How to Manage Pests
UC Pest Management Guidelines
Western Flower Thrips
Scientific Name: Frankliniella occidentalis
(Reviewed 4/10, updated 7/10, pesticides updated 9/15)
In this Guideline:
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.
Nymphs hatch and feed in numbers on tiny fruit, 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.
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.
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.
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
If adult thrips are found on 4 to 5 of the 50 blooms examined or if nymphs are found on 3 blossoms in the sample, 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.
Highly colored varieties can be damaged by thrips feeding just before harvest. Monitor fruit on trees 2 to 3 weeks before harvest (when fruit begins to color) (see PREHARVEST FRUIT SAMPLES). If fruit starts showing damage, a treatment is necessary.
Sample fruit at harvest (see FRUIT EVALUATION AT HARVEST) to assess the effectiveness of the current year's IPM program and to determine the needs of next year's program. Record results for the harvest sample.
UC IPM Pest Management Guidelines: Peach
Insects and Mites
J. K. Hasey, UC Cooperative Extension, Sutter and Yuba counties
Acknowledgment for contributions to the Insects and Mites:W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
R. E. Rice, Kearney Agricultural Center, Parlier
C. Pickel, UC IPM Program, Sutter and Yuba counties