INTERESTING INSECTS

By N A Martin (2016, revised 2017)

Forest shield bug, Sedge seed shield bug

Cimex vittatus Fabricius, 1781

Acanthosoma vittatum Dallas, 1851

Oncacontias bruneipennis Breddin, 1903

This endemic shield bug is present in the North and South Islands of New Zealand. The Forest shield bug is mainly found in native forests. It appears to be restricted to feeding on sedges and trees with small seeds.

Conservation status: The Forest shield bug is widespread in New Zealand forests

The Forest shield bug overwinters as adults that shelter in secluded places. There appears to be a single generation per year with eggs being found from September to January. Each female lays several batches of eggs. The nymphs grow into adults during summer.

Adults are about 4.5-5.5 mm long and more than twice as long as wide. The body is coloured shades of green and brown with a pair of white spots on the forewings. The antennae are mid brown and the mid-brown legs have dark tarsi (feet). The long rostrum that holds the stylets used for feeding, is held between the legs when not used for feeding. Also on the underside is a long abdominal spine that projects forward between the legs.

The pale green, almost spherical eggs are about 1.1 mm long. They appear smooth, but the surface is hexagonally reticulated. They are attached by adhesive. Shortly before hatching, red eye spots can be seen through the shell. Eggs also show dark lines indicating body segments. Several clusters of eggs are probably laid.

The first nymphs that hatch from the eggs are like tiny green, 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 head and dorsal thorax of the second instar are black. The thorax is pale laterally. The abdomen is pale green with black around the scent glands. The antennae and legs are amber coloured. The third instar nymph is similar. The fourth and fifth instar nymphs are green with black and white around the three scent glands. The antennae are wine-red except the terminal segment that is almost black. The joints are white. 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.

Nymphs required high humidity for successful rearing.

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.

Feeding

Like other Hemiptera, the Forest shield bug has sucking mouth parts. The long stylets, special shaped rods, are held in the rostrum. The Forest shield bugs feed on plants. The nymphs appear to be associated grass like plants with developing seeds and shrubs with small 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.

Adult Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCH]

Adult Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCG]

Recently moulted adult Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCB]

Underside of recently moulted adult Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCA]

Eggs of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCI]

Hatching eggs of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae), note the tiny green first instar nymphs. Creator: Minna Personen. © Plant & Food Research. [Image: 2BCJ]

Fifth instar nymph of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCC]

Fifth instar nymph of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCF]

Underside of fifth instar nymph of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCD]

Underside of fifth instar nymph of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCE]

Adult Forest shield bugs have a distinctive elongate appearance and green and brown colouration. The white patches on the wings are also distinctive as is the white tip to the scutellum. The nymphs, which are usually found on grass-like plants with seeds are green with black around the scent glands.

Adult Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCH]

Fifth instar nymph of Forest shield bug, Oncacontias vittatus (Hemiptera: Acanthosomatidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2BCC]

The only natural enemy known is an egg parasite, Trissolcus maori Johnson, 1991 (Hymenoptera: Platygasteridae).

The adults are associated with trees and shrubs, while the nymphs have been found mainly on grass-like plants with developing seeds.

Feeding

Like other Hemiptera, the Forest shield bug has sucking mouth parts. The long stylets, special shaped rods, are held in the rostrum. The Forest shield bugs feed on plants. The nymphs appear to be associated grass like plants with developing seeds and shrubs with small 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.

Why Stink bugs

Pentatomidae and associated families 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.

Myers, JG. 1926. Biological notes on New Zealand Heteroptera. Transactions and Proceedings of the New Zealand Institute. 56: 449-511.

The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.

1 November 2017. NA Martin. Additional photograph of a 5th instar nymph added.