How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Scientific names: Western flower thrips: Frankliniella occidentalis
Greenhouse thrips: Heliothrips haemorrhoidalis
In this Guideline:
Thrips are tiny insects that have four featherlike wings, each
consisting of a thick supporting strut with fine hairs on the front and hind
edges. Thrips go through six life stages: egg, first instar, second instar,
prepupa, pupa, and adult. Thrips insert eggs into plant tissue. The first two
instars and the adults feed by piercing and removing the contents of individual
Western flower thrips. This
thrips has three color forms that vary in abundance depending on the time of
year. There is a pale form that is white and yellow, except for slight brown
spots or blemishes on the top of the abdomen; an intermediate color form with a
dark orange thorax and brown abdomen; and a dark form that is dark brown. The
intermediate form is present throughout the year, but in spring the dark form
predominates while the pale form is most abundant at other times throughout the
year. The dark form is an overwintering form that is usually found in foothill
or mountain areas. Its presence in greenhouses in spring indicates thrips are
migrating into the houses. Western flower thrips usually feed in enclosed
tissues such as flowers, buds, or growing tips. Adults also feed on pollen and
on spider mites. The prepupa and pupal stages take place in the soil beneath
infested plants. Females will lay male eggs if unmated and female eggs are
produced once mating has occurred. Development times to complete one generation
of western flower thrips varies from 11 days (77° to 87°F), to 44 days (50° to
Greenhouse thrips. Adult
thrips are tiny, black, insects with
whitish to translucent wings folded back over their thorax and abdomen. Legs
are also a whitish color. Nymphs are whitish to slightly yellowish in color and
produce a globule of fecal fluid at the tip of their abdomen. These globules of
fluid increase in size until they fall off and another one begins to form, resulting
in a characteristic spotting of the infestation area with black specks of fecal
Western flower thrips primarily feeds on flowers but also sometimes
on new vegetative growth, whereas greenhouse thrips feeds primarily on foliage.
Direct feeding damage includes streaking, spotting, and tissue distortion. On leaves, feeding
often occurs along veins and appears as an outlining of the veins. Western
flower thrips can vector tomato spotted wilt
virus as well as many other viruses.
Western flower thrips may cause premature senescence of flowers, such as
African violets, because they prematurely pollinate the flowers. On orchids,
western flower thrips feeding and egg laying will leave translucent 'pimpling'
spots on petals and leaves. Greenhouse thrips stipple the foliage of numerous
field and greenhouse grown plants. The stippling damage caused by thrips
feeding on individual cells is often confused with mite stippling. Thrips
feeding is often accompanied, however, by black, varnishlike flecks of dried excrement whereas mite stippling
is often accompanied by webbing or shed skins.
Prevention is a good strategy in a thrips management program.
Treat plants with an effective insecticide and move them to a holding area for
inspection and potting.
Three commercially available predators to help control western flower
thrips are the minute
pirate bug, Orius tristicolor, and two predatory mites, Neoseiulus cucumeris and
Hypoaspis miles. Minute pirate bugs are polyphagous and will also feed on
aphids, mites, and small caterpillars. Orius
are released at a rate of 2000 to 4000 per acre, while Neoseiulus cucumeris are released at a rate of 10 to 50 mites per
plant for each of 2 to 3 weeks. These mites will also feed on spider mite eggs,
pollen, and fungi. Hypoaspis miles
are soil-inhabiting predators that feed on thrips prepupae and pupae in the
soil. These mites are generally released in the soil at planting and are most
successful at controlling thrips when there is plant-to-plant contact that
facilitates movement of the predators between plants. A commercially available parasite of greenhouse thrips is Thripobius semileteus. For more
information, see BIOLOGICAL
Because western flower thrips and greenhouse thrips feed on a large
variety of plant species, keep production areas free of weeds, which can serve
as hosts for thrips populations. Most commercially available screens have pore
sizes slightly larger than the width of the western flower thrips thorax (145
microns), meaning that some winged adults can penetrate these openings.
However, covering openings to the greenhouse with fine screens does exclude
most thrips. Be sure that the ventilation system on an existing greenhouse can
accommodate the reduced flow caused by a fine screen or else the system will
need to be modified.
Monitoring and Treatment Decisions
Blue sticky cards are most attractive to western flower thrips.
However, yellow cards are good predictors of western flower thrips populations,
are easier to count and are more commonly used for general-purpose insect
monitoring. Place yellow sticky cards vertically in the crop canopy, with the
lower one-third of the trap in the leaves and the upper two-thirds above the
leaves. As the crop grows, the traps will need to be raised. For more information, see MONITORING
WITH STICKY TRAPS.
There is little research on the most effective trap density to
use or on treatment thresholds. Research in California greenhouse roses
suggests that three traps per cultivar is adequate. In greenhouses with many
different cultivars, place traps in the most sensitive varieties, usually
yellow or white flowers. In large greenhouses of the same or similar cultivars,
there should be at least eight traps per 100,000 square feet. The treatment
threshold for roses is 25 to 50 thrips per card per week (25 for more sensitive
yellow- and white-flowered varieties, 50 for reds). In other crops place one
card per 10,000 square feet. Consider treating if an average of 5 to 10 thrips
per card per week is present.
It is important to note that correct identification of pest
thrips is essential in a monitoring program. There may be several species of
thrips present on a sticky card but only the western flower thrips and greenhouse
thrips should be counted when making treatment decisions.
Most insecticides must be applied at
least two times, 5 to 7 days apart, for efficacy against western flower thrips.
Registered for Use on Greenhouse or Nursery Ornamentals
Read and follow the instructions on the label before using any pesticide.
Before using a pesticide for the first time or on a new crop or cultivar, treat
a few plants and check for phytotoxicity. Also consider pesticide resistance
management and environmental impact.
||Mode of action2
||Beauveria bassiana# (BotaniGard 22WP)
||Treat every 7
days while insects are active. Do not tank mix with most fungicides and wait 48 hours after application to apply a fungicide.
within 10 days of breaking seal. Do not apply to stressed plants or newly transplanted material before roots are established.
(PT Pyrethrum TR)
synthetic pyrethroid that is some-times used as an irritant when mixed with other pesticides.
||Synthetic pyrethroids sometimes used as an irritant when mixed with other pesticides.
||Apply in 50 gal water.
Repeat as necessary up to 4 applications/season. Do not apply with oil or foliar fertilizer. Do not use through any type of irrigation system.
|insect growth regulator
insect. Repeated applications as necessary. Label permits low-volume
Do not exceed 22.5 oz/acre/application
||Use no more
than twice per year and don't exceed 52 oz/acre/year. Don't use on poinsettia.
||Label permits low-volume application. Do not use through any type of irrigation system.
||Not to be used
more than once every 16 weeks. Do not apply to soils that are water logged or saturated. Do not apply to bedding plants intended to be used as food crops.
||(Marathon 60 WP)
||As above, but apply only as a drench.
hydrophobic extract of neem oil#
insect. Repeat applications as necessary. Difficult to get coverage in flowers, best for thrips on foliage.
water/runoff restrictions. Some varieties of chrysanthemum exhibit phytotoxicity. Do not apply more than 3 lb a.i./ acre/season.
||A number of
chrysanthemum varieties have exhibited phytotoxic reactions. In greenhouses
only labeled for use on anthurium, cacti, carnation, rose, orchids, some
foliage plants, young poinsettia, and some varieties of chrysanthemum. Can stunt new growth in roses.
(PT 1300 Orthene TR)
||An aerosol for greenhouse use only.
(PT DuraGuard ME)
(PT Duraplex TR)
insect, so thorough coverage is important. Repeat weekly as needed up to 3
times. Test for phytotoxicity. Do not spray new transplants or newly rooted cuttings. Do not add adjuvants.
||Do not apply
more than 10 times in a 12-month period. Compatible with most beneficials,
but highly toxic to bees and hymenopteran parasites. Direct contact can cause
significant mortality to Phytoseiulus
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Insects and Mites
J. A. Bethke, Entomology, UC Riverside
Acknowledgment for contributions to Insects and Mites:
K. L. Robb, UC Cooperative Extension, San Diego County
H. S. Costa, Entomology, UC Riverside
R. S. Cowles, Connecticut Agricultural Experiment Station, Windsor, CT
M. P. Parrella, Entomology, UC Davis
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