INTERESTING INSECTS

By R J B Hoare and N A Martin (2019)

Golden-brown fern moth

Isopteryx nitidalis Walker, 1866

Diathrausta timaralis Felder & Rogenhofer, 1875

This species was described from Australia under the name Isopteryx nitidalis, and later from New Zealand as Diathrausta timaralis. The Australian and New Zealand moths are currently considered to belong to the same species, but there are slight differences in wing pattern and further studies are warranted. Recently this moth has begun breeding in southern Britain, where the early stages were doubtless imported on ferns.

Within New Zealand, two rather distinct forms of this species occur. The more golden or reddish brown form has larvae feeding on Dennstaedtiaceae, Dryopteridaceae or Pteridaceae. A usually darker and larger form has larvae feeding on Asplenium oblongifolium and occasionally A. bulbiferum. Morphological differences between the two are slight and mostly variable, so it is not clear at present whether one or two species are involved. The Asplenium feeding ‘form’ will be treated on a separate factsheet.

This native moth is found in Australia and throughout New Zealand, including the Chatham Islands and the Subantarctic Islands, where its fern host plants occur. It is found in native ecosystems on ferns in the families Dennstaedtiaceae, Dryopteridaceae and Pteridaceae.

Conservation status: It is found where its host plants grow in native habits, not threatened.

There are several generations per year in warmer areas such as Auckland, but there may be only one generation a year in colder areas.

At rest the adult moths are triangular, about 8-10 mm long and with 15-20 mm wingspan. The upper side of the forewings is rich reddish brown to darker brown with areas of paler brown, white and black; the white is arranged into bead-like marks. The hindwings are paler, whitish with a suffusion of brown and a dark spot on the outer margin. The underside of both wings has markings similar to the upper side but paler. The body is reddish brown on top and paler on the underside, with some white areas on the sides at the base of the abdomen. The area between the base of the legs (underside of thorax) is white. The three pairs of legs are banded brown and white. The head bears a pair of fairly long antennae that are folded back in a narrow V when the moth is at rest. It is presumed that the female moth attracts the male with pheromones (special species-specific scents) after which they mate. The female moth then lays eggs on young fern fronds.

Several eggs may be laid on a young fern frond, probably on the underside. A caterpillar hatches from an egg and feeds on the underside of the fern frond. At first the caterpillar just eats the frond tissue leaving the upper epidermis (skin) intact, creating a ‘window’ in the fern frond. Older caterpillars may eat right through the frond making holes in the frond or notches in the edge of the frond. When the caterpillar is feeding and growing, it moults (changes skin). The young caterpillar is white to pale green and has a black head capsule. Larger caterpillars have glassy, translucent green bodies and the pale head capsule has blackish markings. When fully grown the caterpillar is about 10 mm long. It uses its jaws to bite off pieces of leaf tissue at the edges of the ‘windows’. The caterpillar has three pairs of legs on the thorax, and five pairs of prolegs (false legs) on abdominal segments 3, 4, 5, 6 and 10.

When the caterpillar is full grown it finds a sheltered part of the fern frond and attaches itself by the tip of its abdomen to the frond. It may also be held in place by a silk thread. It then moults into a pupa that turns brown with a few darker markings. Shortly before moth emergence, the colour of the wings and legs can be seen through the skin of the pupa. When the moth is ready to emerge the skin of the pupa at the front splits and the moth pushes its way out. It hangs upside-down from the plant while its body hardens and its wings dry.

Adults of the Golden-brown fern moth, Musotima nitidalis, are similar in appearance to other species of Musotima in New Zealand, especially M. ochropteralis (Australian maidenhair fern moth). Musotima nitidalis differs in having a large, white, more or less comma-shaped mark next to the forewing postmedian line at 2/3 (no such mark in M. ochropteralis) and 2 discrete white discal spots at ½ forewing length (where M. ochropteralis has a single comma-shaped spot). The hindwing is distinctly marked in M. ochropteralis, with clear white postmedian and subterminal lines on a golden-brown ground; M. nitidalis has the hindwing much paler than forewing and with indistinct cross-lines. As mentioned above, fern moths reared from Asplenium oblongifolium and A. bulbiferum may represent a species separate from M. nitidalis; these moths are very similar in appearance, but on average slightly larger, and with the forewing tending to be predominantly blackish brown in colour; the markings are identical to those of M. nitidalis.

The caterpillars of the Golden-brown fern moth are found on ferns belonging to three families, Dennstaedtiaceae, Dryopteridaceae and Pteridaceae. They are the only species of Musotima on these ferns. The related Maidenhair Fern moth, Musotima aduncalis is the only species found on native species of maidenhair ferns, Adiantum species (Pteridaceae) and the Australian maidenhair fern moth, Musotima ochropteralis is found on the introduced European maidenhair fern, Adiantum capillus-veneris and Delta maidenhair fern, A. raddianum.

Musotima caterpillars can be recognised by their glassy, translucent appearance, bright green colour and the distinctive feeding damage done to their host plants. The young caterpillars are pale green with a black head capsule. The older caterpillars are bright green with long dark setae (‘hairs’) and have a pale head capsule with blackish markings. The young caterpillars eat the frond tissue leaving the upper epidermis (skin) intact. Older caterpillars may chew right through the frond.

No pathogens or predators of Golden-brown fern moth have been reported. It is likely that birds, spiders and predatory insects catch and eat the moths.

Parasitoids

Three parasitoids of Golden-brown fern moth, Musotima nitidalis, have been found. One parasitoid wasp, Aucklandella sp. (Hymenoptera: Ichneumonidae), kills the caterpillar after it has pupated. When the adult wasp is ready to emerge, it chews a hole in the moth pupal skin.

A second parasitoid wasp, Diadegma sp. (Hymenoptera: Ichneumonidae), kills the caterpillar when it is fully grown and about to pupate. The wasp larva emerges from the caterpillar and spins an oval, dark grey cocoon in which it pupates. When the adult wasp is ready to emerge it chews a round hole in its cocoon.

The adult female of the third species, the Basket-cocoon wasp, Meteorus pulchricornis, lays an egg in the small caterpillar. When the wasp larva is fully grown it makes a hole in the still live caterpillar and attaches a thread to the fern frond and spins its loose weave basket inside which it spins a cocoon in which it pupates. The basket like cocoon hangs from the fern on the thread.

Table: Natural enemies of Golden-brown fern moth, Musotima nitidalis (Lepidoptera: Crambidae), from Plant-SyNZ database (31 January 2019). The reliability index shows the quality of evidence for the host association (0-10, 10=high quality).

Scientific Name	Common Name	Classification	Enemy Type	Reliability Index	Biostatus
Aucklandella sp.	(Wasp)	Hymenoptera: Ichneumonidae	parasitoid	7	endemic
Diadegma sp.	(Wasp)	Hymenoptera: Ichneumonidae	parasitoid	7	unknown
Meteorus pulchricornis (Wesmael, 1835)	Basket-cocoon parasitoid (Wasp)	Hymenoptera: Braconidae	parasitoid	10	adventive

Caterpillars of the Golden-brown fern moth, Musotima nitidalis, feed on young fronds of ferns in three families (Dennstaedtiaceae, Dryopteridaceae and Pteridaceae). Initially the caterpillars just eat the frond tissue leaving the upper epidermis (skin) intact. Older caterpillars may eat the right through the frond.

Table: Host plants of the Fern moth, Musotima nitidalis (Lepidoptera: Crambidae) from Plant-SyNZ database (31 January 2019). The reliability score shows the quality of evidence for the host association (1-10, 10=high).

Common Name(s)	Scientific Name	Family	Reliability Index	Biostatus
Water fern, Histiopteris, Mātā, Mātātā	Histiopteris incisa (Thunb.) J.Sm.	Dennstaedtiaceae	9	indigenous, non-endemic
	Hypolepis ambigua (A.Rich.) Brownsey & Chinnock	Dennstaedtiaceae	9	endemic
Giant hypolepis	Hypolepis dicksonioides (Endl.) Hook.	Dennstaedtiaceae	9	indigenous, non-endemic
Smooth shield fern	Lastreopsis glabella (A.Cunn.) Tindale	Dryopteridaceae	9	endemic
	Lastreopsis microsora (Endl.) Tindale	Dryopteridaceae	9	indigenous, non-endemic
Hard fern, Scented fern, Lace fern, Ring fern, Mātā, Mātātā	Paesia scaberula (A.Rich.) Kuhn	Dennstaedtiaceae	9	endemic
Austral bracken, Bracken, Bracken fern, Common fern, Manehu, rahurahu, Rārahu, Rarauhe, Rarauhe-mahuika, Tākaka	Pteridium esculentum (G.Forst.) Cockayne	Dennstaedtiaceae	9	indigenous, non-endemic
Sweet fern	Pteris macilenta A. Rich.	Pteridaceae	9	indigenous, non-endemic

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.