Megacopta cribraria

Megacopta cribraria, also called the bean plataspid, kudzu bug, globular stink bug and lablab bug, is a shield bug native to India and China, where it is an agricultural pest of lablab beans and other legumes. The bug, while harmless to houseplants and people, often enters houses. It is attracted to white surfaces such as the walls of houses or white vehicles, because of the high reflectance of the white surfaces as it relates to the bugs' simple eyes. As a defense mechanism, they emit a foul-smelling pheromone that also acts as a congregation pheromone. Aside from smelling foul, the liquid also creates a burning sensation and sometimes leaves a red welt on bare skin. It is similar to other Plataspidae in having a somewhat unusual symbiotic relationship with its gut bacteria. Before laying eggs, females deposit particles containing the symbiont, which are then eaten by newly hatched nymphs under natural conditions. Nymphs experimentally deprived of access to the symbiont exhibited slower growth, smaller body sizes and higher mortality. Megacopta punctatissima, the Japanese common stinkbug, is a name applied to Japanese populations of M. cribraria. It has a gut bacteria that allows it to consume crops such as soybean plants. The two used to be treated as distinct species, but phylogenetic work has found the two to be not distinct. What is called M. cribraria in the United States likely descends from a population of "M. punctatissima" from Kyushu, Japan. The Japanese (JA) population is closest to the East Asian continental (EAC) population. The two diverged 0.71 Mya (million years ago), when the East China Sea land bridge became submerged. The EAC population branched off from the Southeast Asian (SEA) population 1.34 Mya. The bean plataspid gives off an offensive odor when touched, squashed, or poked. Hosted by wisteria, green beans, and other legumes, the insect sucks juice from the stems of soybean plants and reduces crop yield. However, when the insect infests kudzu, another invasive species, it appreciably reduces the growth of that plant.

Anatomy Like other plataspid stinkbugs, this bug has a peculiar anatomy where the digestive tract is disconnected midway. The gut is normally organized in nymphs, but the midgut becomes disconnected during development. The anterior midgut is where food digestion occurs and the posterior midgut becomes enlarged as an organ specialized for holding symbiotic bacteria. Waste is excreted through the Malpighian tubules into the hindgut.

Ecology In 2011 in its invasive range in Georgia, M. cribraria's aggregation score - Taylor's Power Law/Taylor's Law b - had an extremely high slope for adults. The badults was 3.27 ± 0.115 and badults > bnymphs > beggs. By the next year - and continuing at least into 2013 - the adult score was much lower and the order was reversed, with beggs > bnymphs > badults.

Hosts Populations harboring "M. punctatissima"-type microbiomes (Japan; North American invasive) are able to infest Glycine max (soybean) and a few other crop species. North American populations also feed on kudzu, which acts as a major food source and reservoir. Various populations are able to feed on Cajanus cajan (pigeon pea), Vigna unguiculata (cowpea), Lablab purpureus (lablab), Cyamopsis tetragonoloba (guar), and Sesbania bispinosa (danchi).

Microbiome Females are found by Hosokawa et al. (2008) to produce pellets with their own microbiome species and deposit them near their eggs. Larvae then search for and consume these. If these pellets are absent they will search more than those successfully finding pellets, suggesting that microbiome provision is indeed the purpose of this entire process and this is not accidental. The bugs carry Wolbachia and "Ca. Ishikawella" symbioants. Some also carry Arsenophonus.

Ishikawella

M. cribraria lives in symbiosis with a γ-proteobacterium called "Candidatus Ishikawella capsulata". This bacterium lives in the enlarged posterior midgut of the insect without entering insect cells. Hosokawa et al. (2007) finds "M. punctatissima" and not M. cribraria to be naturally able to infest soybean, but that this is solely due to Ishikawaella and can be experimentally induced in M. cribraria by giving it "M. punctatissima"'s symbiont. This sharp difference in function is produced by a very small genetic difference: Hosokawa et al. (2007) finds their 16S ribosomal RNAs to be 99.9% identical and more recent phylogenetic work finds them to be simply the same species. Brown et al. (2014) compared the genomes of Ca. Ishikawella recovered from the bug's eastern North American invasive (NAi) range and found very little variation in their genomes. They carried symbioants very similar to the "M. punctatissima" of Japan, with only 47 fixed differences and one amino acid change. They concluded that the bug must have arrived with a symbioant that gives it the ability to feed on soybean, without needing any switch. This conclusion was supported by phylogenetic work finding that the NAi population to be descended from the JA population. The Ishikawella strains carried by the insect has co-evolved with its host as it radiated from SEA, to EAC, and to JA. Various Ishikawella strains also colonize the guts of other Megacopta species and various Brachyplatys species.

Southeastern United States

In the Southeastern United States, M. cribraria is an invasive species, and was first noticed in northeastern Georgia in 2009. As of 2012, it was spreading rapidly into the surrounding states of Alabama, Florida, North Carolina, South Carolina, Tennessee, and Virginia. It has recently begun to invade Maryland and Mississippi, as well. In 2017, M. cribraria was observed in Texas. The bug causes problems not only for agricultural operations, but is also a major nuisance pest for residents. It "gathers in huge numbers in houses or other structures, seeking shelters in autumn." When disturbed it gives out an unpleasant smell. It also "produces a yellow substance when crushed that can stain cloth, wood and other surfaces." It is also reported to occasionally cause skin irritation.

… excerpt ends here. Continue reading the full article.