By: Nikki Tilley, Author of The Bulb-o-licious Garden
If your onion tops curl up, you may have a case of onion thrips. In addition to affecting onions, however, these pests have also been known to go after other garden crops including:
You may also find thrips feeding on melons and some types of flowers. These insects are most active during spring, but continue their damage throughout fall before overwintering in nearby debris.
The trail of damage left by these pests can be easily seen as they can literally suck the life right of the plants. Typically, thrips prefer to feed on plant tissue from newly emerging leaves.
Besides curling onion leaves, these insects produce silver or white-looking streaks on foliage. The young leaves appear distorted, and severely injured leaves may even turn brown and die.
Bulb growth may be affected as well, being much smaller in size and deformed.
While overhead watering, as well as rain, can help reduce their numbers, other controls are often necessary. Biological control of onion thrips generally includes the introduction of the pest’s natural enemies such as minute pirate bugs, predatory thrips species, and lacewings. Unfortunately, these are only effective with small numbers of thrips, and they are also susceptible to most insect sprays.
Although damage from thrips on onions is most prevalent during early bulbing, it is highly recommended that these pests be controlled well before this. Otherwise, their populations may become large and more difficult to control.
You can evaluate these numbers by counting them on random plants throughout the garden. Pull the leaves apart and check under the leaf folds as well as near the base of the bulb. The nymphs can be recognized by their pale yellow color while the winged adults will be light to dark brown. Having at least 15-30 of these insects means additional control is needed.
Most can be killed off with various insecticides, but contact-residual types or neem oil is more effective. Be sure to thoroughly coat the plant in order to compensate for the shape of the onion leaves.
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Have you seen thrips damage on your plants? Here are tips for identifying and getting rid of thrips in your garden.
Thrips are tiny insects about as fat as a sewing needle that dine on many plants worldwide. Also known as thysanoptera or thunderflies, thrips are sucking insects that can cause some damage to plants. However, their damage can be much worse when they transmit viruses to plants.
Thrips Life Cycle: The life cycle depends on the species of thrips as well as the location, host plant, and other factors. Adult thrips overwinter in plant debris, bark, or other materials. They become active in early spring and lay eggs in plant tissue. These eggs hatch after 3–5 days, and the nymphs then feed for 1–3 weeks before resting to molt in 1–2 weeks. Thrips can have up to 15 generations per year outdoors. Adult thrips live short lives of about one month.
» 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.
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
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
Onion thrips, Thrips tabaci (Order Thysanoptera, Family Thripidae), is a key insect pest in most onion production regions of the world. Immature and adult thrips feed with a punch-and-suck behavior that removes leaf chlorophyll causing white to silver patches and streaks.
Thrips populations increase rapidly under hot, arid conditions and can lead to economic crop loss. The early bulb enlargement stage of onion growth is the most sensitive to thrips feeding.
A complete generation requires 3-4 weeks during the summer months. Five to eight generations may occur each year. Adults overwinter, disperse and feed on young leaves in the center of the onion neck. Eggs are inserted individually into leaves where they are protected. Larvae have both feeding and non-feeding stages. Larvae feed on new leaves in the center of the onion neck.
Sampling should begin when plants have at least 4-5 leaves or by mid June. Open the neck of onion plants and quickly count thrips adults and larvae before they disperse or hide. The majority of thrips will be at the base of youngest leaves in the lower center of the neck.
New York guidelines recommend sampling 5 plants each in 10 different areas of a field for a total of 50 plants per field (Rueda and Shelton 1995). This intensity of sampling could require 45-60 min per field depending on field size and amount of walking time. For feasibility, inspect a reasonable number of plants across different areas of each field. In the early to mid season, thrips numbers tend to be higher near field borders where adults infest first. If this is the case, insecticide sprays could be targeted to borders to slow spread to the entire field.
In the early summer (June to early July) and for highly susceptible onion varieties (e.g., red onions), a threshold of 15 thrips per plant should prevent economic loss during the early bulb growth stage and prevent rapid build-up of populations. In mid to late summer (July and August) and for more thrips-tolerant onion varieties (many yellow and white varieties), an action threshold of 30 thrips per plant has proven effective.
Thrips Management in the Home Garden
Thrips will infest onions, garlic, and other vegetables in the home garden, but small-sized plots are generally not at risk for significant yield loss. Thrips in the home garden are best managed with cultural practices and natural biological control. The following are generally adequate to keep thrips from noticeably reducing onion growth and bulb size in the home garden: add compost to enhance organic matter and soil tilth use mulches plant onions and garlic in small plots surrounded by a diversity of other plants, including garden crops, herbs, and cultivated and wild flowers apply a stiff spray of water from a hose to wash thrips from plants when they are observed and avoid using broad-spectrum, toxic insecticides to preserve natural enemies that keep onion thrips populations low.В В
Insecticides have been the primary tactic for thripsВ management however, repeated applications often lead to resistance in the thrips population, suppression of natural enemies, and unsustainable management.
Recent research has shown that the majority of onion thrips on a plant are in the non-feeding egg stage (60-75% of total population on an onion plant during late June to August), and thus, not exposed to insecticides and other suppressive tactics.
Insecticides grouped by class (i.e., mode of action) that are effective in reducing thrips on onions and registered in Utah as of March, 2008:
pyrethrins + diatomaceous earth (Diatect вЂ“ conventional and organic formulations*)
oxamyl (Vydate)- Section 24(c) label for Utah
Insect Growth Regulator
azadirachtin (Azatin, Neemix*)
spinosad (Success, Entrust*)
methyl parathion (Penncap-M)
kaolin clay (Surround*)
insecticidal soap (SaferвЂ™s*)
stylet oil (JMS*)
permethrin (Ambush, Pounce)
*OMRI approved for organic production.
All brands are registered trademarks. Examples of brands may not be all-inclusive, but are meant to provide examples of products registered on onions in Utah. The availability of insecticides changes rapidly. Always check the label for registered uses, application and safety information, and protection and pre-harvest intervals.
The big question is how do you know if you have thrip damage in your garden. Since these insects are mini, it takes a major thrips infestation to do serious damage.
But since they can easily hide from humans, they can form a large population before you realize it.
They have piercing, sucking mouthparts that they stab into the surface of the leaves and flowers, sucking up the sap and juices. Sometimes, they conceal themselves inside of the flowers, making it even harder to spot them.
Some signs and symptoms of a thrips infestation include:
In most cases, the damage slowly progresses from one thing to the next. It typically starts with yellowing or bleached spots on the leaves or deformed leaves.
Over time, the symptoms move to a silvery look to the leaves and black spots, which are actually fecal matter from the thrips. The final stage involves the leaves wilting and dropping off of the plant.
Assuming that you’ve determined you don’t have the beneficial kind in your garden, it’s time to take care of the problem.
Yes, you can use insecticides, but there’s an issue with that.
The insecticides that can be used to kill thrips also kill the beneficial predator insects in your garden. That’s not a risk most organic gardeners want to take.
Beneficial insects are prized in organic gardens, being far more efficient in the long run at controlling problems when compared to chemical choices.
So, the ideal plan starts with using control strategies that involve the least toxic methods while also using good practices, such as having a consistent watering schedule and removing diseased and dead plant material.
Here are some suggestions to help control and kill a thrips infestation.
When it comes to any pest infestation, catching the problem early will save you a lot of time and frustration. It’s a smart idea to take time each week, twice a week if possible, to carefully inspect your plants, looking for damage and potential pests.
The sooner that you catch the problem, the sooner you can get it under control. It’s easier to put out a small campfire than it is a brush fire, so get to the problem before it’s taking out your entire garden.
These pests don’t spend too much time flying around their wings aren’t very strong. You can still try blue or yellow sticky traps near the plants you’ll catch some of them.
Another option is to put a cloth underneath the plant and shake the branches, forcing the insects to fall off. Remove the cloth quickly and soak it in a bucket of water to kill them.
One good practice is to prune and get rid of injured or damaged parts of your plant regularly. Pests like to focus on damaged and vulnerable areas on the plant.
Rose growers, for instance, can go outside and inspect their plants as they begin to form buds. If the buds look discolored or deformed, trim them off.
If you keep at it, you’ll get rid of the pests and new, healthy buds will form.
You CAN try to use a mild insecticide that isn’t as strong and won’t kill all of the insects in your garden. Two safe alternatives are insecticidal soaps and neem oil.
Keep in mind that these aren’t cure-alls and they can still harm beneficial insects, so you should only turn to them if you’re really struggling to get an infestation under control.
You don’t have to buy soap at the store for your plants. Instead, you can spray or wash your plants don’t with a homemade mixture of soap and water.
All you need is about two teaspoons of soap for each gallon of water – try using regular dish soap.
Gently wash down your plants with this solution, making sure to get it on all of the leaves and other areas of the plant. Don’t forget the underside of the leaves, which is where thrips love to go and hideout.
If you’ve dealt with aphids, then you know this method. Take your hose and, using high-pressure water, focus on knocking pests off of the underside of the leaves.
This method works for aphids and thrips. You can use a blast of water as much and as often as your plant can handle it, but be careful. High-pressure water could damage leaves, so do so with the utmost care.
Shearing, or dramatically cutting back your plant, isn’t a good idea if you’re dealing with thrips. The new growth will attract even more thrips, resulting in the complete opposite of your goals.
Onion thrips (Thrips tabaci) are an important annual pest of onion. They can attack many garden crops, but most commonly cause serious damage to onions, leeks and garlic.
Appearance: Adult onion thrips are about 1 /12 inch long, thin and pale yellow to brown in color. Their wings have only a single, central vein and are fringed with long hairs. Nymphs (immature thrips) resemble adults, but they are smaller and lack wings.
Onion thrips damage to onion leaves (left) and numerous onion thrips adults and nymphs on an onion leaf (right). Phots courtesy of Karen Delahaut (left) and Joe Ogrodnik, Cornell University (right).
Symptoms and Effects: In general, onion thrips prefer tight spaces and cause severe damage on plants that produce tightly packed leaves. Feeding causes whitish blotches that may appear as silvery streaking on leaves. As feeding continues, affected tissue may turn dry and yellow, and may eventually brown and die. On flowering plants, thrips feeding can cause decreased pollen production. Onion thrips can also carry and inoculate plants with viruses such as Iris yellow spot virus (IYSV), Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV). See University of Wisconsin Garden Facts HXT1139, Impatiens Necrotic Spot, for details on this latter virus.
On onions, thrips prefer to feed on the youngest leaves, and the tips of these leaves often brown and die. Thrips feeding can also lead to distorted and undersized bulbs. Cultivars that produce leaves close to the stems are more susceptible to thrips damage than cultivars with more open growth. Onions that have a more circular leaf structure and that have glossier foliage tend to be less prone to damage. Red onions are particularly susceptible to thrips damage, while Spanish onions tend to be somewhat more tolerant.
On cabbage, varieties with dense heads are most susceptible to damage. Heavy thrips feeding can cause cabbage heads to become distorted and leaves to have darkened blisters where feeling has occurred. On cauliflower, thrips damage causes tan or brown streaks on the curds. This damage can provide entry points for bacteria that cause bacterial soft rot (see University of Wisconsin Garden Facts HXT1224, Bacterial Soft Rot, for details).
Life Cycle: Onion thrips overwinter in legume (e.g., alfalfa) and grain (e.g., wheat) fields, in weedy areas, and in onion bulbs not removed from onion fields. Females can reproduce without mating and lay eggs on the undersides of leaves. Eggs hatch after five to 10 days. The first two thrips nymphal stages feed on plants. Two additional non-feeding nymph stages live in the soil or on the soil surface. Nymphs mature into adults after 15 to 30 days, and adults return to plants to feed. Thrips can produce five to eight generations per year, and outbreaks are most likely to occur in the summer during hot, dry periods.
Scouting: Thrips populations can develop very rapidly. Be sure to routinely and carefully monitor crops for thrips during the periods indicated in the table below. Use yellow or white sticky traps and carefully examine plants at the edge of home gardens (or commercial fields) to monitor for the initial movement of thrips early in the growing season. If you find three thrips per onion leaf, consider chemical treatments. However, some onion varieties (e.g., ‘Snow White’, ‘Vega’) are highly tolerant of thrips feeding even with up to 45 thrips per plant.
|When to Scout for Onion Thrips|
Cultural: Remove plant debris from gardens (and commercial fields) as well as any areas surrounding a location where you will be growing onions or other susceptible vegetables.
Chemical: Apply insecticides each year as soon as thresholds (see above) have been exceeded to target the insects before they can reach protected areas (e.g., the insides cabbage heads and onion necks) where insecticides cannot easily penetrate. When using foliar sprays, direct the sprays down into the center of the plants to maximize penetration into protected areas. Also, use sufficient water, as well as a spray additive, to help the insecticide penetrate. Alternate use of at least two insecticide active ingredients to minimize development of insecticide-resistant thrips. See UW-Extension Publication A3422, Commercial Vegetable Production in Wisconsin, for a list of registered insecticides.
For more information on onion thrips: See UW-Extension Bulletin A3422, or contact your county Extension agent.