Insects - Pests of Ornamental Trees, Shrubs and Flowers - page 231

559

LIFE HISTORY
Adults occur from mid-May to mid-June, eggs being
deposited on the shoots of juniper. Larvae mine within
the shoots from late June onwards, and complete their
development in the following spring. They then pupate
on a main shoot, each in a small cocoon hidden beneath
a flake of bark.

DAMAGE
Infested shoots turn brown, and damage to hosts is often
extensive. Particularly severe infestations have occurred
on pencil cedar ( Juniperus virginiana 'Sky Rocket').

Argyresthia sorbiella (Treitschke) ( 559-560 )
A locally common species associated with rowan
( Sorbus aucuparia ). The larvae tunnel within the young
shoots, and cause the tips to wilt and turn black;
infestations are found occasionally on young amenity
trees but are rarely extensive. Larvae are whitish (up to
6 mm long), with a brownish-black head, prothoracic
plate and anal plate; they feed mainly in May, and may
be found if an infested shoot is split open. Adults
( c . 12 mm wingspan) are mainly whitish, with golden-
brown markings on the fore wings; they occur in June
and July.

559 Argyresthia sorbiella damage to shoot of Sorbus .

560

Plutella xylostella (Linnaeus) ( 561 )
Diamond-back moth
This notorious worldwide crop pest occasionally attacks
brassicaceous ornamentals such as Alyssum , candytuft
( Iberis ) and wallflower ( Cheiranthus cheiri ). However,
it is of far greater significance on vegetable brassicas.
The narrow-winged adults (11-16 mm wingspan) are
habitual migrants, and in some years arrive in northern
Europe in considerable numbers. Larvae (up to 12 mm
long) are mainly pale yellowish green, with a light
brown head; the hind set of abdominal prolegs (anal
claspers) are elongate and clearly visible from above.
The larvae feed on the underside of leaves, beneath
flimsy silken webs, and are capable of causing extensive
defoliation. When fully fed they pupate on the
foodplant, each in a net-like cocoon. In particularly
favourable areas, such as southern Europe, there are
several overlapping generations annually, but in
northern Europe there may be no more than two.

560 Larva of Argyresthia sorbiella .

561

561 Diamond-back moth ( Plutella xylostella ).