How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Western flower thrips are slender, very small insects, about 0.03 inch (0.8 mm) long when mature. Adults have feathery wings and vary in color from yellow to dark brown; nymphs are white or yellowish with small dark eyes. In spring, flower thrips populations build up on alfalfa, weeds, ice plant, and other vegetation and move from these hosts when they are cut, stop flowering, or dry up.

Strawberry plantations often have a mixed population of thrips that includes a low percentage of the onion thrips, Thrips tabaci.

DAMAGE

Thrips feeding on strawberry blossoms causes the stigmas and anthers to turn brown and wither prematurely, but not before fertilization has occurred. Although often numerous on berries when cat-facing occurs, western flower thrips do not cause cat-facing, which is a result of lygus bug feeding and possibly other factors. As fruit develops, thrips feeding may cause a russeting (Type I bronzing) of the fruit around the cap, but this injury is seldom economic. (Other types of bronzing are associated with phytotoxicity from sulfur and other types of sprays (Type II) and from plant physiological factors (Type III). The most severe bronzing that covers the entire fruit is believed to have a physiological cause that is associated with hot temperatures occurring from May through July.)

MANAGEMENT

Populations of the western flower thrips build up on a number of crops and weeds. They may migrate into strawberries when other crops are harvested, when second-year strawberries or other perennial hosts stop flowering, or when weeds dry up in spring. Control is not usually necessary because western flower thrips rarely cause economic damage at densities that typically occur in strawberry fields. Sprays applied to control thrips disrupt biological control of other pests such as twospotted spider mites, lygus bugs, whiteflies, and other insects. In addition, because western flower thrips feed on spider mite eggs, at low population levels they can be beneficial. If treatment is necessary, choose the least disruptive insecticide to preserve biological controls agents.

Biological Control
Naturally occurring minute pirate bugs (Orius spp.) feed on thrips. Orius are also available commercially, but release rates and timing have not been determined.

Organically Acceptable Methods
Sprays of the Entrust formulation of spinosad are acceptable for use on organically certified strawberries.

Monitoring and Treatment Decisions
Consider treating only if populations reach 10 thrips per blossom when flowers are shaken onto a flat surface and thrips counted. A better way to sample thrips is to place randomly collected flower blossoms into a glass container with several drops of either ethyl acetate or methyl isobutyl ketone (or with a small amount of either of these materials soaked into cotton or other absorbent material). After at least one-half hour, count the thrips by removing the blossoms and shaking them onto black paper. Because more thrips will be found with this method, the treatment threshold is greater than that indicated for shaking flowers.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
(Note: For increased efficacy, any of the first four insecticides can be tank mixed with either Pyrenone or PyGanic (E or F below). For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i.)
A.	SPINOSAD
	(Entrust)#	1.25–1.5 oz	4	1
	(Success)	6 fl oz	4	1
	MODE OF ACTION GROUP NUMBER1: 5
	COMMENTS: Rotate to an insecticide with a different mode of action after two successive applications. Can be toxic to some natural enemies (e.g. predatory mites, syrphid fly larvae) when sprayed and shortly thereafter (8-24 hours).
B.	SPINETORAM
	(Radiant SC)	6–10 fl oz	4	1
	MODE OF ACTION GROUP NUMBER1: 5
	COMMENTS: Rotate to an insecticide with a different mode of action after two successive applications of either spinetoram or spinosad to help delay the development of resistance to Group 5 insecticides.
C.	MALATHION
	5EC	2–3 pt	12	3
	8E	1.5–2 pt	12	3
	MODE OF ACTION GROUP NUMBER1: 1B
	COMMENTS: Residual activity only about 1 week.
D.	NALED
	(Dibrom) 8EC	1 pt	48	1
	MODE OF ACTION GROUP NUMBER1: 1B
	COMMENTS: Do not use when temperature exceeds 85°F.
E.	PYRETHRIN/PIPERONYL BUTOXIDE
	(Pyrenone)	Label rates	12	0
	MODE OF ACTION GROUP NUMBER1: 3/—
	COMMENTS: Variable efficacy. Not as disruptive of natural enemies as some other options.
F.	PYRETHRIN#
	(PyGanic) 1.4EC	16–64 oz	12	0
	MODE OF ACTION GROUP NUMBER1: 3
	COMMENTS: Apply in sufficient water for thorough coverage.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
#	Acceptable for use on organically grown produce.
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/.

PUBLICATION

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468
Insects and Mites
F. G. Zalom, Entomology, UC Davis
M. P. Bolda, UC Cooperative Extension Santa Cruz County
P. A. Phillips, UC IPM Program/UC Cooperative Extension Ventura County
Acknowledgment for contributions to the Insects and Mites:
N. C. Toscano, Entomology, UC Riverside

Top of page