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Diadromus pulchellus
(Hymenoptera: Ichneumonidae)

By Sam Hitchcock Tilton, Michigan State University

Introduction

Diadromus pulchellusis a solitary wasp that lays its eggs in the pupae (i.e.,pupal endoparasite) of its host, the leek moth Acrolepiopsis assectella. It is native to central Europe, where it was first identified in 1845. Interest in D. pulchellus increased sharply once its host, the leek moth, appeared in Ontario Canada in 1993. Candidates for biological control of this pernicious allium pest were sought in the leek moth’s natural range of central Europe. D. pulchellus was identified as the most appropriate candidate for biological control and studies on its basic biology and ecology have increased. Beginning in 2010, field releases of D. pulchellus were made in the Ottawa area and its impact and spread are being monitored.

Appearance

The family Ichneumonidae is comprised of medium to large parasitic wasps whose antennae have 16 or more segments, and a distinctive “horsehead” shaped cell in the front wing. D. pulchellus females have long ovipositors. D. pulchellus has two bicolor antennae that are curved back towards the head. D. pulchellus is easily confused with D. subtilicornis, a closely related species. But the two can be distinguished by comparing their faces under a hand lens - D. pulchellus does not have wrinkles and has a greater convexity in the middle of the face (at its ‘nose’). See Figure 1.

Habitat

D. pulchellus is native to central Europe, however it was introduced to Canada in 2010 to help protect alliums from the leek moth. Since its introduction, D. pulchellus has successfully overwintered and appears to be naturalizing, with help from continued releases. D. pulchellus benefits from ground and snow cover that helps moderate winter temperatures.

Pests attacked

Though it can successfully parasitize diamondback moth (Plutella xylostella), D. pulchellus attacks leek moth 95% of the time. Leek moth is an introduced pest to Canada that has now spread to New York State and models predict that it can survive in large portions of Eastern US and Southern Canada, where its spread seems inevitable. Leek moth is also predicted to survive in coastal areas of the Pacific Northwest and North-central Mexico.

Life cycle

The lifecycle of D. pulchellus synchronizes well with that of the leek moth. It overwinters as an adult and, following emergence in the spring, mating takes place immediately. After mating females lay eggs inside the pupal host and continue to lay a decreasing amount of eggs until they die four weeks after emergence. Female D. pulchellus lay eggs in leek moth pupae that are ensconced in their cocoons. See Figure 2. Both leek moth and D. pulchellus typically have three generations during the season. D. pulchellus predominantly overwinters as adults.

Relative Effectiveness

Field studies in Canada and Switzerland show parasitism levels of leek moth larvae reach almost 50% when sufficient numbers of D. pulchellus are released. Once established in an area, natural population of D. pulchellus can suppress leek moth populations, although other tactics may also be required.

Pesticide Susceptibility

Pesticide susceptibility studies have not yet been undertaken on this newly introduced species. However, insecticide studies have been conducted on Diadegma insulare, a related species also in the Ichneumonidae family. In these trials all tested insecticides, including spinosad, indoxacarb, esfenvalerate, methomyl, acetamiprid, acephate,and emamectin benzoate, were toxic to D. insulare adults. The neonicotinoid acetamiprid and the insect growth regulator methoxyfenozide also caused significant mortality. This suggests that field applications of any of these materials would likely severely reduce the biological control provided by D. pulchellus.

Commercial Availability

Because D. pulchellus is a new introduction to Canada and has not even been approved for release in the US, there are no North American suppliers.

Acknowledgements

Thanks to Peter Mason from Agriculture Canada who kindly shared his work and that of others, and to Doug Landis and Bill Will for helpful edits.

References

Cordero, R. J., Bloomquist, J. R., and Kuhar, T. P. 2007. Susceptibility of two diamondback moth parasitoids, Diadegma insulare (Cresson) (Hymenoptera; Ichneumonidae) and Oomyzus sokolowskii (Kurdjumov) (Hymenoptera; Eulophidae), to selected commercial insecticides. Bio Control. 42: 48:54.

Jenner, W. H., Kuhlmann, U., Cappuccino, N., and Mason, P. G. 2010. Pre-release analysis of the overwintering capacity of a classical biological control agent supporting prediction of establishment. BioControl. 55(3): 351:362.

Jenner, W.H., Mason, P.G., Cappuccino, N., and Kuhlmann, U. 2010. Native range assessment of classical biological control agents: impact of inundative releases as pre-introduction evaluation. Bulletin of Entomological Research. 100(4): 387:394.

Jenner, W. H., Kuhlmann, U., Miall, J. H., Cappuccino, N., and Mason, P. G. 2014. Does parasitoid state affect host range expression? Bio Control. 78: 15:22.

Mason, P. G., Weiss, R. M., Olfert, O., Appleby, M., and Landry J. –F. 2011. Actual and potential distribution of Acrolepiopsis assectella (Lepidoptera: Acrolepiidae), an invasive alien pest of Allium spp. in Canada. Can Entomologist 143: 185:196.

Mason, P. G., Brauner, A. M., Miall, J.H., and Bennett, A. M. R. 2013. Diadromus pulchellus in North America: field release against leek moth and new characters to distinguish it from Diadromus subtilicornis, a native diamondback moth parasitoid. Biocontrol Science and Technology. 23(3): 260:276

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