Pentatomoids as Vectors of Plant Pathogens 617
Bell 2015) have been sequenced, and these data may assist in providing insight into the vector-microbe
localization relationship. Regarding N. viridula, future research should focus on determining whether
the insect’s internal chemistry and/or morphology affect colonization by the pathogens in the rostrum
and the head. Collectively, these attributes and others likely play a critical role in determining where in
the body the microbes colonize this stink bug.
13.1.6 Objec tives
The role of phytophagous heteropterans in pathogen transmission historically has been underappreciated.
Often, vector screening focuses exclusively on leafhoppers or aphids, and the larger bugs are not even tested
until all other options have been exhausted (Mitchell 2004). This review of the literature will enhance our
understanding of the role of stink bugs as vectors and, perhaps, stimulate further research. The chapter is
organized at the level of the pathogen (e.g., virus, phytoplasma, bacteria, fungi, and trypanosomatids) and
subdivided by crop type. Pentatomid taxonomy follows Rider (2015); the sources for bacterial and fungal
nomenclature are, respectively, Bergey et al. (1974) and Index Fungorum Partnership (2015).
13.2 Viruses
13.2.1 General Overview of Viruses
Virus transmission is associated primarily with aphids, leafhoppers, and planthoppers; few confirmed
cases of virus transmission by Heteroptera are known. Early literature incorrectly attributed many dis-
ease symptoms to “viruses” that subsequently were associated with spiroplasmas, phytoplasmas, or
simply damage by insect saliva and stylets; thus, older reports of mirids and other true bugs as virus
vectors are unreliable. Even today, the term virus may be used in cases where the causative agent is
not known with certainty; for example, beet savoy disease, transmitted by a piesmatid, Piesma cine-
reum (Say), is listed variously in reference books as a virus and a suspected virus (e.g., Narisu 2000,
Ruppel 2003). Nonetheless, two definitive examples of heteropteran transmission of viruses have been
documented: P. quadratum (Fieber) (Piesmatidae) transmits the rhabdovirus that causes beet leafcurl
disease (Proeseler 1980) in Europe, and Engytatus (=Cyrtopeltis) nicotianae (Koningsberger) (Miridae)
transmits velvet tobacco mottle virus in Australia (Gibb and Randles 1991). Modes of transmission in these
two cases are quite different. In P. quadratum, transmission occurs via saliva (Eisbein 1976) whereas in
E. nicotianae, the virus particles are not found in salivary glands but in the gut, hemolymph, and feces. An
ingestion-defecation mode was proposed to explain this vector-virus relationship (Gibb and Randles 1991).
13.2.2 Longan and Lychee
Longan witches’-broom disease affects the shoots, inflorescences, leaves, and seeds of longan,
Dimocarpus longan Lour., in China, Thailand, and Vietnam (Australian Government 2003, Nguyen
et al. 2012). Leaves are deformed, inflorescences fail to expand properly, and no fruits are produced
(Chen et al. 2001). A similar, possibly identical, disease has been reported in China from lychee, Litchi
sinensis Sonn. (Koizumi 1995). The causative agent has been identified as a filamentous virus by sev-
eral Chinese researchers (Chen et al. 2001), but the disease is attributed to a phytoplasma in Thailand
and Vietnam (Chantrasri et al. 1999, Nguyen et al. 2012). The Australian Department of Agriculture,
Fisheries and Forestry thus classifies longan witches’-broom as “aetiological agent unconfirmed”
(Australian Government 2003, 2004). In China, the stink bug Tessaratoma papillosa Drury transmits
the disease in longan and possibly between longan and lychee; however, it is not the only vector; a psyllid
(Cornegenapsylla sinica Yang and Li) also has been implicated and the parasitic plant, dodder, Cuscuta
campestris Yunck., also may play a role (Chen et al. 2001). Studies of T. papillosa (Chen et al. 2001 and
references therein) showed that both nymphs and adults were capable of vectoring the disease agent, and
the virus was present in the bug’s salivary glands. The nymphal transmission rate was 26–45%, whereas
that of adults was 18–36%. This stink bug is not associated with the disease in Thailand or Vietnam.