INTERESTING INSECTS

By N A Martin (2016)

Forestier's ladybird, Black ladybird, Black lady beetle

Platyomus forestieri Mulsant, 1853

Scymnus circularis Sharp, 1889

This adventive ladybird from Australia is found in the North and South Islands of New Zealand. It was released into New Zealand in 1889 to help control scale insects. It has been released in many other countries for the same purpose. In the past it has been confused with the slightly larger, gumtree scale ladybird, Rhyzobius ventralis, but they have different prey preferences and different distribution in Australia.

Conservation status: This adventive ladybird is widespread and helps control scale insects on trees.

The adults are small, about 3.2 millimetres long and 2.3 mm wide. The head, prothorax (first part of the middle body) and elytra (wing covers) are dark brown to black, and covered in short pale setae (hairs). The legs are dark brown and antennae are tan, while the underside is tan coloured. Under the elytra are a pair of wings used for flying. The small head has a pair of compound eyes and two short antennae.

Female ladybirds have a shorter and less strongly sclerotised ovipositor than the closely related gumtree scale ladybird, Rhyzobius ventralis. Eggs are laid singly on, under or against scale insects. The oval, pale yellow eggs are 0.6 m long and 0.25 mm wide and usually placed horizontally.

A larva hatches from each egg. The three pairs of legs are used for walking. As the larva grows, it moults (changes skin). There are four larval instars (stages). The last instar is grey, with a darker band on each abdominal and thoracic segment. The tubercles bear short and medium length seta and a little white wax. The amount of wax varies between individuals. The small head is brown. When the fourth larval instar is fully grown, it attaches itself to a sheltered place on a plant. It may produce more white wax before it moults into a pupa. The pale tan to mid brown pupa is covered with short setae and fewer longer hairs. There is a row of dark spiracular spots along the lateral margin of the abdomen. The moulted larval skin remains at the base of the pupal abdomen. Adults hatch from pupae and mate. The length of time of each life stage depends on temperature, being shorter at higher temperatures.

Annual cycle

Richards (1981) made detailed observations of the development times of each life stage in Sydney, Australia, and more recently in Japan. There are no observations on the annual cycle of this ladybird in New Zealand. There are probably two or three generations in summer in Auckland. They overwinter as adults.

Walking and flying

Both adult and larval stages of Forestier's ladybird have three pairs of legs that can be used for walking. Adults have wings and can fly.

Feeding

The adult and larval ladybirds eat scale insects. The jaws are the primarily structures used for holding and chewing the prey. Legs do not appear to be used for holding food.

Adult Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D70]

Underside of an adult Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D74]

Front view of an adult Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D72]

Side view of an adult Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D73]

Larva of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D6Y]

Larva of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae Creator: Tim Holmes. © Plant & Food Research. [Image: 2D6X]

Pupa of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D75]

Pupa of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D76]

There are several small black Rhyzobius species in New Zealand, Forestier's ladybird is one of the two larger species. These two larger species, Forestier's ladybird, Rhyzobius forestieri and gumtree scale ladybird, Rhyzobius ventralis have often been often confused with each other. A paper by Pope (1981) details how to tell them apart. In addition to differences in genitalia, Forestier's ladybird is smaller and rounder (less elongate). Also the anterior angle of the pronotum is convex not concave, and it often has a lateral groove.

Aola Richards in her 1981 paper describes differences in the appearance of the larvae and pupa of the two species The larvae of the two species are similar in appearance, but the larvae of the gum tree ladybird have an underlying orange or red colour that is visible through the intersegmental membranes. The pupae have several distinctive features. Forestier's ladybird pupa is pale brown with a darker central thorax area The pupa darkens with age. The pupa of the gumtree scale ladybird is dark brown with a strong underlying red colour. In addition, although both are covered with numerous very fine hairs and fewer coarser setae, the latter are more prominent and coarser on the gumtree scale ladybird pupa.

Adult Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D70]

Underside of an adult Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D6Z]

Larva of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D6Y]

Larva of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae Creator: Tim Holmes. © Plant & Food Research. [Image: 2D6X]

Pupa of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D77]

Pupa of Forestier's ladybird, Rhyzobius forestieri (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D75]

No natural enemies of the Forestier's ladybird are known in New Zealand. They are probably preyed upon by birds, spiders and predatory insects.

In Australia, larvae of the mealybug ladybird, Cryptolaemus montrouzieri, fed on Forestier's ladybird larvae and may do so in New Zealand. It is likely that Forestier's ladybird larvae eat each other, because usually only two or three larvae are found near each other.

Adults and larvae of Forestier's ladybird feed on scale insects from several families. Research by Aola Richards in Australia found that the ladybird feeds on species of mealybugs (Pseudococcidae), Coccidae, Eriococcidae and Margarodidae, including species present in New Zealand.

Diverse habits of 'ladybirds'

Not all ladybirds eat insects; some feed on mites. Other species eat plant leaves and are pests especially in some tropical countries, whereas other ladybirds feed on fungi. One of these, Illeis galbula (Mulsant, 1850), from Australia, feeds on powdery mildew fungi. In New Zealand it is common on pumpkins and other cucurbits, plants that are commonly infected by powdery mildews. A plant feeding ladybird, hadda beetle (Epilachna vigintioctopunctata (Fabricius, 1775)) recently established in Auckland feeds on plants in the Solanaceae (potato family).

Biological Control

In New Zealand Forestier's ladybird, Rhyzobius forestieri, contributes to biological control of felted pine scale, Eriococcus araucariae (Hemiptera: Eriococcidae) on Norfolk Island pine and other Araucaria species. It probably also contributes to the control of other scale insects. It has been released in several countries for the biological control of scale insects in the family Coccidae though it is difficult to assess its success. Part of the problem is that for many years its identity was confused with that of the gumtree scale ladybird, Rhyzobius ventralis, which primarily feeds on the gum tree scale, Eriococcus coriaceus. This is an example of the importance careful identification of potential biological agents and the assocaited need to identify their feeding preferences.

Chemical deterrence

Ladybirds have a number of strategies for deterring predators. The larvae of some species produce wax which is a deterrent to some insect predators. The fine white wax threads are extruded from modified setae sockets on the tubercles. Larvae and adults may also exhibit 'reflexive bleeding' when disturbed. Adults and larvae of Forestier's ladybird produce yellow droplets. In the larvae they are produced by the intersegmental membranes and in adults the fluid is produced at the base of their legs. The fluid, which is derived from their blood, is believed to be distasteful to predators and to contain alkaloids.

JongKuk K, Morimoto K. 1995. Biological studies on Rhyzobius forestieri (Mulsant) (Coleoptera: Coccinellidae). Science Bulletin of the Faculty of Agriculture, Kyushu University 50(1/2): 45-50.

Pope RD. 1981. "Rhyzobius ventralis" (Coleoptera: Coccinellidae), its constituent species, and their taxonomy and historical roles in biological control. Bulletin of Entomological Research 71(1): 19-31.

Richards AM. 1981. Rhyzobius ventralis (Erichson) and R. forestieri (Mulsant) (Coleoptera: Coccinellidae), their biology and value for scale insect control. Bulletin of Entomological Research 71(1): 33-46.

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

Alan Flynn for identification of the ladybird.

1 November 2018. NA Martin. Changed symbol used for apostrophes.