Brown shield bug - Dictyotus caenosus
By N A Martin (2016, revised 2017)
Pentatoma caenosus Westwood, 1837
Dictyotus bipunctatus Dallas, 1851
Dictyotus plebejus Stål, 1859
Pentatoma latifrons Walker, 1867
Pentatoma tibialis Walker, 1867
Pentatoma vilis Walker, 1867
Sciocoris polystictica Butler, 1874
Biostatus and Distribution
This adventive shield bug comes from Australia. It is present in the North and South Islands of New Zealand. It is also present in New Caledonia. The Brown shield bug feeds on developing seeds of plants such as plantain (Plantago species) and lucerne.
Conservation status: This Australian shield bug lives in non-forest areas and feeds on seeds of crops and wild plants.
Life Stages and Annual Cycle
The Brown shield bug overwinters as adults that shelter in secluded places. There appears to be a single generation per year with eggs being found in late spring and early summer. Each female lays several batches of eggs. The nymphs grow into adults during summer.
The adults are about 8-10 mm long. The Head, thorax and forewings are mid-brown-brown with small dark punctures. The forewings, which cover the hind wings, also have a network of thin black lines. The black and white edge of the abdomen can be seen protruding from beneath the wings. The underside of the body is pale brown punctures. The three pairs of legs are darkly mottles and have a pale band on the tibia. The antennae are dark brown. On the underside there is a long rostrum that holds the stylets used for feeding.
Several clusters of eggs are laid. Each cluster has about 14 eggs, one for each ovariole. The eggs are white and sculptured. Shortly before hatching red eyes can be seen and a black T-shaped structure, the egg burster, that is used to push up the lid of the egg. The eggs are usually laid in several adjacent rows on plant leaves or similar sites.
The nymphs that hatch from the eggs are like small, black and red, wingless adults. There are five nymphal instars (stages). Nymphs go from one stage to the next by moulting, where the “skin” on the dorsal side splits and the next stage pulls itself out. The first instar nymphs have black heads and thorax. The abdomen is white and red, with a black fringe and black areas in the centre of four segments. Two of these black areas surround the scent glands. The first instar nymphs stay by their eggs after hatching. The second instar is similar, but the background colour of the abdomen is pale brown. The third instar nymphs have a lateral fringe of white on the thorax and small white lateral patches between the abdominal segments. On the fourth and fifth instars the areas that were black are reduced. The rest of the body is shades of brown with fine brown speckles (punctures?). There is a thin lateral white fringe around the thorax and abdomen and there is a short white stripe on the mid-line of the head. The second segment of the antennae is paler than the terminal segment. The wing buds on the fifth instar extend onto the abdomen.
The length of the lifecycle (time from egg to adult) varies with temperature and is faster at higher temperatures.
Walking and flying
The nymphs and adults have six legs (three pairs) that are used for walking. The adults have two pairs of wings. The front pair is modified as covers for the hind wings.
Like other Hemiptera, the Brown shield bug has sucking mouth parts. The long stylets, special shaped rods, are held in the rostrum. The Brown shield bugs feed on plants, especially developing seeds. During feeding the stylets are inserted into the plant. The mandibles hold the rostrum in place. The maxillae are inserted into the plant. They form two tubes, a narrow duct down which saliva is pumped into the plant, and a larger tube up which the partly digested food is sucked.
The first instar nymphs stay by their eggs after hatching and apparently do not feed.
There are several species of Pentatomidae in New Zealand that have brown adults. The nymphs of which are also distinctive.
Brown shield bug, Dictyotus caenosus is a seed feeder. It is more rounded than other species and is a uniform mid brown colour with fine dark punctures and a network of this dark lines on the forewings.
The brown soldier bug, Cermatulus nasalis, is coloured shades of orange-brown to dark brown with black markings and is covered with fine punctations that are usually surrounded by a ring of pale to dark brown or black. The rounded end of the scutellum is usually pale yellow-brown. There is usually a darker patch on the forewing. The antennae are brown.
Schellenberg's soldier bug, Oechalia schellenbergii, is another predator, that has distinct pointed ‘shoulders’. The pale end of the scutellum is pointed.
Adult Pittosporum shield bugs, Monteithiella humeralis, are mainly found on their host plants. The adults are dark brown with pale lateral areas on the pronotum. The basal half the fifth antennal segment is pale.
Fifth instar nymphs
Brown shield bug last stage nymph has a dark grey-brown head and thorax. The abdomen is pale with numerous brown punctures. The thorax and abdomen has a thin white lateral fringe. The head has a pale median line. The areas around the scent gland openings are dark brown. The antennae are dark brown.
The last nymphal stage of the brown soldier bug is mostly black and white. The lateral edge of the pronotum is white and there are white marks on the lateral edges of the abdomen. There is also a prominent white area by the wing buds. The tibia of each leg has a white band. The antennae have red on segments 2, 3, and four. The fifth instar of the subspecies, C. nasalis turbotti, is shiny green instead of black. Australian photos of nymphs are quite different from those illustrated here from around Auckland. Australian specimens are much redder.
Schellenberg's soldier bug last stage nymph is mainly black. There is a thin white lateral edge on the pronotum. On the abdomen there is prominent red area around a central black areas associated with the scent gland openings. The legs also have white bands on the tibia. The antennae are dark.
Pittosporum shield bugs last stage nymph is dark brown and has pale bands on the antennae. The nymphs are normally only found on host plants.
Brown shield bugs have been eaten by several species of birds and the Polynesian rat, Rattus exulans.
Eggs of the Brown shield bug may be parasitized by two species of tiny wasps belonging to the family Platygasteridae. Trissolcus oenone (Dodd, 1913), a native species, parasitizes several native shield bugs. Another egg parasitoid, Trissolcus basalisi (Wollaston 1858), was released into New Zealand in 1949 to control the green vegetable bug, Nezara viridula. It also parasitizes eggs of other shield bugs including the Brown soldier bug. When this wide host range was discovered in the 1960s, it was regarded as beneficial, because at that time protection of crops was regarded as more important than protecting native insects. Eggs parasitized by T. basalis turn black.
In Australia, the brown shield bug is parasitised by a tachinid fly (Diptera: Tachnidae).
|Scientific Name||Common Name||Classification||Enemy Type||Reliability Index||Biostatus|
|Trissolcus basalis (Wollaston, 1858)||Green vegetable bug egg parasitoid (Wasp)||Hymenoptera: Platygasteridae||parasitoid||10||adventive|
|Trissolcus oenone (Dodd, 1913)||Native shield-bug egg parasitoid (Wasp)||Hymenoptera: Platygasteridae||parasitoid||10||native|
|Acridotheres tristis (Linnaeus, 1766)||Indian myna (Bird)||Passeriformes: Sturnidae||omnivore||10||adventive|
|Corvus frugilegus Linnaeus, 1758||Rook (Bird)||Passeriformes: Corvidae||omnivore||10||adventive|
|Rattus exulans (Peale, 1848||Polynesian rat (Mammal)||Rodentia: Muridae||omnivore||10||adventive|
|Sturnus vulgaris Linnaeus, 1758||Starling (Bird)||Passeriformes: Sturnidae||omnivore||10||adventive|
Although the Brown shield bug is widespread there are few recorded host plants. It has mainly been found feeding on ripening fruit and seeds of herbaceous plants and Rubus species.
|Common Name(s)||Scientific Name||Family||Reliability Index||Biostatus|
|Alfalfa, Lucerne||Medicago sativa L.||Leguminosae||10||naturalised|
|English plantain, Lamb's tongue, Narrow-leaved plantain, Rib-grass, Ribwort, Ripple grass||Plantago lanceolata L.||Plantaginaceae||10||naturalised|
|Blackberry||Rubus fruticosus L.||Rosaceae||10||naturalised|
|Boysenberry, Loganberry, Pacific blackberry||Rubus ursinus Cham. & Schltdl.||Rosaceae||10||cultivated|
|Wheat||Triticum aestivum L.||Gramineae||10||naturalised|
Why Stink bugs
Pentatomidae are often called stink bugs because when handled they emit a strong smell. The nymphs have prominent glands on the upper (dorsal) side of their abdomen, while adults have glands between the bases of their legs. The chemicals may deter predators and cause other bugs to drop to the ground, but some of the chemicals produced may also act as aggregation pheromones.
Lariviere M-C. 1995. Cydnidae, Acanthosomatidae, and Pentatomidae (insecta: Heteroptera): systematics, geographical distribution, and bioecology. Fauna of New Zealand. 35: 1-107.
Lariviere M-C, Larochelle A. 2004. Heteroptera (Insecta: Hemiptera): catalogue. Fauna of New Zealand. 50: 1-330.
The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.
Landcare Research New Zealand Limited (Landcare Research) for permission to use photographs.
12 December 2016 N A Martin, Natural Enemies: Corrected biostatus of natural enemies in the table