How to Manage Pests
UC Pest Management Guidelines
Western Flower Thrips
Scientific Name: Frankliniella occidentalis
(Reviewed 11/09, updated 11/09, pesticides updated 9/15)
In this Guideline:
Description of the Pest
Western flower thrips adults 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.
At bloom, adult thrips insert eggs just under the surface of the developing fruit, causing fruit depressions or dimples as the fruit grows; a faint pansy spot may occasionally be visible around the puncture mark on green fruit but fortunately disappears as the fruit colors. Fruit is downgraded only if the egg-laying scars are numerous. Closer to harvest, high populations of thrips have been associated with the appearance of a silvery "halo" spot on mature fruit, particularly where fruit touch. Late-season damage is more common in Northwest growing areas than in California. In general, thrips damage is not common in California cherry orchards but has been seen in years when populations are exceptionally high during critical periods.
Western flower thrips occasionally develop to high population levels in cherry. They 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 on the orchard floor.
Thrips are attracted to blossoms on trees as well as weeds blooming on the orchard floor. To prevent driving thrips into the trees, do not mow or disc the orchard vegetation 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 individual blocks begin to bloom (see MONITORING PESTS AT BLOOM). In nectarines and peaches, thrips are monitored by slapping a shoot with five to ten blossoms against a yellow card or beating tray. A minimum of 50 trees per orchard should be checked for adults. In warm springs, adults may migrate in and out of a block. As thrips damage is uncommon in cherries, economic thresholds have not been established.
Continue to monitor orchards for thrips until fruit coloring. If fruit starts showing damage, a treatment may be necessary.
UC IPM Pest Management Guidelines: Cherry
Insects and Mites
J. A. Grant, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Insects and Mites:J. Colyn, Mid-Valley Ag. Services
M. Devencenzi, Devencenzi Ag. Pest Mgmt. and Research
P. McKenzie, Mid-Valley Ag. Services