Posts Tagged: western flower thrips
Integrated pest management options for the western flower thrips in lettuce
The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is one of the major pests of lettuce in California. It has a wide host range including several vegetable, ornamental, and other cultivated or wild plants. Native to North America, the western flower thrips is also known as alfalfa thrips, California thrips, and maize thrips among others. This article provides a general overview of the pest, its biology, damage, and management.
Biology:
Eggs are small, oval, and inserted into plant tissue. Nymphs are slender and have four instars. The first two - larva I and II – feed on plant tissues while the latter two - prepupa and pupa – are non-feeding stages that are often found in the soil. Larvae are wingless and white initially and turn yellow or orange once they start feeding. Adults are small (< 2 mm), slender, and have two pairs of long, narrow wings with a fringe of hairs. The western flower thrips can occur in different color morphs such as yellow or orange, brown, and black.
Damage:
The western flower thrips prefers flowers, but also feeds on developing buds, fruits, and foliage. Larvae and adults rupture the leaf surface with their rasping mouthparts and feed on plant juices. Feeding damage results in silvery appearance of the leaf surface, which later turns brown. The presence of dark fecal specs indicates thrips occurrence. In lettuce, the western flower thrips transmits Tomato spotted wilt virus and is the sole vector of Impatiens necrotic spot virus. Only the larval stages acquire these tospoviruses and the adults transmit the viruses to other plants as they spread in the field.
Management:
Integrated pest management approach is critical for successful pest management. It involves regular monitoring, exploring the potential of multiple options including cultural and biological solutions, and proper timing and application of various strategies among others. The western flower thrips is one of the pests where insecticide resistance is a common problem. To reduce the risk of resistance development, it is necessary to explore the potential of multiple control options and rotate insecticides with different modes of action. This is essential to suppress pest populations to desired levels and also to maintain control efficacy of existing pesticides.
Cultural control – Remove weed and other hosts that harbor thrips or viruses. Sprinkler irrigation can help reduce thrips populations. Plow down lettuce crop residue to destroy surviving stages. In general, maintaining good plant health with optimal nutrition and irrigation practices helps plants withstand pest damage. Silicate products can improve the structural strength of plant tissues and reduce pest damage and/or populations. Several biostimulants or biological soil amendments can also help activate plant's natural defenses against pest infestations. Consider using them to improve overall plant health and yields, and to protect plants from biotic and abiotic stresses.
Biological control – Predators such lacewings (Chrysopa spp. and Chrysoperla spp.), minute pirate bugs (Orius spp. and Anthocoris spp.), predatory mites (Amblyseius swirski, Ablyseius andersoni, Neoseiulus cucumeris and Stratiolaelaps scimitus), and rove beetles (Dalotia coriaria) attack thrips. Conserve natural enemies with insectary plants and applying safer pesticides, and augment natural populations by releasing commercially reared species.
Microbial control – Entomopathogenic fungi such as Beauveria bassiana and Cordyceps (Isaria) fumosorosea, products based on bacteria such as Burkholderia rinojensis and Chromobacterium subtsugae, and entomopathogenic nematodes such as Heterorhabditis spp. and Steinernema feltiae can be used against one or more life stages. Entomopathogenic nematodes are more effective against pupae in soil because they actively search for and infect their hosts. Entomopathogenic fungi can be used against all life stages.
Botanical control – Azadirachtin alone or in combination with entomopathogenic fungi or insecticides can also be used against multiple life stages. Azadirachtin is an insecticide, antifeedant, and a growth regulator. Similarly, pyrethrins derived from chrysanthemum flowers can be used alone or with other biological or synthetic insecticides. Pyrethrins are nerve poisons. Other botanical insecticides that contain soybean oil, rosemary oil, thymol, and neem oil (which also has a low concentration of azadirachtin) also provide control against thrips through insecticidal, repellency, and antifeedant activities.
Other control options – Insecticidal soaps and mineral oils can be used against different life stages of thrips. Spinosad, a popular insecticide of microbial origin and a mixture of two chemicals spinosyn A and spinosyn D, is very effective against thrips. However, overuse of spinosad can lead to resistance issues in thrips and other insects.
Chemical control – There are several synthetic insecticides that are effective against thrips. It is important to rotate chemicals among different mode of action groups to reduce the risk of insecticide resistance. The following are some synthetic active ingredients and their mode of actions groups in parenthesis that can be used for thrips control: methomyl (1A), bifenthrin (3A), lambda-cyhalothrin (3A), zeta-cypermethrin (3A), clothianidin (4A), spinetoram (5), and cyantraniliprole (28).
Depending on the level of control needed, combinations of products from different categories can improve control efficacy. For example, a combination of entomopathogenic fungi and nematodes can be applied to the soil for controlling prepupae and pupae. While the soil-dwelling predatory mite S. scimitus and the rove beetle, D. coriaria, can be used against pupal stages, other natural enemies can be used against nymphs and adults. A combination of entomopathogenic fungi and azadirachtin can be applied both to the soil or foliage for controlling different life stages. Similarly, various biological and synthetic insecticides can be applied in combination or rotation to obtain desired control.
The categories presented above are based on the source or nature of the active ingredients and do not indicate their organic or conventional label status. Please check the product labels for their appropriateness for managing thrips in lettuce, for use in organic farms, and guidelines for storage, handling, and field use. Entomopathogenic nematodes, fungi, and other biologicals are compatible with several synthetic agricultural inputs, but verify the label guidelines for specific instructions.
Additional resources:
Dara, S. K. 2019. The new integrated pest management paradigm for the modern age. JIPM 10: 1-9. https://doi.org/10.1093/jipm/pmz010
Dara, S. K. 2021. Biopesticides: categories and use strategies for IPM and IRM. UC ANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=46134
Natwick, E. T., S. V. Joseph, and S. K. Dara. 2017. UC IPM pest management guidelines: lettuce. UC ANR Publication 3450. https://www2.ipm.ucanr.edu/agriculture/lettuce/Western-flower-thrips/
Riley, D. G., S. V. Joseph, R. Srinivasan, and S. Diffie. 2011. Thrips vectors of tospoviruses. JIPM 2: I1-I10. https://doi.org/10.1603/IPM10020
Congrats to the Thrips Team!
Congratulations to the international team of scientists, including UC Davis entomologist and...
Professor Diane Ullman of the UC Davis Department of Entomology and Nematology is a co-author of the publication on the Western flower thrips. This image was taken when she was doing research in France.
Diane Ullman to Speak on Western Flower Thrips
Diane Ullman, professor of entomology, UC Davis Department of Entomology and Nematology, will speak...
Recent Abundance of Thrips on Vegetable Crops in the Salinas Valley
Among several species of thrips that invade vegetable crops, western flower thrips [Frankliniella occidentalis (Fig. 1)] is the most destructive species of thrips in the Salinas Valley. They can cause severe feeding injury to all stages of plant development. Early feeding can cause severe stunting or reduce plant development whereas; late feeding can cause visible feeding patches - affecting marketable yield in both the instances. Severe feeding injury is usually associated with very high populations of thrips on the crop. It is likely that recent early surge in thrips populations in the Salinas Valley is related to warmer (high day temperatures) and dry winter.
Thrips is a tiny insect (less than 3 millimeters) and prefers to stay and feed within tight protected areas of the plant such as, between veins or near mid-rib or within the layers or stacks of lettuce leaves or celery stems. Thus, thrips injury was detected in those tight areas of the plant. It is likely that colonizing at those tight areas provide protection from insecticide sprays.
Thrips has a “piercing-sucking” or “punch and suck” mouthpart. Mouthpart of thrips is referred as mouthcone (Fig. 2). Thrips typically feeds using two structures of its mouthcone: (1) a mandible and (2) stylets. As illustrated in the Figure 2, thrips uses the mandible to pierce or punch the plant cell wall and stylets (or needles), which often form a single tube, sucks the liquid food from the plant cell. This feeding apparatus allows thrips to feed on liquid food on a surface or within a plant cell.
As indicated, thrips can cause significant crop loss once its population increases to very high levels. Thrips injury on lettuce may appear as brown streaks, or scarring on the leaves (Fig. 3). If examined closely using magnifying glass, it appears like punctured plant cells and the content removed (Figs. 4 and 5). On celery, the thrips feeding injury is similar but the injured cells appear as raised ridges (Fig. 6). When attacked at the younger stages of the plant development, for e.g., on the growing tips of the cotyledons (Fig. 7), the feeding could deform the true leaves that develop later (Fig. 8).
In addition to feeding injury, western flower thrips are able of transmiting plants viruses (tospoviruses) such as Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV). In the family: Thripidae, there are 1710 species of thrips but only 14 thrips species are currently reported to transmit tospoviruses. Both larval and adult stages of thrips vectors actively feed on the host plants but only early larval instars can acquire tospoviruses and later instar larvae and adults can transmit tospoviruses after a latent period. Adult thrips can acquire tospoviruses, but they do not transmit them because virus could not multiply to sufficient numbers. Also, tospoviruses are not transmitted when the thrips lay eggs into the plant. Thus, each new generation of thrips vectors must acquire the virus as larvae. The weed plants outside the field can be the reservoir for tospoviruses and when the larvae feed on them, they acquire the virus. In the field, larvae feeding on the tospoviruses-infected lettuce plants may also aid virus acquisition. The thrips carrying the virus just need to feed for 10-15 minutes to transmit the virus to uninfected plants.
Typically, bean-shaped eggs are inserted by female western flower thrips into the leaf. Within 5 days, eggs hatch to first instars. If the temperature stays at 86ºF, first instars molt into second instars. This can happen in couple of days in the Salinas Valley. Second instars develop into prepupae within 4-5 days. Most of the prepupae drop to the soil and emerge into adults within 3 days at 86ºF. Pupal stage is the only non-destructive stage of the thrips. Clearly, thrips development is associated to temperature. Adult females lay about 50 eggs and can live up to 4-5 weeks at 86ºF. So, in Salinas Valley due to milder temperature range, western flower thrips may live longer than 5 weeks. Western flower thrips requires a minimum 194 degree days (DD) (min. temp. 49.5oF) to complete a generation, but has been estimated to be as high as 254 DD with a minimum temperature of 43.7oF. Western flower thrips can lay eggs with and without mating. The mated female thrips (fertilized) tend to produce more female offsprings than males whereas, unmated female thrips tend to produce more male offsprings than females.
Thrips are weak flyers but they have fringed wings which help them to get airborne and glide short and long distances. Thrips can stay airborne for about 24 hours in the cooler conditions and can remain without feeding and drinking. They get quickly desiccated if they stay longer in the air. The dispersal of thrips is largely depending on temperature, light, and wind.
To prevent direct feeding injury and viral transmission, it is important that we manage thrips on the crops using the tools such as targeted insecticide sprays. Recent insecticide efficacy studies against western flower thripssuggested that insecticides such as Radiant, Entrust, Lannate, Exirel and Beleaf have decent activity against western flower thrips. Other products, Gladiator and Torac are effective but are not registered for use. Please read the Monterey County crop note (May edition) for details on insecticide efficacy trials. It is important that the growers restrain from repeated use of insecticides within same IRAC class (http://www.irac-online.org/documents/moa-classification/?ext=pdf) in a given season instead rotate insecticides with distinctly different modes of action to reduce development of resistance.
Fig. 8: Thrips feeding injury on young seeding of Swiss Chard causing leaf deformation.
Targeting Thrips
If you grow tomatoes, you ought to be concerned about thrips. They're pests of fruits,...
George Kennedy, the William Neal Reynolds Distinguished Professor of Agriculture at North Carolina State University, stopped to count thrips during a vacation to Mt. St. Helens. (Photo by Scott Kennedy)