloch, G.M., Din, I.M. and Ghani, M.A. (1966). Biology parasites. Entomophaga 36: 95Ð104. and host-plant range of Oeobia verbascalis Schiff. Walker, G.P., Cameron, P.J., Berry, J.A. and Lal, S.N. (in (Pyralidae: Lepidoptera); an enemy of Xanthium press). Parasitoids reared from lepidopteran larvae from strumarium L. Technical Bulletin of the Commonwealth brassicas and associated weeds in Fiji, 1992 and 1995. Institute of Biological Control 7: 81Ð90. Proceedings of the Second Workshop on Biological Bogavic, M. (1953). Some observations on fall webworm Control in the Pacific. parasites. Zastita Bilja 16: 58Ð80. Wang, C-L., Chio, H. and Ho, K-K. (1972). The comparative Cameron, P.J., Hill, R.L., Bain, J. and Thomas, W.P. (1993). study of parasitic potential of the braconid wasp Analysis of importations for biological control of (Apanteles plutellae Kurdj.) to the diamondback moth insects pests and weeds in New Zealand. Biocontrol (Plutella xylostella L.) and rice moth (Corcyra science and technology 3: 387Ð404. cephalonica Staint.). Plant Protection Bulletin (Taipei) Cameron, P.J. and Walker, G.P. (1997). Introduction and 14: 125Ð8. (In Chinese, English summary). evaluation of Cotesia plutellae, a parasitoid of Pieris Waterhouse, D.F. and Norris, K.R. (1987). Biological rapae in New Zealand. In: A. Sivapragasam, W.H. Control-Pacific Prospects. Inkata Press, Melbourne, 454 Loke, A.K. Hussan and G.S. Lim (eds.). Proceedings pp. of the Third International Workshop on Diamondback Wilkinson, D.S. (1939). On two species of Apanteles (Hym. Moth. Kuala Lumpur, Malaysia. Brac.) not previously recognised from the Western Common, I.F.B. (1990). of Australia. E.J. Brill, The Palearctic region. Bulletin of Entomological Research Netherlands, 535 pp. 30: 77Ð84. Delucchi, V., Tadic, M. and Bogavic, M. (1954). Mass rearing of Apanteles plutellae Kurdj. (Hym., Ichneumonidae), endophagous parasites of Plutella maculipennis Curt. and biological observations on these parasites. Zastita Bilja 21: 26Ð41

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