Parasitoid Wasps of the Invasive Brown Marmorated Stink Bug in Utah
Zach Schumm, Arthropod Diagnostician • Mark Cody Holthouse, USU Biology • Yota Mizuno, USU Biology • Diane G. Alston, Extension Entomologist • Lori Spears, Extension Entomologist
The brown marmorated stink bug (BMSB, Halyomorpha halys Stål) (Fig. 1) is an invasive agricultural and nuisance pest native to eastern Asia. It was first confirmed in the U.S. in Allentown, PA, in 1996 and has since spread to 44 U.S. states, many of which have now experienced economic crop damage from this pest (Fig. 2). In Utah, BMSB is now established in five counties (Box Elder, Weber, Davis, Salt Lake and Utah), and has been detected in Cache and Kane counties. While crop damage to peach, apple, squash, and popcorn has been identified, it is currently causing mostly nuisance problems due to overwintering bugs on and inside human structures.
General Parasitoid Information
There are at least two families of stink bug parasitoids in Utah, Eupelmidae and Scelionidae. These are small, typically black wasps that may be mistaken for small gnats or ants. They will fly in search of stink bug egg masses. Once they find the eggs, they will sting them, depositing one of their own eggs into the stink bug egg. The wasp egg will hatch, and the larva wasp will feed and develop within the stink bug egg, effectively killing the host. The adult wasp will emerge a couple of weeks later.
The Eupelmids attacking BMSB are all generalist parasitoids, meaning that they sting the eggs of a wide variety of insects. Native parasitoids in this group are moderately successful at stinging and developing inside BMSB eggs, but are unlikely to control BMSB populations due to their generalist nature.
The second family, Scelionidae, includes some stink bug specialists, meaning they only sting stink bug eggs. Specialists are more promising as a control agent for BMSB. Although many of the native Scelionid species will sting BMSB eggs, some will not develop into an adult and emerge. Those that can complete development within BMSB eggs have the potential to be more effective control agents.
Stink bug eggs are usually bright in color (Fig. 3) and take 5 to 7 days to develop and hatch. Eggs will develop a triangular egg “burster” shortly before stink bugs emerge from the egg (Fig. 4). However, if parasitized by a wasp, the eggs will turn dark brown or black after about a week. As the wasps develop, the eggs will continue to darken until the adult wasps emerge about 14 days later (Fig. 3).
There is usually a skewed sex ratio in emerging wasps. In a typical stink bug egg mass that consists of 14-28 eggs, one to three wasps will be male, and the rest will be female. Male wasps will emerge first and wait for the females to emerge. Once mated, the females fly off in search of new egg masses to sting.
Fig. 3. Left: An adult BMSB with a freshly laid egg mass; Right top: A parasitoid stings a stink bug egg mass; Right bottom: eggs darken as parasitoids develop.
Fig. 4. A BMSB egg mass with triangular egg bursters. The nymphal stink bugs inside are close to hatching.
Parasitoid Wasp Families in Utah
Eupelmidae – Generalist Egg Parasitoids
Eupelmids are small (3-5 mm), slender wasps that are generalist egg parasitoids. One genus, Anastatus, will parasitize BMSB, as well as other stink bugs and insects. They can resemble ants at first glance. Females often have a white band or white triangles on the wings. There are three species of Anastatus known to attack BMSB in Utah (Table 1). When seen on a stink bug egg mass, a general rule is that these wasps are much larger than an individual egg (Fig. 5). Females (Fig. 6) are typically larger than males, and under direct light can exhibit brown, green, or blue iridescence. Males are typically all black, smaller (< 4 mm), and lack wing patterns, making males indistinguishable to species without a microscope.
Scelionidae – Specialist Egg Parasitoids
Scelionids are very small (1-2 mm), but often robust wasps that are specialists on different insect groups (Fig. 7). One genus within this family, Trissolcus, only stings stink bug eggs. The wasps attacking BMSB can only be identified to species by using microscopes, as they are entirely black, small, and lack wing patterns or other characteristics to separate them with the naked eye. However, they can be generally identified in the field to family or genus using the tool that they are as small as or smaller than a stink bug egg (Fig. 8). There are at least two genera of stink bug parasitoids in the family Scelionidae in Utah (Trissolcus and Telenomus), with at least eight different species between these two genera (Table 1).
Surveys are ongoing for other species of parasitoid wasps in Utah, particularly Trissolcus japonicus (samurai wasp) (Fig. 9). This wasp is native to eastern Asia, the native range of BMSB. In its native range, BMSB causes minimal economic damage, presumably due to effective biological control by the samurai wasp.
Samurai wasp was collected in China and is undergoing host range testing in U.S. quarantine facilities to assess non-target effects for release in the U.S. However, samurai wasp has arrived on its own to the U.S. It has been found in 12 states as of January 2019 (Maryland, Pennsylvania, New Jersey, New York, Delaware, Oregon, Ohio, Virginia, West Virginia, Michigan, California, and Washington).
If samurai wasp is found in Utah, it can be reared and redistributed throughout the state to contribute to biological control of BMSB. Until samurai wasp is located in Utah, its release is prohibited.
Samurai wasp is more likely to be found in areas where BMSB are abundant (urban areas of Salt Lake and Utah valleys). However, it could be found in any location with established BMSB populations, making widespread surveys highly valuable.
Surveying for Parasitoids
Methods used to survey for stink bug parasitoids include physical placement of stink bug egg masses on host plants in the field, finding naturally-laid stink bug egg masses laid directly on host plants, and deployment of yellow sticky cards.
Physical Egg Mass Placements:
Lab-reared stink bug eggs are attached to small squares of cardstock paper. These cards are then clipped to the underside of leaves on common hosts of stink bugs in Utah (fruit trees, vegetables, and ornamental trees such as northern catalpa [Catalpa speciosa]) (Fig. 10). Cards are left for 3 to 4 days to attract parasitoids. When collecting cards, parasitoids guarding the eggs are also collected to further assess their efficacy in stinging and developing in eggs.
Finding Naturally-laid Egg Masses:
Stink bug egg masses can be found on the underside of leaves, on the fruiting structures, and occasionally on the stems of host plants (Fig. 11). Just as with deployed egg masses, parasitoids guarding the egg masses are collected to assess efficacy in killing and developing in stink bug eggs.
Yellow Sticky Cards:
Yellow sticky cards attract various insects. Cards are deployed for two weeks on the trunks and branches of ornamental and agricultural host plants, after which the wasps are removed from the card and identified (Fig. 12). When placed in areas with high parasitoid wasp diversity, cards are an effective tool for monitoring wasp diversity and density. While cards are effective at locating parasitoid wasps, information regarding the wasp’s behavior on BMSB eggs, or their effectiveness at stinging, killing, and sustaining populations within them cannot be determined.
If You See a Parasitoid Wasp:
If you see a parasitoid on an egg mass, be mindful that it is beneficial for your garden/crops, as the wasps are potentially killing the stink bugs that would normally emerge and begin eating your plants. You can choose to either leave the stink bug eggs and parasitoids on the plant (preferred), or chase off the parasitoid, remove the stink bug eggs from the plant, and freeze or destroy them before they hatch. It is possible that if the parasitoid wasps are left on the egg masses, then you may end up with more wasps, and consequently fewer stink bugs negatively impacting your plants. Parasitoids that are known to attack BMSB eggs in Utah can be seen in Figure 13.
Report Parasitoid Wasps
Reports of stink bug egg masses with parasitoid wasps on them are valuable to protect all Utah communities and property owners, and could help find new wasp species that may assist with biological control of BMSB. USU Extension has created a Report an Invasive Pest in Utah website to report a finding, upload a photo, and find contact information for submitting a specimen for identification. Collect the egg mass and wasp in a bag or sealable container for specimen submission.
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- Herlihy M.V., Talamas E.J., Weber D.C. 2016. Attack and success of native and exotic parasitoids on eggs of Halyomorpha halys in three Maryland habitats. PLOS ONE 11(3): e0150275.
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- Spears, L.R., Alston, D.G., and Murray, M. 2018. Brown marmorated stink bug management for fruits and vegetables in Utah (6 pp). Utah State University Extension Fact Sheet Ent-197-18, Logan, UT.
- Spears, L.R., Davis, R., Alston, D.G., and Ramirez, R. 2016. First detector guide to invasive insects: biology, identification, and monitoring (53 pp). Utah State University Extension, Logan, UT.
- Spears, L.R., Davis, R., and Ramirez, R. 2015. Invasive insect look-alikes: mistaken insect identity (6 pp). Utah State University Extension Fact Sheet Ent175-15-PR, Logan, UT.
- Stopbmsb.org A comprehensive website on BMSB identification, management, and new research.
- Talamas, E., Herlihy, M., Dieckhoff, C., Hoelmer, K., Buffington, M., Bon, M-C., and Weber, D. 2015. Trissolcus japonicus (Ashmead) (Hymenoptera, Scelionidae) emerges in North America. Journal of Hymenoptera Research. 43. 119-128.
1 Jeff Wildonger, USDA-ARS-BIIR
3 Zach Schumm, Utah State University
4 Rutgers New Jersey Agricultural Experiment Station
5 Ashley Jones, University of Maryland
6 John Rosenfeld, bugguide.net
7 John Rosenfeld, bugguide.net
8 Elizabeth Beers, Washington State University
9 Elijah J. Talamas, USDA ARS
10 Zach Schumm, Utah State University
11 Cody Holthouse, Utah State University
12 Zach Schumm, Utah State University
13 Cody Holthouse and Zach Schumm, Utah State University
14 Zach Schumm, Utah State University
Funding for this publication was made possible by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Specialty Crop Research Initiative (under award number 2016-51181-25409), U.S. Department of Agriculture, Utah Agricultural Experimental Station, Animal and Plant Health Inspection Service, Plant Protection and Quarantine (USDA-APHIS-PPQ) (under cooperative agreement number AP18PPQFO000C197), and USU Extension.