Western Flower Thrips

May 9, 2008

DESCRIPTION Adults: Western flower thrips is about 1 mm long, with the female larger than the male.
The female varies from yellow to dark brown, and has a more rounded abdomen. The male is always pale
yellow and has a narrower abdomen.

Eggs: Yellowish eggs cannot be seen because they are laid into the plant tissue.

Larvae: The larvae develop through two instars and are distinctly yellow. Second instars become
whitish prior to molting.

Prepupae and Pupae: Both prepupa and pupa are yellowish, quiescent non-feeding stages. The
antennae and wing pads are typical for most thrips species.
BIOLOGY Distribution: The distribution was thought to be limited to west of the Mississippi
River prior to 1980. However, this thrips has become the most prevalent species attacking
greenhouse flowers throughout the United States and Canada, and many countries in Europe and Asia.

Host Plants: This thrips feeds on almost any flowering plant. Carnations, chrysanthemums,
gerberas, geraniums, marigolds, pansies, and roses are the major host plants.

Damage: The western flower thrips feeds on the flowers and foliage by inserting its modified
left mandible into the tissue, and sucking the fluids from cells. Oviposition and feeding
scars reduce the aesthetic quality and marketability of ornamental plants. When thrips feed
on developing tissues, affected cells are unable to expand, and mature leaves and petals are
distorted. When thrips feed on expanded tissue, effected cells become filled with air, which
imparts a silvery appearance. This thrips also is an important vector of tomato spotted wilt
virus and impatiens necrotic spot virus.

Life Cycle: Females lay eggs in tender plant tissue. The eggs hatch in 2 to 14 days, depending
on temperature. First instar larvae begin feeding on egg eclosion. Second-instar larvae also
feed on plant tissue, usually in flowers. These larvae are found in the protection of perianth
of the flower or within developing terminal foliage. Late in the second instar they stop
feeding and move down the plant to pupate. Thrips develop through two quiescent, non-feeding
pupal stages in the soil, plant litter or in a protected area on the plant. Adults emerge and
resume feeding on flowers, buds, and terminal foliage. The entire life cycle from oviposition
to adult emergence can take 12 days in hot weather to 44 days in cool weather.

CONTROL: The thrips are difficult to manage with pesticides because of their thigmotactic
behavior causes them to feed deep in the flowers and buds where they are sheltered from chemicals.
Chemical management of western flower thrips has received much attention, but control remains difficult.
Natural enemies have been investigated and biological control programs using insidious plant bugs
and predaceous mites in the genus Amblyseius have been used in greenhouses. Screening has been shown
to effectively exclude western flower thrips.

Written by Mike · Filed Under Pests