Insects - Pests of Ornamental Trees, Shrubs and Flowers - page 40

LIFE HISTORY
Adult whiteflies first appear in late May and early June,
depositing eggs in clusters on the underside of leaves.
The eggs, which are liberally dusted with white waxen
powder, hatch in 2-3 weeks. Nymphs then develop on
the leaves. Individuals 'pupate' a few weeks later.
A second generation of adults appears in August and
September. Nymphs of this second generation
overwinter, and complete their development in the
spring. In favourable, southerly regions of mainland
Europe there are three or more generations annually.

DESCRIPTION
Adult: 1 mm long; body pale yellow; wings pure white
and held relatively flat when in repose. Egg: broadly
conical, yellowish but soon becoming grey. Nymph:
light green, oval, flat and scale-like. Pseudo-pupa: oval
and whitish, with relatively short marginal wax
processes and several pairs of long waxen dorsal tubes.

LIFE HISTORY
Infestations occur on the underside of leaves of many
cultivated greenhouse plants, as well as on various
weeds. Adults usually congregate on the foliage towards
the tops of the plants and fly rapidly if disturbed. Each
female is capable of depositing over 200 eggs during
her lifetime, individuals usually surviving for 3-6
weeks. The eggs are laid in distinctive circular groups
on smooth leaves but tend to be more scattered on hairy
ones. They darken soon after laying and hatch in about
nine days at greenhouse temperatures of 21ºC. Newly
emerged nymphs crawl over the leaf surface for a short
while but soon settle down to feed, inserting their
mouthparts into the leaf tissue and remaining
completely immobile throughout the rest of their
development. They pass through three feeding stages
and a non-feeding pseudo-pupal stage; new adults
appear about 18 days later. Breeding, which is mainly
parthenogenetic, is continuous under favourable
conditions, and there are several overlapping
generations annually. Between cropping, the pest
commonly survives on greenhouse weeds or on outdoor
plants in the near vicinity. Adults also hibernate
throughout the winter on outdoor weeds, but they are
unable to survive if weather conditions are severe.

DAMAGE
Infestations are usually unimportant, although foliage is
contaminated by whitish wax and by honeydew upon
which sooty moulds develop.

Trialeurodes vaporariorum (Westwood) ( 60 )
Glasshouse whitefly
A tropical and subtropical species, introduced
accidentally into northern Europe where it is now a
widespread and notorious greenhouse pest. Infestations
occur on various ornamentals, including Asparagus
plumosus, Begonia, Calceolaria, Cineraria, Coleus,
Dahlia , diviner's sage ( Salvia divinorum ), Freesia ,
Fuchsia , Gerbera , Hibiscus , Pelargonium , poinsettia
( Euphorbia pulcherrima ), primrose ( Primula vulgaris ),
Solanum , tobacco plant ( Nicotiana ) and many others;
Chrysanthemum is a poor host and less commonly
attacked. Infestations also occur on seedling trees and
shrubs raised under protection and on some, such as
London plane ( Platanus hispanica ), growing outdoors.

DAMAGE
Attacked plants are weakened and growth checked, the
leaves sometimes becoming spotted with yellow or
otherwise discoloured; heavy infestations often lead to
premature leaf fall. Plants also become contaminated
with sticky honeydew and covered in sooty moulds.

60

60 Colony of glasshouse whitefly ( Trialeurodes vaporariorum ).