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Best Management Practices for Thrips (Thysanoptera: Thripidae) in Cotton

Introduction

Thrips (Thysanoptera: Thripidae) are the most consistent and predictable insect pests of Upland cotton, Gossypium hirsutum L., in South Carolina and much of the southeastern United States. These small insects have piercing-sucking and rasping mouthparts and feed on almost all portions of the cotton plant, with the most significant injury occurring on seedlings (plant emergence to five true leaves).1,2 Excessive feeding injury can produce severely stunted plants (figure 1), often resulting in loss of yield or, at least, a delay in crop maturity. The predominant species infesting and causing injury to seedling cotton in the Southeast is the tobacco thrips, Frankliniella fusca (Hinds) (figure 2), as it readily reproduces on and colonizes the crop, but other species of less importance are present.3,4

Rows of planted cotton in a field.

Figure 1. A row of unprotected cotton (foreground) with extensive injury from thrips, compared with rows protected with insecticide at planting (background). Image credit: Jeremy Greene, Clemson University.

Adult female and male thrips lined up next to a lowercase 12-point font letter i to compare their size.

Figure 2. Lowercase ‘i’ in Times New Roman 12-point font for comparison with (from left to right) two adults (female with macropterous [large, functioning wings] and male with brachypterous [rudimentary, non-functioning wings]), pupa, prepupa, and two immatures (also referred to as ‘nymphs’ or ‘larvae’) of tobacco thrips. Image credit: Sophia Conzemius, Clemson University.

Identification

Tobacco thrips can be identified using genetic techniques3 or by visual characteristics using a microscope.3,5 As adults, tobacco thrips have large, fully functioning (macropterous) or rudimentary (brachypterous) wings and bodies about 1 to 2 mm in length that can be dark brown or tan (figures 2 and 3). Immatures of the tobacco thrips are wingless, yellow, and smaller than adults (figures 2 and 4). Eggs of thrips (figure 5) are partially inserted into leaf tissue. Other species of importance in cotton closely resemble tobacco thrips, with the most notable species being the western flower thrips, Frankliniella occidentalis Pergande.

Two adult female thrips.

Figure 3. Adult females (macropterous, left; brachypterous, right) of tobacco thrips. Image credit: Sophia Conzemius, Clemson University.

Two immature tobacco thrips.

Figure 4. Immature stages of tobacco thrips. Image credit: Sophia Conzemius, Clemson University.

A thrips egg on the tip of a thumbtack.

Figure 5. Thrips egg on the tip of a thumbtack. Image credit: Sophia Conzemius, Clemson University.

Life Cycle

Adult females of tobacco thrips deposit eggs primarily into the cotyledons (seed leaves) and initial true leaves of cotton seedlings.6 Larvae emerge from hatched eggs in less than a week, depending on temperature,7 feed on leaf cells for approximately another week before undergoing a short (a few days) resting stage, and then turn into newly formed adults that seek mates and start the entire process over again.8 Tobacco thrips can have multiple generations per year,9 with only a generation or two having an impact on cotton. Tobacco thrips and related species overwinter as adults, and possibly as immatures, on weeds, winter crops, and plant debris (i.e., litter) in the southeastern United States.10

Management

Chemical Control

Because thrips can significantly injure cotton and reduce yields or at least delay maturity, most cotton planted in the southeastern United States receives a prophylactic insecticide treatment. Preventative control with insecticides used at planting is the most common method of addressing tobacco thrips in cotton. Insecticides used at planting for thrips are applied to the seed or placed in the furrow with seed as liquid or granular formulations. These insecticides systemically enter a seedling cotton plant through water uptake in the root system and control thrips that feed on above-ground tissue. The neonicotinoids11 thiamethoxam and imidacloprid, the organophosphate11 acephate, and the carbamate11 thiodicarb are commonly used as seed treatments. Acephate and imidacloprid can also be applied as in-furrow liquid sprays, and the organophosphate phorate and the carbamate aldicarb are applied as in-furrow granular insecticides. This preventative, at-plant strategy for chemically controlling thrips on seedling cotton (figure 6) has worked well for decades, but some resistance to insecticides, specifically the widely used neonicotinoids, has been reported.12

The left image shows unhealthy cotton seedlings, and the right image shows healthy cotton seedlings that had insecticide applied.

Figure 6. Cotton seedlings (7-leaf stage) without (left) and with (right) preventative insecticide used at planting. Image credit: Jeremy Greene, Clemson University.

As cotton grows, insecticides applied at planting become diluted and are metabolized into derivatives in plants and lose effectiveness. Foliar insecticide applications are often required to control thrips in cotton as the plant continues to grow, representing a shift to reactive control rather than preventative. Many states have published guidelines regarding thrips control on cotton. The guidelines recommend sampling for thrips and assessing feeding injury (figure 7)13 on seedling cotton and comparing results with treatment thresholds. For example, the Cotton Insect Management section of the South Carolina Pest Management Handbook recommends that insecticides be applied as foliar sprays to control thrips in seedling cotton (up to four or five true leaves) when populations meet or exceed two or more thrips per plant, with particular attention paid to when immature thrips appear, signaling that insecticides used at planting are no longer providing control.14 Shaking plants in large white containers, such as drinking cups, is a good way to count dark-colored adult thrips. Shaking plants over or onto something dark, such as a black drop cloth or clipboard, is an excellent method for enumerating light-colored immature thrips (watch a thrips YouTube video that shows the methods of sampling for thrips). When thresholds are met or exceeded, options for foliar sprays of insecticides include, but are not limited to, the organophosphates acephate, dicrotophos, dimethoate, and the spinosyn11 spinetoram as the most efficacious materials available for post-planting control of thrips in seedling cotton. Non-chemical options (presented hereafter) for managing thrips in cotton should be used when possible to slow the development of resistance to insecticides.

Six images of cotton seedlings with varying degrees of advancing injury from thrips.

Figure 7. Numerical ratings of progressive feeding injury to cotton seedlings caused by thrips. Ratings ‘1’ through ‘5’ represent advancing injury to true leaves, with ‘0’ indicating no visible injury. Image credit: Jeremy Greene, Clemson University.

Cultural Control

Other strategies for controlling thrips in cotton include practices deployed as cultural techniques. For example, delaying the planting date can reduce the risk of thrips injury.13 A mathematical model9 and web-based tool,15 the Cotton Infestation Predictor Tool, is available to help cotton producers determine appropriate planting dates to minimize risk from tobacco thrips. This new tool can also be used to organize fields by risk, allowing for variable scouting efforts and control options based on the intended planting date. Modifying planting date as a cultural control tactic could reduce the use of chemical controls, slow development of insecticide resistance, benefit the environment, and potentially save input costs for producers.

Another cultural control strategy involves using the inherent properties of cotton varieties to withstand injury from thrips. Host-plant resistance (HPR) deters colonization of and injury from thrips by making the plant less palatable to thrips (antixenosis) and/or increasing mortality or reducing longevity and reproduction of thrips on the resistant plant (antibiosis). Having this built-in protection can be very effective when identified and deployed. However, Upland cotton is generally considered highly susceptible to thrips, with no known thrips-resistant commercial varieties.2,16 Some cotton species appear to be more resistant or tolerant of thrips, such as Pima cotton, Gossypium barbadense L.16,17 Contradictive traits such as highly pubescent18 and hairless19 phenotypes have been linked with reduced thrips populations, yet both have been reported to be susceptible characteristics in other studies.20,21 The high glanding breeding line ‘Arkot 8727’ has been registered with thrips resistance22, though glandless cotton was found to have lower thrips damage.23 Additional genetic and chemical analyses are needed before traditional plant breeding techniques will yield HPR characteristics in cotton that can be used as a successful management practice for thrips in the Southeast. Despite these hurdles, advances in genetic engineering have produced a cotton trait that uses antixenosis to deter injury from thrips.24 Cotton with this trait is expected to be commercially available in 2022.

Heavy plant residue at the time of cotton planting can also reduce thrips populations and damage to cotton. Heavy residue can be achieved by incorporating a winter cover crop into the rotation as well as reducing tillage.25,26 Cover crops have been adopted because of numerous other benefits, including suppressed weed pressure,27 conserved soil moisture,28 and reduced nematode injury.28 Sanitation (also known as clean culture) is commonly used as a cultural control technique for numerous insect pests, but bare soil (without any plant residue) promotes higher populations of thrips and does not decrease thrips injury on young cotton plants.

Additional cultural control practices for managing thrips in seedling cotton are contradictive or deficient. Skip-row plant spacing does not influence the abundance of thrips in seedling cotton.29 However, within-row plant spacing might have an impact, with decreasing numbers of thrips reported with an increase in plant spacing in cotton30 and onions31 (another crop susceptible to thrips injury). Heavy rainfall events often reduce populations of thrips on cotton32 and onions33 and can promote rapid plant growth to decrease the time plants are susceptible to injury from thrips. Overhead sprinkler irrigation can also reduce populations of thrips in cotton.34

Physical/Mechanical Control

The use of barriers, sound waves, electricity, radiation, temperature, and other physical control tactics has not been reported as effective in controlling thrips in seedling cotton. Of these strategies, mulching films or other reflective ground surfaces have been shown to reduce thrips, aphids, and whiteflies in tomatoes35 and other crops.36 Screens and materials blocking ultraviolet light37 can provide greenhouse crops with some protection, but the implementation of physical or mechanical control strategies in field crops, such as cotton, are challenging, cost-prohibitive, and questionably effective.

Biological Control

Parasitoids, predators, and disease organisms can provide natural biological control of thrips, but it is almost always not enough protection to prevent infestations of thrips from having an economic impact. Manipulations of predaceous arthropods are expensive, and ratios of costs to benefits regularly preclude the effective use of introducing or augmenting predators of thrips on cotton.38 Despite some natural enemies of thrips in seedling cotton, deliberate modification of beneficial organisms is not currently a practical strategy for managing thrips in the crop.

Other Control Options

Additional best management practices (BMPs) for thrips in seedling cotton include the use of starter fertilizer39 and minimizing plant stress, such as that caused by herbicide injury, to help seedlings grow normally and quickly outgrow feeding injury. A good integrated pest management (IPM) strategy will use several of the aforementioned control tactics and BMPs concurrently.40

Summary

Thrips are consistent and predictable insect pests of seedling cotton in the southeastern United States. Although a complex of species infests seedling cotton in the region, tobacco thrips are the predominant species requiring management in the crop. Control strategies rely heavily on chemical control used at planting, but issues with insecticide resistance are emerging that compel the use of additional tactics. Cultural control strategies, such as the use of cover crops, reduced tillage operations, delayed planting date, targeted irrigation, and starter fertilizer, can complement chemical control and likely help slow the development of insecticide resistance. Producers and managers of cotton in the southeastern United States should consider using a multi-tactic approach that uses several BMPs in an overall IPM approach for managing thrips.

References Cited

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  2. Cook D, Herbert A, Akin DS, Reed J. 2011. Biology, crop injury, and management of thrips (Thysanoptera: Thripidae) infesting cotton seedlings in the United States. Journal of Integrated Pest Management. 2011 Oct. 2(2):1–9. doi:10.1603/IPM10024.
  3. Wang H, Kennedy G, Reay-Jones FPF, Reisig DR, Toews MD, Roberts PM, Herbert DA, Taylor S, Jacobson AL, Greene JK. Molecular identification of thrips species infesting cotton in the southeastern USA. Journal of Economic Entomology. 2018 Apr:1(2):892–898. doi:10.1093/jee/toy036.
  4. Stewart S D, Akin DS, Reed, J, Bacheler J, Catchot A, Cook D, Gore J, Greene JK, Herbert A, Jackson RE, Kerns DL, Leonard BR, Lorenz GM, Micinski S, Reisig D, Roberts P, Studebaker G, Tindall K, Toews M. Survey of thrips species infesting cotton across the southern US Cotton Belt. J. Cotton Sci. 17(4):263–269. http://journal.cotton.org/journal/2013-17/4/upload/JCS17-263.pdf.
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  39. Olson DM, Davis RF, Brown SL, Roberts P, Phatak SC. Cover crop, rye residue and in‐furrow treatment effects on thrips. Journal of applied entomology. 2006 Jun;130(5):302-8.J. App. Entomol. 130(5):302–308. doi:10.1111/j.1439-0418.2006.01066.x.
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