INTERESTING INSECTS

By N A Martin (2019)

Cabbage tree mealybug

Trionymus diminutus cordylinidis Brittin, 1938

The endemic Cabbage tree mealybug lives on Cabbage trees, Cordyline species in the North & South Islands. They live at the base of leaves and high numbers of the mealybugs are associated with the death of young, usually unbranched, cabbage trees.

Conservation status: This native mealybug is not endangered and is found in parks, gardens and native ecosystems in the North & South Islands.

Diagramme of the life cycle of a typical mealybug (Hemiptera: Pseudococcidae). Creator: JM Cox. © Drawing published in Fauna of New Zealand 11:1-228, Fig. 1. [Image: 2F19]

The Cabbage tree mealybug appears to breed all year. There do not appear to be discrete generations. It is not known how long it takes from egg to adult. All stages live on the leaves of cabbage trees. Initially the mealybugs are found at the base of the lower live leaves, but populations can increase greatly and the mealybugs colonise the innermost young leaves. High numbers feeding on the leaves is associated with the death of young cabbage trees.

The adult female is oval, about 3-4 mm long and about 2 mm wide. The body is orange pink with a covering of powdery white wax. The mature female has a short pair of 8 segmented antennae and three pairs of legs. There is no distinct division between the head or thorax (middle section of the body) and abdomen. On the underside of the head there is a short rostrum that guides the feeding stylets. The adult female mealybug live in a space between the base of leaves or amongst the litter that accumulates between the base of leaves. She forms a white wax chamber around herself.

The nymphs look like small adult females. There are three female nymphal instars (stages) and two male nymphal instars. These feeding stages grow by moulting (changing skin). The actively feeding nymphs appear to surround themselves with white wax. The second instar male makes a fluffy white cocoon in which develop two pre-adult non-feeding stages, a prepupa and a pupa. The prepupa and pupa have wing buds. The adult male emerges from the pupa. The moulted prepupal and pupal skins are pushed out the end of the cocoon. The adult male does not have a rostrum or stylets and does not feed. When it is ready to emerge from the cocoon, the back end of the cocoon is pushed open and the male backs out. After it has opened the back of the cocoon, its wings (1 pair) expand and harden. It also grows a pair of long wax tails. It is presumed that the wax tails help balance the insect in flight. The male may mate with females of the same colony or fly to another colony to mate.

Feeding and honeydew.

Mealybug adult females and nymphs have sucking mouthparts. Specially shaped rods called stylets are held in the short sheath-like rostrum. When it wishes to feed, the mealybug moves the tip of the rostrum onto the surface of the plant leaf or stem. The stylets are then gradually pushed into the plant and manoeuvred into the phloem (nutrient transport vessels) of the plant. The mealybugs suck the plant’s sap, which is high in sugars and low in other nutrients. Mealybugs have a short white wax anal tube through which they excrete the excess sugary liquid, which is called honeydew.

Walking, flying and dispersal.

The adult male has legs and wings. It can walk around the leaves where its cocoon was and it can fly to other leaves or to different trees. Adult females and nymphs also have legs and can walk. They may move about the group of leaves where they were born. In other insects with a none flying adult female, the first stage larvae or nymphs are able to disperse to new trees. They usually do this using the wind. It is likely that some first instar nymphs climb to a prominent place on a leaf or branch and await a gust of wind.

Female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) exposed on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the white wax surrounding the insect. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302L]

Female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the short antenna (right). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302N]

Female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the lack of white wax filaments around its body. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302M]

Small nymph and underside of a female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) walking on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the short antennae and legs. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302R]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) amongst the debris at the base of a leaf of a Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302T]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) at the base of a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the white wax surrounding the insects. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302S]

Two images of a fluffy white wax cocoon made by Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae). The undisturbed cocoon (right) had some wax pushed off to expose the nymph inside. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302P]

A winged adult male (right) Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) partly under a fluffy white cocoon. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302W]

Two images of a winged adult male (right) Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302X]

A winged adult male Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) by cocoons with nymphs. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302Y]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) squeezed in between leaves in the crown of a Cabbage tree, Cordyline australis (Asparagaceae): note the white wax surrounding the insects, Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302D]

winged adult male (left) Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note its two white wax tails. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302Z]

Mealybugs (Pseudococcidae) require specialist skills for their identification. However, when some species are on their host plants, they have distinct features that enables them to be named with confidence. The Cabbage tree mealybug is one such species. Three species of mealybugs have been found on Cabbage trees, Cordyline species and can be distinguished as follows:

. The Cabbage tree mealybug is mainly found at the base of leaves, often amongst the debris that accumulates there. The mealybug is covered by a thin layer of powdered wax, and only has short caudal (terminal) wax filaments.

. The native Long egg-sac mealybug, Paracoccus glaucus (Maskell, 1879), has short filaments of white wax around its body and one pair of strong white wax filaments at the back. The other distinguishing feature is the long egg sac made by the female.

. The adventive species, the Long-tailed mealybug, Pseudococcus longispinus (Targioni Tozzetti, 1867), has long white wax filaments all around its body with two pairs of very long posterior filaments.

Severe damage to the crown of juvenile cabbage trees may be a sign that Cabbage tree mealybugs are present. Examination of the base of the oldest leaves may reveal the white wax around a single mealybug or a colony. The presence of ants on the stem or in the crown may also indicate that mealybugs are present.

Juvenile Cabbage trees, Cordyline australis (Asparagaceae) in Travis Wetland, Christchurch, with leaves infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note that one tree has many yellow dying leaves. Creator: Grahame Bell. © Grahame Bell. [Image: 305J]

A juvenile Cabbage tree, Cordyline australis (Asparagaceae) infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note the small crown of green leaves and the brown dying leaves at the base of the crown. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3030]

A healthy juvenile Cabbage tree, Cordyline australis (Asparagaceae) with a crown of green leaves extending down the stem Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3034]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) exposed at the base of lower leaf a juvenile Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3032]

Ants and Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) exposed at the base of the lower leaves a juvenile Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3036]

Female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the white wax. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302C]

Female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of a Cabbage tree, Cordyline australis (Asparagaceae): note the lack of white wax filaments around its body. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302M]

Two female Long egg-sac mealybugs, Paracoccus glaucus (Hemiptera: Pseudococcidae) on a leaf of New Zealand flax, Phormium tenax (Hemerocallidaceae): note the pair of white wax tails. Creator: Tim Holmes. © Plant & Food Research. [Image: 2RE0]

A female Long egg-sac mealybug, Paracoccus glaucus (Hemiptera: Pseudococcidae) with an egg sac and first instar (stage) nymphs on a leaf of New Zealand flax, Phormium tenax (Hemerocallidaceae): note the white wax covering the old female. Creator: Tim Holmes. © Plant & Food Research. [Image: 2RE5]

Adult female Long-tailed mealybug, Pseudococcus longispinus (Hemiptera: Pseudococcidae) on a leaf of Queensland kauri, Agathis robusta (Araucariaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300D]

An adult female and nymphs of Long-tailed mealybugs, Pseudococcus longispinus (Hemiptera: Pseudococcidae) on a leaf of a cabbage tree, Cordyline australis (Asparagaceae): note one nymph (bottom left hand corner) has just moulted. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300J]

No pathogens or parasitoids of the Cabbage tree mealybug are known.

Predators

Only one predator of the Cabbage tree mealybug has been observed feeding on the mealybug, a hoverfly, Allograpta ventralis (Miller, 1921) (Diptera: Syrphidae). The larva of a gall fly, (Diptera: Cecidomyiidae), was found associated with a colony of the mealybug. The larvae of a species of gall fly have been found feeding on mealybugs.

Other potential predators include ladybirds (Coleoptera: Coccinellidae).

Gall fly larva (Diptera: Cecidomyiidae) by a colony of Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302H]

Gall fly larva (Diptera: Cecidomyiidae) by a colony of Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302I]

Gall fly larva (Diptera: Cecidomyiidae) by a colony of Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf of Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302J]

The Cabbage tree mealybug has only been found on two species of Cabbage tree, Cordyline species. It may well be on the other species of cabbage tree, but remains to be discovered. It is rarely found on the most abundant species, Cordyline australis.

Several times heavy infestations of the cabbage tree mealybug have been observed to be associated with the death of juvenile trees. However, it is not known if mature cabbage trees are also killed by this mealybug. Several mature trees that have small branches infested with Cabbage tree mealybugs are currently under observation.

Feeding and honeydew

Mealybug adult females and nymphs have sucking mouthparts. Specially shaped rods called stylets are held in the short sheath-like rostrum. When it wishes to feed, the mealybug moves the tip of the rostrum onto the surface of the plant leaf or stem. The stylets are then gradually pushed into the plant and manoeuvred into the phloem (nutrient transport vessels) of the plant. The mealybugs suck the plant’s sap, which is high in sugars and low in other nutrients. Mealybugs have a short white wax anal tube through which they excrete the excess sugary liquid, which is called honeydew.

A healthy juvenile Cabbage tree, Cordyline australis (Asparagaceae) with a crown of green leaves extending down the stem Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3034]

An old Cabbage tree, Cordyline australis (Asparagaceae) with a few dead branches. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3038]

A juvenile Cabbage tree, Cordyline australis (Asparagaceae) infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note the small crown of green leaves and the brown dying leaves at the base of the crown. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3030]

Three months later the juvenile Cabbage tree, Cordyline australis (Asparagaceae) infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) has fewer green leaves and the central leaves are dead. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 303A]

The dead central leaves of a juvenile Cabbage tree, Cordyline australis (Asparagaceae) with a very high infestation of Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 303C]

Side shoots of an old Cabbage tree, Cordyline australis (Asparagaceae) that have a low infestation of Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3039]

Why is there so much white wax?

Most mealybugs produce much white flocculent wax with which they are covered and which also covers the areas of plants they inhabit. To the human eye this makes it much easier to find the colonies of mealybugs. However, does it make it easier for predators and parasitoids to find them, or is the white wax some kind of deterrent and warning colouration? Other insects with a scale stage also cover themselves with white wax. This suggests to me that it may be some kind of deterrent and warning.

Why are Cabbage tree mealybugs killing their host plants?

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) have usually been found on single juvenile plants or small groups of juvenile plants that are isolated from mature plants. These isolated plants have usually been found when the mealybug population is well developed and the plants die within one or two years of the mealybugs being discovered. It seems unlikely that the Cabbage tree mealybug kills all the plants on which it lives. So what is going on?

My guess is that young first instar larvae colonise new plants by chance, being blown there by the wind. Male and female nymphs need to colonise the same isolated plant(s) so that the mealybugs can breed and multiply. Where the cabbage tree mealybug occurs on plants in a mature stand of cabbage trees, I expect that populations are kept under control by their natural enemies. At present only two predators are known. These fly larvae can get down between the cabbage tree leaves and feed on the more exposed mealybugs. Because the mealybugs are at the base of the leaves in the narrow spaces between leaves and amongst the debris, they are probably hard to access by many other mealybug predators like ladybirds. They may also be protected by the dense white wax with which they surround themselves.

It appears that the natural enemies of the cabbage tree mealybug are not good at locating new colonies that are a long way from an existing colony. So Cabbage tree mealybug colonies on isolated plants are at risk of dying out when they kill their host plants.

Observations on some mature trees during the next few years may provide some answers.

Juvenile Cabbage trees, Cordyline australis (Asparagaceae) in Travis Wetland, Christchurch, with leaves infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note that one tree has many yellow dying leaves. Creator: Grahame Bell. © Grahame Bell. [Image: 305J]

A juvenile Cabbage tree, Cordyline australis (Asparagaceae) infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note the small crown of green leaves and the brown dying leaves at the base of the crown. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3031]

White wax surrounding an adult female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) on a leaf at the base of the crown of a Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3033]

An adult female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) exposed by pulling down on a leaf at the base of the crown of a Cabbage tree, Cordyline australis (Asparagaceae): note the dense white wax that enclosed the mealybug. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3035]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) exposed by pulling down on a leaf at the base of the crown of a Cabbage tree, Cordyline australis (Asparagaceae): note the dense white wax that enclosed the mealybugs. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302F]

Ants and Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) exposed at the base of the lower leaves a juvenile Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 3037]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) squeezed in between leaves in the crown of a Cabbage tree, Cordyline australis (Asparagaceae): note the white wax surrounding the insects. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302E]

Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) in a groove in a leaf in the crown of a Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302Q]

Adult female Cabbage tree mealybug, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) in a groove in a leaf in the crown of a Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302U]

A juvenile Cabbage tree, Cordyline australis (Asparagaceae) that is heavily infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). It has few green leaves and the central leaves are dead. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 303B]

The dead central leaves of a juvenile Cabbage tree, Cordyline australis (Asparagaceae) that is heavily infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 303D]

A juvenile Cabbage tree, Cordyline australis (Asparagaceae) in Travis Wetland, Christchurch, with leaves heavily infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note the few upright green leaves. Creator: Grahame Bell. © Grahame Bell. [Image: 305I]

Juvenile Cabbage trees, Cordyline australis (Asparagaceae) in Travis Wetland, Christchurch, with leaves heavily infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note the few upright green leaves. Creator: Grahame Bell. © Grahame Bell. [Image: 305H]

The top of a dying juvenile Cabbage tree, Cordyline australis (Asparagaceae) heavily infested with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). It has few green leaves and the central leaves are dead. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302G]

Heavily infestation of Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae) amongst the base of leaves of a dying juvenile Cabbage tree, Cordyline australis (Asparagaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 302K]

A juvenile Cabbage tree, Cordyline australis (Asparagaceae) that has died after a heavy infestation with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 305F]

The base of leaves of a juvenile Cabbage tree, Cordyline australis (Asparagaceae) that has died after a heavy infestation with Cabbage tree mealybugs, Balanococcus cordylinidis (Hemiptera: Pseudococcidae): note the white wax produced by the mealybugs. Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 305G]

Bowie MH. 2001. Ecology and morphology of Allograpta ventralis (Diptera: Syrphidae) a predator of the cabbage tree mealybug Balanococcus cordylinidis (Hemiptera: Pseudococcidae). New Zealand Natural Sciences. 26: 1-11.

Cox JM. 1987. Pseudococcidae (Insecta: Hemiptera). Fauna of New Zealand. 11: 1-230.

Plant-SyNZ: Invertebrate herbivore-host plant association database. plant-synz.landcareresearch.co.nz/.

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

Auckland Regional Park Rangers for assisting with the study of the cabbage tree mealybug.

Grahame Bell, Christchurch, for supplying photos of badly damaged trees and leaves from infested trees.