Green vegetable bug

The green vegetable bug (GVB) is a major pest of soybeans, especially in coastal areas. It attacks other bean crops such as mungbeans, navy beans and azuki beans, as well as cotton.

Scientific name

Nezara viridula

Description of adult

GVBs are bright green, shield-shaped, 13–15mm long with 3 small white spots between their shoulders. Over-wintering adults are purple-brown. Yellow or orange GVB variants are occasionally seen. GVBs emit a foul smell when disturbed to deter predators.

GVB eggs are laid in rafts (50–100 eggs per raft) and are circular in cross section. Newly-laid eggs are cream but turn bright orange prior to hatching. Parasitised GVB eggs are black. GVB nymphs vary in colour. Newly hatched nymphs (1.5mm long) are orange and brown (sometimes black). Later instars are either green or black, with white, cream, orange and red markings.

Final (fifth) instar nymphs have reduced patterning (i.e. more base colour, green to black), and have prominent wing buds. Younger nymphs are round or oval rather than shield shaped and usually aggregate in large clusters. Older nymphs are more widely dispersed.

Similar species

Smaller podsucking bugs with which GVB adults may be confused include:

• the redbanded shield bug, which has a red or white band across its 'shoulders'
• the green stink bug, which is green and brown.

Life history

GVB typically invade summer legumes at flowering and commence feeding and egg laying. The eggs are laid in rafts containing 50–100 eggs and take 6 days to hatch at 25°C.

Nymphs do not usually reach a damaging size until mid to late podfill. Usually only one generation develops per summer legume crop. Nymphs require pods containing seeds to complete their development and the podding phase of most summer legumes is only slightly longer than GVB's life cycle. There are five nymphal instars with a total development time of 30 days. Development is faster at temperatures higher than 25°C but there is considerable nymphal and adult mortality at temperatures greater than 35°C. In Queensland, GVB has 3 to 4 generations during the summer.

GVB over-winters as adults, often sheltering (but not feeding) in yet to be harvested maize crops, under bark on trees, or in farm buildings. It is possible that GVB overwinters in cane crops, but the coast may be warm enough for bugs to keep feeding and breeding year-round.

Distribution

GVB was first recorded in Australia in 1916 and is now found throughout the country. It is a cosmopolitan pest found in all tropical, subtropical and warmer temperate regions of the world.

Host range

Soybean, mungbean, navy bean, sunflower, cotton, maize, peanut, linseed, azuki bean.

Soybeans in particular are a favoured host but GVB attack all summer and winter pulses (except chickpea). GVB is also a major pest in cotton and many horticultural crops.

Damage

Major, widespread and regular. This species is the most damaging podsucking bug in pulses, by virtue of its abundance, widespread distribution, rate of damage and rate of reproduction. It is one of the most recognised agricultural pests in Australia. Very high populations are frequently encountered in coastal Queensland.

Adult bugs typically invade summer legumes at flowering, but GVB is primarily a pod feeder with a preference for pods with well-developed seeds. Summer legumes remain at risk until pods are too hard to damage (i.e. very close to harvest). Damaging populations are typically highest in late summer crops during late podfill (when nymphs have reached or are near adulthood).

GVB use their long thin mouthpart to suck nutrients from the seed (stinging the seed). Legume pods most at risk are those containing well-developed seeds. While GVB also damages buds and flowers, soybeans can compensate for this early damage. 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.

Control

Crops should be inspected for GVB twice weekly from budding until close to harvest. Sample for GVB in the early to mid morning when bugs bask at the top of the crop canopy.

Beat sheet sampling is the most efficient monitoring method for podsucking bugs including GVB.

Thresholds

For edible soybeans (destined for human consumption), thresholds are governed by the need to not exceed maximum seed damage limits, usually 2%. As a result, the GVB threshold in typical edible soybeans crops is a low 0.3 adult GVB per square metre. For crushing and stockfeed soybeans with lesser quality requirements, the threshold is raised to 1.0 GVB per square metre.

Cultural

Where possible avoid sequential plantings of summer legumes as bug populations will move progressively from earlier to later plantings, eventually building to very high levels (e.g. greater than 20 per square metre). Also avoid cultivar and planting time combinations that are more likely to lengthen the duration of flowering and podding (e.g. early plantings of the soybean cultivar Melrose).

Biological

GVB eggs are frequently parasitised by a tiny introduced wasp Trissolcus basalis (green vegetable bug egg parasite). Parasitised eggs are easily recognised as they turn black. Parasitised GVB eggs may be confused with eggs of the predatory shield bugs but lack the spines that ring the top of the eggs of these species.

GVB nymphs are attacked by ants, spiders and predatory bugs. Final (fifth) instar and adult GVB are parasitised by the recently introduced tachinid fly, Trichopoda giacomellii.

Chemical

Bugs should be controlled during early podfill before nymphs reach a damaging size. Pesticides are best applied in the early to mid morning to contact bugs basking at the top of the canopy.

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.