276 Invasive Stink Bugs and Related Species (Pentatomoidea)
These authors examined the response of Trissolcus erugatus Johnson and T. podisi egg volatiles emitted
by the eggs of Euschistus conspersus Uhler and H. halys. In the case of H. halys, hexadecanal, octadec-
tanal, and eicosanal were detected. Female T. eugatus and T. podisi were also repelled by egg extracts and
aldehyde blends identified from H. halys eggs. It is not yet known whether populations of native parasit-
oids will adapt over time to overcome the egg defense, but this is a question worth studying.
When compared with wild egg masses, sentinel eggs may underestimate parasitism rates because they
lack kairomone cues left on leaf surfaces by the movement of parent stink bugs that help lead parasitoids
to egg masses. In ornamental tree nurseries, wild egg masses had significantly greater levels of parasit-
ism by native Anastatus species, which typically have very broad host ranges (Jones et al. 2014). In other
habitats, differences between sentinel and wild egg parasitism rates have been less pronounced (Kim
Hoelmer, unpublished data). As with predation, variability among years, geographic locations, and crop-
ping systems regarding the most common parasitoids encountered has been reported.
Parasitism of Halyomorpha halys by indigenous parasitoids in the United States is substantially lower
than parasitism levels of indigenous stink bugs (e.g., Koppel et al. 2009, Tillman 2010). However, because
parasitism of H. halys by indigenous parasitoids is high in its native Asian range, a classical biological
control program has been considered for suppression of H. halys. Exploratory surveys were conducted
in China, South Korea, and Japan to collect natural enemies of H. halys, and the candidate agents were
returned to United States quarantine facilities for evaluation as potential biological control agents for
field release. Collected material was identified using extracted DNA in conjunction with morphological
characters (Talamas et al. 2015a; Marie-Claude Bon and Kim Hoelmer, unpublished data). These sur-
veys and one conducted by Zhang et al. (2017) in northern China confirmed that Trissolcus japonicus
was the most common and effective parasitoid of H. halys throughout its Asian range, and Trissolcus
cultratus was the next most abundant species. Several geographic populations of Trissolcus obtained
from these collections (Trissolcus japonicus, Trissolcus cultratus, Trissolcus mitsukurii, and Trissolcus
itoi) have been evaluated for their host specificity. To date, the species considered to be most promising
is T. japonicus, especially because of its high parasitism rates in China, Korea, and Japan (Yang et al.
2009; Tim Haye and Kim Hoelmer, unpublished data). An unexpected impact of H. halys on parasitoids
is its interference with plant tritrophic signaling. In a study examining the impact of H. halys feeding on
V. faba on the parasitism of Nezara viridula (L.) eggs by Trissolcus basalis (Wollaston), Martorana et al.
(2017) demonstrated that it interfered with the attraction of T. basalis to its host’s eggs.
Laboratory host range evaluations in the United States (no-choice and choice assays) of nearly 60 spe-
cies of phytophagous and predatory pentatomoids have shown that Trissolcus japonicus and Trissolcus
cultratus are oligophagous in their host acceptance; although many of the tested species were not
attacked at all, or attacked only at low rates, other species in a variety of host subfamilies were attacked,
including some predatory stink bugs such as Podisus maculiventris (Say) (Christine Dieckhoff and Kim
Hoelmer, unpublished data). Laboratory choice/no choice assays are by design highly conservative and
are predictive of a natural enemy’s physiological host range, but behavioral and ecological factors often
reduce the realized host range under natural conditions (Van Lentern et al. 2006). Further studies are
now needed to examine these factors.
Recently, an adventive population of Trissolcus japonicus was discovered in the wild during the con-
duct of sentinel egg mass surveys for native parasitoids in Beltsville, Maryland, during 2014 (Talamas
et al. 2015a). Expanded surveys during 2015 found the parasitoid to be present at several other sites in
Maryland, Washington, D.C., and adjacent states of Virginia and Delaware. The Asian parasitoid was
found in both wild and sentinel egg masses in arboreal habitats, but not in nearby crop fields (Herlihy et al.
2016). A second adventive population of T. japonicus also was discovered during sentinel surveys in 2015
in Vancouver, Washington, on the opposite coast of the United States (Milnes et al. 2016). It is unknown
how and when T. japonicus arrived in North America, but it is presumed to have been accidental (Talamas
et al. 2015a). Genetic analyses have shown that both east and west coast adventive populations are distinct
from T. japonicus populations held in North American quarantine culture and from each other (Marie-
Claude Bon, unpublished data). Continued surveys are planned to document the distribution and spread
of T. japonicus in the United States and to determine its impact on stink bug populations. The unexpected
introduction of T. japonicus creates questions about the impact it may have not only on Halyomorpha
halys populations but on those of native parasitoids and whether or not they can coexist with T. japonicus.