INTERESTING INSECTS

By N A Martin (2016)

Gumtree scale ladybird

Scymnus ventralis Erichson, 1842

Scymnus restituor Sharp, 1889

Scymnus halli Broun, 1914

This adventive ladybird from Australia is found in the North Island, and Canterbury and Nelson in the South Island of New Zealand. In 1898 it was deliberately released into New Zealand to control its primary prey, the gum tree scale insect, Eriococcus coriaceus (Hemiptera: Eriococcidae), a pest on some species of Eucalyptus trees. In the past it has been confused with the slightly smaller Forestier's ladybird, Rhyzobius forestieri, but they have different prey preferences and distribution in Australia.

Conservation status: This adventive ladybird is widespread, and contributes to the biological control of the gum tree scale on Eucalyptus trees.

The adult ladybirds are about 4 millimetres long and 3 mm wide. The head, prothorax (first part of the middle body) and elytra (wing covers) are black, and covered in short pale setae (hairs). The legs are black, but the tarsi (feet) and antennae and mouth parts are reddish brown as is the abdomen and parts of the area between the legs. Under the elytra is a pair of wings used for flying. The small head has a pair of compound eyes and two short antennae.

The female has a long thin, but heavily sclerotised ovipositor that can be inserted into the ovisac of the female scale. The pale yellow egg is oval, 0.7 mm long and 0.3 mm wide and usually placed horizontally. A single egg is either placed singly against the female scale or amongst the wax produced by the scale nymphs.

A larva hatches from each egg. The newly hatched larvae are orange-grey and covered with short hairs on tubercles with a few longer setae especially laterally (along their sides). 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 dark grey to almost black with an underlying orange to red colour. There is white wax on the tubercles though the amount of wax varies between individuals. The red colour of the larvae may be associated with the scale insects they are feeding upon. The small head is dark brown. When the fourth larval instar is fully grown, it attaches itself to a sheltered place on a plant. It may produce more wax before it moults into a pupa. The red-brown pupa is covered with fine and coarse setae. The moulted larval skin with white fluffy wax 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, but there are no observations on the annual cycle of this ladybird in New Zealand. The adults probably overwinter and there are probably two or three generations in summer.

Walking and flying

Both adult and larval stages of the gumtree scale 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 gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) with eaten gum tree scale Eriococcus coriaceus (Hemiptera: Eriococcidae) on Eucalyptus leaf. Creator: Tim Holmes. © Plant & Food Research. [Image: 2D5T]

Side view of adult gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D5W]

Underside of adult gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2D5V]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus leaf. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5N]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus twig. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5L]

Larva of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus twig. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5K]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus twig, note the moulted larval skin and the dark last instar (stage) larva with white wax at the base of some setae on the tubercles. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5Q]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus leaf, note the moulted larval skin and the dark faecal pellets on the leaf. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5X]

Underside of larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae). Creator: Minna Personen. © Plant & Food Research. [Image: 2D5Z]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae), note the white wax. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5Y]

Prepupal larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae), note the white wax. Creator: Minna Personen. © Plant & Food Research. [Image: 2D62]

Pupa of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae), note the moulted larval skin with fine white wax at the base of the pupa. Creator: Tim Holmes. © Plant & Food Research. [Image: 2D5S]

Pupa of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae), note the moulted larval skin without white wax at the base of the pupa. Creator: Minna Personen. © Plant & Food Research. [Image: 2D60]

There are several black Rhyzobius species in New Zealand, the gumtree scale 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, gumtree scale is larger and more elongate. Also the anterior angle of the pronotum is concave and not convex with a lateral groove.

Aola Richards in her 1981 paper describes some of the differences in the appearance of the larvae and pupa of the two species The larvae of the two species are similar in appearance though 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 at first pale brown with a darker central thorax area. It then becomes darker brown. 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, these are more prominent and coarser on the gumtree scale ladybird pupa.

Adult gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) about 4 mm long. Creator: Tim Holmes. © Plant & Food Research. [Image: 2D5U]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus leaf. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5R]

Larvae of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae) feeding on gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on a Eucalyptus leaf. Creator: Minna Personen. © Plant & Food Research. [Image: 2D5P]

Pupa of gumtree scale ladybird, Rhyzobius ventralis (Coleoptera: Coccinellidae), note the moulted larval skin with fine white wax at the base of the pupa. Creator: Tim Holmes. © Plant & Food Research. [Image: 2D5S]

In New Zealand the gumtree scale ladybird is preyed upon by silvereye (Zosterops lateralis, Aves). They are probably preyed upon by other birds, spiders and predatory insects. In Australia, larvae may be attacked by parasitoid wasps.

In New Zealand the gumtree scale ladybird is an important predator of gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae). It also feeds on other kinds of scale insects, including a mealybug. Adults and larvae both feed on scale insects. However, some published records of prey may refer to Forestier's ladybird, Rhyzobius forestieri, because there was confusion about the identity of these two species. Adults and larvae of either of the two ladybirds fed on beetle eggs in captivity.

In Australia, gumtree scale ladybird also principally feeds on gum tree scale. Less frequently eaten prey include scale insects from several families including Margarodidae, Diaspididae and Pseudococcidae.

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 gumtree scale ladybird, Rhyzobius ventralis, contributes to the successful biological control of gum tree scale, Eriococcus coriaceus (Hemiptera: Eriococcidae) on Eucalyptus species. It was released in several countries for the biological control of gum tree scale and scale insects in the family Coccidae. In these other countries, it was not successful. Part of the problem is that for many years its identity was confused with that of the Forestier's ladybird, Rhyzobius forestieri, which feeds on other scale insects. 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 setal sockets on the tubercles. Larvae and adults may also exhibit 'reflexive bleeding' when disturbed. Adults and larvae of Forestier's ladybird produce tomato red, orange brown or 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.

Clark AF 1938. A survey of insect pests of eucalypts in New Zealand. New Zealand Journal of Science and Technology 19(12): 750-761.

Moeed A.1979. Foods of the silvereye (Zosterops lateralis; Aves) near Nelson, New Zealand. New Zealand Journal of Zoology 6(3): 475-479.

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.

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