How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic

Thrips

Scientific names: Onion thrips: Thrips tabaci
Western flower thrips: Frankliniella occidentalis

(Reviewed 1/07, updated 6/08, pesticides updated 6/16)

In this Guideline:


DESCRIPTION OF THE PESTS

Thrips are very small, slender insects that are best seen with a hand lens: mature onion thrips are about 0.05 inch (1.3 mm) long and flower thrips are slightly larger at 0.06 inch (1.5 mm) long. The most distinctive characteristic of thrips are two pairs of wings that are fringed with long hairs. Adults are pale yellow to light brown in color. The immature stages have the same body shape as adults but are lighter in color and are wingless. When viewed under a microscope, western flower can be distinguished from onion thrips by its red eyes and 8-segmented antennae, while onion thrips' eyes are gray and its antennae are 7-segmented.

Both onion thrips and western flower thrips have a very extensive range of hosts, including cereals and broadleaved crops. Both species attack onions, but onion thrips are believed to be more prevalent and injurious. They also can be a problem on garlic, but generally are not as serious a pest as they are on onion. Onion thrips thrive in hot, dry conditions and are usually more damaging in areas where these climatic conditions prevail for most of the production season.

DAMAGE

Thrips are the most common and serious insect pest of onions, and are found wherever onions are grown in California. High populations of thrips can reduce both yield and keeping quality of onions. Thrips are most damaging when they feed during the early bulbing stage of plant development. Scarring of leaves is a serious problem on green onions.

Thrips have rasping-sucking mouthparts and feed by rasping the surface of the leaves and sucking up the liberated plant fluid. They feed under the leaf folds and in the protected inner leaves near the bulb. When population levels are high, thrips can also be found feeding on exposed leaf surfaces. Both adults and nymphs cause damage. When foliage is severely damaged, the entire field takes on a silvery appearance. Severe scarring also creates an entry point for foliar leaf diseases.

MANAGEMENT

Biological Control

Natural enemies, including predaceous mites, minute pirate bugs, and lacewings, are often found feeding on thrips. These beneficials are very susceptible to insecticide sprays, however, and may not be important in fields where insecticides have been used.

Cultural Control

Avoid planting onions near grain fields, if possible, because thrips numbers often build up in cereals in spring. Overhead irrigation and rainfall provide some suppression of thrips populations, but treatments are often still necessary.

Organically Acceptable Methods

Biological and cultural controls as well as sprays of the Entrust formulation of spinosad are acceptable for use on organically certified crops.

Monitoring and Management Decisions

Although thrips feeding during the early bulbing stage is the most damaging to yields, thrips must be controlled before onions reach this stage so that populations do not exceed levels that can be adequately controlled. Onions can tolerate higher thrips populations closer to harvest; however, in the case of hand-topped onions, thrips can be extremely annoying to harvest crews and treatment closer to harvest may be desirable.

To make a cursory evaluation of thrips infestation levels, randomly sample leaves and evaluate thrips numbers and damage under leaf folds. A far more reliable means of evaluating thrips populations, however, is to randomly sample entire onion plants. This way leaves can be pulled apart and, using a hand lens, all the thrips on the inner leaves near the bulb can be counted as well as those under the leaf folds. Sample at least five plants from four separate areas of the field. A reliable treatment threshold has not been developed; however, a threshold of 30 thrips per plant mid-season (lower for very young plants and higher for larger mature plants) has been used successfully for dry bulb fresh market and drying onions.

For processing onions, monitor thrips by examining the entire top growth of the onion plant and counting the number of thrips. Sample 10 plants from four areas of the field. Sample weekly, or more frequently when counts exceed 20 thrips per plant. Calculate the average number of thrips per plant on two successive sample dates. Divide the average by the number of days between samples to get the number of thrips per plant per day or thrips-days. Add up the thrips-days on the sample day to get the cumulative thrips-days (CTD) during crop growth. Research indicates that significant yield loss occurs when 500 to 600 CTD or more accumulate. This is the equivalent of 50 to 60 thrips per plant per day for 10 days, 25 to 30 thrips per plant per day for 20 days, and so on.

The marketability of green onions (those marketed fresh with the leaves attached) is severely reduced by thrips scarring; apply treatments at the first sign of thrips feeding. On onions grown for seed, thrips can reduce yield and quality of seed production during seed set, but no treatment thresholds have been established.

While resistance to organophosphate insecticides has not been evaluated in California, it has been documented in other states and is suspected in California. For this reason, alternate insecticides from different chemical families when multiple treatments are needed during a season. Thorough coverage is essential for control, as most thrips feed in protected areas of the plant.

Common name Amount per acre REI‡ PHI‡
(Example trade name)   (hours) (days)

UPDATED: 6/16
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
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. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. SPINOSAD
  (Entrust)# 1.25–2.5 oz 4 1
  (Success) 4–8 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 5
  COMMENTS: Provides suppression. Do not use more than 29 fl oz Success/acre per season or 9 oz Entrust/acre per season. Use allowed under supplemental label. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
B. SPINETORAM
  (Radiant SC) 6–10 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 5
  COMMENTS: Control may be improved with the addition of a spray adjuvant. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
C. SPIROTETRAMAT
  (Movento) 5 fl oz 24 3
  MODE-OF-ACTION GROUP NUMBER1: 23
 
D. ABAMECTIN
  (Agri-Mek SC)* 1.75–3.5 fl oz 12 30
  MODE-OF-ACTION GROUP NUMBER1: 6
  COMMENTS: Apply when thrips have reached the economic threshold; do not use as rescue treatment. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
E. METHOMYL*
  (Lannate SP) Onions: 1 lb 48 7
    Garlic: 0.5 lb 48 7
  (Lannate LV) Garlic: 1.5–3 pt 48 7
    Onions: 1.5 pt 48 7
  MODE-OF-ACTION GROUP NUMBER1: 1A
  COMMENTS: Add a wetting agent to improve coverage. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
F. DIAZINON*
  (Diazinon AG600) Label rates 72 (3 days) See label
  MODE-OF-ACTION GROUP NUMBER1: 1B
  COMMENTS: Only for use on bulb and green onions. Do not apply more than one application per year. Avoid drift and tailwater runoff into surface waters. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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.
* Permit required from county agricultural commissioner for purchase or use.
# 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).

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453

Insects and Mites

S. Orloff, UC Cooperative Extension, Siskiyou County
E.T. Natwick, UC Cooperative Extension, Imperial County
L. D. Godfrey, Entomology, UC Davis
S. K. Dara, UC Cooperative Extension, Santa Barbara County

Acknowledgment for contributions to Insects and Mites:
W. E. Chaney, UC Cooperative Extension, Monterey County
R. L. Coviello, UC Cooperative Extension, Fresno County
G. J. Poole, UC Cooperative Extension, Los Angeles County

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