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Small brown bean bug

  • Small brown bean bug, Melanacanthus scutellaris, adult, showing cream marking in the middle of the back and cream stripes along each side
    Small brown bean bug, Melanacanthus scutellaris, adult, showing cream marking in the middle of the back and cream stripes along each side
  • Small brown bean bug, Melanacanthus scutellaris, nymph, showing ant-like appearance
    Small brown bean bug, Melanacanthus scutellaris, nymph, showing ant-like appearance

General information

Small brown bean bugs can be an infrequent but major pest of many legume crops, such as mung beans, soybeans, navy beans and adzukis. Care needs to be taken while monitoring fields as the nymphs of the small and large brown bean bugs can be mistaken for ants.

Scientific nameMelanacanthus scutellaris
Description

An elongated brown bug 10-12 mm in length (not including legs and antennae) with long antennae and with a cream stripe along each side. This stripe is often less distinct in females, which are ´rounder´ than males. Males also have a prominent pale patch in the scutellum. Melanacanthus has a short spine on each ´shoulder´ (less pronounced than on Riptortus sp. - the large brown bean bugs), and has moderately robust and spiny hind legs (thinner than those of Riptortus sp.).

The nymphs are dark brown to black and are similar in outline to ants. However, close inspection shows they lack the very narrow ´waist´ that is typical of ants.

Eggs are laid in small clusters and are shiny olive green. They are slightly elliptical with a flat top, a rounded base and are 1 mm long.

Similar speciesMay be confused with Riptortus sp., in particular, the large brown bean bug .
DistributionNative to Australia. Reported from New South Wales, Queensland, Western Australia and probably the Northern Territory.
Crops attackedAll summer and winter pulses (except chickpeas). It is also a minor pest in cotton and many horticultural crops.
Life cycleAdults typically invade summer legumes at flowering and commence feeding and egg laying; they lay scattered single eggs. There are five nymphal stages and nymphs usually reach a damaging size to coincide with mid to late podfill. Development times for eggs and nymphs are about six and 20 days respectively at 26°C. Under laboratory conditions, individual females have laid up to 300 eggs over a 58-day period (about five eggs a day). Potentially two generations could develop per summer pulse crop.
DamageThey are primarily pod feeders with a preference for pods containing well-developed seeds, but they also damage buds and flowers. Damage to young pods produces deformed and shrivelled seeds, reducing yield. Some pulse cultivars can compensate for early damage, but seeds damaged in older pods are blemished and difficult to grade out, reducing harvested seed quality, particularly that destined for human consumption.
MonitoringBeat sheet sampling may underestimate numbers of M. scutellaris, as they quickly fly away when disturbed. Crops should be sampled during the early morning and crop scouts should familiarise themselves with the appearance of flying (and escaping) M. scutellaris and include these in sampling counts.
Chemical controlSmall brown bean bugs are likely to be controlled by synthetic pyrethroids registered against green vegetable bug, but no products are specifically registered against this pest in mung beans. For current chemical control options see Pest Genie or APVMA.