The common blossom thrips or tomato thrips, Frankliniella schultzei (Trybom) (Thysanoptera: Thripidae), is an emerging pest of various vegetable crops in south Florida. Although this thrips species has been present in the region for a long period, not much attention was given to it until recently when it was found to be associated with increased cases of tomato chlorotic spot tospovirus (TCSV) (Londoño et al. 2012) and groundnut ring spot tospovirus (Webster et al. 2010) in Florida. Worldwide F. schultzei has been reported feeding on 83 species of ornamental and vegetable plants in 55 families (Palmer 1990; Milne et al. 1996). In the south Florida, it has been reported to infest tomato, squash cucumber, and bean (Kakkar et al. 2012a, b). Frankliniella schultzei causes direct and indirect damage on leaves, flowers and fruits of its host plants. Direct damage is caused by feeding and oviposition on leaves, flowers and fruits resulting in necrotic spots on tender leaves and dark dotted blemishes in fruits (Kakkar et al. 2010). Oviposition causes brown lesions on fruits and flowers, and at high densities it may lead to premature abscission of flowers and fruits. Indirect damage by this thrips is caused by its transmission of viral diseases to the host plants. Frankliniella schultzei is a known vector of several plant damaging virus including Tomato spotted wilt tospovirus (TSWV) (Sakimura 1969; Wijkamp et al. 1995), Tomato chlorotic spot tospovirus (TCSV), groundnut ring spot tospovirus (GRSV) (Wijkamp et al. 1995), Tobacco streak ilarvirus (TSV) (Klose et al. 1996), Stem necrosis virus and Capsicum chlorosis virus (Horticulture Australia 2005).

In Miami-Dade County in 2012, F. schultzei caused economic damage to some tomato and pepper plantings by transmitting Groundnut ring spot virus. It is the first record in the United States of F. schultzei causing large scale destruction (30–60%) of tomato and pepper production due to virus transmission (Londoño et al. 2012). During our study we found that F. schultzei can cause significant damage to tomato and pepper by transmitting virus even at the low population density of 1 adult/10 plants) (D.R.S. personal observation). Plants affected in an early stage of development may die before bearing fruits; while infection at a late stage causes malformed, unmarketable fruits with ring like lesions. Considering the increased cases of F. schultzei in the area and its wide host range, it is imperative to initiate the development of effective management practices to avoid economic losses from thrips-induced plant diseases.

Thrips are opportunistic insects; small size is advantageous for them to acquire microhabitats on both primary and alternative hosts. In Brazil, Guimaraes et al. (1997) observed higher incidence of TSWV when tomato was planted near a maize windbreak, which sheltered this pest. In such cases, roguing of infected plants has been found to be useful in reducing rates of new infestations (D.R.S. personal observation). Cultural practices can be helpful in providing some control over thrips and associated virus damage to the crops, for example, temporary flooding was reported to cause reduction of F. schultzei population by killing soil inhabiting pupae (Bournier 1994). Biological control using generalist predators such as minute pirate bug or phytoseiid mites could be effective but their potential as a solution against this pest still needs to be evaluated. In commercial farming with high value crops and low aesthetic thresholds, a rescue program is essential to reduce economic losses. Moreover, when dealing with an insect pest whose low population abundance can cause significant damage by transmitting viral diseases, it is important to control such a pest species at the incipient stage of an infestation.

In this study we evaluated the efficacy of various conventional and novel insecticides of diverse mode of actions in managing F. schultzei. Specific objectives of the study were to evaluate 1) effectiveness of insecticides of diverse mode of action in rotation, 2) methods of application of these insecticides, and 3) timing of applications during cropping season. The outcome of this study might provide better management of this thrips, reduce the transmission of the viral disease, and avoid the rapid development of insecticide resistance.

Materials and Methods

Three chemical trials were conducted varying in host crops, insecticide treatments, application methods, and application time. Two studies were conducted on tomato (Solanum lycopersicum L.) and one on bell pepper (Capsicum annuum L.). All studies were conducted in the research plots of Tropical Research and Education Center (TREC), Homestead, Florida. The soil type of all experimental plots was Krome gravelly loam (loamy-skeletal, carbonatic hyperthermic lithic Udorthents), which consists of about 33% soil and 67% pebbles > 2mm.

Trial 1: Effectiveness on Tomato of Diamide Insecticides vs a Neonicotinoid Insecticide Applied by Soil Drenching followed by Drip Application

Table 1.

Various treatments applied in the first trial on ‘BHN 585’ tomato along with their brand names, manufacturers, rates, mode and timing of application.

This study was designed to determine effectiveness of application of a new class (anthanilic diamide) of insecticides against the standard (neonicotinoid) in achieving control of common blossom thrips. Application methods of insecticides were at planting soil drench followed by drip application.

Field preparation and planting. Raised soil beds each 6 in (15 cm) high and 36 in (91 cm) wide with 6 feet (183 cm) spacing between bed centers were prepared and covered with black on white 1 mL polyethylene mulch (Grower's Solution LLC., 1211 A Boyd Farris Rd., Cookeville, Tennessee). Beds were provided with 2 parallel lines of drip tape (T-systems, DripWorks, Inc., 190 Sanhedrin Circle, Willits, California) having 5 in (13 cm) spacing to supply 1,500 gallons (5,678 L) of water/acre/day. The T-tapes were placed 12 in (30 cm) apart straddling the center of each bed to irrigate and fertigate the plants. At the time of preparation of beds, granular fertilizer 8:16:16 (N: P: K) at the rate of 1,200 pounds/acre (1360 Kg/ha) was broadcast on the upper surface of a bed and incorporated mechanically within the upper 4 in (10 cm) of the soil. ‘BHN 585’ tomato seedlings were planted 18 in (46 cm) apart within rows. Plants were drip irrigated and fertigated with 4-0-8 (N: P: K) by applying 0.5 lb (0.227 Kg) N/day/acre starting at 4 weeks after planting and progressively increasing this rate by the increment of 0.25 lb (0.113 Kg) every 2 weeks until 4.0 lb (1.81 Kg) N/acre/ day when plants were bearing fruit. Each treatment plot consisted of 2 beds, 30 ft long (99.14 m) and was arranged in a Randomized Complete Block (RGB) design with 4 replications. A 5 ft (1.52 m) wide fallow area separated the blocks from each other.

Treatments: Three insecticides - imidacloprid (neonicotinoid, IRAC Group 4A; Admire®' Bayer CropScience, Research Triangle Park, North Carolina), cyazypyr (anthrnilic diamide, IRAC Group 28; Ve rim ark®, DuPont Crop Protection, Wilmington, Delaware) and rynaxypyr (anthranilic diamide; IRAC Group 28; Coragen®, DuPont Crop Protection, Wilmington, Delaware) were evaluated either alone or in rotation with each. Information about rate, date and mode of application has been included in Table 1. Methods of applying insecticide treatments were soil drenching at planting delivering 100 GPA (909.1 L/ha), and drip application post planting by delivering 120 GPA (1,090.1 L/ha). Evaluation of treatments was made once a week for a period of 6 weeks by randomly collecting 10 full grown leaves from each treatment plot. All leaves from a treatment plot were placed in a zip-lock bag and were marked with date, treatment and block number. While collecting in the field, the samples were temporarily placed in an icebox (28 × 16 × 16 in [71 × 41 × 41 cm]) to avoid desiccation. At the end of collection, all samples were transported to the laboratory. Leaf sample in each zip-lock bag was soaked in 50 mL of 70% ethanol for 15–25 min to separate thrips from the leaf/flower samples. Leaf samples were rinsed out of alcohol by continuous and smooth swirling to avoid any escape of thrips from the sample. All thrips left in the alcohol were separated by a 500 mesh (26 micrometer) nematode extraction sieve (W. S. Tyler® Industrial Group, Mentor, Ohio). Finally, thrips specimens in the sieve were transferred to a Petri dish with 5–10 mL ethanol (70%) to count numbers of adults and larvae by a binocular microscope at 10X–20X. Numbers of tomato plants infected with GRSV per plot were recorded on 2 dates: 13 and 20 May.

Trial 2: Effectiveness on Tomato of an Imidacloprid Soil Drench Followed by Foliar Sprays of Either Cyazypyr or Dinotefuran

Table 2.

Insecticide treatments applied in the second trial on ‘BHN 585’ tomato along with their brand names, manufacturers, rates, mode of action and timing of application.

The study was conducted to determine effectiveness of 2 classes of insecticides (anthranilic diamide and neonicotinoid) in a program for achieving control of the common blossom thrips for an extended period of time. In each program, neonicotinoid (imidacloprid) was applied at planting as a soil drench followed by cyazypyr and dinotefuran as a foliar spray.

‘BHN631’ tomato seedlings were transplanted in the soil beds. Preparation of beds, maintenance of crops, application of treatments, collection of samples and their further processing were the same as described in the first study. A RGB design was employed to provide 5 replicates each for 5 treatments using a plot size of 3 rows, each 50 ft (15.25 m) long. Three insecticides - imidacloprid, cyazypyr and dinotefuran were tested in this study (Table 2). Imidacloprid was applied as a soil drench, and cyazypyr and dinotefuran were applied as foliar sprays. Three rates of cyazypyr (13.46, 16.82 and 20.5 oz/acre) were used to determine rate response in controlling F. schultzei. Soil application of imidacloprid was accomplished by delivering a volume of 120 GPA (1,090.1 L/ha) at the time of planting. Cyazypyr (Exirel®) and dinotefuan were each applied on the foliage on 4 dates at weekly intervals after transplanting. Foliar treatment of cyazypyr was applied by a CO₂ backpack sprayer with 2 nozzles at 30 psi delivering 70–100 GPA (636–909 L/ha). Evaluation of treatments was initiated 10 days after planting and continued at 5-day intervals for 40 days. Treatments were evaluated by randomly collecting 10 leaves per treatment plot for common blossom thrips. On each sampling date of thrips all plants in a plot were thoroughly checked for the presence of GRSV.

Trial 3: Effectiveness on Pepper of Foliar Applications of the Combination of 3 Insecticides vs. Spinetoram

Table 3.

Various treatments applied in trial #3 on ‘bell’ pepper along with their brand names, manufacturers, rates, mode of action and timing of application.

This study was conducted to determine effectiveness of joint foliar applications of spiromesifen, Chenopodium amhrosioide compared to spinetoram in controlling common blossom thrips and its transmitted GRSV on pepper.

‘Bell’ pepper seedlings were transplanted 12 in (30 cm) apart on 8 in (20 cm) high and 36-in (90 cm) wide beds of Rockdale soil. The beds were supplied with drip irrigation lines and covered with 1.5-mL thick black polyethylene mulch. Pepper plants were irrigated once daily using a drip system as described in the first study. Fertilizer (N-P-K mix) was applied at 200-50-240 lbs/acre. To control weeds trifluralin (Treflan EC, 24 lbs/A) was used once at 10 days before planting, supplemented during the middle of the season with mechanical cultivation.

This study involved the use of 3 insecticides in 2 treatments were:1) spirotetramat (IRAC Group 23; Movento®, Bayer CropScience) in combination with Induce (Helena Chemical Company) followed by Chenopodium ambrosioides extract,a terpenoid (Requiem®) (IRAC Group not established, Bayer CropScience) and 2) spinetoram (Radiant®) (IRAC Group 5, Dow Agro Science). Treatment's rates/ acre, application methods and application timings are shown in Table 3. Treatment plots consisted of 2 beds, each 30 ft (9 m) long and 3 ft wide. Treatments were arranged in a RGB design with 4 replications. A non-planted 5 ft (1.52 m) area separated each block. Treatments were sprayed on 5 dates at weekly intervals by a CO₂ backpack sprayer with 2 flat (even) nozzles (Spraying Systems Co., Wheaton, Illinois 60189-7900, USA)/row delivering 50-70 GPA (454-636 L/ha) depending on the canopy of the volume of experimental pepper plants. Evaluation of treatments was made 24 h after each application by randomly selecting 20 leaves (one leaf/ plant) from each treatment plot. All procedures from collection of leaf samples to counting of thrips were as described in the first study. In this study, data presented in the tables represent combination of adults and larvae.

Results

Trial 1: Effectiveness on Tomato of Diamide Insecticides vs a Neonicotinoid Insecticide Applied by Soil Drenching followed by Drip Application

Low population abundance of F. schultzei was reported during the entire study season (Table 4). During first few samplings, no significant difference in mean numbers of adults was observed in different treatment plots. On the 5th sampling date mean numbers of adults in all treated plots, except imidacloprid treated plots, were significantly lower than control plots. The effect of treatments on larval count was observed in the 6th sampling (Table 5) where mean numbers of larvae in all treated plots except imidacloprid treated plots were significantly lower than the non-treated control

We observed GRSV infected plants in all treatment plots on the first sampling date (Table 6). Only imidacloprid applied at planting as a soil drench followed by 2 drip applications of cyazypyr at 14 and 28 days after planting (Treatment# 1) and cyazypyr (drench) followed by drip application of imidacloprid 14 days after planting (Treatment# 5) significantly reduced GRSV incidence when compared with the non-treated control. Other treatments did not differ from the non-treated control in reducing GRSV incidence. On the second sampling date, Treatment# 1 and Treatment# 2 had significantly fewer incidence of GRSV than the non-treated plants. Other treatments did not differ from the non-treated control.

Trial 2: Effectiveness on Tomato of an Imidacloprid Soil Drench Followed by Foliar Sprays of Either Cyazypyr or Dinotefuran

Population abundance of F. schultzei (adults+ larvae) was low during this study as in the first study (Table 7). F. schultzei adults + larvae were recorded on all sampling dates in treated and non-treated tomato plants. Imidacloprid applied as a soil drench followed by weekly foliar applications of cyazypyr at the highest rate (20.5 oz/acre) consistently provided significant suppression of F. schultzei population after the first sampling date (10 DAP) compared to the untreated control. However, imidacloprid treatments with low rates of cyazypyr (16.82 and 13.46 oz/acre) showed significant reduction in F. schultzei population after second (15 DAP) and fourth (30 DAP) dates, respectively suggesting that high rate of cyazypyr can provide effective control against this pest. Imidacloprid followed by weekly foliar application of dinotefuran significantly reduced F.schultzei populations after the second sampling date (15 DAP) when compared with the non-treated control.

The mean numbers of GRSV infected plants in various insecticide treatments for first 3 sampling did not differ from the untreated control (Table 8). Fourth sampling date (25 DAP) onwards, mean number of GRSV infected plants were significantly lower in the plots drenched with imidacloprid and sprayed with highest rate of cyazypyr (20.5 oz/acre) than the non-treated control. Imidacloprid followed by 2 lower rates of cyazypyr (13.46 and 16.82 oz/acre) showed significant reduction in GRSV incidence on the fifth sampling date (30 DAP) onward compared to the untreated control. Effect of dinotefuran treatment in suppressing GRSV incidence was slower than cyazypyr treatments and was found to be effective 35 DAP.

Table 4.

Mean numbers of Frankliniella schultzei adults/10 leaf sample of tomato in the six insecticide Treatments in Trial #1.

Table 5.

Mean numbers of Frankliniella schultzei larvae/10-leaf sample of tomato in the various insecticide Treatments in Trial #1.

Table 6.

Progression of groundnut ring spot virus infection of tomato plants treated with various insecticide treatments in Trial #1.

Trial 3: Effectiveness on Pepper of Foliar Applications of the Combination of 3 Insecticides vs. Spinetoram

Table 7.

Mean numbers of Frankliniella schultzei (adults + larvae)/10-leaf sample of tomato treated with Imidacloprid as a soil drench, Cyazypyr by foliar application and Dinotefuran by foliar application in trial #2. Data pertaining to adults and larv ae were combined.

Population abundance of F. schultzei adults was low during this study (Table 9). Adults were recorded on all sampling dates in the non-treated control plants. In this trial, none of the insecticide treatments was found to be effective in regulating F. schultzei population on any sampling date. Mean numbers of adults in the treated plants, irrespective of insecticides, did not differ from the non-treated control on any of the sampling dates. Corresponding to adults' abundance, treatments had no significant effect on the larval abundance in plots (Table 10).

We found that at the low population level of F. schultzei it was still able to spread GRSV (Table 11) to pepper crop. Thus, no significant difference in GRSV incidence was observed in insecticide treated and non-treated control plots.

Discussion

We conducted 3 trials on tomato and pepper using 7 insecticides belonging to 5 IRAC groups. Imidacloprid and dinotefuran are neonicotinoid insecticide belonging to IRAC Group 4A; Cyazypyr and Rynaxypyr are diamide insecticides belonging to IRAC Group 28; Spirotetramat is a Tetronic acid derivative belonging to IRAC Group 23; Spinetoram is a spinosyn insecticide belonging to IRAC Group 5, and Chenopodium amhrosioides is a plant extract that not been assigned to an IRAC Group. These insecticides have modes of action (MOA). Neonico tinoid insecticides are neuro toxic, while the diamide insecticide chlorantraniliprole acts on calcium channels in muscles causing uncontrolled calcium release resulting in muscle contraction. Spiro tetramat is a new insecticide, which inhibits lipid production causing growth inhibition in immatures and reduced reproduction in adults. Spinetoram (active ingredient spinosad) acts in a manner similar to neonicotinoids. Chenopodium ambrosioides is a unique product having multisite mode of action. It degrades the exocuticle and destroys the tracheal lining of soft bodied insects.

In the first trial, none of the treatments provided season long control of F. schultzei adults and larvae. Regardless of the inconsistent effectiveness of insecticides in reducing F. schultzei population, imidacloprid as a plant drench followed by 2 drip applications of diamide insecticides provided significant reduction of GRSV infection. We speculate that increased thrips abundance at the later stages of the study could be due to the movement of thrips population from neighboring vegetation. Imidacloprid alone did not control F. schultzei adults and larvae on tomato, but when imidacloprid treatment was followed by drip application of cyazypyr, greater suppression of the F. schultzei population was achieved. Cyazypyr alone applied at planting inconsistently reduced F. schultzei. However, use of cyazypyr in a program with imidacloprid was more effective than any insecticide applied alone. In the second trial, application of imidacloprid at planting followed by cyazypyr on foliage provided better control of F. schultzei adults and larvae and less transmission of GRSV than soil application of cyazypyr. Also, the higher rate of cyazypyr performed better than the lower rate. In trials involving foliar application, the contact of thrips with the insecticide could be a reason for improved control of thrips and less transmission of the virus. In the last trial conducted on pepper, Spinetoram, Spirotetramat and Chenopodium ambrosioides applied on foliage neither reduced the thrips population nor the transmission of GRSV to the plants. The present study result differs from Dow AgroScience's study in Australia where foliar application of Spinetoram and Spiro tetramat provided significant reduction of F. schultzei population and its transmission of the tomato spotted wilt virus (Dow AgroSciences 2013). In this region, Spinetoram has been used for controlling multiple pests of vegetable, ornamental and fruit crops for more than 15 yr. Due to such prolonged use, reduced efficacy of Spinetoram in controlling melon thrips and other thrips species has been experienced in various studies conducted in South Florida (D.R.S. unpublished data). Thus, in order to delay resistance development in the target thrips species and to keep effective chemistries on the market it is very important to rotate the use various insecticides with different modes of action.

Table 8.

Mean numbers of ground nut ring spot virus (GRSV) infected tomato plants/plot that were treated with Imidacloprid (soil drench), Cyazypyr(foliar application) and Dinotefuran (foliar application) in Trial #2.

Table 9.

Mean numbers in Trial #3 of Frankliniella schultzei adults/20 leaf of ‘bell’ pepper treated with Spirotetramat, Chenopodium ambrosioides, and Spinetoram. All insecticides were sprayed on the foliage.

Table 10.

Mean numbers in Trial #3 of Frankliniella schultzei larvae/20 leaf sample of ‘bell’ pepper treated with spirotetramat, chenopodium ambrosioides, and spinetoram. all insecticides were applied on foliage.

Table 11.

Mean numbers of groundnut ring spot virus infected ‘bell’ pepper plants per plottreated with Spirotetramat, Chenopodium ambrosioides, and Spinetoram in Trial #3.

In summary, diamide insecticide (cyazypyr) showed efficacy in managing F. schultzei. However, diamide insecticides in a program with neonicotinoid (Imidacloprid) provided better management of F. schultzei and GRSV. Foliar application of diamide insecticide was more effective than soil application of the same. Further research should be conducted by using both foliar and soil application of diamide in the same study.

Disclaimer

Federal and Florida laws require that all pesticides must be handled and applied in strict accordance with the label and worker protection standards (re-entry times, protective clothing, etc.). For complete information pertaining to use of any insecticides, follow the label. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the University of Florida.