Thrips Management In Onion
Thrips Management In Onion
- 1 Thrips Management In Onion
- 2 Thrips Management In Onion
- 3 Onion thrips — control measures?
- 4 How to Control Onion Thrips
- 5 Onion-Thrips
Thrips Management In Onion
» Thrips are some of the most damaging insect pests of onions.
» Thrips are most problematic during warm and dry conditions.
» Insecticides are the primary method for controlling thrips on onions.
DAMAGE TO ONIONS
Thrips are some of the most damaging insect pests to the leaves of onions world-wide. Although there are many species of thrips, the western flower and onion thrips are the most common species in North America. 1 Both species have fairly wide host ranges, feeding on both broadleaf and grass plants, including alfalfa, common bean, grains, grasses, and various weed species. 2
Thrips thrive when conditions are hot and dry. Thus, they tend to be more problematic in the western growing regions of the U.S. and are considered to be the most damaging insect pest of onions in California. 1 Cool weather slows their development, and heavy rain or overhead irrigation can significantly lower populations in an onion planting.
Thrips feed on the leaves of onion plants, causing the leaves to turn white (Figure 1). High-level infestations cause significant leaf damage that results in a reduction of photosynthetic area and the plant’s ability to produce food for the developing bulb. Infestations that develop during the early stages of bulb formation have the largest impact on bulb size and quality. Infestations later in the season are less problematic, as onions can tolerate higher populations of thrips when they are closer to harvest. Thrips feeding damage affects the leaf quality of green onions because of the formation of feeding scars. 1 Thrips are also a vector of the Iris yellow spot virus on onions.
Figure 1. Whitening of onion leaves from the feeding of thrips. Howard F. Schwartz, Colorado State University, Bugwood.org
REPRODUCTION OF THRIPS
Adult females lay eggs in plant tissue, and the larvae that hatch feed on the leaf tissue (Figure 2). 2 There are two stages of feeding larvae (instars I and II) that collect under the folds of onion leaves or in the densely packed area where the leaves emerge from the neck of the bulb. The adults also feed on and damage onion leaves. The cycle of reproduction is completed in two to three weeks under favorable conditions.
Figure 2. Thrips larvae feeding on an onion leaf. Whitney Cranshaw, Colorado State University, Bugwood.org
Cultural practices that help reduce damage from thrips include tilling in plant residue, destroying volunteer onion plants, 3 and not planting onions near cereal grain fields that can serve as a source of thrips. 1 As cereal crops mature and senesce, the adult thrips will migrate off of the cereal plants and seek out neighboring green plants, including onions. Heavy rains and overhead irrigation can wash thrips off of onion leaves. Research has shown that onion fields that are overhead irrigated have lower thrips populations than neighboring fields receiving furrow or drip irrigation. 2
Natural predators can help manage thrips populations, most effectively later in the season when thrips are more exposed. Early in the season, thrips larvae are somewhat protected from predators under folded leaves and in the tight area where leaves emerge from the neck. However, predators are often killed by insecticide applications, so they may not be a factor in thrips management in these settings. 1
The primary method used for managing thrips is the application of insecticides. Thorough coverage of plant tissues with insecticides is needed to provide good control. This requires moderate spray pressures and high application volumes. Because thrips hide under folded leaves and near the base of the leaves, it can be difficult to get insecticides into those protected spaces where thrips are feeding. 3
Insecticide applications should be scheduled based on scouting observations. Scouting should be done by randomly sampling leaves or whole plants from four or more locations in the field, including field edges where colonization by thrips is likely to occur first. At least five plants should be evaluated at each location. Leaf bases and areas under leaf folds should be inspected for feeding damage and the presence of larvae. 1
No reliable action thresholds have been established for thrips management. 1 However, action thresholds are recommended for various situations. In New York, the recommended action threshold is an average of three larvae per green leaf. In California, the recommended threshold is 30 larvae per plant during the mid-season. This threshold should be lower when plants are young and higher as plants near maturity because they become less susceptible to damage. In Colorado, recommendations are based on the relative susceptibility of the onion varieties planted. An action threshold of 35 larvae per plant is recommended for thrips tolerant varieties, 25 for moderately susceptible varieties, and 15 per plant for highly susceptible varieties. On sweet onions grown in the southern U.S. the recommended action threshold is 5 to 10 larvae per plant, and for green onions, applications should begin at the first sign of feeding damage. 1,2,3
Processors may have specific recommendations for scouting for thrips. The University of California IPM guidelines for processing onion recommend evaluating ten plants from four different areas of the eld, counting the numbers of thrips on all leaves of each plant. Plants should be evaluated weekly when thrips counts are low, but more frequently when they exceed 20 per plant. A cumulative thrips-days (CTD) value is calculated by averaging the number of thrips per plant over two successive rating dates; then that number is divided by the number of days between ratings to determine the number of thrips per plant per day (thrips-days). The CTD is calculated by adding up the thrips-days on the latest sampling date. Studies have found that CTD values over 500 are associated with significant yield losses, which is equivalent to 50 or more thrips per plant per day for 10 days, or 25 or more thrips per plant per day for 20 days. 1
Several different insecticide products are available for controlling thrips (Table 1). These products differ in important characteristics such as their impact on bees and other beneficial insects, the minimum re-entry interval (REI), the minimum pre-harvest interval (PHI), the maximum number of applications per season, recommended frequencies of application, and the availability and use restrictions in each state. Product registrations and guidelines for use change frequently, so growers should consult the most recent label of a product before use. 1,3,4
Resistance to insecticides has become a major problem. Thrips have developed resistance to commonly used insecticides in some locations, and these products no longer provide adequate levels of control in those areas. Therefore, it is recommended that applications of insecticides should alternate between products with active ingredients belonging to different mode of action groups (insecticide classes) to help prevent or slow the development of insecticide resistant thrips populations. 1,2,3,4
1 Orlo , S., Natwick, E.T., Godfrey, L.D., Dara, S.K. 2016. Thrips. UC IPM pest management guidelines: Onion and garlic. UC ANR Publication 3453.
2 Schwartz, H.F. and Mohan, S.K. 2008. Compendium of onion and garlic diseases and pests, Second Edition. The American Phytopathological Society, St. Paul.
3 Reiners, S. and Seaman, A. 2016. Cornell integrated crop and pest management guidelines for commercial vegetable production.
4 Egel, D.S. 2016. Midwest vegetable production guide for commercial growers.
For additional agronomic information, please contact your local seed representative. Developed in partnership with Technology Development & Agronomy by Monsanto.
Individual results may vary, and performance may vary from location to location and from year to year. The information provided in this communication may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about growing onions. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with this specific crop.
SEMINIS DOES NOT WARRANT THE ACCURACY OF ANY INFORMATION OR TECHNICAL ADVICE PROVIDED HEREIN AND DISCLAIMS ALL LIABILITY FOR ANY CLAIM INVOLVING SUCH INFORMATION OR ADVICE. 160908110525 092316DME
Frankliniella occidentalis– known as Western Flower Thrips or Californian Thrips, is a small insect originating on the West Coast of North America, which spread through much of Europe in the 1980’s as a consequence of the international trade in plants. It is now established as a major pest in most parts of the world.
Adult thrips have delicate, hair-fringed wings that allow them to move into and through crops. Western Flower Thrips adults are attracted by flower odours. Once in flowers, they feed on pollen and on developing petals, causing blemishes that reduce the value of cut flowers. Some flower varieties and colours are more prone to damage than others.
Eggs are generally laid in leaf tissue or developing fruit, and can cause small, hard, ‘warts’. Feeding on developing fruit also causes damage, typically seen as a roughening and hardening of the surface. In sweet peppers this generally shows as a ‘corkiness’ of the surface, whilst in strawberries the affected areas fail to develop, and can appear golden: the fruit becomes distorted, and market value is reduced.
Larvae are small, usually orange in colour, and have only two feeding stages before dropping to the ground. In the upper layers of the soil they form a non-feeding pre-pupa, then a pupa, and at this stage they are vulnerable to some soil dwelling predators. Some pupation can also occur amongst foliage.
Frankliniella occidentalis is also responsible for transmission of numerous virus diseases to plants, the most important of which are protected crops are Tomato Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot Virus (INSV), both affecting peppers and a wide range of ornamental crops. Virus diseases are generally acquired by first instar larvae feeding on infected leaf tissue. These larvae are unable to transmit the virus until they become adults. Later larval stages and adults are unable to acquire the virus, but infected adults can transmit virus to fresh, uninfected plants, thus spreading the disease from plant to plant within the crop, and into new crops.
Other species of thrip are also major crops pests, including Onion thrips (Thrips tabaci).
Onion thrips — control measures?
In a Nutshell
- Small silver patches on upper leafside.
- Yellowing of leaves.
- Deformation of leaves, flowers and fruits.
Larvae and adults feed on plant tissues and produce small silver patches on the upper side of leaf blades, an effect known as ‘silvering’. The same patches can appear on petals where the pigment has been removed. On the underside of the leaves, the thrips and their larvae sit together in groups alongside their black dung spots. Leaves of affected plants yellow, wither, deform or shrivel. Feeding during bud or flower development later results in scarred, stunted or deformed flowers or fruits respectively and loss of yield.
Thrips are 1-2 mm long, yellow, black or both in colour. Some varieties have two pairs of wings, others do not have wings at all. They hibernate in plant residues or in the soil or on alternative host plants. They are also vectors for a broad range of viral diseases. Thrips do infest a broad variety of plants. Dry and warm weather conditions favor population growth. Humidity reduces it. Adults can be easily carried by winds, clothes, equipment, and containers not properly cleaned after fieldwork.
Some biological control measures have been developed for specific thrips. The insecticide spinosad is generally more effective against thrips than any of the chemical or other biological formulations. It lasts 1 week or more and moves short distances into sprayed tissue. It can, however, be toxic to certain natural enemies (e.g., predatory mites, syrphid fly larvae) and bees. Therefore, do not apply spinosad to plants that are flowering. In the case of flower thrips infestation, a combination of garlic extracts with some insecticides also seems to work well. Against species that attack the leaves and not the flower, try neem oil or natural pyrethrins, especially on the undersides of the leaves. The use of highly reflective UV mulch (metalized reflective mulch) has been recommended.
Always consider an integrated approach with preventive measures together with biological treatments if available. Due to the high reproductive rates and their life cycles, thrips have developed resistance to different classes of pesticides. Effective contact insecticides include fipronil, imidacloprid, or acetamiprid, which in many products are combined with piperonyl butoxide to enhance their effect.
How to Control Onion Thrips
Onion thrips cause leaves to turn a silvery color.
Onion thrips are miniscule pests that cause leaf damage by sucking valuable moisture, chlorophyll and nutrients from onion plants. These tiny pests generally hide under the leaf folds and in the inner leaves near the onion bulb, making them difficult to detect. Onion thrips are most common in areas with hot dry weather during the growing season and can be quite damaging to entire crops of onions. Take action as soon as you notice damage to protect your onions and ensure a harvest.
Spray your onion plants with a garden hose. Position your garden hose, with a sprayer attachment, over the top of the onion plants. Irrigate them well, which knocks thrips from the plants and helps to prevent their return.
Apply a nitrogen-based fertilizer around the base of your onion plants. A consistent availability of nitrogen in the soil is associated with reduced populations of onion thrips. Reapply the fertilizer, as necessary, throughout the growing season.
Put a layer of mulch on the ground around the onion plants. Mulch might increase natural predators, such as minute pirate bugs or lacewings, that feed on thrips. It also provides a barrier for thrip pupa to prevent them from reaching the plant and causing damage. Mulch cools the soil temperature as well, which decreases thrip populations.
Spray insecticide on the onion plants. Follow the package directions exactly. Use a insecticide with spinosad, such as Entrust, if you grow an organic garden.
Plant appropriate crops adjacent to your onions. Avoid planting grains around your onions because they attract onion thrips. Surround onions with other vegetables, herbs and flowers instead.
Onion thrips (Thrips tabaci)
Western flower thrips (Frankliniella occidentalis)
Pest description and crop damage
Thrips infestations can reduce onion yield by as much as 20%. Onion thrips also vector Iris Yellow Spot Virus. Onion thrips thrive in hot, dry conditions and are usually more damaging where these conditions prevail most of the production year.
Biology and life history
Pest monitoring Although thrips feeding during the early bulbing stage is the most damaging to yields, thrips must be controlled before onions reach this stage. Otherwise, populations might exceed levels that can be controlled adequately.
In California, a reliable means of evaluating thrips populations is to randomly sample entire onion plants. Larva stages tend to congregate around the newest leaves of the plant. Sample at least five plants from four separate areas of the field. Early in the season, an average of 2 thrips per plants warrants control measures. A threshold of 20 thrips per plant mid-year (lower for very young plants and higher for larger mature plants) has been used successfully for dry bulb fresh market and drying onions. 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.
Avoid planting onions near grain fields, if possible, because thrips numbers often build up in cereals in spring. Onion grown with overhead irrigation tend to have lower thrips numbers as thrips tend to be washed from the plant to some extent with heavy irrigation or rains.
Management-chemical control: HOME USE
- azadirachtin (neem oil)-Some formulations are OMRI-listed for organic use.
- Beauveria bassiana -Some formulations are OMRI-listed for organic use.
- insecticidal soap-May require several applications. Some formulations are OMRI-listed for organic use.
- pyrethrins-Some formulations are OMRI-listed for organic use.
- spinosad-Some formulations are OMRI-listed for organic use.
Management-chemical control: COMMERCIAL USE
Thorough coverage is essential for control, as most thrips feed in protected areas of the plant.
- abamectin (Agri-Mek 0.15EC ) at 0.009 to 0.019 lb ai/a. PHI 30 days. REI 12 hr. Retreatment interval 7 days. Do not exceed 0.019 lb ai/a per season.
- acetamiprid (Assail SG) at 0.094 to 0.15 lb ai/a. PHI 7 days. REI 12 hr. Retreatment interval 7 days. Do not exceed 0.6 lb ai/a per season.
- Chenopodium ambrosioides extract (Requiem 25EC) at 3 to 6 pints formulated product per acre. REI 4 hr. Use high rate when conditions are favorable for heavy pressure. OMRI-listed for organic use.
- cyantraniliprole (Exirel) at 0.088 to 0.133 lb ai/a. PHI 1 day. REI 12 hr. Retreatment interval 5 days. Rotate with other modes of action. Begin applications early for best results. Do not exceed 0.4 lb ai/a per season.
- cyantraniliprole + abamectin (Minecto Pro) at 7 to 10 fl oz/A. PHI 30 days. Do not apply more than two times in a row. Retreatment interval 7 days. Do not apply more than 20 fl oz/A per season (0.18 lb ai/a of cyantraniliprole and 0.038 lb ai/a of abamectin).
- cypermethrin (Holster) at 0.08 to 0.1 lb ai/a. PHI 7 days. Dry bulb onion only. REI 12 hr. Retreatment interval 7 days. Do not exceed 0.5 lb ai/a per season. Do not graze or feed crop residues to livestock.
- deltamethrin (Delta Gold) at 0.018 to 0.028 lb ai/a. PHI 1 day. REI 12 hr. Retreatment interval 5 days. Do not exceed 0.112 lb ai/a per season.
- gamma-cyhalothrin (Declare) at 0.010 to 0.015 lb ai/a. PHI 14 days. REI 24 hr. Retreatment interval 5 days. Do not exceed 0.12 lb ai/a per season. An oil or nonionic surfactant improves control.
- lambda-cyhalothrin (Warrior II) at 0.02 to 0.03 lb ai/a. PHI 14 days. Bulb onion only. Retreatment interval 5 days. Do not exceed 0.24 lb ai/a per season. An oil or surfactant adjuvant improves control.
- malathion (Fyfanon 8E) at 1 to 1.56 lb ai/a. PHI 3 days. REI 12 hr. Retreatment interval 7 days. Limit 2 treatments per season.
- Metarhizium anisopliae (Met 52EC) at 40 to 80 fl oz/100 gal as drench or 0.25 to 2 quarts/a. PHI 0 days. REI 4 hr.
- methomyl (Lannate SP) at 0.9 lb ai/a. PHI 7 days. REI 48 hr. Do not exceed, per season 5.4 lb ai/a for green onion or 3.6 lb ai/a for dry bulb onion. Limit 8 treatments per season. Retreatment interval 5 days.
- oxamyl (Vydate L) at 0.5 to 1 lb ai/a. PHI 14 days. REI 48 hr. Dry bulb only. Do not harvest tops. Do not exceed 4.5 lb ai/a per season. Suppression only.
- oxydemeton methyl (MSR) at 0.375 to 0.5 lb ai/a. PHI 30 days. REI 10 days. Do not exceed 1 lb ai/a per season. Retreatment Interval 14 days. Additional 2ee registration for Idaho for side dress application.
- permethrin (Ambush 25WP) at 0.15 to 0.3 lb ai/a. PHI 1 day. REI 12 hr. Dry bulb onion only. Do not exceed 2 lb ai/a per season.
- pyriproxyfen (Knack) at 0.067 lb ai/a. PHI 3 days. REI 12 hr. Do not exceed 0.134 lb ai/a per season. Retreatment interval 14 days.
- spinetoram (Radiant SC) at 0.047 to 0.078 lb ai/a. PHI 1 day. Retreatment interval 4 days. Do not exceed five applications or 0.234 lb ai/a per season. Follow resistance management procedures on the label.
- spinosad (Success) at 0.063 to 0.125 lb ai/a. PHI 1 day. REI 4 hr. Retreatment interval 4 days. Do not exceed 0.45 lb i/a per season. For suppression. Some formulations are OMRI-listed for organic use.
- spirotetremat (Movento) at 5 fl oz/a. PHI 3 days. Retreatment interval 7 days. REI 24 hr.
- zeta-cypermethrin (Mustang) at 0.0375 to 0.05 lb ai/a. PHI 7 days. REI 12 hr. Retreatment interval 7 days. Do not graze or cut for feed. Do not exceed 0.25 lb ai/a per season.
Pesticide resistance management Resistance to organophosphate and pyrethroid insecticides has been documented in several states and is suspected in California. For this reason, alternate insecticides from different chemical families when multiple treatments are needed during a year. Be sure to use surfactants when recommended by the label as thrips reside down in the neck of the onion plant and contact of the chemistry to thrips is important.