How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Adult citrus thrips are small, orange-yellow insects with fringed wings. During spring and summer, females lay about 25 eggs in new leaf tissue, young fruit, or green twigs; in fall, overwintering eggs are laid mostly in the last growth flush of the season. Overwintered eggs hatch in March about the time of the new spring growth. First-instar larvae are very small, whereas second-instar larvae are about the size of adults, spindle-shaped, and wingless. They feed actively on tender leaves and fruit, especially under the sepals of young fruit. Third- and fourth-instar (propupa and pupa) thrips do not feed and complete development on the ground or in the crevices of trees. When adults emerge, they move actively around the tree foliage.

Citrus thrips do not develop below 58°F (14°C). They can produce up to eight generations during the year if the weather is favorable.

When monitoring citrus thrips, you must be able to distinguish them from flower thrips, which feed on flower parts but do not damage citrus. Shortly after petal fall, immature flower thrips can be seen moving around young fruit, but they soon pupate and adults disperse to other plants, consequently they are only concentrated in citrus orchards for a short period in spring. For more information on distinguishing citrus thrips from other thrips, see UC ANR Publication 3303, Integrated Pest Management for Citrus, 2nd edition.

DAMAGE

Citrus thrips is of greatest economic importance on San Joaquin navel oranges, desert citrus, and coastal lemons. On fruit, the citrus thrips punctures epidermal cells, leaving scabby, grayish or silvery scars on the rind. Second-instar larvae do the most damage because they feed mainly under the sepals of young fruit and are larger than first instars. As fruit grow, damaged rind tissue moves outward from beneath the sepals as a conspicuous ring of scarred tissue. Fruit are most susceptible to scarring from shortly after petal fall until they are about 1.5 inch (3.7 cm) in diameter. Thrips damage is higher on fruit located on the outside canopy where fruit is also susceptible to wind damage and sunburn.

MANAGEMENT

Citrus thrips populations can vary greatly from year to year. Monitor to determine if treatments are needed in a particular year. Navel oranges are more susceptible to damage than are Valencia oranges, which often do not require treatment.

Treatment of young, nonbearing trees in a grove is not recommended except in severe cases. Although the citrus foliage is often heavily damaged by citrus thrips, healthy trees can withstand the damage and frequent treatments can lead to the development of insecticide resistance, making control of thrips on fruit more difficult in later years.

Citrus thrips is less of a problem in orchards that receive minimal broad-spectrum pesticide treatments than in orchards that are heavily treated. Because of pesticide-induced hormoligosis (i.e. stimulation of thrips reproduction), thrips populations tend to increase after treatments with organophosphates, carbamates, pyrethroids, foliar neonicotinoids, and the miticide pyridaben (Nexter).

Biological Control
A number of natural enemies attack citrus thrips, including the predaceous mite Euseius tularensis, spiders, lacewings, dustywings, and minute pirate bugs. Some controversy exists regarding the degree of citrus thrips control afforded by E. tularensis populations; they provide some control but are also a very good "indicator" species, giving an indication of the level of general natural enemies present in an orchard. Citrus thrips population levels are aggravated when broad-spectrum pesticides are used, probably because of both a reduction in natural enemy levels and pesticide-induced hormoligosis.

In some years, when citrus thrips densities are excessively high, no amount of E. tularensis or other natural enemies in combination with selective pesticides can keep citrus thrips below an economic threshold.

Organically Acceptable Methods
Biological control is acceptable for use in organically managed orchards as well as sprays of the Entrust formulation of spinosad with an organically approved oil.

Resistance
Citrus thrips has a history of rapidly developing resistance to chemicals that are used repeatedly and frequently for its control. For example, resistance to dimethoate and formetanate hydrochloride (Carzol) has developed in a number of citrus thrips populations in the San Joaquin and Coachella valleys; cyfluthrin (Baythroid) and fenpropathrin (Danifol) resistance has appeared in several groves in Kern County. With the limited number of pesticides available for control of citrus thrips now and in the foreseeable future, it is wise to monitor citrus thrips levels carefully, to limit treatments only to populations that are causing or are expected to cause significant levels of fruit scarring (treatments to prevent foliar damage are not recommended), and to time and apply treatments optimally so that reapplications are not required. Although citrus thrips disperse a good deal, citrus thrips resistance problems are often localized. Thus, growers using repeated applications for citrus thrips control are most likely to experience resistance problems at a later date.

Selectivity
The botanical insecticide sabadilla (Veratran) and spinetoram (Delegate), spinosad (Success or Entrust) and abamectin (Agri-Mek, etc.) are relatively nontoxic to beneficial insects and mites. The broad-spectrum organophosphate (dimethoate), carbamate (formetanate hydrochloride–Carzol), and pyrethroids (cyfluthrin–Baythroid, fenpropathrin-Danitol) insecticides are toxic and fairly persistent against both beneficial mites and beneficial insects and disrupt biological control.

Monitoring
Check young fruit for immature thrips and monitor the undersurface of inside foliage for predaceous mites. Monitor from petal fall until fruit is greater than 1.5 inches in diameter. For oranges, the monitoring time is about 6 to 8 weeks in spring. For lemons, monitor June through October.

Monitoring fruit for citrus thrips. Select trees that are three to four rows in from the outside edge of the block. Sample 25 young fruit from each corner of the block for a total of 100 fruit. Take only one to two healthy, dark green fruit from outside, sunny branches of each tree. Look for thrips on the stem end of the fruit under the calyx. Count fruit as infested only if it has one or more wingless first-or second-instar nymphs (ignore pupae and adults). Record the total fruit infested with immature citrus thrips and calculate the percentage of infested fruit (example form—102 KB, PDF).On very susceptible varieties, such as San Joaquin Valley navels, monitor fruit at least twice a week after petal fall, and continue monitoring as long as susceptible fruit is on the tree.

Monitoring predatory mites. Examine the underside of twenty 5-leaf terminals with fully expanded leaves from shady areas of the canopy (a total of 100 leaves), and count the number of adult predatory mites. Calculate and record the average number of predatory mites per leaf (example form—102 KB, PDF). A minimum of 0.5 predatory mites per leaf is needed to assist with biological control of citrus thrips.

Treatment Decisions
Treatment thresholds vary by growing region, cultivar, beneficial mite populations, and the type of miticide that will be applied. A significant factor affecting threshold levels is whether the orchard is sheltered from wind damage (lower threshold) or has a history of outside fruit scarring from seasonal winds (higher threshold). As fruit get larger, treatment thresholds go up. Less susceptible varieties, such as Valencia oranges, may not require monitoring or treatment.

Sabadilla (Veratran), spinetoram (Delegate), spinosad (Entrust, Success), or abamectin (Agri-Mek, etc.) are recommended to avoid severe mortality of natural enemies. Sabadilla is a stomach poison that contains sugar or molasses as a bait and must be consumed by the thrips in order to be effective. When a treatment of sabadilla, spinetoram, spinosad, or abamectin is planned, beneficial mite populations are considered significant if you have more than 0.5 predators per leaf. Just after petal fall, treatment thresholds are as follows: Valencia oranges—10% of fruit sampled with one or more immature citrus thrips and few predators present, or 20% infested in the presence of significant levels of beneficial mites; navel oranges—5% of fruit sample infested and few predators present, or 10% infested with significant levels of beneficial mites. Raise these thresholds as fruit grow.

Dimethoate, formetanate hydrochloride (Carzol), cyfluthrin (Baythroid), and fenpropathrin (Danitol) are contact poisons and are most effective when applied shortly before the majority of citrus thrips hatch (when 5% or less of the fruit are infested with first instar citrus thrips). On very susceptible varieties, such as San Joaquin Valley navel oranges, monitor fruit at least twice a week after petal fall. Less susceptible varieties such as Valencia oranges may not require treatment. Optimal timing of these contact materials is usually shortly after petal fall but can be delayed depending on weather and on thrips development. If an application of dimethoate, formetanate hydrochloride, cyfluthrin, or fenpropathrin is planned, the threshold is 1 to 5% infested fruit on navel oranges. Do not treat citrus thrips prebloom or after fruit exceed 1.5 inches in diameter, unless severe populations are present.

Because of the continuous fruiting nature of coastal lemons, a treatment threshold of between 10 to 20% infested fruit is used, depending on whether the orchard is sheltered from wind damage (lower threshold) or has a history of outside fruit scarring from seasonal winds (higher threshold).

When monitoring indicates a treatment may be needed, it is essential to properly time and apply a treatment in order to reduce the likelihood of a second treatment being needed, and thus reduce the long-term development of resistance. Apply the treatment using outside coverage (OC) by reducing spray blower wind velocity. Ground application is more effective than air application and 200 gallons per acre is more effective than lower or higher gallonage, except with the sugar or molasses bait treatments using sabadilla. Because of their smaller size, coastal lemon trees receive adequate control with an aerial application. Firm data on optimal gallonage with sugar baits are not available, but some growers believe that lower gallonage is more effective because the sugar concentration is increased. Don't apply sabadilla and a sugar bait just before or during periods of heavy dew, fog, or drizzle. Such weather conditions cause the sugar bait to separate from the toxin, rendering the treatment ineffective.

• Summary of Important Links

IMPORTANT LINKS

• Citrus thrips monitoring form (102 KB, PDF)
• Photos of citrus thrips and western flower thrips

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Insects, Mites, and Snails
E. E. Grafton-Cardwell, Kearney Agricultural Center, Parlier
J. G. Morse, Entomology, UC Riverside
N. V. O‘Connell, UC Cooperative Extension, Tulare County
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County
C. E. Kallsen, UC Cooperative Extension, Kern County
D. R. Haviland, UC Cooperative Extension, Kern County
Acknowledgments for contributions to Insect, Mites, and Snails:
J. Barcinas, E. S. I., Corona, CA
R. Dunn, Badger Farming County, Exeter, CA
J. Gorden, Pest Management Associates, Exeter, CA
H. Griffiths, E. S. I., Corona, CA
D. Machlitt, Consulting Entomology Services, Moorpark, CA
C. Musgrove, retired entomologist, Riverside, CA
K. Olsen, S & J Ranch, Pinedale, CA
T. Roberts, E. S. I., Corona, CA
J. Stewart, Pest Management Associates, Exeter, CA
P. Washburn, Washburn & Sons Citrus Pest Control, Riverside, CA
K. Godfrey, USDA Biological Control, Sacramento
D. Headrick, California Polytechnic State University, San Luis Obispo
B. Faber, UC Cooperative Extension, Ventura County
J. Kabashima, UC Cooperative Extension, South Coast Research and Extension Center

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