INSECT CULTURE AND HOST PLANTS

The S. dorsalis population used in various studies was derived from a colony established in 2005 from an infestation on Knock-Out® rose, Rosa hybrid Radrazz in south Florida. The colony was maintained in a greenhouse at the Tropical Research and Education Center, University of Florida, Homestead, Florida (25.50°N, 80.49°W), for several generations on various hosts (i.e., cotton, Gossypium hirsutum L. [Malvaceae]; peanut, Arachis hypogea L. [Fabaceae]; capsicum pepper, Capsicum annum L. [Solanaceae]; and Knock-Out® rose). The pest population was maintained on all hosts during the study by periodically replacing the old plants with young host plants. The host plants for the current studies were grown from seed in seedling trays in Fafard® 2 Mix (Canadian Formula; Sun Gro Horticulture, Agawam, Massachusetts) and placed into a plexiglass screened cage (61 cm length × 91 cm width × 61 cm height) for about 7 wk. Plants in seedling trays were then transplanted into 3.78 L plastic pots. Potted plants were irrigated as needed (about 3 times per wk) and fertilized once each wk with 5 g of nitrogen-phosphorus-potassium (N-P-K): 20-20-20 Multi-Purpose Plus (Diamond R Fertilizer, Fort Pierce, Florida).

THRIPS SPECIES DETERMINATION

The thrips specimens collected from Miami-Dade and Hillsborough counties, Florida; Los Angeles and Orange counties, California; Harris County, Texas; and Barnstable County, Massachusetts, were identified according to morphological characteristics described by Hoddle et al. (2009) using a dissecting microscope at 10× magnification. The detailed information on the thrips samples received is included in Table 1. The identity of thrips specimens was reconfirmed by USDA-APHIS entomologist Thomas Skarlinsky (Thysanoptera specialist in the eastern USA, Miami, Florida). Molecular characterization of thrips samples was done following protocols of Dickey et al. (2015) using the hTPG primer set (heat shock protein 83) at the United States Horticulture Research Laboratory, USDA-Agricultural Research Services, Fort Pierce, Florida (27.43°N, 80.40°W). Before sequencing, the amplified products were cleaned using Montage® PCR clean-up filters (Millipore, Billerica, Massachusetts). A quantity of 50 ng of total thrips nuclear DNA was used in BigDye® sequencing reactions. All sequencing was performed bidirectionally with the amplification primers and BigDye® Terminator v.3.1 cycle sequencing kits (Applied Biosystems, Foster City, California). Sequence reactions were analyzed on an Applied Biosystems 3730XL DNA sequence analyzer and were then compared and edited using Sequencher software (Gene Codes, Ann Arbor, Michigan). Species determination was based on direct sequence comparisons using the webbased National Center for Biotechnology Information BLAST sequence comparison application ( http://blast.ncbi.nlm.nih.gov/Blast.cgi).

Table 1.

Details of the thrips specimens collected from different host plants and locations within the USA.

INSECTICIDE EFFICACY

All studies were conducted either in a greenhouse or in a research field at the Tropical Research and Education Center. Insecticide active ingredient, trade name, and formulations, Insecticide Resistance Action Committee classification, manufacturer, application rates, and application methods used in the greenhouse and field trials are listed in Table 2.

Greenhouse Efficacy Trials

Greenhouse trials were conducted to find one or more alternative to standard products used for thrips control. The experimental trials served to evaluate the efficacy of 10 chemical insecticides and 3 mycoinsecticides against S. dorsalis on pepper when 1) applied directly to the pest population as a curative management approach, and 2) used to cause residual or persistence impact on the incipient thrips population as a preventive management approach. In trial 1, chemical insecticides were evaluated when applied as foliar application; in trial 2, chemical insecticides were evaluated when applied as drench application; and in trial 3, the foliar-applied mycoinsecticides Metarhizium anisopliae (Metschnikoff) Sorokin (Clavicipitaceae), Isaria fumosorosea Wize (Cordycipitaceae), and Beauveria bassiana (Bals.-Criv.) Vuill. (Cordycipitaceae) were evaluated against S. dorsalis. Spinetoram (Radiant ®) was used as a chemical standard in trial 3.

Pepper plants at the incipient stage of thrips infestation but with no thrips damage were selected for the preventive studies, whereas plants with established populations of thrips larvae and adults were used for the curative experiments. The treatments in each of the 3 trials (chemical: foliar and drench; mycoinsecticide: foliar) were arranged in a randomized complete block design wherein treated pots were spaced 0.45 m apart. In various trials, depending upon the availability of pepper plants, treatments were replicated 4, 5, or 12 times for each experiment; each replicate consisted of 3 plants. Treatments were applied at the recommended rate using a small hand-held sprayer delivering 65.5 mL/m² at 211 kPa for foliar insecticides, whereas in the soil drench each pot received about 100 mL solution of each tested material per 3.78 L pot. The leaf turn method (Tomson et al. 2017) was used for adult and larval population estimates, and no leave was removed from the plant. A population estimate per replicate was made by counting the number of larvae and adult thrips on 5 top leaves selected randomly per plant. Treatment assessments in curative experiments (foliar and drench) were made 3, 5, 7, 10, and 15 d after application of treatments, and in preventive experiments (foliar and drench) assessments were made 3, 7, 10, 15, and 20 d after treatment. Because fungal pathogens take time to germinate and show effect, in trial 3 the treatment assessments were done 5, 10, 15, and 20 d after treatment in both curative and preventive experiments.

Table 2.

Commercially formulated insecticides evaluated against Scirtothrips dorsalis on Jalapeno pepper in greenhouse and field trials.

STATISTICAL ANALYSES

Scirtothrips dorsalis data from the greenhouse trials and insecticides rotation field experiments were analyzed independently using a generalized linear mixed model with the SAS® (SAS Institute 2009) procedure GLIMMIX. The model was used to determine the effect of insecticide treatments, sampling period (time), and their interaction on S. dorsalis larval and adult counts, separately (Table 3). Because the response variable was count data with no upper bound, in the model statement distribution was specified as Poisson. The autoregressive correlation structure was applied to account for the correlation in data generated by re-sampling the same experimental unit over time. Differences among treatment means were separated using Fisher's LSD test (α = 0.05) in the repeated measures model. One-way ANOVA (PROC GLM) was used to compare S. dorsalis feeding damage in different treatment plots and to assess the effect of insecticides on number of pepper flowers and fruits. The Abbott formula (Abbott 1925) and the Henderson—Tilton formula (Henderson & Tilton 1955) were used for estimation of thrips mortality in preventive and curative experiments, respectively.

Table 3.

ANOVA statistics for Scirtothrips dorsalis post-treatment populations in the greenhouse and field trials to evaluate various insecticidal treatments applied curatively or preventively on Jalapeno peppers.

THRIPS SPECIES DETERMINATION

The thrips samples collected from different locations in Florida, Texas, and California were S. dorsalis South Asia 1, whereas specimens collected from Massachusetts were East Asia 1 (Table 1). This is the first report of S. dorsalis detected in Massachusetts and the second reported state in the USA (after New York) that has East Asia 1. Sequences obtained from S. dorsalis samples collected from different parts of the USA were deposited in GenBank, and their accession numbers are available in Table 1. Apart from S. dorsalis, the other thrips species that were detected on roses were Frankliniella tritici Fitch and Frankliniella occidentalis Pergande (Thysanoptera: Thripidae).

THRIPS ABUNDANCE IN GREENHOUSE TRIALS

In the greenhouse efficacy trials with chemical insecticides applied to foliage or as a soil drench, both of the main effects (treatment and time) had significant effects on S. dorsalis (larval and adult) densities (Table 3). In the trial with mycoinsecticides, there was a significant effect of treatment on S. dorsalis population, but no significant effect of sampling period was detected, except for larval abundance in the preventive experiment. No significant effect of the treatment*time interaction was observed on thrips populations in any of the experiments (Table 3). Scirtothrips dorsalis population sizes varied greatly; however, significant reductions in thrips numbers were observed in all treatments as compared with the untreated control. For making recommendations to growers, an insecticide was considered effective when mortality exceeded 70% of the thrips life stages during a sampling period.

Field Rotational Program

Overlapping generations of S. dorsalis existed on pepper throughout the study, which explained the significant effects of treatment and sampling period. There was no effect of treatment*time interaction on the abundance of either S. dorsalis larvae or adults (Table 3). Mycoinsecticides and the Chenopodium extract applied alone did not significantly reduce S. dorsalis larval abundance on any of the sampling dates when compared with the untreated control (Fig. 1). However, when rotated with spinetoram, mycoinsecticides and the Chenopodium extract provided a significant suppression of S. dorsalis larvae (compared with the untreated control) on all sampling dates (wk 2: F_11,144 = 2.79, P = 0.0026; wk 4: F_11,144 = 1.58, P = 0.0118; wk 6: F_11,144 = 2.14, P = 0.0207), and the performance of rotational treatments did not differ from spinetoram applied alone (mortality >90%) (Table 7). Similar results for S. dorsalis adults were observed where the performance of mycoinsecticides and Chenopodium extract each applied alone was not pronounced (Fig. 2), and spinetoram alone or in rotation with mycoinsecticides performed better and provided >85% mortality of thrips adults on all the sampling dates (Table 7). Significant suppression of S. dorsalis adults compared with the untreated control was observed in all the spinetoram-treated plots (applied alone or in rotation) in wk 2 (F_11,144 = 1.95, P = 0.0380) and wk 6 (F_11,144 = 1.59, P = 0.0169).

All treatments applied alone or in rotation significantly reduced S. dorsalis feeding damage on pepper when compared with the untreated control (Table 7). Mean numbers of flowers were significantly higher in all treated plots (except I. fumosorosea [NoFly™]) than in the untreated plots (Table 7). Plants treated with the Chenopodium extract and mycoinsecticides in rotation with spinetoram showed more flowers per treated plot than plants treated with a mycoinsecticide alone. As was the case with flowers, mean numbers of fruits before harvest (irrespective of size and age) were significantly greater on plants treated with Chenopodium extract and each of the mycoinsecticides in rotation with spinetoram than on plants in treatments without spinetoram (Table 7). Plants treated with spinetoram alone had significantly more flowers and fruits than plants in any other treatments.

Table 4.

Results of foliar applications of chemical insecticides. Mean numbers of Scirtothrips dorsalis larvae and adults on 5 leaves per plant in each replicate.

Table 5.

Results of drench applications of chemical insecticides. Mean numbers of Scirtothrips dorsalis larvae and adults on 5 leaves per plant in each replicate.

Table 6.

Results of foliar applications of mycoinsecticides. Mean numbers of Scirtothrips dorsalis larvae and adults on 5 leaves per plant in each replicate.

SCIRTOTHRIPS DORSALIS SPECIES COMPLEX IN THE INFESTED REGIONS

The small size (<1.5 mm), thigmotactic behavior, and adaptability of this thrips to a wide range of climatic conditions help explain its high invasiveness and on exotic pest lists of many countries. Results of our previous study (Dickey et al. 2015) confirmed that S. dorsalis South Asia 1 has invaded at least 2 agriculturally important states of the USA, namely Florida and Texas, and the current study confirmed its presence in California. The existence of established populations of South Asia 1 on multiple hosts in S. dorsalis infested regions within the USA suggests that this species is more prevalent compared with another member of the S. dorsalis species complex, S. dorsalis East Asia 1, which has only been reported twice in the past 5 yr on the same host (hydrangea) in 2 neighboring eastern states (New York and Massachusetts). Based on the invasion history of members of this thrips complex around the globe, it appears that South Asia 1 is a thrips with great invasion potential, unlike East Asia 1, which apparently displays a much smaller invasion potential (Dickey et al. 2015). To this date, S. dorsalis South Asia 1 occurs in India, Pakistan, Bangladesh, Myanmar, Japan, Vietnam, Thailand, Israel, and the USA. Within a few years of its introduction in Florida, S. dorsalis (later characterized to be South Asia 1) was established from the southern to the central region of Florida (Kumar et al. 2013), and Osborne (2008) reported damage on >50 plant taxa that included bedding plants, cut flowers, foliage plants, and perennials (mostly ornamental hosts). In south Florida, South Asia 1 was found on 11 fruit hosts, some of which had never been reported before as reproductive host, suggesting that the host range of this insidious pest is continuing to expand as it invades new regions (Kumar et al. 2012).

INSECTICIDE PERFORMANCE

Synthetic insecticide applications have always been the primary mode of pest management amongst ornamental growers because of the low damage threshold of ornamental plants and zero tolerance for export items. Because S. dorsalis is a serious pest of ornamental plants in many counties of Florida and Texas, and an emerging pest in California, determination of materials that can be integrated in the current management practices for this pest is of paramount importance. The current study was initiated when local nurseries in south and central Florida had severe infestations of S. dorsalis on multiple ornamental and tropical fruit crops. To reduce impacts on various hosts, we emphasized the evaluation of potentially suitable insecticides with different modes of action that could be used as curative and preventive measures for the suppression of S. dorsalis South Asia 1, and some of these materials were assessed for rotational use.

In the greenhouse trials, we evaluated 10 chemical insecticides (14 formulations) representing 5 modes of action as defined by the Insecticide Resistance Action Committee (IRAC), and 3 mycoinsecticides. We found a range of products that were effective in regulating S. dorsalis South Asia 1 populations. To make recommendations to growers, insecticides resulting in >70% thrips mortality were considered effective, and their properties are summarized in Table 8. Based on the results, insecticides belonging to IRAC groups 4A: acetamiprid (Assail®), clothianidin (Belay®), and thiamethoxam (Actara®); 4D: flupyradifurone (Sivanto™); 21A: tolfenpyrad (Hachi-Hachi®); 28: cyantraniliprole (Cyazypyr™); and a premix formulation of 4A and 28: thiamethoxam + chlorantraniliprole (Voliam Flexi®) can be recommended to growers for use as foliar sprays in suppressing adult and larval S. dorsalis populations for >7 d. Among these insecticides, similar efficacy during drench application was observed only for imidacloprid (Admire® Pro), whereas other products were effective only in controlling thrips larvae. For the purpose of preventing populations from developing (prophylactic measures), foliar applications of insecticides belonging to IRAC groups 4A: thiamethoxam, clothianidin; 4D: flupyradifurone; 21A: tolfenpyrad; 28: cyantraniliprole; and a premix formulation of 4A and 28: thiamethoxam + chlorantraniliprole, and drench application of flupyradifurone and imidacloprid, can be recommended.

Fig. 1.

Mean numbers of Scirtothrips dorsalis larvae per 10 leaf samples of Jalapeno pepper treated with different insecticides. Solid lines represent treatments where 1 insecticide was applied alone, and dashed lines show treatments of 2 insecticides applied in rotation. Same color solid and dashed lines represent same insecticide applied alone or in rotation with spinetoram.

Foliar applications of spinetoram (Radiant®) (IRAC group 5) can be used in both curative and preventive management programs for S. dorsalis, because it was effective for more than 10 d and was one of the few insecticides that eliminated the S. dorsalis population from treated plants during the study period. Similar results on the efficacy of spinetoram for S. dorsalis management were reported by Seal & Kumar (2010), indicating it is still one of the most effective tools growers have for management of S. dorsalis.

Table 7.

Mortality of Scirtothrips dorsalis larvae and adults on Jalapeno pepper treated with various regimes of insecticides.

Thrips mortality due to mycoinsecticides was lower than that caused by the chemical standard (spinetoram), but mycoinsecticides provided suppressive control ranging from 47 to 70%, 55 to 83%, and 68 to 86% when used as a curative measure in 3 treatments involving B. bassiana (mean larval mortality: 54.7%; adult mortality: 67.7%), M. anisopliae (larval mortality: 60.7%; adult mortality: 78.7%), or I. fumosorosea (larval mortality: 72.7%; adult mortality: 84.0%), respectively. Greater thrips mortalities in curative applications than in preventive applications could be due to the active spores on the upper leaf surface (adaxial side) and their spread (via conidia or blastospores) from affected individuals to the unaffected individuals on both abaxial and adaxial leaf surfaces. However, in preventive applications, the mortality occurs only due to the residual spores present on the leaf surfaces, which can dehydrate or become inviable before the insects occur, resulting in low rates of infection in arriving thrips.

Fig. 2.

Mean numbers of Scirtothrips dorsalis adults per 10 leaf samples of Jalapeno pepper treated with different insecticides. Solid lines represent treatments where 1 insecticide was applied alone, and dashed lines show treatments of 2 insecticides applied in rotation. Same color solid and dashed lines represent same insecticide applied alone or in rotation with spinetoram.

Greater mortality among larvae than adults may be explained by adults being able to escape mycoinsecticide treatments, unlike thrips larvae, which roam around the spore-treated plant surface and have a higher probability of becoming exposed to the spores than adults. Our results indicate that mycoinsecticides are better suited for curative than preventive applications. Among the 3 mycoinsecticides tested, I. fumosorosea was the most effective against S. dorsalis, suggesting that in the greenhouse and nursery environments, I. fumosorosea can be efficiently integrated into management of this pest, and can serve as an important tool for organic growers.

In its native region, S. dorsalis populations have been found to be resistant to a range of chemical classes including organochlorine (dichlorodiphenyltrichloroethane, benzene hexachloride, and endosulfan), organophosphate (acephate, dimethoate, phosalone, methylo-demeton, and triazophos), and carbamate insecticides (carbaryl) (Reddy et al. 1992; Sridhar & Rani 2003; Vanisree et al. 2011). Thus, it is imperative to evaluate newer classes of insecticides and integrate them in a management program to minimize the risk of the progressive assembly of genes for resistance through selection.

Thiamethoxam is a neonicotinoid precursor that is converted to clothianidin in insects and plants (Nauen et al. 2003). It is a second generation neonicotinoid compound that disrupts the binding of the insect neurotransmitter acetylcholine at its nicotinic acetylcholine receptors present at the post-synaptic cell junctures. Thiamethoxam is a broad-spectrum insecticide known for its activity against many pests, including thrips (Teague et al. 1999; Jacobson & Hara 2003; Aslam et al. 2004; Larral & Ripa 2007; Seal & Kumar 2010; McKenzie et al. 2015). Chlorantraniliprole and cyantraniliprole are novel anthranilic diamide insecticides that selectively bind to the ryanodine receptors in muscle cells, resulting in the uncontrolled release of calcium stores (Lahm et al. 2005). They cause depletion of calcium, feeding cessation, lethargy, muscle paralysis, and ultimately death of target organisms (Cordova et al. 2006; Lahm et al. 2007). Flupyradifurone is the first member of the newly created IRAC subgroup 4D (Nauen et al. 2015), which belongs to the butenolide chemical class. It contains a novel bioactive scaffold isolated from the medicinal plant Stemona japonica (Blume) Miq. (Stemonaceae). Tolfenpyrad is a broad-spectrum contact insecticide with a pyrazole carboxamide structure. It acts as a mitochondrial electron transport inhibitor and affects respiration of the target pests resulting in rapid insecticidal responses such as cessation of feeding, cessation of movement, and lack of fecundity. From the current study, it was apparent that the S. dorsalis South Asia 1 population was susceptible to several groups of insecticides, among which the older insecticides acetamiprid, clothianidin, and imidacloprid had a long residual impact. So, to keep these chemicals in the marketplace, it is important to use them in rotation programs. The premix insecticide chlorantraniliprole + thiamethoxam showed promising results against larval and adult stages, but such insecticides should be used with care, as the non-judicious use of such premixes can result in the selection of resistance to 2 mode of action classes. In the experiments where foliar-applied (nonsystemic) insecticides such as spinetoram and tolfenpyrad were used as drench applications, effective suppression of the thrips population was observed for the next few days. We speculate that such thrips suppression could be due to the movement of thrips to the soil in search of food and in preparation for pupation, leading to direct and indirect contact with the product, causing death.

Table 8.

Choices of insecticides for rotational use against Scirtothrips dorsalis South Asia 1. An insecticide was considered effective when mortality exceeded 70% for thrips life stages.

In the insecticide rotation field trial, we found that the Chenopodium extract and the 3 mycoinsecticides when used alone did not provide effective suppression of S. dorsalis South Asia 1 populations; consequently, flower and fruit numbers in these plots were sparse, relative to the chemical standard (spinetoram). But when alternated with spinetoram, the effectiveness of these treatments was comparable to the spinetoram treatment. This finding suggests that the use of such effective chemistries can be reduced by half or even more by incorporating mycoinsecticides in the thrips management program, provided weather conditions are conducive to mycoinsecticides spore germination and growth. The use of chemical applications for thrips management can also be reduced by integrating biological control agents such as Amblyseius swirskii Athias-Henriot (Mesostigmata: Phytoseiidae) and Orius insidiosus (Say) (Hemiptera: Anthocoridae), which have been found effective in regulating several thrips species in both protected and open field conditions in Florida agroecosystems (Arthurs et al. 2009; Srivastava et al. 2014; Kakkar et al. 2016). The older chemistries tested in this study (acetamiprid, imidacloprid) have been reported to have moderate to harmful impact on aforementioned 2 thrips predators (Studebaker & Kring 2003; Cloyd & Bethke 2011; Srivastava et al. 2014; Roubos et al. 2014; Biobest 2017), which further encourages the integration of novel groups with low toxicity to natural enemies in the thrips management program. Further research needs to be conducted to develop the best management program to reduce the application frequency of spinetoram, including its integration with the use of mycoinsecticides.

In conclusion, our limited survey records from the S. dorsalis infested regions confirmed that in the USA, South Asia 1 has a wider distribution than East Asia 1. At present, multiple insecticides are available from different chemical classes for regulating their populations. However, to maintain their efficacy for a long period, growers should consider rotating insecticides with different modes of action. We believe that this study can serve as a springboard for implementing management strategies for S. dorsalis South Asia 1 and help growers to integrate insecticides in the preventive and curative tactics used for thrips management in the affected regions.