614 Invasive Stink Bugs and Related Species (Pentatomoidea)
ingestion may occur on a variety of plant parts, including leaves, stems, buds, fruits, developing seeds,
or mature seeds (McPherson and McPherson 2000, Silva et al. 2012).
Several methods have been used to characterize and measure heteropteran feeding, but determining
preferred target tissue after the stylets penetrate the plant tissue has always been a challenge. Counts of
the number of surface flanges of gelling saliva (often called “stylet sheaths”) have been used frequently
since first introduced by Bowling (1979). In most cases, these counts have accurately reflected crop
damage caused by mechanical or enzymatic injury and pathogen introduction (Zeilinger et al. 2015 and
references therein), but not always (e.g., blueberries [Wiman et al. 2015]). In contrast, the relationship
between counts of flanges and food consumption or preference is inconsistent and unreliable, depend-
ing on the species and stage (Zeilinger et al. 2015). Growth rate is related significantly to flange counts
only for fifth instars of Euschistus servus (Say) but not for Nezara viridula nymphs nor for adults of
either species (Zeilinger et al. 2015). Histological studies have been in use for over a century to fol-
low stylets or the course of sheath material in plant tissue (e.g., Horsfall 1923 and references therein)
but are time-consuming. A model for estimating potential stylet penetration by pentatomids and other
heteropterans has been developed by Esquivel (2011, 2015, and see Section 13.1.3.4, Stylet Penetration
Potential). Major advances in our understanding of pathogen transmission by hemipterans have come
from electrical-penetration graphing (EPG) techniques. Fereres and Moreno (2009) reviewed the role
of EPG in interpreting virus transmission by aphids; the technique has also been invaluable for study-
ing auchenorrhynchan vectors (e.g., sharpshooters [Backus 2016]). Few electrical-monitoring studies of
heteropterans have been published to date; however, interest in this technique is growing as indicated by
recent publications on pentatomids (Wiman et al. 2014, Lucini and Panizzi 2016), and the potential for
application to vector studies certainly exists.
13.1.3 Pentatomid Mouthparts
13.1.3.1 Structure of Mouthparts
Pentatomids possess the typical heteropteran mouthparts described previously (see Section 13.1.2,
Heteropteran Feeding Mechanisms), composed of interlocking needle-like stylets to pierce food sources
and obtain nutritional resources. This unique mode of feeding involves complex linkages among the
individual stylets to form a stylet bundle, and deliberate changes in insect posture while feeding that
affect stylet-penetration potential (Esquivel 2011, 2015).
13.1.3.2 Formation of the Stylet Bundle
Butt (1943), in examining Chinavia hilaris (Say) [as Acrosternum hilare (Say)] and Oncopeltus fasciatus
as representatives of the “terrestrial plant-feeding forms” of Hemiptera, provided a detailed descrip-
tion of the development of the individual maxillary and mandibular stylets that form the stylet bundle.
Briefly, within the insect head and on either side of the precibarium and alimentary canal, the maxil-
lary and mandibular stylets arise from “walls of the bristle pouches.” Levers are associated with these
pouches to aid in protraction and retraction of stylets. Additionally, at their bases, stylets are flattened
with attached protractor and retractor muscles (Butt 1943). Similar flattening of the stylets is observed
in Nezara viridula (Jesus F. Esquivel [JFE], unpublished data). These muscles presumably interact with
the levers for the protraction and retraction of the stylets from the labium (further clarified in Section
3.1.3.3). Dendritic nerve bundles are present in the neural canals of each stylet, and these nerve bundles
innervate along the length of the stylet as suggested by the reduced number of bundles visible in cross-
sections at the distal end of the maxillary stylets (JFE, unpublished data).
Smith (1926) presented a general diagram of the configuration of the mouthparts for hemipterans. As
described above, the stylets originate from pouches within the head on either side of the precibarium and
salivary duct. From their origin, and moving towards the apex of the head, the maxillary and mandibular
stylets begin to converge upon the precibarium and salivary duct. This convergence brings together the
maxillary stylets from either side of the head to form the food and salivary canals which align with the
precibarium and salivary duct, respectively (Figure 13.1). The salivary canal is used to egest saliva and