Semiochemistry of Pentatomoidea 709
resources such as host plants, based on nutritional benefits of gregarious feeding (Karban and Agrawal
2002). For piercing-sucking insects, this translates to the preferential supply of nutrients by the host
plant, increasing fecundity (Dixon and Wratten 1971, Lopez et al. 1989, Awmack and Leather 2002).
Third, aggregations can result in increased protection from natural enemies by providing more apparent
aposematic visual or allomonal chemical signals. Benefits also can arise from being in groups where
natural enemies are not density-dependent in their attack, causing a dilution of risk of attack, especially
to individuals surrounded by other conspecifics (Wertheim et al. 2005). Finally, there may be some fit-
ness improvement of abiotic environmental conditions under higher density; for example, Lockwood and
Story (1986) found acceleration of development with higher density under cool (but not warm) conditions
for early instars of Nezara viridula, as well as better protection from most predators. This effect is not
only weather-dependent, but instar-dependent; mortality increased with density for later instars (Kiritani
1964). These potential mechanisms in favor of aggregation are not mutually exclusive, and may be quite
complex, raising important questions of function and teleology (Cardé 2014). Nevertheless, to be selected
for, there must be a net benefit, and there are clearly potential negative effects of higher density, including
competition for food and mates, increased disease transmission, increased apparency to natural enemies,
and the metabolic costs of aggregating, including pheromone production and the energetic costs of flying
or walking to join an aggregation.
With pheromone emission under control of the sender, excessive densities also can be discouraged,
with an inverse density response observed, for instance, with Murgantia histrionica males in rates of
emission of aggregation pheromone (Zahn et al. 2008). This is likely a mechanism to avoid aggregation
of too many bugs on host plants or patches. For this same species, Cabrera Walsh et al. (2015) found that
although a synthetic mixture of the stereoisomers of murgantiol containing the aggregation pheromone
was highly attractive to bugs, it did not retain bugs on host plants, and numbers of M. histrionica per
baited plant reached a plateau due to a net balance of immigration and emigration.
Many factors are potentially significant in selection for aggregation pheromone production, release,
and response. There must be a net benefit within species both to the sender and receiver, which accrues
frequently enough, and also at low enough densities, for aggregation pheromones to be selected for over
evolutionary time, presumably from pre-existing odor production and detection mechanisms (Wertheim
et al. 2005). However, given the flexibility in production, release, and response to pentatomoid phero-
mones, evolution of this capability does not oblige individuals to send or to respond to the signals at any
given time, particularly as adult bugs are relatively long-lived and can replenish their energy stores by
feeding. This flexibility is an asset to stink bugs, even as it makes studying their chemical ecology more
difficult!
15.5.1.1 Cross-Species Attraction
One of the most curious and still unexplained phenomena in pentatomid semiochemistry is the cross-
attraction of a number of species to the pheromones of other species. This has been reported for both
adults and nymphs (Table 15.4). In some cases, this may represent overlap in chemistry of the phero-
mones of the respective species, but, in other cases, it clearly does not. One of the better-known examples
is the attraction of Halyomorpha halys to the pheromone of Plautia stali, methyl (2E,4E,6Z)-2,4,6-
decatrienoate. This compound also attracts the Asian species Glaucias subpunctatus, for which no
pheromone is known, and the North American species Chinavia hilaris, which, like H. halys, does not
produce this compound (Aldrich et al. 2009, Tillman et al. 2010). There are other examples attributable
to chemical overlap, for instance the Japanese native Nezara antennata and the exotic N. viridula, which
share their two pheromone components (Moraes et al. 2008b, Shimizu and Tsutsumi 2011). There are
still other cases in which either imprecise stereochemistry or field isomerization of pheromone compo-
nents (Khrimian et al. 2008) may account for cross-attraction, leaving open the possibility that there may
or may not be cross-attraction between living bugs in the field. Depending on the diel pattern of natural
emissions, UV-light-driven isomerization may occur during daylight yet be undetected by laboratory
collections in which pheromone-emitting substrates are subjected to lower light intensity and aerations
often are conducted in UV-opaque collection vessels. Diel cycles of pheromone emission and response
may make cross-attraction more or less likely in the field (discussion in Moraes et al. 2005a).