Western Flower Thrips

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.

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

Begin monitoring thrips as individual blocks begin to bloom (see EARLY SEASON MONITORING). Monitor for thrips by examining one blossom from 50 trees located throughout the orchard. Slap the flower in the palm of your hand and look for dislodged adult thrips or use a light yellow background to hit the flowers against. 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 flowers 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 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 (PDF) sample.

Common name		Amount per acre**		REI‡	PHI‡
(Example trade name)		(conc.)	(dilute)	(hours)	(days)
Pesticide precautions Protect water Calculate VOCs Protect bees
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
A.	SPINOSAD
	(Entrust)#	1.25–2.5 oz	0.42–0.83 oz	4	1
	(Success)	4–8 fl oz	1.3–2.7 fl oz	4	1
	MODE-OF-ACTION GROUP NUMBER1: 5
	COMMENTS: Do not apply more than 29 oz/acre per year of Success or 9 oz/acre per year of Entrust. To avoid development of insect resistance, do not treat successive generations of the same pest with the same product. Control may be improved by addition of an adjuvant. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
B.	SPINETORAM
	(Delegate WG)	4.5–7 oz		4	1
	MODE-OF-ACTION GROUP NUMBER1: 5
	COMMENTS: Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
C.	METHOMYL*
	(Lannate SP)	0.5–1 lb		96 (4 days)	4
	MODE-OF-ACTION GROUP NUMBER1: 1A
	COMMENTS: The use of this material causes mite problems.
D.	ABAMECTIN*
	(Agri-Mek SC)	2.25–4.25 fl oz	0.5–1 fl oz	12	21
	MODE-OF-ACTION GROUP NUMBER1: 6
	COMMENTS: May be combined with oil. Do not make more than 2 applications per growing season and allow at least 21 days between treatments. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging. To avoid illegal residues, see label for required spray additives.

**	For dilute applications, rate is per 100 gal water to be applied in 300-500 gal water/acre, according to label; for concentrate applications, use 80-100 gal water/acre, or lower if label allows.
‡	Preharvest interval. Do not apply within this many days of harvest.
*	Permit required from county agricultural commissioner for purchase or use.
—	Not recommended or not on label.
1	Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

• Fruit sampling for pest damage at harvest (PDF)