Frankliniella occidentalis (Pergande), the Western
flower thrips, is one of the premier insect
pest species. It occurs worldwide,
and causes problems leading to crop losses in the field and in
greenhouses.
Female Western flower thrips are slightly larger than females
of other flower infesting species. Females range in color from
yellow to dark grayish-brown. Yellow forms usually have gray bands
on each abdominal segment,
and may have areas of the head and thorax
shaded with gray. Males are generally paler and smaller than the
females. Western flower thrips may be confused with Frankliniella
williamsi (Hood), which primarily infests grasses, especially
corn. The females of F. williamsi are generally
paler yellow with lighter-colored body setae
than in the western flower thrips.
The antennae are eight segmented. The apex of the second antennal
segment is simple. The pedicel
of the third antennal segment is simple, or may have a slightly
sinuous outline. The postocular
setae are about as long as the interocellar
setae.
The anteromarginal and anteroangular
setae are well developed and of about the same length. Wings are
well developed, with dark gray to black fringe cilia. These may
form a dark line down the center of the abdomen when the wings
are folded.
The comb on abdominal segment VIII is complete.
The western flower thrips infests a wide range of wild and cultivated
hosts. It is particularly troublesome
in tomatoes, peppers, some leafy vegetables and cucurbits. Ornamental
crops, especially cut flowers, such as roses and chrysanthemums
can be devastated (Frantz & Mellinger 1990, Yuding, et al.
1988, Robb & Parrella 1991).
Western flower thrips are important vectors
of tomato spotted wilt virus (Peliwal
1976, Sakimura 1962, Salguero Navas 1991). Crops especially susceptible
to the virus are tomatoes, peppers, and leafy vegetables, notably
lettuce in Hawaii.
This thrips destructively feeds on many vegetable crops, as well
as on cotton. Feeding damage can occur on both fruits and foliage.
Damage in peppers can be severe when the infest the walls of developing
pods during high population periods. Feeding damage in these situations
occurs where fruits rest against each other or against stems or
leaves. Feeding damage on pepper pods takes the form of ringspots
around points of fruit-to-fruit contact, or bands along the point
of contact with the stem. In cases where thrips have congregated
under leaves, the outline of the leaf is often etched into the
fruit wall by their feeding.
Among thrips infesting tomatoes, western flower thrips are unusual
in that their egg laying punctures can cause economic damage.
These wounds develop into white bumps 2-3 mm in diameter. USDA
grading standards currently reduce the grade quality if 5-7 oviposition
wounds are present on a fruit. Similar wounds are also known from
some fruit crops, such as white seedless grapes.
Managing western flower thrips is challenging for several reasons.
Years of exposure to many insecticides
has led to a high level of resistance to all but a few active
ingredients. Which insecticide to use may vary from one farm site
to the next, based on past pesticide use patterns. Contacting
the local Cooperative Extension agent or University personnel
for the latest recommendations in suggested.
Some of the crops most affected by western flower thrips are harvested
on relatively short intervals, or may require workers to be in
the field or greenhouse frequently. In such cases, the use of
synthetic insecticides may pose personal safety or crop residue
problems. Alternative control measures may be practical in such
situations.
These include releasing predators,
such as minute pirate bugs or the predatory
phytoseiid mites, Amblyseius(=Neoseiulus) cucumeris
or A. degenerans. A pathogen, currently under development,
Beauveria bassinia, has also shown good efficacy
against western flower thrips in greenhouse cut flowers. Releasing
such biological control agents
may be most effective in a closed setting, such as a greenhouse.
Releases in outdoor settings may have some value, but the economics
of purchasing and releasing beneficials
on a commercial basis becomes prohibitive in all but the smallest
fields. The preferred approach is to closely monitor the numbers
of thrips and which species are present. In crops where TSWV is
not a threat, a certain number, for example, 2-3 per pepper bloom,
may be acceptable if other species are also present to compete
for resources. Following a biologically-based pesticide
program will also promote the activity of naturally occurring
predators, such as minute pirate bugs. (See the discussion of
management practices under Frankliniella bispinosa).