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Western Flower Thrips Frankliniella occidentalis

Western flower thrips are minute insects, about 0.03 inch long, with two pairs of fringed wings. The adult 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 an 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.

First-instar nymphs are opaque or light yellow, turning to golden yellow after the first molt. The nymphal stage lasts from 5 to 20 days.

Damage

Nymphs hatch and feed in numbers on fruits, often under the drying calyx or flower parts. Their feeding scars the surface of the fruit. These scars enlarge as the fruit grows, and may cause fruit deformity. Thrips can also cause silvering just before fruits mature.

Although some feeding does take place on blossoms, little damage results until fruit forms. Thrips can damage terminal shoots and cause them to stop growing. Usually one to two small dead leaves cling to the terminal. Buds just below the terminal grow, giving the branch a bushy appearance.

Management

Western flower thrips overwinter as adults in weeds, grasses, alfalfa, and other hosts, either in the orchard floor or nearby. In early spring, if overwintering sites are disturbed or dry up, thrips migrate to flowering trees and plants and deposit eggs in the tender portions of the host plant, e.g. shoots, buds, and flower parts

Cultural Control

Thrips are often attracted to weeds blooming on the orchard floor. To prevent driving thrips into the trees, do not disc the cover crop when trees are in bloom. Open, weedy land adjacent to orchards should be disced as early as possible to prevent thrips development and migration of adults into orchards.

Organically Acceptable Methods

Cultural controls, clean cultivation, and sprays of the Entrust formulation of spinosad are organically acceptable tools.

Monitoring and Treatment Decisions
Begin monitoring thrips as trees begin to bloom. Monitor for thrips by examining blossoms from trees by slapping a shoot with five to ten blossoms against a yellow card or look for the immature stages within the blossoms. Often nymphs are not dislodged by the slapping method so also dissect individual flowers and examine them with a hand lens for nymphs. First instar nymphs are white in color and often difficult to see, so be sure to check carefully. Check a minimum of 50 trees per orchard for nymphs. In warm springs, adults will often migrate in and out of a block without being detected so it is important to always sample for nymphs.

If two or more adult thrips are present or if any nymphs are found, a treatment is warranted. If a treatment is applied, make it before the calyx becomes tight around the developing ovary. If nymphs are found under the jacket after it tightens around the fruit, use methomyl.

Literature from UC IPM Davis:  http://ipm.ucanr.edu/PMG/r540300411.html
K. R. Day, UC Cooperative Extension, Tulare County
K.Tollerup, UC IPM Program, Kearney Agricultural Center, Parlier

In FieldClimate we use following conditions for calculation of the risk model: 

Risk model for The western flower thrips [Frankliniella occidentalis (Pergande) based on daily values:

Risk increases, when  air temperatures higher than 18°C and smaller than 32°C and relative humidities higher than 70% by 20%
Risk decreases, when air temperature higher than 32°C for more than 8 hours or
if relative hiumidity is never higher than 70% or if all temperatures below 15°C reduce risk by 10%