Stink Bugs

Pests of Field Crops in Southern Africa

STINKBUGS

(Hemiptera, Heteroptera: Pentatomidae)

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Text Box: Stinkbugs (particularly the green variety, Nezara viridula) need little introduction. They belong to the shield bug family (so named because of their shape) and produce the evil-smelling defensive fluid that is associated with all bugs of this family. The bugs themselves are shades of green, and about 15 mm by 8 mm in dimensions. The long, piercing proboscis (or "beak") lies, when at rest, between the forelegs underneath the body. The nymphs of N. viridula appear rather different from the adults, lacking the green wings. Their bodies are at first coloured in a series of black, yellow and white dots, and they are more rounded in shape. Later they are more predominantly green, but still with the coloured spots. They grow from fairly tiny to about 10 mm long, with the wings becoming more apparent at each moult. The eggs are barrel-shaped, about 1 mm long, creamy to pinkish in colour, and laid in batches stuck to the leaves . TOP

Text Box: Green stinkbugs are cosmopolitan insects, and have a very wide host range. They have been recorded on a number of fruits, crops, vegetable and garden ornamentals, but their mobility may mean that some of the plants on which they have been found are not true hosts. Cotton, deciduous fruit, tomatoes, legumes of all sorts (particularly soyabeans and garden beans), wheat and nut crops, such as macadamias are amongst recorded hosts. TOP

Text Box: These bugs usually feed on developing fruit. Their feeding punctures cause local necrosis, presumably due to a toxin in their salivary juices, with resulting fruit spotting, deformation or even shedding, if inflicted early enough. Sharp indentations are quite commonly seen on still-green peaches, and the bugs can inflict damage on soyabeans, causing discolouration of the developing seeds. On cotton they generally occur in low numbers on the green bolls, and are seldom of economic importance. In wheat they are often found in quite large numbers, probably because at this time of year (mid-winter), there may be little else that is green and succulent to feed on. They are capable of piercing the rachis between the developing grains, with the result that the wheat ear turns white and dies from that point upwards. Obviously the level of damage depends on the population level. In tobacco they sometimes feed on new leaves within the shoot, which later results in wilting. TOP

Text Box: The development of N. viridula is relatively slow, and probably two to three generations are produced in a year. Eggs are laid in batches of 50 to 60, stuck together in rafts on the undersides of leaves. They soon hatch into tiny first instar nymphs, which cluster by the eggs and do not feed. There are five instars, with the preferred food being developing seeds or fruit, until, after some eight weeks, the bugs reach adulthood. After mating, they disperse and feed by piercing soft plant tissues and sucking the sap. The female may live for a few months, laying over 500 eggs in that time. At the end of the wet season, large numbers of bugs can be found, often on soyabean crops, and particularly those that were late-planted. During the winter months (May to September), there are often reports of large numbers of bugs in houses, sheltering in secluded places such as amongst pot plants. They can be found similarly in wheat, and may begin feeding on the developing grains as the weather warms up in September. TOP

Text Box: Control of stinkbugs in a field crop is probably seldom necessary. Stinkbugs are sap-suckers, but do not succumb rapidly to systemic insecticides, because their large size and short feeding period (often not in the main sap stream) probably mean that they do not pick up enough chemical. This means that stinkbugs must be controlled with a chemical that has contact properties as well. Monocrotophos will provide this function but has a longish harvest interval which must be observed. Since the bugs feed on the developing fruit (at least on beans), this may not be possible. Trichlorfon, carbaryl, some of the synthetic pyrethroids and endosulfan, which are shorter-lived, may be useful here, acting purely as contact chemicals. In Macadamia nut crops grown in South Africa, a number of synthetic pyrethroids are recommended when the nuts are green and marble-sized to prevent damage which later appears as brownish lesions in the kernels . TOP

Green stink bug.

Green stinkbug on sunflower.

Nymph.