Host specificity assessments of Cotesia plutellae, a parasitoid of diamondback moth 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 Lepidoptera 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 insects: 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 Arthropod 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 8–12 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 11–13 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 insect per exposure per oviposition per oviposition per exposure Yponomeutidae Plutella xylostella (DBM) 182/190 32/41 21/23 60/110 Plutella antiphona – – 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 – 1/12 – Basaris itea ex Australia 0/30 – – – Arctiidae Nyctemera amica 17/24 – – 7/22 Noctuidae Agrotis ipsilon – 0/3 5/12 1/50 Diarsia intermixta – 2/6 1/8 9/50 Diarsia intermixta ex Australia – – 2/31 – Graphania mutans 15/30 – 3/17 3/50 Graphania ustistriga – – – 30/50 Helicoverpa armigera 16/30 2/16 0/5 – Neumichtis saliaris ex Australia – – 0/20 – Spodoptera litura 14/24 1/8 0/17 0/47 Thysanoplusia orichalcea 4/10 – 0/4 – 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.