HortFACT
Identification
Adults are 18-20mm long, rest with the wings held roof-wise, and have the outline broken by crests on the thorax. The wings are purplish-brown or brassy-brown shaded; the forewings always have a central, creamy-silver, broken 'Y' mark. Adults fly readily by day, especially during overcast conditions.
The eggs are small, round, and pallid, and are usually laid singly or in small clusters on the undersides of leaves.
The larva [caterpillar] is a "semi-looper", having only two pairs of abdominal prolegs, instead of the normal four. It walks by "looping", alternately extending the front of the body and drawing up the hind part to almost meet it. All instars [growth stages] of the larva are green, and later instar (larger) larvae have thin white lines along the body, which is characteristically stouter towards the back end. Where populations are crowded, the forelegs and some body stripes may be blackened.
The pupa is enclosed in a cocoon of flimsy white silk in a fold on the underside of a leaf. It is at first bright green, later darkening along the back so that it appears bicoloured, with the back and part of the wing case black and the rest yellowish green.
Host Plants
The main hosts of concern are tomato, bean, potato, and tamarillo; other horticultural hosts are tobacco, mint, passionfruit (black and banana). Foliage and fruit are attacked. Many annual and herbaceous perennial ornamentals, as well as shrubs, e.g., Fatsia and even rangiora, are defoliated. Various large-leaved weeds (Digitalis, Verbascum, Myosotis, Echium) are hosts in wasteland areas.
Damage
The adult moth sucks nectar from flowers. Damage is done by the larva. Early instars feed on the underside of the leaf, scraping "windows" between the veins. Later instars chew right through the leaf, usually leaving main veins intact. On tomato and beans, larvae chew into green fruit. Over 50% of the leaf surface may be destroyed, but there is some evidence that the actively growing parts of the host plant are not readily attacked.
Distribution
The silver Y moth is generally distributed throughout India and the Pacific. In New Zealand its occurrence is sporadic south of Christchurch, but is common from Blenheim (latitude 42oS) northwards in all horticultural areas. It was previously considered to be the same as the Palaearctic (England to Japan) and African silver Y. Ch. Calcites (Esper).
Life cycle
| Egg |  |
| Larvae | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
There are at least four partly overlapping generations per year. In warmer parts of the country (e.g., north of Hamilton, Bay of Plenty coast, Hawke's Bay coast, Tasman and Golden Bays) all stages may be found at all times. Adults are particularly common in Nelson during July and August. As with the light brown apple moth (HortFact 401003) there is no specific winter resting stage; eggs, larvae, and pupae develop more slowly than in summer. Adults are highly mobile, and probably colonise each spring those areas with a rigorous winter.
Eggs take 7-10 days to hatch, larvae 28-42 days to attain full size (35-40mm), and pupae 10-14 days to produce an adult, which can live for over 21 days.
J. S. DUGDALE