Previous PageTable Of Contents

Parasitism of Nyctemera amica (White) (Lepidoptera: Arctiidae) and Plutella xylostella (L.) (Lepidoptera: Plutellidae) by Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae)

Nancy Endersby1 and Peter Cameron2

1Department of Primary Industries, Knoxfield, Private Bag 15, Ferntree Gully Delivery Centre, Victoria 3156, Australia
2
20 Westminster Road Mt Eden, Auckland 1003, New Zealand
Corresponding author: cameronp@xtra.co.nz

Abstract

The parasitoid, Cotesia plutellae, is used as a biological control agent for diamondback moth, Plutella xylostella, in many countries and has been evaluated as a candidate for release in New Zealand. C. plutellae was originally released in Australia in 1951, but is rarely found. A glasshouse host specificity trial was conducted in Australia to assess whether C. plutellae would parasitise Nyctemera amica, magpie moth, on the noxious weed ragwort (Senecio jacobæa L.) in the presence of P. xylostella on cabbage. Although P. xylostella was expected to be the preferred host of C. plutellae, a greater proportion of the N. amica larvae was parasitised. It is likely, therefore, that C. plutellae would parasitise N. amica on ragwort in the vicinity of Brassica plants in the field. The closely related moth, Nyctemera annulata (Boisduval), is valued as a native species in New Zealand and significant parasitism of this insect would not be acceptable. The proposal to release C. plutellae will not be pursued until further information on host specificity is obtained.

Keywords

biological control, host specificity, Senecio jacobæa

Introduction

Cotesia plutellae (Hymenoptera: Braconidae) is being assessed as a potential biological control agent for diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), in New Zealand. This parasitoid has been released in most Brassica growing regions worldwide to augment natural populations or as a new introduction (Talekar & Shelton 1993). P. xylostella is the major pest of vegetable brassicas in New Zealand and Diadegma semiclausum (Hellén), the only larval parasitoid present, is not well synchronised with pest populations (Cameron et al. 1998). C. plutellae has been released in many countries for biological control of P. xylostella (Fitton & Walker 1990) and was imported from Italy and released in Australia for this purpose in the 1950s (Wilson 1960). Michael Keller of the University of Adelaide made a further introduction from Taiwan in the 1990s.

In the laboratory, host specificity assessments using no-choice tests (Cameron & Walker 1997) demonstrated that C. plutellae is capable of developing in several species of Lepidoptera other than P. xylostella, including a naturally occurring hybrid of Nyctemera amica (White) and N. annulata (Boisduval) (Lepidoptera: Arctiidae). However, in a flight tunnel, C. plutellae was equally attracted to species that were unsuitable for development (Cameron & Walker 1997). These observations showed that laboratory arenas produced results that are difficult to relate to the field host range of C. plutellae. A summary of laboratory and field records (Fitton & Walker 1990) noted that C. plutellae has been reared from 20 species of Lepidoptera, but most of these are rare occurrences. These records included two from the Family Arctiidae, based on five specimens of Diacrisia urticae (Esp.) (Wilkinson 1939) and <0.01% parasitism of Hyphantria cunea Drury (Bogavic 1953). The present study was initiated to provide further test data on the host status of Nyctemera spp.

The magpie moth, N. amica, occurs from central Queensland to Tasmania and into south-western Australia (Common 1990). Larvae of N. amica feed on the introduced weed, ragwort, Senecio jacobæa L., as well as on native species of Senecio. There has been no assessment of the impact of feeding by larvae of native moths, such as N. amica, on introduced weeds, but it is presumed to have some benefit for weed control (Common 1990). Biological control programs for ragwort, in Australia, are based on the use of exotic insects (Field 1990).

The presence of N. amica and the availability of cultures of C. plutellae provided a useful test situation for the New Zealand biological control programme, as well as providing an assessment of the potential interactions between an exotic parasitoid and an endemic Australian species of Lepidoptera. This investigation was conducted using semi-field conditions to verify previous records of host specificity of C. plutellae. The trial was designed to determine whether C. plutellae would locate and parasitise N. amica in the glasshouse and the proportion of parasitism of N. amica and P. xylostella was compared.

Materials and methods

P. xylostella larvae were reared on cabbage seedlings (Brassica oleracea cv. Green Coronet) seedling leaves in the laboratory. First to III instar larvae of P. xylostella were placed on cabbage and I to II instar larvae of N. amica were placed on ragwort seedlings (S. jacobæa) before each release, with the exception of the release on 22.vi.99 in which host larvae were carried over from the release on 17.vi.99 (Table 1). Cotesia plutellae, obtained from the University of Adelaide colony that originated in Taiwan, was reared on P. xylostella larvae at 22°C. Three glasshouse releases of C. plutellae were made between 28.v.99 and 22.vi.99 at the Institute for Horticultural Development, Knoxfield, Victoria, Australia. Adult C. plutellae (released on 28.v.99, 11.vi.99 and 22.vi.99) were released from plastic vials opened in the vicinity of the plants. Cocoons (released on 17.vi.99) in plastic vials were placed between the pots until wasps emerged. Wasps were free to move throughout the full glasshouse space.

Table 1. Numbers of potted plants, potential host larvae and wasps used in three glasshouse releases for host specificity testing of Cotesia plutellae

 

Release 1

Release 2

Release 3a

Release 3b

   

28.v.99

11.vi.99

17.vi.99

22.vi.99

Plants

Ragwort

20

50

50

50

 

Cabbage

20

50

50

50

Insects

Nyctemera amica

103

350

170

 
 

Plutella xylostella

140

350

170

 
 

Cotesia plutellae

180

340

200 cocoons

100

Cabbage seedlings were planted into PVC pots, 140 mm in diameter, and allowed to establish. At the time of Release 1 (28.v.99), cabbage seedlings had at least ten leaves. Ragwort plants were grown from seed or transplanted from infested sites into PVC pots of 120 mm diameter. All potting mix contained pine bark and sand (3:1). At the time of Release 1, each ragwort plant had at least five leaves. For later releases, plants were correspondingly larger.

Releases were confined to one section of a glasshouse with dimensions of 7.5 x 5.9 m. Cabbage and ragwort plants were arranged in a chequerboard pattern of alternating species on a bench measuring 2.3 x 1.4 m. Automatic sprinkler irrigation occurred twice per day. Glasshouse temperature was measured with a Hobo® data logger. The average temperature recorded was 15.3°C. Temperature extremes of 6.0°C and 22.6°C were recorded during the study period.

From six days onward after release of C. plutellae, larvae were collected and placed into individual Solo® cups for rearing on fresh leaves in the laboratory at 22°C. The fate of all larvae was noted and parasitoids were retained for identification.

Results

Larvae of N. amica tended to move away from the plants to sheltered areas (e.g. under the rims of the plastic pots) between feeding sessions. Larvae of P. xylostella rarely moved from the cabbage seedlings and, consequently, more P. xylostella larvae were re-collected than N. amica larvae (χ2=186.6, df=1, P<0.05) (Table 2).

Table 2. Numbers of potential host larvae (Nyctemera amica and Plutella xylostella) placed on plants before release of Cotesia plutellae and numbers collected after parasitoid release

 

Nyctemera amica

Plutella xylostella

Total released

623

660

Total collected

340

586

% collected

55

89

χ2 test for association between species and potential to be recollected: χ2=186.6, df=1, P<0.05

Both N. amica and P. xylostella were parasitised by C. plutellae in the glasshouse (Table 3). Parasitism of N. amica was significantly higher than parasitism of P. xylostella2=22.1, df=1, P<0.05). There was a low background level of parasitism of P. xylostella by D. semiclausum.

Table 3. Total parasitism of Nyctemera amica and Plutella xylostella by Cotesia plutellae in three glasshouse releases

 

Nyctemera amica

Plutella xylostella

Total collected from 3 releases

340

586

Total parasitised by Cotesia

73

60

% parasitised by Cotesia (of those collected)

21.5

10.2

% parasitised by Diadegma (of those collected)

0

4.5

χ2 test for association between species and susceptibility to parasitism: χ2=22.1, df=1, P<0.05

Some predators were present in the glasshouse throughout the experiment. Spiders (Salticidae) were common and one was observed taking a larva of P. xylostella. Low numbers of ants were present and one was observed taking an adult C. plutellae.

Discussion

Cotesia plutellae had a choice of P. xylostella and N. amica as potential hosts in the glasshouse. Nyctemera amica was the preferred host for C. plutellae under these experimental conditions and this result augments laboratory host records from a Nyctemera hybrid in New Zealand (Cameron & Walker 1997). This hybrid of the Australian species N. amica and the New Zealand species N. annulata occurs naturally in the field in the Auckland region (Kay 1980). The combined results of the semi-field tests and the previous laboratory tests, suggest that C. plutellae would parasitise Nyctemera spp. that were near Brassica plants in the field.

Cotesia plutellae parasitised several other species of Lepidoptera in no-choice tests in the laboratory (Cameron & Walker 1997), which suggests that it may have a broader host range than the two species tested in the glasshouse. It was also attracted to several species of Lepidoptera in flight tunnel tests, including species that were unsuitable for development of the parasitoid (Cameron & Walker 1997). The latter results suggest that C. plutellae is attracted to insect damaged foliage rather species-specific stimuli. Similar attraction has been reported for Cotesia rubecula, a specialist parasitoid of Pieris rapae (Agelopoulos & Keller 1994), indicating that there is still difficulty in designing and interpreting laboratory tests that will predict host specificity in the field. As host location may be disrupted in small-scale laboratory experiments due to confinement (Sands 1993), field or semi-field experiments such as those used here could be expected to give a better prediction of host range in the field.

Field data on the host range of C. plutellae indicate that it rarely parasitises Lepidoptera other than P. xylostella. Extensive field collections in Malaysia and Fiji where C. plutellae is abundant (Cameron et al. 1998) detected only rare instances of alternative hosts. In addition, the pyralid, Crocidolomia binotalis Zeller, which is a known laboratory host for C. plutellae (Lim 1982), was not detected as a field host in these regions. These results are consistent with those of Fitton and Walker (1990), who suggested that variable information on host specificity may be explained by the existence of different parasitoid strains.

To resolve these issues and provide information for the potential introduction of C. plutellae from Australia to New Zealand, we considered that the semi-field experiment provided the best prediction for the host status of Nyctemera spp. As N. annulata is valued as a native species in New Zealand and significant parasitism of this insect is not acceptable, the proposal will not be pursued until further information is obtained.

In Victoria, the significance of any interaction between N. amica and C. plutellae is restricted by the virtual absence of the parasitoid in this state. Cotesia plutellae was released in Australia in the 1950s and was subsequently recovered from P. xylostella in the ACT, New South Wales and Queensland (Wilson 1960). It is now rarely collected in any state. Surveys for C. plutellae have mostly been based on collections of P. xylostella, but collections of other Lepidoptera from brassicas in South Australia have not revealed C. plutellae (Keller & Cameron unpublished data). Parasitoids of N. amica identified by Clarke (1996) in Tasmania also did not include C. plutellae.

Acknowledgements

Thanks to Mike Keller and Wang Xin-geng of the University of Adelaide for supplying Cotesia plutellae, Karen Green for growing the plants and rearing the insects used in the trial, David McLaren of the Keith Turnbull Research Institute for the supply of Nyctemera amica, Jo Berry of Landcare Research for identifying Cotesia plutellae and Peter Ridland, Joanna Petkowski, Ian Porter and Peter Merriman for commenting on the manuscript. The glasshouse study was funded by the New Zealand Foundation for Research Science and Technology contract C02618 to Crop & Food Research.

References

Agelopoulos NG & Keller MA. 1994. Plant-natural enemy associations in the tri-trophic system Cotesia rubecula-Pieris rapae-Brassicaceae (Cruciferae) II: Preference of C. rubecula for landing and searching. Journal of Chemical Ecology 20, 1735-1748.

Bogavic M. 1953. Some observations on fall webworm parasites. Zastita Bilja 16, 58-80.

Cameron PJ & Walker GP. 1997. Host specificity of Cotesia rubecula and Cotesia plutellae, parasitoids of white butterfly and diamondback moth. In: Proceedings of the 50th New Zealand Plant Protection Conference, Lincoln University, Canterbury, New Zealand, 18–21 August 1997, 236-241.

Cameron PJ, Walker GP & Keller MA. 1998. Comparative measures of the host specificity of Cotesia rubecula and Cotesia plutellae (Hymenoptera: Braconidae). Sixth Australasian Applied Entomological Research Conference, 511-515.

Clarke AR. 1996. Parasitoids associated with a Tasmanian population of Nyctemera amica (White) (Lepidoptera: Arctiidae). The Australian Entomologist 23, 17-20.

Common IFB. 1990. Moths of Australia. Melbourne University Press, Carlton, Victoria.

Field RP. 1990. Progress towards biological control of ragwort in Australia. Proceedings of the VII International Symposium on Biological Control of Weeds 1990, 315-322.

Fitton M & Walker A. 1990. Hymenopterous parasitoids associated with diamondback moth: the taxonomic dilemma. In: Diamondback moth and other crucifer pests (ed NS Talekar). Proceedings of the Second International Workshop, Tainan, Taiwan, 10-14 December 1990, Asian Vegetable Research and Development Center, Shanhua, Taiwan, AVRDC Publication No. 92-368, pp. 10-14.

Kay M. 1980. Nyctemera amica x N. annulata colony at Woodhill (Lepidoptera: Arctiidae). New Zealand Entomologist 7, 154-158.

Lim GS. 1982. The biology and effects of parasites on diamondback moth, Plutella xylostella (L.). PhD thesis, University of London, 317 pp.

Sands DPA. 1993. Effects of confinement on parasitoid/host interactions: Interpretation and assessment for biological control of arthropod pests. In: Pest Control and Sustainable Agriculture (eds S Corey, D Dall & W Milne) pp. 196-199. CSIRO Australia.

Talekar NS & Shelton AM. 1993. Biology, ecology, and management of the diamondback moth. Annual Review of Entomology 38, 275-301.

Wilkinson DS. 1939. On two species of Apanteles (Hym. Brac.) not previously recognised from the Western Palearctic region. Bulletin of Entomological Research 30, 77-84.

Wilson F. 1960. A review of the biological control of insects and weeds in Australia and Australian New Guinea. Technical Communication No. 1, Commonwealth Institute of Biological Control, Ottawa, Canada.

Previous PageTop Of Page