Yponomeuta malinellus (Apple Ermine Moth) - Fact Sheet

Background

Apple ermine moth is a defoliator of apple trees throughout Europe and Asia, as far east as Japan. It was introduced into New York State in 1909, into New Brunswick in 1917 and into Ontario in 1957, but was eradicated in all cases.

In 1981 a single colony was collected on Vancouver Island near Duncan. The following year another single colony was collected at Lantzville, also on Vancouver Island. There were no further detection of apple ermine moth until 1985 when it was found to be widely distributed at Bellingham in Washington State. Subsequently it was found at Cloverdale, B.C. the same year. Surveys conducted in 1985-1986 established that the pest occurred over large areas of southeastern Vancouver Island and the southwestern corner of the B.C. mainland through to adjacent Washington State. In 1989-1990 the apple ermine moth for the first time was found at many locations in the interior of B.C. For regulatory purposes the whole province is now regarded as infested. It is not known How, When or Where the apple ermine moth was introduced into western North America.

Host

Malus spp. (apple)

Pest Distribution

WORLD DISTRIBUTION: British Isles, North Korea, South Korea, Japan, Sweden,Finland, Russia (Siberia), China (Manchuria), Iran, Pakistan
North America: Canada (British Columbia) and USA (WA, OR)

Life Cycle and Biology

One generation occurs each year. Oviposition may occur from July through September. Hatching from the eggs takes place in early autumn and the first-instar larvae remain under the protective coating of the egg mass until early spring. After the winter, the larvae move to the unfolding leaves of the bud adjacent to the egg mass. First-instar larvae mine the leaves but towards the end of blossoming, larvae cease mining and feed within a communal web. Second and third-instar larvae feed on the upper epidermis and parenchyma of apple leaves; fourth and fifth-instar larvae devour the entire leaf, leaving only the larger veins and the petiole. The communal webs are extended as necessary to include more leaves. Individual colonies may migrate "en masse" to other areas in the tree after defoliation of a particular area. In June and July, the final-instar larvae spin greyish-white elongated cocoons which are suspended in rows or clusters within or adjacent to the feeding webs. Adult emergence occurs from July to early September. The adults are active at dusk and at night. Females mate 1 to 2 weeks after emergence, releasing a pheromone that attracts males.

Detection & Identification

Symptoms

In shipments of foreign nursery stock, inspectors should look for the small, oval egg masses on the bark near a bud. If the apple trees are not in the dormant state, look for webbing or cocoons. When surveying, attention should be paid to the crown areas of mature trees.

The communal webs are characteristic, occurring from spring into July, with typical spotted larvae feeding only on Malus leaves, or with typical long white cocoons hanging close together, all within the webs. Tent caterpillar (Malacosoma spp.) webs occur at the same time but are in the crotches of branches, the larvae are large and striped and they feed outside of the webs. Fall webworm (Hyphantria cunea) webs normally occur later in the season, the larvae are striped and the cocoons do not occur inside the webs. Ugly-nest caterpillar (Archips cerasivoranus) webs can be somewhat similar to apple ermine moth webs but the larvae and pupae are quite different. Small webs of the apple ermine moth may resemble damage caused by other web-spinning or leaf-tying larvae, but the combined characteristics of web, larvae and pupae, on Malus, as described above, are believed to be unique in Canada.

The early leaf-mining damage is insignificant but developing larvae soon begin to feed communally within a tent, typically beginning with a few leaves at the tip of a twig and extending the tent to cover more leaves as necessary (in B.C. in 1985, these tents appeared most often high in the crown areas). Old literature reports from Europe indicate that in heavy infestations the webs coalesce and may envelop the entire tree; such infestations result in total defoliation. Often tender twigs are also damaged by the starving larvae affecting fruit production for several years following the outbreak.

Identification

ADULT: Small moths (16-20 mm) with snowy-white forewings and grey or leaden hind wings. The wings have long fringes on the lateral and posterior margins. The upper sides of the forewing usually have 15 to 30 small, black dots arranged or distributed in 3 or 4 rows. Head, palpi and antennae white. Thorax white with a few black dots (Fig. 1)

EGG: The individual egg has the appearance of a flattened, yellow, soft disc with the centre area slightly raised, and marked with longitudinal ribbings. Ten to eighty eggs are deposited in overlapping rows to form a flattened, slightly convex, oval egg mass. At the time of deposition, the egg mass is covered with a glutinous substance, which on exposure to air forms a resistant, protective coating. This coating not only acts as an egg-shield but provides an ideal overwintering site for the diapausing first-instar larvae. The egg mass is yellow at first but then darkens until eventually it is grey-brown and resembles the bark of apple twigs. Egg masses average 3-10 mm in length and 4 mm in width but vary considerably in size and shape. The egg mass is usually placed on a new shoot or in axils near a bud (or on two-year old wood) (Figures 2-4)

LARVA: Grey, yellowish-grey, greenish-brown, and greyish-green larvae have been reported. The mature larva is approximately 15-20 mm in length; the anterior and posterior extremities are much narrower than the remainder of the body. There are 2 conspicuous laterodorsal black dots on each segment from the mesothorax to the 8th abdominal segment. Head, thoracic shield and anal plate are black (Fig. 5)

PUPA: Length 6-8 mm, width 2 mm. Head, thorax and abdominal segments 8-10 or 9-10 are dark brown. Remainder of pupa orange-yellow, yellow-brown. In white cocoon, 10-12 mm long and 3 mm wide; densely woven and opaque (Fig. 6)