Leek moth parasitoid–Diadromus pulchellus

Biocontrol Agent Factsheet

D. pulchellus can reduce populations of leek moths damage to Allium crops, however, it is not sufficient as a standalone method for leek moth control.

Common Names

Leek moth parasitoid

Relative effectiveness

D. pulchellus can reduce populations of leek moths, and therefore damage to Allium crops, however, it is not sufficient as a standalone method for leek moth control. Although it has been reported that D. pulchellus can parasitize diamondback moths (P. xylostella), D. pulchellus attacks leek moth 95% of the time.

Where to use

Vegetables in the allium family (leeks, onions, scallions, garlic, etc.)

About Leek moth parasitoid

Diadromus pulchellus, is a species of parasitoid wasp in the family, Ichneumonidae. These wasps are endoparasites, which are parasites that live in the tissues and organs of their hosts. Since the species appeared in Ontario Canada in 1993, it has been identified as an effective parasitoid of the leek moth (Acrolepiopsis assectella), and its impact and spread are still being monitored. It is now an established species in the Northeastern United States and Canada and can be found from early spring into the fall.

  • Native/Non-native: Non-native
  • Preferred climate: temperate, cold tolerant
  • Region: Northeast United States and Canada
  • Established: Yes
  • Where established: Northeastern United States

Leek moth parasitoid Appearance

Diadromus pulchellus adults have long, curved bicolor abdomens with yellowish-brown legs and long, curved black and yellow antennae that have 16 or more segments. Females are medium to large in size averaging 8-10 mm in length, and have a curved ovipositor, which is a long tubed organ protruding from the female’s abdomen that is used to deposit eggs. Males are slightly smaller in size averaging 7-9 mm in length and do not have an ovipositor.

Black wasp with long, black and yellow antennae and yellow legs.

Figure 3. Adult Diadromus pulchellus, leek moth parasitoid.

Black wasp with long, black and yellow antennae and yellow legs.

Figure 4. Diadromus pulchellus females parasitizing (laying eggs in) a leek moth pupa.

How to Use Leek moth parasitoid

Biocontrol category:  Classical - released once and persists

When to use: The spring emergence of first generation D. pulchellus adults is well synchronized with the population peak of pupating leek moths.

Maximizing effectiveness: Due to the complex nature of their reproduction, D. pulchellus (and many other parasitoid wasps) are not available for purchase from commercial rearing companies and therefore, support of naturally occurring populations is the best method of supporting this species. To maximize the effects of parasitism, D. pulchellus should be used as a biological control agent in areas with temperate weather and widespread allium production. Additionally, higher relative humidity and abundant sources of adult food (nectar) is also critical to many parasitoid wasp species.

Pest stage: Immature (pupal stage) while overwintering. In its native habitat, the leek moth overwinters as an adult or as a pupa in protected areas such as plant debris, hedges and row covers. In New York, we have observed overwintering adults but not pupae in the field. Overwintering leek moths pupate and emerge in the spring as moths with brown and cream colored wings.

Mode of action: Parasitoid

Conservation: Because D. pulchellus is already established in the Northeastern United States and Canada, biological control programs have been focused on supporting populations and beginning in 2010, field releases of D. pulchellus were made in the Ottawa area and its impact and spread are still being monitored. In general, parasitoid wasp species can be supported by providing adequate sources of nectar-producing plants as well as efforts to reduce the use of broad-spectrum insecticides near their habitats. D. pulchellus adults are able to successfully overwinter since their introduction to North America in 1993 and benefits from ground and snow cover that moderate winter temperatures. 

Compatibility: D. pulchellus is likely not compatible with most commercially available insecticides, and therefore population support requires careful consideration of broad-spectrum insecticide use. Additionally, the use of herbicides should also be considered when supporting populations of natural enemies, as herbicides can reduce floral and nectar availability that is key to parasitoid survival. (Any time you use a pesticide, you must read and follow the label directions and comply with all applicable laws and regulations related to pesticide use. Also be sure that any pesticide used is approved for use in your country and state/province.)

Risk: Parasitoid wasps are not a sting risk to humans.

Commercially available: No

About Leek moth (Acrolepiopsis assectella)

Mature leek moth larvae reach about 13-14 mm in length, and are yellow to gray-brown in color. Larvae have a small, yellow-brown head capsule. They are marked by rows of small grey dots running the length of the body. Eggs of Diadromus pulchellus are laid in the body cavity of pupating leek moths  that are ensconced in their cocoons, and emerge in the spring. See Figure 1.

Leek moth Damage

Feeding and tunneling damage by leek moth (larvae) can occur in all parts of Allium (e.g. leeks, onions, scallions, etc.) plants except the flowers. The primary damage of concern happens in the center of the plant where larvae feed and tunnel, causing pinholes and leaving behind frass (droppings) that contaminate the plant and result in rejected, unmarketable leeks, onion, scallions, etc.

Greenish-yellow caterpillar with dark spots in a web.

Figure 1. An immature (larva) Leek moth (A. assectella) in silk webbing.

Greenish-yellow caterpillar feeding on green stem.

Figure 2. Leek moth (A. assectella) feeding damage.

Diadromus pulchellus (Hymenoptera: Ichneumonidae)

The leek moth, Acrolepiopsis assectella Zeller (Lepidoptera: Acrolepiidae), is an important pest of Allium spp. crops (e.g. leeks, onions, scallions, etc.) in North America (Mason et al. 2010). The leek moth is native to Europe, however, it was first reported in Ottawa, Canada in 1993. It has since expanded its range into most temperate regions of the world (Landry 2007, Mason et al. 2010). Populations of leek moths in North America constrain Allium spp. production by feeding and tunneling in all parts of Allium plants, except the flowers (Olmstead et al. 2012). The primary damage of concern happens in the center of the plant where larvae feed and tunnel, causing pinholes and leaving behind frass that contaminate the plant and result in rejected, unmarketable leeks, onions, scallions, etc. This lack of marketable yield due to leek moth damage has severe economic consequences for Allium producers. Therefore, focus has been placed on integrated pest management programs to find effective insecticide chemistries, estimate correct timing of applications, polycultural planting techniques, and biological control programs. Biological control programs in particular have focused on identifying natural enemies of leek moths that can effectively reduce populations. 

One species of parasitoid wasp that has been identified as a successful parasitoid of overwintering leek moth pupae is Diadromus pulchellus, an endoparasitic Ichneumonid wasp (Mason et al. 2013). Leek moths overwinter as adults or pupa in protected areas such as plant debris, hedges and row covers, moths pupate and emerge in the spring as moths with brown and cream colored wings. Female D. pulchellus are able to take advantage of this overwintering behavior by laying eggs in the body cavity of pupating leek moths that are ensconced in their cocoons, where they develop and emerge in the spring (Shelton 1992). 

Since the species appeared in Ontario Canada in 1993, populations of D. pulchellus are now established in the Northeastern United States and Canada and can be found from early spring into the fall. Candidates for biological control of leek moths were sought in the moth’s natural range of central Europe. However, D. pulchellus was identified as the most appropriate candidate for biological control of leek moths when it has been reported by Mason et al. (2013) that D. pulchellus can parasitize leek moth 95% of the time as opposed to other species of moth (e.g. diamondback, Plutella xylostella). Despite its reduction of leek moth populations, and therefore damage to Allium crops, populations of D. pulchellus alone are not sufficient as a standalone method for leek moth control. 

Species of D. pulchellus can be recognized by their long, curved bicolor abdomens with yellowish-brown legs and long, curved bicolor antennae that have 16 or more segments. Females are medium to large in size averaging 8-10 mm in length, and have a curved ovipositor. Males are slightly smaller in size averaging 7-9 mm in length and do not have an ovipositor. Females live between 3 and 4 weeks under ideal conditions. Adult D. pulchellus can be easily identified by their distinctive horsehead shaped cell in the front wing, but can be easily confused with D. subtilicornis, a closely related species (Mason et al. 2013). However, the two species can be distinguished by comparing their faces under a hand lens D. pulchellus species do not have wrinkles and have a greater convexity in the middle of the face (Shelton 1992; Mason et al. 2013). 

Because D. pulchellus is already established in the Northeastern United States and Canada, biological control programs have been focused on supporting populations and beginning in 2010, field releases of D. pulchellus were made in the Ottawa area where impact and spread are still being monitored (Mason et al. 2011, 2013). Adults have been able to successfully overwinter in North America and benefit from ground and snow cover that moderate winter temperatures. Populations of D. pulchellus are not commercially available for release, therefore, support of D. pulchellus populations is the best method of maximizing parasitism. At the farm level, the presence of Allium crops can frequently be critical to attract populations of D. pulchellus. Although pesticide susceptibility studies have not yet been undertaken, it is safe to assume that broad-spectrum insecticides will have negative effects on D. pulchellus populations, as well as herbicides that reduce floral and nectar availability which is often critical to D. pulchellus as well as many parasitoid wasp species survival.

Author

Lidia Komondy
Cornell University Department of Entomology

Date: December 2021

I would like to thank the Cornell University Department of Entomology Extension Outreach Program.

Modified from an article written by Sam Hitchcock Tilton: Tilton, S.H. 2014. Diadromus pulchellus (Hymenoptera: Ichneumonidae), Biological Control: A guide to Natural Enemies of North America. 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.
 

  • 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. Biological 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. BioContro. 55(3): 351:362.
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  • 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. Canadian 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.
  • Figure 3. Adult Diadromus pulchellus, leek moth parasitoid. Credit: 2011 CNC/BIO Photography Group, Centre for Biodiversity Genomics; Sample ID: CNCHYM 06107. Attribution Non-Commercial Share-Alike 2.0, Generic (CC BY-NC-SA 2.0)
  • Figure 3. Adult Diadromus pulchellus, leek moth parasitoid. Credit: 2011 CNC/BIO Photography Group, Centre for Biodiversity Genomics; Sample ID: CNCHYM 06107. Attribution Non-Commercial Share-Alike 2.0, Generic (CC BY-NC-SA 2.0)
  • Figure 4. Diadromus pulchellus females parasitizing (laying eggs in) a leek moth pupa. Credit: Andrea Brauner, Agriculture and Agri-Food Canada.
  • Figure 1. An immature (larva) Leek moth (A. assectella) in silk webbing. Credit: 2017, Patrick Clement, Flickr. 
  • Figure 2. Leek moth (A. assectella) feeding damage. Credit: 2009, jimmygod via Flickr. 
Portrait of Amara Dunn
Amara Dunn-Silver

Senior Extension Associate

NYS Integrated Pest Management

Amara Dunn-Silver